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A
MANUAL
ELEMENTARY GEOLOGY.
By the same Author.
THE PRINCIPLES OF GEOLOGY; or, the Moprrn CHANGES of the Earta and its INHABITANTS, as illustrative of Geology. Ninth and thoroughly revised Edition. With Woodcuts. 8vo. 18s.
TRAVELS IN NORTH AMERICA: Canapa and Nova Scotia.
With GEOLOGICAL OBSERVATIONS. Second Edition. Maps and Plates. 2 vols. Post 8vo. 12s.
A SECOND VISIT TO NORTH AMERICA. Third Edition. 2vols. Post 8vo. 12s.
Lonpon: A. and G. A. SPOTTISWOODE, New-street-Square.
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MANUAL
OF
ELEMENTARY GEOLOGY:
OR,
THE ANCIENT CHANGES OF THE EARTH AND ITS INHABITANTS
AS ILLUSTRATED BY GEOLOGICAL MONUMENTS.
+
By SIR CHARLES LYELL, M.A. F.R.S.
AUTHOR OF “PRINCIPLES OF GEOLOGY,” ETC.
“Tt is a philosophy which never rests — its law is progress: a point which yesterday was invisible is its goal to-day, and will be its starting-post to-morrow.” EDINBURGH Review, July, 1887.
\ AMMONITE. TRILOBITE.
TERTIARY. SECONDARY. PRIMARY.
FIFTH EDITION, GREATLY ENLARGED, AND ILLUSTRATED WITH 750 WOODCUTS.
7
LONDON: JOHN MURRAY, ALBEMARLE STREET. © 1856.
The right of translation is reserved.
PREFACE TO THE FIFTH EDITION.
It is now more than three years since the appearance of the last Edition of the Manual (published January, 1851). In that interval the science of Geology has been advancing as usual at a rapid pace, making it desirable to notice many new facts and opinions, and to consider their bearing on the previ- ously acquired stock of knowledge. In my attempt to bring up the information contained in this Treatise to the present state of the science, I have added no less than 200 new Illus- trations and 140 new pages of Text, which, if printed separately and in a less condensed form, might have constituted alone a volume of respectable size. To give in detail a list of all the minor corrections and changes would be tedious; but I have thought it useful, in order to enable the reader of former editions to direct his attention at once to what is new, to | offer the following summary of the more important additions and alterations.
Principal Additions and Alterations in the present Edition.
Cumar. IX. —“ The general Table of Fossiliferous strata,” for- merly placed at the end of Chapter XXVII, is now given at P. 105., that the beginner may accustom himself from the first to refer to it from time to time when studying the numerous sub- divisions into which it is now necessary to separate the chrono- logical series of rocks. The Table has been enlarged by a column of Foreign Equivalents, comprising the names and localities of some of the best known strata in other countries of contemporaneous date with British Formations.
Cumar. XIV.—XVI. — The classification of the Tertiary formations has been adapted to the information gained by me during a tour made in the summer of 1851 in France and Belgium. The results of my survey were printed in the Quarterly Journal of the Geological
A 3
vi PREFACE TO THE FIFTH EDITION.
Society of London for 1852. In the course of my investigations I enjoyed opportunities of determining more exactly the relations of the Antwerp and the Suffolk crag, p. 174.; the stratigraphical place of the Bolderberg beds near Hasselt, p. 179.; that of the Limburg or Kleyn Spawen strata, p. 189.; and of other Belgian and French deposits. In reference to some of these, the questions so much con- troverted of late, whether certain groups should be called Lower Miocene or Upper Eocene, are fully discussed, p. 184. et seg.
Tn the winter of 1852, I had the advantage of examining the north- ern part of the Isle of Wight, in company with my friend the late lamented Professor Edward Forbes, who pointed out to me the discoveries he had just made in regard to the true position of the Hempstead series (pp. 186 —193.), recognized by him as the equi- valent of the Kleyn Spawen or Limburg beds, and his new views in regard to the relation of various members of the Eocene series between the Hempstead and Bagshot beds. An account of these discoveries, with the names of the new subdivisions, is given at pp. 209. et seg.; the whole having been revised when in print by Edward Forbes.
The position assigned by Mr. Prestwich to the Thanet sands, as an Eocene formation inferior to the Woolwich beds, is treated of at p. 222., and the relations of the Middle and Lower Eocene of France to various deposits in the Isle of Wight and Hampshire at p. 223. et seg. In the same chapters, many figures have been introduced of characteristic organic remains, not given in previous editions.
Cuar. XVIL—In speaking of the Cretaceous strata, I have for the first time alluded to the position of the Pisolitic Limestone in France, and other formations in Belgium intermediate between the White Chalk and Thanet beds, p. 236.
Cuar. XVILIIL.—The Wealden beds, comprising the Weald Clay and Hastings Sands apart from the Purbeck, are in this chapter for the first time considered as belonging to the Lower Cretaceous Group, and the reasons for the change are stated at p. 264.
Cuar. XIX.— Relates to “the denudation of the Weald,” or of the country intervening between the North and South Downs. It has been almost entirely rewritten, and some new illustrations in- troduced. Many geologists have gone over that region again and again of late years, bringing to light new facts, and speculating on the probable time, extent, and causes of so vast a removal of rock. I have endeavoured to show how numerous have been the periods of denudation, how vast the duration of some of them, and how little the necessity to despair of solving the problem by an appeal to ordi- nary causation, or to invoke the aid of imaginary catastrophes and paroxysmal violence, pp. 272—291.
Cuar. XX.—XXI.—On the strata from the Oolite to the Lias inclusive. The Purbeck beds are here for the first time considered
PREFACE TO THE FIFTH EDITION. vil
as the uppermost member of the Oolite, in accordance with the opinions of the late Professor E. Forbes, p. 295. Many new figures of fossils characteristic of the subdivisions of the three Purbecks are introduced; and the discovery, in 1854, of a new mammifer alluded to, p. 296.
Representations also of fossils of the Upper, Middle, and Lower Oolite, and of the Lias, are added to those before given.
Cuar. XXI.—XXIIIL.— On the Triassic and Permian forma- tions. The improvements consist chiefly of new illustrations of fossil remains.
Cumar. XXIV.—XXV.— Treating of the Carboniferous group, I have mentioned the subdivisions now generally adopted for the classification of the Irish strata (p. 362.), and I have added new figures of fossil plants to explain, among other topics, the botanical characters of Calamites, Sternbergia, and Trigonocarpum, and their relation to Conifere (pp. 367,868, 371.). The grade also of the Conifers in the vegetable kingdom, and whether they hold a high or a low position among flowering plants, is discussed with reference to the opinions of several of the most eminent living botanists; and the bearing of these views on the theory of progressive development, p. 373.
The casts of rain-prints in coal-shale are represented in several woodcuts as illustrative of the nature and humidity of the carboni- ferous atmosphere, p. 384. The causes also of the purity of many seams of coal, p. 3885., and the probable length of time which was required to allow the solid matter of certain coal-fields to accumulate, p. 386., are discussed for the first time.
_ Figures are given of Crustaceans and Insects from the Coal, pp. 388, 389.; and the discovery of some new Reptiles is alluded to, p. 405.
_ T have also alluded to the causes of the rarity of vertebrate and invertebrate air-breathers in the coal, p. 405.
That division of this same chapter (Chap. XXV.) which relates to the Mountain Limestone has been also enlarged by figures of new fossils, and among others by representations of Corals of the Paleo- zoic, as distinguishable from those of the Neozoic, type, p- 407.3 also by woodcuts of several genera of shells which retain the patterns of their original colours, p. 410. The foreign equivalents of the Mountain Limestone are also alluded to, p. 413.
Cuar. XXVI.—In speaking of the Old Red Sandstone, or De- vonian Group, the evidence of the occurrence of the skeleton of a Reptile and the footprints of a Chelonian in that series are recon- sidered, p.416. New plants found in Treland in this formation are figured, p. 418.; also the Pterygotus, or large crustacean of Forfar- shire, p. 419. ; and, lastly, the division of the Devonian series in North Devon into Upper, Middle, and Lower, p. 424., the fossils of
A 4
Vill PREFACE TO THE FIFTH EDITION.
the same (p. 425. e¢ seq.), and the equivalents of the Devonian beds in Russia and the United States, are treated of, p. 429. and 432.
Cuar. XXVII.—The classification and nomenclature of the Si- lurian rocks of Great Britain, the Continent of Europe, and North America, and the question whether they can be distinguished from the Cambrian, and by what paleontological characters, are discussed in this chapter, pp. 483. 451. and 457.
The relation of the Caradoc Sandstone to the Upper and Lower Silurian, as inferred from recent investigations (p. 441.), the vast thickness of the Llandeilo or Lower Silurian in Wales (p. 446.), the Obolus or Ungulite grit of St. Petersburg and its fossils (p. 447.), the Silurian strata of the United States and their British equivalents (p. 448.), and those of Canada, the discoveries of M. Barrande re- specting the metamorphosis of Silurian and Cambrian trilobites (pp. 445. 454.), are among the subjects enlarged upon more fully than in former editions, or now treated of for the first time.
The Cambrian beds below the Llandeilo, and their fossils, are like- wise described as they exist in Wales, Ireland, Bohemia, Sweden, the United States, and Canada, and some of their peculiar organic remains are figured, p. 451. to p. 457.
Lastly, at the conclusion of the chapter, some remarks are offered respecting the absence of the remains of fish and other vertebrata from the deposits below the Upper Silurian, p. 457., in elucidation of which topic a Table has been drawn up of the dates of the successive discovery of different classes of Fossil Vertebrata in rocks of higher and higher antiquity, showing the gradual progress made in the course of the last century and a half in tracing back each class to more and more ancient rocks. The bearing of the positive and negative facts thus set forth on the doctrine of progressive develop- ment is then discussed, and the grounds of the supposed scarcity both of vertebrate and invertebrate air-breathers in the most ancient formation considered, p. 460.
Cumar. XXVIII. — With the assistance of an able mineralogist, M. Delesse, I have revised and enlarged the glossary of the more abundant volcanic rocks, p. 476., and the table of analyses of simple minerals, p. 479.
Cuar. XXIX.—In consequence of a geological excursion to Madeira and the Canary Islands, which I made in the winter of 1853-4, I have been enabled to make larger additions of original matter to this chapter than to any other in the work. The account of Teneriffe and Madeira, pp. 514. 522., is wholly new. Formerly I gave an abstract of Von Buch’s description of the island of Palma, one of the Canaries, but I have now treated of it more fully from my own observations, regarding Palma as a good type of that class of volcanic mountains which have been called by Von Buch “craters of elevation,” pp. 498—512. Many illustrations, chiefly from the pencil of my companion and fellow-labourer, Mr. Hartung, have been introduced. In reference to the above-mentioned sub-
PREFACE TO THE FIFTH EDITION. `
jects, citations are made from Dana on the Sandwich Islands, P. 493., and from Junghuhn’s Java, p. 496.
Cmar. XXXV.—XXXVII.— The theory of the origin of the metamorphic rocks and certain views recently put forward by some geologists respecting cleavage and foliation have made it desirable to recast and rewrite a portion of these chapters. New proofs are cited in favour of attributing cleavage to mechanical force, p. 610., and for inferring in many cases a connection between foliation and cleavage, p. 615. At the same time, the question—how far the planes of foliation usually agree with those of sedimentary depo- sition, is entered into, p. 614.
Cuar. XXXVIII.—To the account formerly published of mineral veins some facts and opinions are added respecting the age of the rocks and alluvial deposits containing gold in South America, the United States, California, and Australia.
I have already alluded to the assistance afforded me by the late Professor Edward Forbes towards the improvement of some parts of this work. His letters suggesting corrections and additions were continued to within a few weeks of his sudden and unexpected death, and I felt most grateful to him for the warm interest, which, in the midst of so many and pressing avocations, he took in the success of my labours. His friendship and the power of referring to his sound judgment in ~ cases of difficulty on palontological and other questions were among the highest privileges I have ever enjoyed in the course of my scientific pursuits. Never perhaps has it been the lot of any Englishman, who had not attained to political or literary eminence, more especially one who had not reached his fortieth year, to engage the sympathies of so wide a circle of admirers, and to be so generally mourned. The untimely death of such a teacher was justly felt to be a national loss; for there was a deep conviction in the minds of all who knew him, that genius of so high an order, combined with vast acquirements, true independence of character, and so many social and moral ex- cellencies, would have inspired a large portion of the rising generation with kindred enthusiasm for branches of knowledge hitherto neglected in the education of British youth.
As on former occasions, I shall take this opportunity of stating that the “‘ Manual” is not an epitome of the “ Principles of Geology,” nor intended as introductory to that work. So much confusion has arisen on this subject, that it is desirable
xX PREFACE TO THE FIFTH EDITION.
to explain fully the different ground occupied by the two pub- lications. ‘The first five editions of the “ Principles” comprised a 4th book, in which some account was given of systematic geo- logy, and in which the principal rocks. composing the earth’s crust and their organic remains were described. In subsequent editions this 4th book was omitted, it having been expanded, in 1838, into a separate treatise called the “ Elements of Geo- logy,” first re-edited in 1842, and again recast and enlarged in 1851, and entitled « A Manual of Elementary Geology.” Of this enlarged work another edition, called the Fourth, was published in 1852.
Although the subjects of both treatises relate to Geology, as their titles imply, their scope is very different ; the “ Principles ” containing a view of the modern changes of the earth and its inhabitants, while the “‘ Manual” relates to the monuments of ancient changes. In separating the one from the other, I have endeavoured to render each complete in itself, and independent ; but if asked by a student which he should read. first, I would recommend him to begin with the “ Principles,” as he may then proceed from the known to the unknown, and be provided beforehand with a key for interpreting the ancient phenomena, whether of the organic or inorganic world, by reference to changes now in progress.
Tt will be seen on comparing “ The Contents” of the “ Prin- ciples” with the abridged headings of the chapters of the pre- sent work (see the following pages), that the two treatises have but little in common; or, to repeat what I have said in the Preface to the “Principles,” they have the same kind of con- nection which Chemistry bears to Natural Philosophy, each being subsidiary to the other, and yet admitting of being con- sidered as different departments of science.*
CHARLES LYELL. 53. Harley Street, London, February 22. 1855.
* As it is impossible to enable the reader to recognize rocks and minerals at sight by aid of verbal descriptions or figures, he will do well to obtain a well- arranged collection of specimens, such as may be procured from Mr. Tennant (149. Strand), teacher of Mineralogy at King’s College, London. :
CONTENTS.
CHAPTER I.— On the different Classes of Rocks.
Geology defined — Successive formation of the earth’s crust — Classification of rocks according to their origin and age— Aqueous rocks — Volcanic rocks — Plutonic rocks — Metamorphic rocks— The term primitive, why erroneously applied to the crystalline formations - = a i wi e “ Page 1
CHAPTER TI. — Aqueous Rocks — Their Composition and Forms of Stratification.
Mineral composition of strata— Arenaceous rocks — Argillaceous — Calcareous — Gypsum — Forms of stratification — Diagonal arrangement — Ripple-mark - 10
CHAPTER II. — Arrangement of Fossils in Strata — Freshwater and Marine. `
Limestones formed of corals and shells — Proofs of gradual increase of strata derived from fossils — Tripoli and semi-opal formed of infusoria — Chalk derived principally from organic bodies— Distinction of freshwater from marine formations — Alter- nation of marine and freshwater deposits = - - - - 21
CHAPTER IV. — Consolidation of Strata and Petrifaction of Fossils.
Chemical and mechanical deposits — Cementing together of particles — Concretionary nodules — Consolidating effects of pressure — Mineralization of organic remains — Impressions and casts how formed— Fossil wood— Source of lime and silex in solution - - ES - - - - - - 33
Cuarrer V.— Elevation of Strata above the Sea — Horizontal and Inclined Stratification. _
Position of marine strata, why referred to the rising up of the land, not to the going down of the sea — Upheaval of horizontal strata — Inclined and vertical stratification — Anticlinal and synclinal lines — Theory of folding by lateral movement — Creeps —Dip and strike — Structure of the Jura — Inverted position of disturbed strata — Unconformable stratification — Fractures of strata — Faults - = - 4
CHAPTER VI.— Denudation.
Denudation defined — Its amount equal to the entire mass of stratified deposits in the earth’s crust — Levelled surface of countries in which great faults occur — Denuding power of the ocean — Origin of Valleys — Obliteration of sea-cliffs — Inland sea-cliffs and terraces - - -~ - - - - z re
CHAPTER VII.— Alluvium.
Alluvium described — Due to complicated causes — Of various ages — How distin- guished from rocks in situ — River-terraces — Parallel roads of Glen Roy - 79
Cuaprer VIII.— Chronological Classification of Rocks.
Aqueous, plutonic, volcanic, and metamorphic rocks, considered chronologically — Lehman’s division into primitive and secondary — Werner's addition of a transition class — Neptunian theory — Hutton on igneous origin of granite—The name of “primary” for granite and the term “transition” why faulty — Chronological no-
Menclature adopted in this work, so far as regards primary, secondary, and ter- tiary periods Sy 3 z K - - $ - - 89
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xii CONTENTS.
CHAPTER IX.— On the different Ages of the Aqueous Rocks.
On the three tests of relative age — superposition, mineral character, and fossils — Change of mineral character and fossils in the same formation — Proofs that distinct species of animals and plants have lived at successive periods — Distinct provinces of indigenous species — Similar laws prevailed at successive geological periods— Test of age by included fragments — Frequent absence of strata of intervening periods — General Table of Fossiliferous strata - - - - Page 96
CHAPTER X.— Classification of Tertiary Formations. — Post Pliocene Group.
General principles of classification of tertiary strata — Difficulties in determining their chronology — Increasing proportion of living species of shells in strata of newer origin — Terms Eocene, Miocene, and Pliocene — Post-Pliocene recent strata - ET i
CHAPTER XI. — Newer Pliocene Period. — Boulder Formation.
Drift of Scandinavia, northern Germany, and Russia — Fundamental rocks polished, grooved, and scratched — Action of glaciers and icebergs — Fossil shells of glacial period—Drift of eastern Norfolk—Ancient glaciers of North Wales—Irish drift - 121
CHAPTER XII. — Boulder Formation — continued.
Effects of intense cold in augmenting the quantity of alluvium — Analogy of erratics and scored rocks in North America, Europe, and Canada — Why organic remains so rare in northern drift— Many shells and some quadrupeds survived the glacial cold— Alps an independent centre of dispersion of erratics — Meteorite in Asiatic drift - - ne - = - * > - 131
Cuaprer XIII. — Newer Pliocene Strata and Cavern Deposits.
Pleistocene formations — Freshwater deposits in valley of Thames — In Norfolk cliffs — In Patagonia — Comparative longevity of species in the mammalia and testacea — Crag of Norwich — Newer Pliocene strata of Sicily — Osseous breccias and cavern- deposits — Sicily — Kirkdale — Australian cave-breccias — Relationship of geogra- phical provinces of living vertebrata and those of Pliocene species— Teeth of fossil quadrupeds = - A = = s z - = 146
CHAPTER XIV. — Older Pliocene and Miocene Formations.
Red and Coralline crags of Suffolk — Fossils, and proportion of recent species — Depth of sea, and climate — Migration of many species of shells southwards during the gla- cial period — Antwerp crag — Subapennine beds — Miocene formations — Faluns of Touraine — Depth of sea and littoral character of fauna — Climate — Proportion of recent species of shells — Miocene strata of Bordeaux, Belgium, and North Germany — Older Pliocene and Miocene formations in the United States — Sewalik Hills in India 3 = - - - - à a - 161
Crapter XV.— Upper Eocene Formations. (Lower Miocene of many authors.)
Remarks on classification, and on the line of separation between Eocene and Miocene — Whether the Limburg strata in Belgium should be called Upper Eocene— Strata of same age in North Germany — Mayence basin — Brown Coal of Germany— Upper Eocene of Isle of Wight —Of France —Lacustrine strata of Auvergne and the Cantal — Upper Eocene of Bordeaux, &c. — Of Nebraska, United States ü - 184
Cyuapter XVI. — Middle and Lower Eocene Formations.
‘Middle Eocene strata of England — Fluvio-marine series in the Isle of Wight and "Hampshire — Successive groups of Eocene Mammalia — Fossils of Barton Clay — Of the Bagshot and Bracklesham beds— Lower Eocene strata of England — London Clay proper—Strata of Kyson in Suffolk — Fossil monkey and opossum — Plastic clays and sands — Thanet sands — Middle and Lower Eocene formations of France — Nummulitic formations of Europe and Asia — Eocene strata at Claiborne, Alabama — Colossal cetacean — Orbitoid limestone — Burr stone - $ 6 - 208
CONTENTS.
CHAPTER XVII. — Cretaceous Group.
Lapse of time between the Cretaceous and Eocene periods — Formations in Belgium and France of intermediate age — Pisolitic limestone — Divisions of the Cretaceous series in North-Western Europe — Maestricht beds — Chalk of Faxoe — White chalk —How far derived from shells and corals — Chalk flints— Fossils of the Upper Cretaceous rocks — Upper Greensand and Gault — Chalk of South of Europe— Hip- purite limestone — Cretaceous rocks of the United States - = Page 235
CHAPTER XVIII. — Lower Cretaceous and Wealden Formations.
Lower Greensand — Term “Neocomian”— Fossils of Lower Greensand — Wealden formation — Weald Clay and Hastings Sand — Fossil shells and fish — Their relation to the Cretaceous type — Flora of Lower Cretaceous and Wealden periods - ~ 257
CHAPTER XIX. — Denudation of the Chalk and Wealden.
Physical geography of certain districts composed of Cretaceous and Wealden strata — Lines of inland chalk-cliffs on the Seine in Normandy — Denudation of the chalk and wealden in Surrey, Kent, and Sussex — Chalk once continuous from the North to the South Downs — Rise and denudation of the strata gradual— At what period the _ Weald valley was denuded, and by what causes — Elephant-bed, Brighton — San- gatte cliff—Conclusion ~ - - - - - - - 268
CHAPTER XX.— Jurassic Group.— Purbeck Beds and Oolite.
The Purbeck beds a member of the Upper Oolite — New fossil Mammifer— Dirt-bed — Fossils of the Purbeck beds — Portland stone and fossils — Middle Oolite — Coral Rag — Zoophytes — Nerinzan limestone — Diceras limestone — Oxford Clay, Ammonites and Belemnites— Lower Oolite, Crinoideans — Great Oolite—Stonesfield Slate — Fossil mammalia — Yorkshire Oolitic coal-field — Brora coal — Fuller’s Earth — In- ferior Oolite and fossils - = - = - - ~ - 292
CHAPTER XXI. — Jurassic Group, continued. — Lias.
Mineral character of Lias— Fossil shells and fish — Radiata — Ichthyodorulites — Reptiles — Ichthyosaur and Plesiosaur — Fluvio-marine beds in Gloucestershire, and Insect limestone — Fossil plants — Origin of the Oolite and Lias — Oolitic coal-field of Virginia a - - 5 - - - - - 818
Cuarrer XXII. — Trias or New Red Sandstone Group.
Distinction between New and Old Red Sandstone— The Trias and its three divisions in Germany — Keuper and its fossils— Muschelkalk and fossils — Fossil plants of the Bunter — Triassic group in England — Footsteps of Cheirotherium — Osteology of the Labyrinthodon — Triassic mammifer — Origin of Red Sandstone and Rock-salt — New Red Sandstone in the United States — Fossil footprints of birds and reptiles in the valley of the Connecticut - - - = - - - 334
Cuaprer XXIII.— Permian or Magnesian Limestone Group.
Fossils of Magnesian Limestone — Term Permian — English and German equivalents — Marine shells and corals—Palwoniscus and other fish—Thecodont saurians— Permian Flora — Its generic affinity to the carboniferous— Psaronites or tree- ferns - - - - ~- ~ ~ s n - 853
Cuaprer XXIV. — The Coal, or Carboniferous Group.
Carboniferous strata in England — Coal-measures and Mountain limestone — Carboni- ferous series in Ireland and South Wales — Underclays with Stigmaria — Carboni-
———
=e se RSE Sa
xiv CONTENTS. |
ferous Flora — Ferns, Lepidodendra, Calamites, Sigillariee — Conifer — Sternbergia — Trigonocarpon — Grade of Conifere in the Vegetable Kingdom — Absence of Angiosperms — Coal, how formed — Erect fossil trees — Rain-prints — Purity of the Coal explained —Time required for its accumulation — Crustaceans and insects
Page 361 Cuaprer XXV.— Carboniferous Group — continued.
Coal-fields of the United States — Section of the country between the Atlantic and Mississippi — Uniting of many coal-seams into one thick bed — Vast extent and continuity of single seams of coal — Ancient river-channel in Forest of Dean coal- field — Climate of Carboniferous period — Insects in coal — Great number of fossil fish — First discovery of the skeletons of fossil reptiles — First land-shell of the Coal found — Rarity of air-breathers, whether vertebrate or invertebrate, in Coal-measures — Mountain limestone— Its corals and marine shells - “ m - 391
Cuarrer XXVI. — Old Red Sandstone or Devonian Group.
Old Red Sandstone of the borders of Wales — Scotland and the South of Ireland — Fossil reptile of Elgin — Fossil Devonian plants at Kilkenny — Ichthyolites of Clashbinnie — Fossil fish, &c., crustaceans, of Caithness and Forfarshire — Distinct lithological type of Old Red in Devon and Cornwall—Term “Devonian ”—Devonian series of England and the Continent — Old Red Sandstone of Russia — Devonian strata of the United States - - - - - - ~ 415
CHAPTER XXVII.— Silurian and Cambrian Groups.
Silurian strata formerly called “ Transition ” — Subdivisions — Ludlow formation and fossils — Ludlow bone-bed, and oldest known remains of fossil fish — Wenlock form- - ation, corals, cystideans, trilobites — Caradoc sandstone —Pentameri and Tentaculites — Lower Silurian, rocks — Llandeilo flags — Cystideæ — Trilobites — Graptolites — Vast thickness of Lower Silurian strata in Wales — Foreign Silurian equivalents in Europe — Ungulite grit of Russia — Silurian strata of the United States — Canadian equivalents — Deep-sea origin of Silurian strata — Fossiliferous rocks below the Llandeilo beds — Cambrian group — Lingula flags— Lower Cambrian — Oldest known fossil remains — “ Primordial group” of Bohemia — Metamorphosis of trilo- bites— Alum schists of Sweden and Norway — Potsdam sandstone of United States and Canada — Trilobites on the Upper Mississippi — Supposed period of invertebrate animals — Absence of fish in Lower Silurian — Progressive discovery of vertebrata in older rocks — Doctrine of the non-existence of vertebrata in the older fossiliferous periods premature - - - - - ~ -= - 433
Cuarrer XXVII. — Volcanic Rocks.
Trap rocks — Name, whence derived — Their igneous origin at first doubted — Their general appearance and character — Mineral composition and texture — Varieties of felspar — Hornblende and augite — Isomorphism — Rocks, how to be studied — Basalt, trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuff — Agglomerate — Laterite — Alphabetical list, and explanation of names and synonyms of volcanic rocks — Table of the analyses of minerals most abundant in the volcanic and hypo- gene rocks x = x rs R x - 464
CHAPTER XXIX. — Volcanic Rocks—continued.
Trap dikes— Strata altered at or near the contact — Conversion of chalk into marble — Trap interposed between strata — Columnar and globular structure — Relation of trappean rocks to the products of active voleanos— Form, external structure, and origin of volcanic mountains— Craters and Calderas — Sandwich Islands — Lava flowing underground — Truncation of cones — Javanese Calderas— Canary Islands — Structure and origin of the caldera of Palma — Aqueous conglomerate in Palma — Hypothesis of upheaval considered — Slope on which stony lavas may form—
CONTENTS. -ORV
Island of St. Paul in the Indian Ocean — Peak of Teneriffe, and ruins of older cone — Madeira — Its volcanic rocks, partly of marine, and partly of subaerial origin — Central axis of eruptions— Varying dip of solid lavas near the axis, and further from it — Leaf-bed and fossil land-plants — Central valleys of Madeira how formed
Page 480
CHAPTER XXX. — On the Different Ages of the Volcanic Rocks.
Tests of relative age of volcanic rocks — Test by superposition and intrusion — Test by alteration of rocks in contact— Test by organic remains — Test of age by mineral cha- racter — Test by included fragments— Volcanic rocks of the Post-Pliocene period — _ Basalt. of Bay of Trezza in Sicily — Post-Pliocene volcanic rocks near Naples—
Dikes of Somma — Igneous formations of the Newer Pliocene period — Val di Noto in Sicily - S i X < = - - - - 523
Cuaprer XXXI. — On the different. Ages of the Volcanic Rocks—continued.
Volcanic rocks of the Older Pliocene period — Tuscany — Rome — Volcanic region of Olot in Catalonia — Cones and lava-currents — Miocene period — Brown-coal of the Eifel and contemporaneous trachytic rocks — Age of the brown-coal — Peculiar cha- racters of the volcanos of the Upper and Lower Eifel—Lake craters — Trass— Hun- garian volcanos - - - * - - - -~ = 585
Cuarrer XXXII. — On the different Ages of the Volcanie Rocks — continued,
Volcanic rocks of the Pliocene and Miocene periods continued — Auvergne — Mont Dor — Breccias and alluviums of Mont Perrier, with bones of quadrupeds — Mont Dome --Cones not denuded by general flood — Velay — Bones of quadrupeds buried in | scoriæ — Cantal — Eocene volcanic rocks — Tuffs near Clermont — Hill of Gergovia —
. Trap of Cretaceous period — Oolitic period —New Red Sandstone period — Carboni- ferous period —Old Red Sandstone’ period — Silurian period — Cambrian volcanic rocks ~ - - - = - - y LES SA - 550
i
CuarteR XXXIII. — Plutonic Rocks — Granite.
General aspect of granite — Analogy and difference of volcanic and plutonic formations — Minerals in granite— Mutual penetration of crystals of quartz and felspar— Syenitic, talcose, and schorly granites — Eurite— Passage of granite into trap — Granite veins in Glen Tilt, and other countries — Composition of granite veins — Metalliferous veins in strata near their junction with granite — Quartz veins — Whe- ther plutonic rocks are ever overlying—Their exposure at the surface due tO denudation = - - - - - - - - 565
CHAPTER XXXIV. — On the different Ages of the Plutonic Rocks.
Difficulty in ascertaining the age of a plutonic rock — Test of age by relative position — Test by intrusion and alteration — Test by mineral composition — Test by included fragments — Recent and Pliocene plutonic rocks, why invisible — Tertiary plutonic rocks in the Andes— Granite altering Cretaceous rocks— Granite altering Lias — Granite altering Carboniferous strata — Granite of the Old Red Sandstone period — Syenite altering Silurian strata in Norway — Oldest plutonic rocks—Granite pro- truded in a solid form — Age of the granites of Arran, in Scotland = - 579
CHAPTER XXXV. — Metamorphic Rocks.
General character of metamorphic rocks — Gneiss —-Hornblende-schist —Mica-schist — Clay-slate — Quartzite — Chlorite-schist — Metamorphic limestone — Alphabetical list and explanation of the more abundant rocks of this family — Origin of the metamorphic strata — Their stratification — Fossiliferous strata near intrusive masses of granite converted into different members of the metamorphic series — Objections to the metamorphic theory considered — Partial conversion of Eocene slate into &neiss < - - - - - 2 E Bie - - 594
`
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CONTENTS.
CuartER XXXVI. — Metamorphic Rocks —continued.
Origin of the metamorphic rocks, continued — Definition of joints, slaty cleavage, and foliation — Causes of these structures — Mechanical theory of cleavage —Supposed combination of crystalline and mechanical forces — Lamination of some volcanic rocks due to motion — Whether the foliation of the crystalline schists be usually parallel with the original planes of stratification - - - Page 607
Carrer XXXVII.— On the different Ages of the Metamorphic Rocks,
Age of each set of metamorphic strata twofold — Test of age by fossils and mineral character not available — Test by superposition ambiguous — Conversion of fossili- ferous strata into metamorphic rocks — Limestone and shale of Carrara — Metamor-. phic strata older than the Cambrian rocks — Others of Lower Silurian origin —Others of the Jurassic and Eocene periods — Why scarcely any of the visible crystalline strata are very modern — Order of succession in metamorphic rocks — Uniformity of mineral character —Why the metamorphic. strata are less calcareous than the fossiliferous - - 2 ž n i - 618
Cuaprer XXXVIII. — Mineral Veins.
Werner’s doctrine that mineral veins were ‘fissures filled from above — Veins of segre- gation — Ordinary metalliferous veins or lodes — Their frequent coincidence with faults — Proofs that they originated in fissures in solid rock — Veins shifting other veins— Polishing of their walls or “slicken-sides ” — Shells and pebbles in lodes — Evidence of the successive enlargement and reopening of veins — Why some veins alternately swell out and contract — Filling of lodes by sublimation from below —. Chemical and electrical action — Relative age of the precious metals — Copper and lead veins in Ireland older than Cornish tin — Lead veins in Lias, Glamorganshire— Gold in Russia, California, and Australia — Connection of hot springs and mineral veins — Concluding remarks - - - - ~- - - 626
f
| ry 4 p FET tE? EE HA
EASO
MANUAL
OF
ELEMENTARY GHOLOGY,
CHAPTER I. ON THE DIFFERENT CLASSES OF ROCKS.
‘Geology defined — Successive formation of the earth’s crust— Classification of rocks according to their origin and age — Aqueous rocks— Their stratification and im- bedded fossils — Volcanic rocks, with and without cones and craters — Plutonic rocks, and their relation to the voleanic— Metamorphic rocks, and their probable origin — The term primitive, why erroneously applied to the crystalline formations — Leading division of the work.
OF what materials is the earth composed, and in what manner are these materials arranged? ‘These are the first inquiries with which Geology is occupied, a science which derives its name from the Greek Yñ, ge, the earth, and Noyoe, logos, a discourse. Previously to experience we might have imagined that investigations of this kind would relate exclusively to the mineral kingdom, and to the various rocks, soils, and metals, which occur upon the surface of the earth, or at various depths beneath it. But, in pursuing such researches, we soon find ourselves led on to consider the successive changes which have taken place in the former state of the earth’s surface and interior, and the Causes which have given rise to these changes; and, what is still More singular and unexpected, we soon become engaged in researches into the history of the animate creation, or of the various tribes of animals and plants which have, at different periods of the past, in- habited the globe.
All are aware that the solid parts of the earth consist of distinct Substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like; but previously to observation it is commonly imagined that all these had remained from the first in the state in which we now see them,—that they were created in their present form, and in their present position. The geologist soon comes to a different con- clusion, discovering proofs that the external parts of the earth were not all produced in the beginning of things in the state in which we now behold them, nor in an instant of time. On the contrary, he ĉan show that they have acquired their actual configuration and con- dition gradually, under a great variety of circumstances, and at suc- cessive periods, during each of which distinct races of living beings
B
PE PEON ? : SRC rag a ns
TE
2 CLASSIFICATION OF ROCKS, [Cr. I.
have flourished on the land and in the waters, the remains of these creatures still lying buried in the crust of the earth,
By the “earth’s crust,” is meant that small portion of the exterior of our planet which is accessible to human observation, or on which we are enabled to reason by observations made at or near the surface. These reasonings may extend to a depth of several miles, perhaps ten miles; and even then it may be said, that such a thickness is no more than zby part of the distance from the surface to the centre. The remark is just; but although the dimensions of such a crust are, in truth, insignificant when compared to the entire globe, yet they are vast, and of magnificent extent in relation to man, and to the or- ganic beings which people our globe. Referring to this standard of magnitude, the geologist may admire the ample limits of and admit, at the same time, that not only the exterior o but the entire earth, is but an atom in the midst of t worlds surveyed by the astronomer.
The materials of this crust are not thrown together confusedly ; but distinct mineral masses, called rocks, are found to occupy definite spaces, and to exhibit a certain order of arrangement. The term rock is applied indifferently by geologists to all these substances, whether they be soft or stony, for clay and sand are included in the term, and some have even brought peat under this denomination, Our older writers endeavoured to avoid offering such violence to our language, by speaking of the component materials of the earth as consisting of rocks and soils. But there js often so insensible a pas- sage from a soft and incoherent state to that of stone, that geologists of all countries have found it indispensable to have one technical term to include both, and in this sense we find roche applied in French, rocca in Italian, and Jelsart in German. The beginner, however, must constantly bear in mind, that the term rock by no means implies that a mineral mass is in an indurated or stony con- dition.
The most natural and convenient mode of classifying the various rocks which compose the earth’s crust, is to refer, in the first place, to their origin, and in the second to their relative age. I shall therefore begin by endeavouring briefly to explain to the student how all rocks may be divided into four great classes by reference to their different origin, or, in other words, by reference to the different circumstances and causes by which they have been produced,
- The first two divisions, which will at once be understood ag natural, are the aqueous and volcanic, or the products of watery and those of igneous action at or near the surface.
Aqueous rocks.—The aqueous rocks, sometimes called the sedi- mentary, or fossiliferous, cover a larger part of the ea than any others. These rocks are stratified, or divided into distinct layers, or strata. The term stratum means simply a bed, or any thing spread out or strewed over a given surface ; and we infer that these strata have been generally spread out by the action of w from what we daily sec taking place near the mouths of rivers,
his domain, f the planet, he countless
rth’s surface
ater, or on
Cx. L] AQUEOUS ROCKS. 3
the land during temporary inundations. For, whenever a running Stream charged with mud or sand, has its velocity checked, as when It enters a lake or sea, or overflows a plain, the sediment, previously held in suspension by the motion of the water, sinks, by its own gravity, to the bottom. In this manner layers of mud and sand are thrown down one upon another.
If we drain a lake which has been fed by a small stream, we fre- quently find at the bottom a series of deposits, disposed with consi- derable regularity, one above the other; the uppermost, perhaps, may be a stratum of peat, next below a more dense and solid variety of the same material; still lower a bed of shell-marl, alternating with Peat or sand, and then other beds of marl, divided by layers of clay: Now, if a second pit be sunk through the same continuous lacustrine formation, at some distance from the first, nearly the same series of beds is commonly met with, yet with slight variations; some, for ex- ample, of the layers of sand, clay, or marl, may be wanting, one or more of them having thinned out and given place to others, or some- times one of the masses first examined is observed to increase in thickness to the exclusion of other beds.
The term “ formation,” which I have used in the above explana- tion, expresses in geology any assemblage of rocks which have some character in common, whether of origin, age, or composition. Thus we speak of stratified and unstratified, freshwater and marine, aqueous and volcanic, ancient and modern, metalliferous and non-metallifer- ous formations.
Tn the estuaries of large rivers, such as the Ganges and the Missis- sippi, we may observe, at low water, phenomena analogous to those of the drained lakes above mentioned, but on a grander scale, and extending over areas several hundred miles in length and breadth. When the periodical inundations subside, the river hollows out a channel to the depth of many yards through horizontal beds of clay and sand, the ends of which are seen exposed in perpendicular cliffs. These beds vary in their mineral composition, or colour, or in the fineness or coarseness of their particles, and some of them are occa- sionally characterized by containing drift wood. At the junction of the river and the sea, especially in lagoons nearly separated by sand bars from the ocean, deposits are often formed in which brackish- water and salt-water shells are included.
The annual floods of the Nile in Egypt are well known, and the fertile deposits of mud which they leave on the plains. This mud is stratified, the thin layer thrown down in one season differing slightly in colour from that of a previous year, and being separable from it, as has been observed in excavations at Cairo, and other places.*
When beds of sand, clay, and marl, containing shells and vegetable matter, are found arranged in a similar manner in the interior of the earth, we ascribe to them a similar origin ; and the more we examine their characters in minute detail, the more exact do we find the re- semblance. ‘Thus, for example, at various heights and, depths in the
* See Principles of Geology, by the Author, Index, “ Nile,” « Rivers,” &e. + B 2
ENE PT NIT “aH iia:
ee
4 AQUEOUS ROCKS.
earth, and often far from seas, lakes, and rivers, we meet with layers of rounded pebbles composed of flint, limestone, granite, or other rocks, resembling the shingles of a sea-beach or the gravel in a torrent’s bed. Such layers of pebbles frequently alternate with others formed of sand or fine sediment, just as we may see in the channel of a river descending from hills bordering a coast, where the current sweeps down at one season coarse sand and gravel, while at another, when the waters are low and less rapid, fine mud and sand alone are carried seaward.*
If a stratified arrangement, and the rounded form of pebbles, are alone sufficient to lead us to the conclusion that certain rocks origi- nated under water, this opinion is farther confirmed by the distinct and independent evidence of fossils, so abundantly included in the earth’s crust. By a fossil is meant any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes. Now the remains of animals, especially of aquatic species, are found almost everywhere imbedded, in stratified rocks, and sometimes, in the ease of limestone, they are in such abundance as to constitute the entire mass of the rock itself, Shells and corals are the most frequent, and with them are often associated the bones and teeth of fishes, fragments of wood, im- pressions of leaves, and other organic substances. Fossil shells, of forms such as now abound in the sea, are met with far inland, both near the surface, and at great depths below it. They occur at all heights above the level of the ocean, having been observed at eleva- tions of more than 8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above 18,000 feet in the Himalaya.+
These shells belong mostly to marine testacea, but in some places exclusively to forms characteristic of lakes and rivers. Hence it is concluded that some ancient strata were deposited at the bottom of the sea, and others in lakes and estuaries.
When geology was first cultivated, it was a general belief, that these marine shells and other fossils were the effects and proofs of the deluge of Noah; but all who have carefully investigated the phenomena have long rejected this doctrine. A transient flood might be supposed to leave behind it, here and there upon the surface, scattered heaps of mud,.sand, and shingle, with shells confusedly in- termixed ; but the strata containing fossils are not superficial depo- sits, and do not simply cover the earth, but constitute the entire mass of mountains. Nor are the fossils mingled without reference to the original habits and natures of the creatures of which they are the memorials ; those, for example, being found associated together which lived in deep or in shallow water, near the shore or far from it, in brackish or in salt water.
It has, moreover, been a favourite notion of some modern writers, who were aware that fossil bodies could not all be referred to the deluge, that they, and the strata in which they are entombed, might
* See p. 18. fig. 7. + Capt. R. J. Strachey found oolitic fossils 18,400 feet high in the Himalaya.
Cu. LJ VOLCANIC ROCKS. 5
have been deposited in the bed of the ocean during the period which, intervened between the creation of man and the deluge. They have imagined that the antediluvian bed of the ocean, after having been’ the receptacle of many stratified deposits, became converted, at the time of the flood, into the lands which we inhabit, and that the ancient continents were at the same time submerged, and became the bed of the present seas. This hypothesis, although preferable to the diluvial theory before alluded to, since it admits that all fossiliferous Strata were successively thrown down from water, is yet wholly inadequate to explain the repeated revolutions which the earth has undergone, and the signs which the existing continents exhibit, in most regions, of having emerged from the ocean at an era far more remote than four thousand years from the present time. Ample proofs of these reiterated revolutions will be given in the sequel, and it will be seen that many distinct sets of sedimentary strata, hundreds and sometimes thousands of feet thick, are piled one upon the other in the earth’s crust, each containing peculiar fossil animals and plants of species distinguishable for the most part from all those now living. The mass of some of these strata consists almost entirely of corals, others are made up of shells, others of plants turned into coal, while some are without fossils. In one set of strata the species of fossils are marine; in another, lying immediately above or below, they as clearly prove that the deposit was formed in a lake or in a brackish estuary. When the student has more fully examined into these appearances, he will become convinced that the time required for the origin of the rocks composing the actual continents must have been far greater than that which is conceded by the theory above alluded to; and likewise that no one universal or sudden Conversion of sea into land will account for geological appearances.
We have now pointed out one great class of rocks, which, however they may vary in mineral composition, colour, grain, or other cha- racters, external and internal, may nevertheless be grouped together as having a common origin. They have all been formed under water, in the same manner as modern accumulations of sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all characterised by stratification or fossils, or by both.
Volcanic rocks. — The division of rocks which we may next con- Sider are the voleanic, or those which have been produced at or near the surface whether in ancient or modern times, not by water, but by the action of fire or subterranean heat. These rocks are for the most part unstratified, and are devoid of fossils. They are more par- tially distributed than aqueous formations, at least in respect to hori- zontal extension. Among those parts of Europe where they exhibit characters not to be mistaken, I may mention not only Sicily and the country round Naples, but Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire, and Ardéche, towards the centre and south of France, in which are several hundred conical hills having the forms of modern volcanos, with craters more or less perfect on many of their summits. These cones are composed more>
B 3
6 VOLCANIC ROCKS. [Cu. I.
over of lava, sand, and ashes, similar to those of active volcanos. Streams of lava may sometimes be traced from the cones into the adjoining valleys, where they have choked up the ancient channels of rivers with solid rock, in the same manner as some modern flows of lava in Iceland have been known to do, the rivers either flowing peneath or cutting out a narrow passage on one side of the lava. Although none of these French volcanos have been in activity within the period of history or tradition, their forms are often very perfect. Some, however, have been compared to the mere skeletons of vol- canos, the rains and torrents having washed their sides, and removed all the loose sand and scorie, leaving only the harder and more solid materials. By this erosion, and by earthquakes, their internal struc- ture has occasionally been laid open to view, in fissures and ravines ; and we then behold not only many successive beds and masses of porous lava, sand, and scoriz, but also perpendicular walls, or dikes, as they are called, of volcanic rock, which have burst through the other materials. Such dikes are also observed in the structure of Vesuvius, Etna, and other active volcanos. They have been formed by the pouring of melted matter, whether from above or below, into open fissures, and they commonly traverse deposits of volcanic tuff; a substance produced by the showering down from the air, or in- cumbent waters, of sand and cinders, first shot up from the interior of the earth by the explosions of volcanic gases.
Besides the parts of France above alluded to, there are other countries, as the north of Spain, the south of Sicily, the Tuscan territory of Italy, the lower Rhenish provinces, and Hungary, where spent volcanos may be seen, still preserving in many cases a conical form, and having craters and often lava-streams connected with them.
There are also other rocks in England, Scotland, Ireland, and almost every country in Europe, which we infer to be of igneous origin, although they do not form hills with cones and craters. Thus, for example, we feel assured that the rock of Staffa, and that of the Giant’s Causeway, called basalt, is volcanic, because it agrees in its columnar structure and mineral composition with streams of lava which we know to have flowed from the craters of volcanos. We find also similar basaltic and other igneous rocks associated with beds of tuff in various parts of the British Isles, and forming dikes, such as have been spoken of; and some of the strata through which these dikes cut are occasionally altered at the point of contact, as if they had been exposed to the intense heat of melted matter.
The absence of cones and craters, and long narrow streams of superficial lava, in England and many other countries, is principally to be attributed to the eruptions having been submarine, just as a considerable proportion of volcanos in our own times burst out beneath the sea. But this question must be enlarged upon more fully in the chapters on Igneous Rocks, in which it will also be shown, that as different sedimentary formations, containing each their characteristic fossils, have been deposited at successive periods, so also volcanic sand and scoriæ have been thrown out, and lavas
Cx. I] PLUTONIC ROCKS. 7
have flowed over the land or bed of the sea, at many different epochs,
or have been injected into fissures; so that the igneous as well as - the aqueous rocks may be classed as a chronological series of monu-
ments, throwing light on a succession of events in the history of the
earth.
Plutonic rocks (Granite, &c.).—We have now pointed out the existence of two distinct orders of mineral masses, the aqueous and the volcanic: but if we examine a large portion of a continent, especially if it contain within it a lofty mountain range, we rarely fail to discover two other classes of rocks, very distinct from either of. those above alluded to, and which we can neither assimilate to de- posits such as are now accumulated in lakes or seas, nor to those generated by ordinary volcanic action. The members of both these divisions of rocks agree in being highly crystalline and destitute of organic remains. The rocks of one division have been called plu- tonic, comprehending all the granites and certain porphyries, which are nearly allied in some of their characters to volcanic formations. The members of the other class are stratified and often slaty, and have been called by some the erystalline schists, in which group are included gneiss, micaceous-schist (or mica-slate), hornblende-schist, statuary marble, the finer kinds of roofing slate, and other rocks afterwards to be described.
As it is admitted that nothing strictly analogous to these crystalline productions can now be seen in the progress of formation on the earth’s surface, it will naturally be asked, on what data we can find a place for them in a system of classification founded on the origin of rocks, I cannot, in reply to this question, pretend to give the student, in a few words, an intelligible account of the long chain of facts and reasonings by which geologists have been led to infer the analogy of the rocks in question to others now in progress at the surface. The result, however, may be briefly stated. All the various kinds of granite which constitute the plutonic family, are supposed to be of igneous origin, but to have been formed under great pressure, at a considerable depth in the earth, or sometimes, perhaps, under a certain weight of incumbent water. Like the lava of volcanos, they have been melted, and have afterwards cooled and crystallised, but with extreme slowness, and under conditions very different from those of bodies cooling in the open air. Hence they differ from the volcanic rocks, not only by their more crystalline texture, but also by the absence of tuffs and breccias, which are the products of eruptions at the earth’s surface, or beneath seas of inconsiderable depth. They differ also by the absence of pores or cellular cavities, to which the expansion of the entangled gases gives rise in ordinary lava.
Although granite has often pierced through other strata, it has rarely, if ever, been observed to rest upon them, as if it had over- flowed. But as this is continually the case with the volcanic rocks, they have been styled, from this peculiarity. “ overlying” by Dr. Mac Culloch; and Mr. Necker has proposed the term “underlying ” for
B4
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| | | | f iF |
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8 METAMORPHIC ROCKS.
the granites, to designate the opposite mode in which they almost invariably present themselves.
Metamorphic, or stratified crystalline rocks. —The fourth and last great division of rocks are the crystalline strata‘and slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other three classes. They contain no pebbles, or sand, or scorie, or angular pieces of imbedded stone, and no traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corre- sponding in form and arrangement to those of sedimentary formations, and are therefore said to be stratified. The beds sometimes consist of an alternation of substances varying in colour, composition, and thickness, precisely as we see in stratified fossiliferous deposits. Ac- cording to the Huttonian theory, which I adopt as the most probable, and which will be afterwards more fully explained, the materials of these strata were originally deposited from water in the usual form of sediment, but they were subsequently so altered by subterranean heat, as to assume a new texture. It is demonstrable, in some cases at least, that such a complete conversion has actually taken place, fossiliferous strata having exchanged an earthy for a highly erys- talline texture for a distance of a quarter of a mile from their contact with granite. In some cases, dark limestones, replete with shells and corals, have been turned into white statuary marble, and hard clays, containing vegetable or other remains, into slates called mica-schist or hornblende-schist, every vestige of the organic bodies having been obliterated.
Although we are in a great degree ignorant of the precise nature of the influence exerted in these cases, yet it evidently bears some analogy to that which volcanic heat and gases are known to pro- duce ; and the action may be conveniently called plutonic, because it appears to have been developed in those regions where plutonic rocks are generated, and under similar circumstances of pressure and depth in the earth. Whether hot water or steam permeating stratified masses, or electricity, or any other causes have co-operated to produce the crystalline texture, may be matter of speculation, but it is clear that the plutonic influence has sometimes pervaded entire mountain masses of strata.
In accordance with the hypothesis above alluded to, I proposed in the first edition of the Principles of Geology (1833), the term “Metamorphic” for the altered strata, a term derived from pera, meta, trans, and poppn, morphe, forma.
Hence there are four great classes of rocks considered in reference to their origin,—the aqueous, the volcanic, the plutonic, and the metamorphic. In the course of this work it will be shown, that portions of each of these four distinct classes have originated at many successive periods. They have all been produced contem- poraneously, and may even now be in the progress of formation on a large scale. It is not true, as was formerly supposed, that all granites, together with the crystalline or metamorphic strata, were first formed,
Cu. 1] FOUR CLASSES OF ROCKS CONTEMPORANEOUS. 9
and therefore entitled to be called “ primitive,” and that the aqueous and volcanic rocks were afterwards super-imposed, and should, there- fore, rank as secondary in the order of time. This idea was adopted 1n the infancy of the science, when all formations, whether stratified or unstratified, earthy or crystalline, with or without fossils, were alike regarded as of aqueous origin. At that period it was naturally argued, that the foundation must be older than the superstructure ; but it was afterwards discovered, that this opinion was by no means ™m every instance a legitimate deduction from facts; for the inferior parts of the earth’s crust have often been modified, and even entirely changed, by the influence of volcanic and other subterranean causes, while Super-imposed formations have not been in the slightest degree altered. In other words, the destroying and renovating processes. have given birth to new rocks below, while those above, whether crystalline or fossiliferous, have remained in their ancient condition. “ven in cities, such as Venice and Amsterdam, it cannot be laid down as universally true, that the upper parts of each edifice, whether of brick or marble, are more modern than the foundations on which they rest, for these often consist of wooden piles, which may have Totted and been replaced one after the other, without the least injury to the buildings above; meanwhile, these may have required scarcely any repair, and may have been constantly inhabited. So it is with the habitable surface of our globe, in its relation to large masses of rock immediately below: it may continue the same for ages, while sub- jacent materials, at a great depth, are passing from a solid to a fluid State, and then reconsolidating, so as to acquire a new texture.
As all the crystalline rocks may, in some respects, be viewed as belonging to one great family, whether they be stratified or un- Stratified, plutonic or metamorphic, it will often be convenient to Speak of them by one common name. It being now ascertained, as
. above stated, that they are of very different ages, sometimes newer than the strata called secondary, the terms primitive and primary which were formerly used for the whole must be abandoned, as they would imply a manifest contradiction. It is indispensable, therefore, to find a new name, one which must not be of chronological import, and must express, on the one hand, some peculiarity equally attribu- table to granite and gneiss (to the plutonic as well as the altered rocks), and, on the other, must have reference to characters in which those rocks differ, both from the volcanic and from the unaltered Sedimentary strata. I proposed in the Principles of Geology (first edition, vol, iii.), the term “hypogene” for this purpose, derived from ùro, under, and yopa to be, or to be born; a word implying the theory that granite, gneiss, and the other crystalline formations are alike netherformed rocks, or rocks which have not assumed their Present form and structure at the surface. They occupy the lowest place in the order of superposition. Even in regions such as the Alps, where some masses of granite and gneiss can be shown to be of com- paratively modern date, belonging, for example, to the period here- after to be described as tertiary, they are still underlying rocks,
NB ONS ET MEF
E
10 ; COMPONENTS OF STRATA. (Ca. IT.
They never repose on the volcanic or trappean formations, nor on strata containing organic remains. They are hypogene, as “ being under” all the rest.
From what has now been said, the reader will understand that each of the four great classes of rocks may be studied under two distinct points of view; first, they may be studied simply as mineral masses deriving their origin from particular causes, and having a certain composition, form, and position in the earth’s crust, or other characters both positive and negative, such as the presence or absence of organic remains. In the second place, the rocks of each class may be viewed as a grand chronological series of monuments, attesting a succession of events in the former history of the globe and its living inhabitants.
I shall accordingly proceed to treat of each family of rocks; first, in reference to those characters which are not chronological, and then in particular relation to the several periods when they were formed.
CHAPTER II.
AQUEOUS ROCKS — THEIR COMPOSITION AND FORMS OF STRATIFI- CATION.
Mineral composition of strata— Arenaceous rocks — Argillaceous — Calcareous — Gypsum — Forms of stratification — Original horizontality — Thinning out — Dia- gonal arrangement — Ripple mark.
In pursuance of the arrangement explained in the last chapter, we shall begin by examining the aqueous or sedimentary rocks, which are for the most part distinctly stratified, and contain fossils. We may first study them with reference to their mineral composition, external appearance, position, mode of origin, organic contents, and other characters which belong to them as aqueous formations, inde- pendently of their age, and we may afterwards consider them chrono- logically or with reference to the successive geological periods when they originated.
I have already given an outline of the data which led to the belief that the stratified and fossiliferous rocks were originally deposited under water; but, before entering into a more detailed investigation, it will be desirable to say something of the ordinary materials of which such strata are composed. These may be said to belong principally to three divisions, the arenaceous, the argillaceous, and the calca- reous, which are formed respectively of sand, clay, and carbonate of lime. Of these, the arenaceous, or sandy masses, are chiefly made up of siliceous or flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter, with a certain proportion, about a fourth in weight, of aluminous earth; and, lastly, the calcareous rocks or limestones consist of carbonic.acid and lime.
11
Arenaceous or siliceous rocks. — To speak first of the sandy divi- sion: beds of loose sand are frequently met with, of which the grains consist entirely of silex, which term comprehends all purely siliceous minerals, as quartz and common flint. Quartz is silex in its purest form, Flint usually contains some admixture of alumine and oxide of iron. The siliceous grains in sand are usually rounded, as if by the action of running water. Sandstone is an aggregate of such grains, which often cohere together without any visible cement, but more commonly are bound together by a slight quantity of siliceous or calcareous matter, or by iron or clay.
Pure siliceous rocks may be known by not effervescing when a drop of nitric, sulphuric or other acid is applied to them, or by the grains not being readily scratched or broken by ordinary pressure. In nature there is every intermediate gradation, from perfectly loose sand, to the hardest sandstone. In micaceous sandstones mica is very abundant; and the thin silvery plates into which that mineral divides, are often arranged in layers parallel to the planes of strati- fication, giving a slaty or laminated texture to the rock.
When sandstone is coarse-grained, it is usually called grit. If the grains are rounded, and large enough to be called pebbles, it becomes a conglomerate or pudding-stone, which may consist of pieces of one or of many different kinds of rock. A conglomerate, therefore, is Simply gravel bound together by a cement.
Argillaceous rocks. —Clay, strictly speaking, is a mixture of silex or flint with a large proportion, usually about one fourth, of alumine, or argil; but in common language, any earth which possesses suffi- cient ductility, when kneaded up with water, to be fashioned like paste by the hand, or by the potter’s lathe, is called a clay; and such clays vary greatly in their composition, and are, in general, nothing more than mud derived from the decomposition or wearing down of rocks. The purest clay found in nature is porcelain clay, or kaolin, which results from the decomposition of a rock composed of felspar and quartz, and it is almost always mixed with quartz.* Shale has also the property, like clay, of becoming plastic in water: it is a more Solid form of clay, or argillaceous matter, condensed by pressure. It usually divides into lamine more or less regular.
One general character of all argillaceous rocks is to give out a peculiar, earthy odour when breathed upon, which is a test of the presence of alumine, although it does not belong to pure alumine, but, apparently, to the combination of that substance with oxide of iron. i
Calcareous rocks. — This division comprehends those rocks which, like chalk, are composed chiefly of lime and carbonic acid. Shells
Cu. II.] MINERAL COMPOSITION OF STRATIFIED ROCKS.
and corals are also formed of the
* The kaolin of China consists of 71°15 parts of silex, 15°86 of alumine, 1°92 of lime, and 6°73 of water (W. Phillips, Mineralogy, p. 33.); but other porcelain Clays differ materially, that of Cornwall being composed, according to Boase, of
same elements, with the addition
nearly equal parts of silica and alumine, with 1 per cent. of magnesia. (Phil. Mag. vol. x. 1837.)
T See W. Phillips’s Mineralogy, “ Alu- mine.”
=
k | 17 f i
ine i an EN ets cee, E ra REDS
12 MINERAL COMPOSITION OF STRATIFIED ROCKS. [Cz. IL
of animal matter. To obtain pure lime it is necessary to calcine these calcareous substances, that is to say, to expose them to heat of sufficient intensity to drive off the carbonic acid, and other volatile matter. White chalk is sometimes pure carbonate of lime; and this rock, although usually in a soft and earthy state, is occasionally sufficiently solid to be used for building, and even passes into a compact stone, or a stone of which the separate parts are so minute as not to be distinguishable from each other by the naked eye.
Many limestones are made up entirely of minute fragments of shells and coral, or of calcareous sand cemented together. These last might be called “calcareous sandstones ;” but that term is more properly applied to a rock in which the grains are partly calcareous and partly siliceous, or to quartzose sandstones, having a cement of carbonate of lime.
The variety of limestone called “ oolite ” is composed of numerous small egg-like grains, resembling the roe of a fish, each of which has usually a small fragment of sand as a nucleus, around which con- centric layers of calcareous matter have accumulated.
Any limestone which is sufficiently hard to take a fine polish is called marble. Many of these are fossiliferous ; but statuary marble, which is also called saccharine limestone, as having a texture re- sembling that of loaf-sugar, is devoid of fossils, and is in many cases a member of the metamorphic series.
Siliceous limestone is an intimate mixture of carbonate of lime and flint, and is harder in proportion as the flinty matter predominates.
The presence of carbonate of lime ina rock may be ascertained by applying to the surface a small drop of diluted sulphuric, nitric, or muriatic acids, or strong vinegar ; for the lime, having a greater chemical affinity for any one of these acids than for the carbonie, unites immediately with them to form new compounds, thereby be- coming a sulphate, nitrate, or muriate of lime. The carbonic acid, when thus liberated from its union with the lime, escapes in a gaseous form, and froths up or effervesces as it makes its way in small bubbles through the drop of liquid. This effervescence is brisk or feeble in proportion as the limestone is pure or impure, or, in other words, according to the quantity of foreign matter mixed with the carbonate of lime. Without the aid of this test, the most experienced eye cannot always detect the presence of carbonate of lime in rocks.
The above-mentioned three classes of rocks, the siliceous, argil- laceous, and calcareous, pass continually into each other, and rarely occur in a perfectly separate and pure form. Thus it is an exception to the general rule to meet with a limestone as pure as ordinary white chalk, or with clay as aluminous as that used in Cornwall for porcelain, or with sand so entirely composed of siliceous grains as the white sand of Alum Bay in the Isle of Wight, or sandstone so pure as the grit of Fontainebleau, used for pavement in France. More commonly we find sand and clay, or. clay and marl, intermixed in the same mass. When the sand and clay are each in considerable quantity, the mixture is called loam. If there is much calcareous
Cx. IL] FORMS OF STRATIFICATION. 13
. Matter in clay itis called marl; but this term has unfortunately been used so vaguely, as often to be very ambiguous. It has been applied to substances in which there is no lime; as, to that red loam usually called red marl in certain parts of England. Agriculturists were in the habit of calling any soil a marl, which, like true marl, fell to pieces readily on exposure to the air. Hence arose the confusion of using this name for soils which, consisting of loam, were easily Worked by the plough, though devoid of lime.
Marl slate bears the same relation to marl which shale bears to clay, being a calcareous shale. It is very abundant in some countries, as in the Swiss Alps, Argillaceous or marly limestone is also of Common occurrence. —
There are few other kinds of rock which enter so largely into the Composition of sedimentary strata as to make it necessary to dwell
ere on their characters. I may, however, mention two others,— Magnesian limestone or dolomite, and gypsum. Magnesian limestone is composed of carbonate of lime and carbonate of magnesia; the Proportion of the latter amounting in some cases to nearly one half. Tt effervesces much more slowly and feebly with acids than common limestone. In England this rock is generally of a yellowish colour ; but it varies greatly in mineralogical character, passing from an earthy state to a white compact stone of great hardness. Dolomite, So common in many parts of Germany and France, is also a variety of magnesian limestone, usually of a granular texture.
Gypsum,— Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a soft whitish-yellow rock, with a texture resembling that of loaf-sugar, but sometimes it is entirely composed of lenticular crystals. Itis insoluble in acids, and does not effervesce like chalk and dolomite, because it does not contain carbonic acid Sas, or fixed air, the lime being already combined with sulphuric acid, for which it has a stronger affinity than for any other. An- hydrous gypsum is a rare variety, into which water does not enter as a component part. Gypseous marl is a mixture of gypsum and marl, Alabaster is a granular and compact variety of gypsum found 1n masses large enough to be used in sculpture and architecture. It 1S sometimes a pure snow-white substance, as that of Volterra in Tuscany, well known as being carved for works of art in Florence and Leghorn. It is a softer stone than marble, and more easily Wrought, |
Forms of stratification. — A series of strata sometimes consists of one of the above rocks, sometimes of two or more in alternating beds.
Thus, in the coal districts of England, for example, we often pass through several beds of sandstone, some of finer, others of coarser grain, some white, others of a dark colour, and below these, layers of shale and sandstone or beds of shale, divisible into leaf-like lamine, and containing beautiful impressions of plants. Then again we meet With beds of pure and impure coal, alternating with shales and sand- Stones, and underneath the whole, perhaps, are calcareous strata, or beds of limestone, filled with corals and marine shells, each bed dis-
14 i ALTERNATIONS. [Cu. ID.
tinguishable from another by certain fossils, or by the abundance of particular species of shells or zoophytes.
This alternation of different kinds of rock produces the most dis. tinct stratification; and we often find beds of limestone and marl, conglomerate and sandstone, sand and clay, recurring again and again, in nearly regular order, throughout a series of many hundred strata. The causes which may produce these phenomena are various, and have been fully discussed in my treatise on the modern changes of the earth’s surface.* It is there seen that rivers flowing into lakes and seas are charged with sediment, varying in quantity, composition, colour, and grain according to the seasons; the waters are sometimes flooded and rapid, at other periods low and feeble; different tribu- taries, also, draining peculiar countries and soils, and therefore charged with peculiar sediment, are swollen at distinct periods. It was also shown that the waves of the sea and currents undermine the cliffs during wintry storms, and sweep away the materials into the deep, after which a season of tranquillity succeeds, when nothing but the finest mud is spread by the movements of the ocean over the same submarine area.
It is not the object of the present work to give a description of these operations, repeated as they are, year after year, and century after century; but I may suggest an explanation of the manner in which some micaceous sandstones have originated, namely, those in which we see innumerable thin layers of mica dividing layers of fine quartzose sand. I observed the same arrangement of materials in recent mud deposited in the estuary of La Roche St. Bernard in Brit- tany, at the mouth of the Loire. The surrounding rocks are of gneiss, which, by its waste, supplies the mud: when this dries at low water, it is found to consist of brown laminated clay, divided by thin seams of mica. The separation of the mica in this case, or in that of mica- ceous sandstones, may be thus understood. If we take a handful of quartzose sand, mixed with mica, and throw it into a clear running stream, we see the materials immediately sorted by the water, the grains of quartz falling almost directly to the bottom, while the plates of mica take a much longer time to reach the bottom, and are carried farther down the stream. At the first instant the water is turbid, but immediately after the flat surfaces of the plates of mica are seen all alone reflecting a silvery light, as they descend slowly, to form a dis- tinct micaceous lamina. The mica is the heavier mineral of the two; but it remains a longer time suspended in the fluid, owing to its greater extent of surface. It is easy, therefore, to perceive that where such mud is acted upon by a river or tidal current, the thin plates of mica will be carried farther, and not deposited in the same places as the grains of quartz; and since the force and velocity of the stream varies from time to time, layers of mica or of sand will be thrown down successively on the same area.
Original horizontality.—It is said generally that the upper and
* Consult Index to Principles of Geology, “ Stratification,” “ Currents,” “ Deltas,” “ Water,” &c.
Cu. II] HORIZONTALITY OF STRATA. 15
under surfaces of strata, or the “planes of stratification,” are parallel. Although this is not strictly true, they make an approach to parallelism, for the same reason that sediment is usually deposited at first in nearly horizontal layers. The reason of this arrangement can by no means be attributed to an original evenness or horizontality in the bed of the Sea: for it is ascertained that in those places where no matter has been recently deposited, the bottom of the ocean is often as uneven as that of the dry land, having in like manner its hills, valleys, and ravines. Yet if the sea should sink, or the water be removed near the mouth of a large river where a delta has been forming, we should see extensive plains of mud and sand laid dry, which, to the eye, would appear perfectly level, although, in reality, they would slope gently from the land towards the sea.
: This tendency in newly-formed strata to assume a horizontal posi- tion arises principally from the motion of the water, which forces along particles of sand or mud at the bottom, and causes them to Settle in hollows or depressions where they are less exposed to the force of a current than when they are resting on elevated points. The velocity of the current and the motion of the superficial waves diminish from the surface downwards, and are least in those depres- Sions where the water is deepest.
A good illustration of the principle here alluded to may be Sometimes seen in the neighbourhood of a volcano, when a section, whether natural or artificial, has laid open to view a succession of various-coloured layers of sand and ashes, which have fallen in Showers upon uneven ground. Thus let A B (fig. 1.) be two ridges, with an intervening valley. These original inequalities of the Surface have been gradually effaced by beds of sand and ashes cĉ, d, e, the surface at e being quite level. It will be seen that, although the materials of the first layers have accommodated them-
selves in a great degree to the shape | of the ground A B, yet each bed is
= = thickest at the bottom. At first a
AaB] great many particles would be carried
sa by their own gravity down the steep
Sides of A and B, and others would afterwards be blown by the wind as they fell off the ridges, and would settle in the hollow, which Would thus become more and more effaced as the strata accumulated from c to e. This levelling operation may perhaps be rendered more Clear to the student by supposing a number of parallel trenches to be dug in a plain of moving sand, like the African desert, in which case the wind would soon éause all signs of these trenches to disappear, and the surface would be as uniform as before. Now, water in Motion can exert this levelling power on similar materials more Casily than air, for almost all stones lose in water more than a third of the weight which they have in air, the specific gravity of rocks eing in general as 24 when compared to that of water, which is estimated at 1. But the buoyancy of sand or mud would be still Sreater in the sea, as the density of salt water exceeds that of fresh.
| $ i | LE i$ | l
a a aaa eA em mm Set ee a aa > Sess ea =
16 DIAGONAL OR CROSS STRATIFICATION. [Cu. II.
Yet, however uniform and horizontal may be the surface of new deposits in general, there are still many disturbing causes, such as eddies in the water, and currents moving first in one and then in another direction, which frequently cause irregularities. We may sometimes follow a bed of limestone, shale, or sandstone, for a dis- , tance of many hundred yards continuously; but we generally find at length that each individual stratum thins out, and allows the beds which were previously above and below it to meet. If the materials are coarse, as in grits and conglomerates, the same beds can rarely be traced many yards without varying in size, and often coming to an end abruptly. (See fig. 2.)
Section of strata of sandstone, grit, and conglomerate.
Diagonal or cross stratification. — There is also another phe- nomenon of frequent occurrence. We find a series of larger strata, each of which is composed of a number of minor layers placed
Fig. 3.
Section of sand at Sandy Hill, near Biggleswade, Bedfordshire. Height 20 feet. (Green-sand formation.) obliquely to the general planes of stratification. To this diagonal arrangement the name of “false or cross stratification” has been given. Thus in the annexed section (fig. 3.) we see seven or eight large beds of loose sand, yellow and brown, and the lines a, b, c, mark some of the principal planes of stratification, which are nearly horizontal. But the greater part of the subordinate laminae do not conform to these planes, but have often a steep slope, the inclination being sometimes towards opposite points of the compass. When the sand is loose and incoherent, as in the case here represented, the
Cz. IL] CAUSES OF DIAGONAL STRATIFICATION. å 17
deviation from parallelism of the slanting laminz cannot possibly be accounted for by any re-arrangement of the particles acquired during the consolidation of the rock. In what manner then can such irre- Sularities be due to original deposition? We must suppose that. at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots instead of being spread out uniformly over a wide area. Sometimes, when banks are thus formed, currents may cut passages through them, just as a river forms its bed. Suppose the bank A (fig. 4.) to be thus formed with
a steep sloping side, and the water being in a tranquil state, the layer of sediment No. 1. is thrown down upon it, conforming nearly to its surface. Afterwards the other layers, 2, 3, 4, may be deposited in Succession, so that the bank B C D is formed. If the current then increases in velocity, it may cut away the upper portion of this mass down to the dotted line e (fig. 4.), and deposit the materials thus removed.farther on, so as to form the layers 5, 6, 7, 8. We have now the bank B C D E (fig. 5.), of which the surface is almost level
Fig. 5.
t 4 E
and on which the nearly horizontal layers, 9, 10, 11, may then accumulate. It was shown in fig. 3. that the diagonal layers of suc- cessive strata may sometimes have an opposite slope. This is well Seen in some cliffs of loose sand on the Suffolk coast. A portion Fig. 6. of one of these is represented in
fig. 6., where the layers, of which there are about six in the thick- ness of an inch, are composed of quartzose grains. This arrange- ment may have been due to the altered direction of the tides and
Cliff between Mismer and Dunwich. currents in the same place. The description above given of the slanting position of the minor ayers constituting a single stratum is in certain cases applicable on a much grander scale to masses several hundred feet thick, and many miles in extent. A fine example may be seen at the base of the aritime Alps near Nice. The mountains here terminate abruptly c
aS
EOL REFS OT
rarer
18 CAUSES OF DIAGONAL STRATIFICATION. [Cm. Il.
in the sea, so that a depth of many hundred fathoms is often found within a stone’s throw of the beach, and sometimes a depth of 3000 feet within half a mile. But at certain points, strata of sand, marl, or conglomerate, intervene between the shore and the mountains, as in the annexed fig. (7.), where a vast succession of slanting beds
Monte Calvo. Fig. 7.
c d
SSS 4
Section from Monte Calvo to the sea by the valley of Magnan, near Nice.
A. Dolomite and sandstone. (Green-sand formation ?) a, b, d. Beds of gravel and sand. c. Fine marl and sand of St. Madeleine, with marine shells.
of gravel and sand may be traced from the sea to Monte Calvo, a distance of no less than 9 miles in a straight line. The dip of these beds is remarkably uniform, being always southward or towards the Mediterranean, at an angle of about 25°. They are exposed to view in nearly vertical precipices, varying from 200 to 600 feet in height, which bound the valley through which the river Magnan flows. Although, in a general view, the strata appear to be parallel and uniform, they are nevertheless found, when examined closely, to be wedge-shaped, and to thin out when followed for a few hundred feet or yards, so that we may suppose them to have been thrown down originally upon the side of a steep bank where a river or alpine torrent discharged itself into a deep and tranquil sea, and formed a delta, which advanced gradually from the base of Monte Calvo to a distance of 9 miles from the original shore. If subsequently this part of the Alps and bed of the sea were raised 700 feet, the coast would acquire its present configuration, the delta would emerge, and a deep channel might then be cut through it by a river.
It is well known that the torrents and streams, which now descend from the alpine declivities to the shore, bring down annually, when the snow melts, vast quantities of shingle and sand, and then, as they subside, fine mud, while in summer they are nearly or entirely dry ; so that it may be safely assumed, that deposits like those of the valley of the Magnan, consisting of coarse gravel alternating with fine sediment, are still in progress at many points, as, for instance, at the mouth of the Var. They must advance upon the Mediterranean in the form of great shoals terminating in a steep talus; such being the original mode of accumulation of all coarse materials conveyed into deep water, especially where they are composed in great part of pebbles, which cannot be transported to indefinite distances by cur- rents of moderate velocity. By inattention to facts and inferences of this kind, a very exaggerated estimate has sometimes been made
Cx. IL] RIPPLE MARK. 19
of the Supposed depth of the ancient ocean. There can be no doubt,
‘r example, that the strata a, fig. 7., or those nearest to Monte Calvo, are older than those indicated by b, and these again were ormed before c; but the vertical depth of gravel and sand in any one place cannot be proved to amount even to 1000 feet, although it may perhaps be much greater, yet probably never exceeding at any point 3000 or 4000 feet. But were we to assume that all the Strata were once horizontal, and that their present dip or inclination was due to subsequent movements, we should then be forced to con- clude, that a sea 9 miles deep had been filled up with alternate layers of mud and pebbles thrown down one upon another.
In the locality now under consideration, situated a few miles to the west of Nice, there are many geological data, the details of which Cannot be given in this place, all leading to the opinion, that when the deposit of the Magnan was formed, the shape and outline of the alpine declivities and the shore greatly resembled what we now behold at many points in the neighbourhood. That the beds, a, b, e, d, are of comparatively modern daté is proved by this fact, that in seams of loamy marl intervening between the pebbly beds are fossil shells, half of which belong to species now living in the Mediterranean.
Ripple mark. —The ripple mark, so common on the surface of Sandstones of all ages (see fig. 8.), and which is so often seen on the
Slab of ripple-marke (new red) sandstone from Cheshire.
Sea-shore at low tide, seems to originate in the drifting of materials
along the bottom of the water, in a manner very similar to that which
may explain the inclined layers above described. This ripple is not
entirely confined to the beach between high and low water mark, but
IS also produced on sands which are constantly covered by water. c 2
20 FORMATION OF RIPPLE MARK. [Cu. II.
Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand. The following is the manner in which I once observed the motion of the air to produce this effect on a large extent of level beach, exposed at low tide near Calais. Clouds of fine white sand were blown from the neighbour- ing dunes, so as to cover the shore, and whiten a dark level sur- face of sandy mud, and this fresh covering of sand was beautifully rippled. On levelling all the small ridges and furrows of this ripple over an area of several yards square, I saw them perfectly restored in about ten minutes, the general direction of the ridges being always at right angles to that of the wind. The restoration began by the ap- pearance here and there of small detached heaps of sand, which soon lengthened and joined together, so as to form long sinuous ridges with intervening furrows. Each ridge had one side slightly inclined, and the other steep ; the lee-side being always steep, as b, c,—d, e; the windward-side a gentle slope, as a, b, —c, d, fig. 9. When a gust of
Fig. 9,
a —> e — 7 e
wind blew with sufficient force to drive along a cloud of sand, all the ridges were seen to be in motion at once, each encroaching on the furrow before it, and, in the course of a few minutes, filling the place which the furrows had occupied. The mode of advance was by the continual drifting of grains of sand up the slopes a b and ¢ d, many of which grains, when they arrived at 6 and d, fell over the scarps 6c and d e, and were under shelter from the wind; so that they remained stationary, resting, according to their shape and mo- mentum, on different parts of the descent, and a few only rolling to the bottom. In this manner each ridge was distinctly seen to move slowly on as often as the force of the wind augmented. Occasionally part of a ridge, advancing more rapidly than the rest, overtook the ridge immediately before it, and became confounded with it, thus causing those bifurcations and branches which are go common, and two of which are seen in the slab, fig. 8. We may observe this con- figuration in sandstones of all ages, and in them also, as now on the sea-coast, we may often detect two systems of ripples interfering with each other; one more ancient and half effaced, and a newer one, in which the grooves and ridges are more distinct, and in a different direction. This crossing of two sets of ripples arises from a change of wind, and the new direction in which the waves are thrown on the shore.
The ripple mark is usually an indication of a sea-beach, or of water from 6 to 10 feet deep, for the agitation caused by waves even during storms extends to a very slight depth. To this rule, however, there are some exceptions, and recent ripple marks have been ob- served at the depth of 60 or 70 feet. It has also been ascertained that currents or large bodies of water in motion may disturb mud and
Cu. III.] GRADUAL DEPOSITION INDICATED BY FOSSILS. 21
sand at the depth of 300 or even 450 feet.* Beach ripple, however, may usually be distinguished from current ripple by frequent changes dm its direction. In a slab of sandstone, not more than an inch thick, the furrows or ridges of an ancient ripple may often be seen in several Successive lamin to run towards different points of the compass.
CHAPTER III. ARRANGEMENT OF FOSSILS IN STRATA — FRESHWATER AND MARINE.
Successive deposition indicated by fossils — Limestones formed of corals and shells — Proofs of gradual increase of strata derived from fossils — Serpula attached to Spatangus — Wood bored by teredina— Tripoli and semi-opal formed of infusoria —Chalk derived principally from organic bodies—Distinction of freshwater from marine formations— Genera of freshwater and land shells— Rules for recognizing Marine testacea—Gyrogonite and chara— Freshwater fishes — Alternation of marine and freshwater deposits — Lym-Fiord.
Havine in the last chapter considered the forms of stratification so far as they are determined by the arrangement of inorganic matter, we may now turn our attention to the manner in which organic re- mains are distributed through stratified deposits. We should often be unable to detect any signs of stratification or of successive deposi-
tion, if particular kinds of fossils did not occur here and there at Certain depths in the mass. At one level, for example, univalve shells of some one or more species predominate ; at another, bivalve shells; and at a third, corals; while in some formations we find layers of vegetable matter, commonly derived from land plants, separating Strata.
Tt may appear inconceivable to a beginner how mountains, several thousand feet thick, can have become filled with fossils from top to bottom ; but the difficulty is removed, when he reflects on the origin of stratification, as explained in the last chapter, and allows sufficient time for the accumulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer Was once the uppermost, and covered immediately by the water in Which aquatic animals lived. Each stratum in fact, however far it may now lie beneath the surface, was once in the state of shingle, or loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped.
By attending to the nature of these remains, we are often enabled to determine whether the deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish, or fresh. Some limestones Consist almost exclusively of corals, and in many cases it is evident
* Edin. New Phil. Journ. vol, xxxi.; and Darwin, Vole. Islands, p. 134. c 3
p TAi a aa
22 GRADUAL DEPOSITIONS (Cu. IH.
that the present position of each fossil zoophyte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downwards. This arrange- ment is sometimes repeated throughout a great saccession of strata. From what we know of the growth of similar zoophytes in modern reefs, we infer that the rate of increase was extremely slow, and some
_of the fossils must have flourished for ages like forest trees, before they attained so large a size. During these ages, the water remained
clear and transparent, for such corals cannot live in turbid water.
- In like manner, when we see thousands of full-grown shells dis- persed every where throughout a long series of strata, we cannot doubt that time was required for the multiplication of successive generations ; and the evidence of slow accumulation is rendered more striking from the proofs, so often discovered, of fossil bodies having lain for a time on the floor of the ocean after death before they were imbedded in sediment. Nothing, for example, is more common than to see fossil oysters in clay, with serpulz, or barnacles ( acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the creatures whose re- mains now adhere to it, grew from an embryo to a mature state. Attached shells which are merely external, like some of the ser- pule (a) in the annexed figure (fig. 10.), may often have grown upon an oyster or other shell while the animal within was still living;
but if they are found on the inside, it could only happen after the death of the inhabitant of the shell which affords the support. Thus, in fig. 10., it will be seen that two serpulz have grown on the inte- ‘rior, one of them exactly on the place where the adductor muscle of the Gryphea (a kind of oyster) was fixed. å i Some fossil shells, even if simply attached to the, outside of others, bear full testimony to the conclu- sion above alluded to, namely, that an interyal elapsed between the death of the creature to whose shell they adhere, and the burial of the same in mud or sand. The sea- urchins or Echini, so abundant in white chalk, afford a good illustra-
; i a E ° ben ele a a i area tion. It is well known that these
Cu. IIL] INDICATED BY FOSSILS. 23
animals, when living, are invariably covered with numerous suckers, or gelatinous tubes, called “ambulacral,” because they serve as organs of motion. They are also armed with spines supported by rows of tubercles. These last are only seen after the death of the sea-urchin, when the spines have dropped off. In fig. 12. a living species of Spatangus, common on our coast, is represented with one half of its
Serpula attached to ; Recent Spatangus with the spines a fossil Spatangus removed from one side. from the chalk. b. Spine and tubercles, nat. size. a. The same magnified. shell stripped of the spines. In fig. 11. a fossil of the same genus from the white chalk of England shows the naked surface which the individuals of this family exhibit when denuded of their bristles. The full-grown Serpula, therefore, which now adheres externally, could not have begun to grow till the Spatangus had died, and the Spines were detached.
Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea- urchin in the chalk (see fig. 13.), which has fixed to it the lower valve of a Crania, a genus of bivalve mollusca. The upper valve
(b, fig. 13.) is almost invariably wanting, though.
occasionally found in a perfect state of preservation.
im white chalk at some distance. In this case, we
see clearly that the sea-urchin first lived from youth
to age, then died and lost its spines, which were
carried away. Then the young Crania adhered
za to the bared shell, grew and perished in its turn; Ea igs romthe chalk after which the upper valve was separated from lee of the the lower before the Echinus became enveloped in
Crania detached. chalky mud.
It may be well to mention one more illustration of, the manner in which single fossils may sometimes throw light on a former state of things, both in the bed of the ocean and on some adjoining land. We Meet with many fragments of wood bored by ship-worms at various depths in the: clay on which London is built. Entire branches and Stems of trees, several feet in length, are sometimes dug out, drilled all over by the holes of these borers, the tubes and shells of the mol- lusk still remaining in the cylindrical hollows. In fig. 15. e, a re- presentation is given of a piece of recent wood pierced by the Teredo navalis, or common ship-worm, which destroys wooden piles and Ships. When the cylindrical tube d has been extracted from the wood, a shell is seen at the larger extremity, composed of two pieces, as shown atc. In like manner, a piece of fossil wood (a, fig. 14.)
c4
24 SLOW DEPOSITION OF STRATA.
has been perforated by an animal of a kindred but extinct genus,
called. Teredina by Lamarck. The calcareous tube of this mollusk
was united and as it were soldered on to the valves of the shell (b), Fig. 14. .
Fossil and recent wood drilled by perforating Mollusca.
Fig. 14. a. Fossil wood from London clay, bored by Teredina. b. Soen and tube of Teredina personaia, the right-hand figure the ventral, the left the orsal view.
Fig. 15. e. Recent wood bored by Teredo. d. Shell and tube of Teredo navalis, from the same. c. Anterior and posterior view of the valves of same detached from the tube,
which therefore cannot be detached from the tube, like the valves of the recent Teredo. The wood in this fossil specimen is now con- verted into a stony mass, a mixture of clay and lime; but it must once have been buoyant and floating in the sea, when the Teredine lived upon it, perforating it in all directions. Again, before the infant colony settled upon the drift wood, the branch of a tree must have been floated down to the sea by a river, uprooted, perhaps, by a flood, or torn off and cast into the waves by the wind: and thus our thoughts are carried back to a prior period, when the tree grew for years on dry land, enjoying a fit soil and climate.
It has been already remarked that there are rocks in the interior of continents, at various depths in the earth, and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such masses may be compared to modern oyster-beds and coral-reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stony deposits in the earth’s crust, now proved to have been derived from plants and animals of which the organic origin was not suspected until of late years, even by naturalists. Great surprise was therefore created by the recent discovery of Professor Ehrenberg, of Berlin, that a certain kind of siliceous stone, called tripoli, was entirely composed of mil- lions of the remains of organic beings, which the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants. They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomacez by those naturalists who believe in their vegetable origin. The substance alluded to has
Cu. II1.] INFUSORIA OF TRIPOLI. 25
long been well known in the arts, being used in the form of powder for polishing stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no less than 14 feet thick. This stone, when ex- amined with a powerful microscope, is found to consist of the sili-
Bacillaria Gaitllonella Gaillonella vulgaris ? distans. Jferruginea.
These figures are magnified nearly 300 times, except the lower figure of G. ferruginea (fig. 18. a), which is magnified 2000 times.
ceous plates or frustules of the above-mentioned Diatomaceæ, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness; but Ehrenberg estimates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans (see fig. 17 -) in every cubic inch, which weighs about 220 grains, or about 187 millions in a single grain. At every stroke, therefore, that we make with this polishing powder, several millions, perhaps tens of millions, of perfect fossils are crushed to atoms.
The remains of these Diatomacee are of pure silex, and their forms are various, but very marked and constant in particular genera and species. ‘Thus, in the family Bacillaria (see fig. 16.), the fossils preserved in tripoli are seen to ex- hibit the same divisions and transverse lines which characterize the living spe- cies of kindred form. With these, also, the siliceous spicule or iniernal sup- ports of the freshwater sponge, or Spongilla of Lamarck, are sometimes in- termingled (see the needle- shaped bodies in fig- 20.). These flinty cases and spi- cule, although hard, are ) gèl very fragile, breaking like iy dill glass, and are therefore all admirably adapted, when
“it | rubbed, for wearing down il I into a fine powder fit for
Fig. 19,
j
Fragment of semi-opal from the great bed of tripoli, Bilin. polishing the surface of
Fig. 19. Naturalsize. - s metals. Fig. 20. The same magnified, showing circular articula- Besides the tripoli, je |
tions of a species of Gazllonella, and spicule of
Spongilla, exclusively of the fossils
FOSSIL INFUSORIA. ' (Ce. HE
above described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi- opal, in which innumerable parts of Diatomacea and spicule of. the Spongilla are filled with, and cemented together by, siliceous matter, It is supposed that the siliceous remains of the most delicate Dia- tomacex have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quantity.
In the Bohemian tripoli above described, as in that of Planitz in Saxony, the species of Diatomacez (or Infusoria, as termed by Ehren- berg) are freshwater ; but in other countries, as in the tripoli of the Isle of France, they are of marine species, and they all belong to formations of the tertiary period, which will be spoken of hereafter.
A well-known substance, called bog-iron ore, often met with in peat-mosses, has also been shown by Ehrenberg to consist of innu- merable articulated threads, of a yellow ochre colour, composed partly of flint and partly of oxide of iron. These threads are the cases of a minute microscopic body, called Gaillonella Serruginea (fig. 18.).
It is clear that much time must have been required for the accu- mulation of strata to which countless generations of Diatomacer have contributed their remains; and these discoveries lead us naturally to suspect that other deposits, of which the materials have usually been supposed to be inorganic, may in reality have been derived from microscopic organic bodies. That this is the case with the white chalk, has often been imagined, this rock having been observed to abound in a variety of marine fossils, such as echini, testacea, bryozoa, corals, Sponges, crustacea, and fishes. Mr. Lonsdale, on examining, in Oct. 1835, in the museum of the Geological Society of London, portions of white chalk from different parts of England, found, on carefully pulverizing them in water, that what appear to the eye simply as white grains were, in fact, well preserved fossils, He obtained above a thousand of these from each pound weight of chalk, some being fragments of minute bryozoa and corallines, others entire Foraminifera and Cytheride. The annexed drawings will give an idea of the beautiful forms of many of these bodies. The figures a a represent their natural size, but, minute as they seem, the
Cytheride and Foraminifera from the chalk, Fig. 21; Fig. 22. Fig. 23.
Cythere, Müll. Portion of Cristellaria Rosalina. Cytherina, Lam, Nodosaria. rotulata.
smallest of them, such as a, fig. 24., are gigantic in comparison with the cases of Diatomacew before mentioned. It has, moreover, been lately discovered that the chambers into which these Foraminifera
Cu. IIL] FRESHWATER AND MARINE FOSSILS. 27
are divided are actually often filled with thousands of well-preserved organic bodies, which abound in every minute grain of chalk, and are especially apparent in the white coating of flints, often accom- panied by innumerable needle-shaped spicule of sponges. After reflecting on these discoveries, we are naturally led on to conjecture that, as the formless cement in the semi-opal of Bilin has been derived from the decomposition of animal and vegetable remains, so also many chalk flints in which no organic structure can be re-
Cognized may nevertheless have constituted a part of microscopic animalcules.
“The dust we tread upon was once alive !”— BYRON.
How faint an idea does this exclamation of the poet convey of the real wonders of nature! for here we discover proofs that the calcareous and siliceous dust of which hills are composed has not only been once alive, but almost every particle, albeit invisible to the naked eye, still retains the organic structure which, at periods of time incalculably remote, was impressed upon it by the powers of life. :
Freshwater and marine fossils. — Strata, whether deposited in salt or fresh water, have the same forms; but the imbedded fossils are very different in the two cases, because the aquatic animals which frequent lakes and rivers are distinct from those inhabiting the sea. In the northern part of the Isle of Wight formations of marl and limestone, more than 50 feet thick, occur, in which the shells are principally, if not all, of extinct species. Yet we recognize their freshwater origin, because they are of the same genera as those now abounding in ponds and lakes, either in our own country or in warmer latitudes.
In many parts of France, as in Auvergne, for example, strata of limestone, marl, and sandstone are found, hundreds of feet thick, which contain exclusively freshwater and land shells, together with the remains of terrestrial quadrupeds. The number of land shells Scattered through some of these freshwater deposits is exceedingly great ; and there are districts in Germany where the rocks scarcely Contain any other fossils except snail-shells (helices) ; as, for instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In Order to account for this phenomenon, the geologist has only to €xamine the small deltas.of torrents which enter the Swiss lakes when the waters are low, such as the newly-formed plain where the Kander enters the Lake of Thun: He there sees sand and mud Strewed over with innumerable dead land shells, which have been brought down from valleys in the Alps in the preceding spring, during the melting of the snows. Again, if we search the sands on the borders of the Rhine, in the lower part of its course, we find countless land shells mixed with others of species belonging to lakes, Stagnant pools, and marshes. These individuals have been washed
-
E ES REET
ENER
28 DISTINCTION OF FRESHWATER LOR III.
away from the alluvial plains of the great river and its tributaries, some from mountainous regions, others from the low country.
Although freshwater formations are often of great thickness, yet they are usually very limited in area when compared to marine deposits, just as lakes and estuaries are of small dimensions in com- parison with seas.
We may distinguish a freshwater formation, first, by the absence of many fossils almost invariably met with in marine strata. For example, there are no sea-urchins, no corals, and scarcely any zoo- phytes; no chambered shells, such as the nautilus, nor microscopic Foraminifera. But it is chiefly by attending to the forms of the mollusca that we are guided in determining the point in question. In a freshwater deposit, the number of individual shells is often as great, if not greater, than in a marine stratum; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of recent freshwater and land shells are few when contrasted with the marine. Thus, the genera of true mollusca according to Blainville’s system, excluding those of extinct species and those without shells, amount to about 200 in number, of which the terrestrial and freshwater genera scarcely form more than a sixth.*
Almost all bivalve shells, or those of acephalous mollusca, are marine, about ten only out of ninety genera being freshwater.
Cyclas obovata ; fossil. Hants. Cyrena consobrina ; fossil. Grays, Essex.
Among these last, the four most common forms, both recent and fossil, are Cyclas, Cyrena, Unio, and Anodonta (see figures); the
Fig. 28.
Anodonta Cordierii ; Anodonta latimarginatus ; Unio littoralis ; fossil, Paris. recent. Bahia. recent. Auvergne.
two first and two last of which are so nearly allied as to pass into each other.
* See Synoptic Table in Blainville’s Malacologie.
FROM MARINE FORMATIONS. 29
Lamarck divided the bivalve mollusca into the Dimyary, or those having two large mus- cular impressions in each valve, as a b in the Cyclas, fig. 25., and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in fig. 30. Now, as none of these last, or the unimuscular bivalves, are freshwater, we may at once pre- sume a deposit in which we find any of them
Gryphea incurva, Sow. (G. ar- to be marine.
cuata, Lam.) upper valve. Lias. The univalve shells most characteristie of fresh-water deposits are, Planorbis, Lymnea, and Paludina. (See
Planorbis euomphalus ; Lymnea longiscata ; Paludina lenta ; fossil. Isle of Wight. fossil. Hants. fossil. Hants.
figures.) But to these are occasionally added Physa, Succinea, Ancylus, Valvata, Melanopsis, Melania, and Neritina. (See figures.) Fig. 34. Fig. 35. Fig. 36. Fig. 37.
Succinea amphibia ; Ancylus elegans ; Valvata ; Physa hypnorum ; fossil. Loess, Rhine. fossil. Hants. fossil. recent. Grays, Essex.
In regard to one of these, the Ancylus (fig. 35.) Mr. Gray observes that it sometimes differs in no respect from the marine Fig. 40.
Auricula ; Melania Physa colum- ; Melanopsis buc- recent. Ava. inquinata. naris. Paris. cinoidea ; recent. Paris basin. basin. Asia. Siphonaria, except in the animal. The shell, however, of Ancylus is usually thinner.* * Gray, Phil. Trans., 1835, p. 302.
i
30 DISTINCTION OF FRESHWATER [Cu. III.
Some naturalists include Neritina (fig. 42.) and the marine Nerita (fig. 43.) in the same genus, it being scarcely possible to
Fig 43. Fig. 44.
Neritina globulus. Paris basin, Nerita granulosa. Paris basin.
distinguish the two by good generic characters. But, as a general rule, the fluviatile species are smaller, smoother, and more globular than the marine; and they have never, like the Nerite, the inner margin of the outer lip toothed or crenulated. (See fig. 43.)
A few genera, among which Cerithium (fig. 44.) is the most abundant, are common both to rivers and the sea, having species peculiar to each. Other genera, like Auri- Cerithium cula (fig. 38.), are amphibious, frequenting marshes, espe- vin ori cially near the sea.
The terrestrial shells are all univalves. The most abundant genera among these, both in a recent and fossil state, are Helix (fig. 45.), Cyclostoma (fig. 46.), Pupa (fig. 47.), Clausilia (fig. 48.),
Fig. 45. Fig. 46. Fig. 47. Fig. 48, Fig. 49.
Helix Turonensis. Cyclostoma Pupa Clausilia Bulimus lubricus, Faluns, Touraine. elegans. tridens. bidens. Loess, Rhine. Loess. Loess. Loess.
Bulimus (fig. 49.), and Achatina ; which two last are nearly allied and pass into each other. :
The Ampullaria (fig. 50.), is another genus of shells, inhabiting rivers and ponds in hot countries. Many fossil Species have been referred to this genus, but they have been found chiefly in marine formations, and are suspected by some conchologists to belong to
Natica and other marine genera. All univalve shells of land and freshwater spe- cies, with the exception of Melanopsis (fig. 41.), and Achatina, which has a slight indentation, have ‘venthe Joma’ entire mouths; and this circumstance may often serve as a convenient rule for distinguishing freshwater from marine strata; since, if any univalves occur of which the mouths are not entire, we may presume that the formation is marine. The aper- ture is said to be entire in such shells as the Ampullaria and the land shells (figs. 45 — 49.) when its outline is not interrupted by an indentation or notch, such as that seen at b in Ancillaria
SPS OFS Te
oe
Serra
Ca. II] FROM MARINE FORMATIONS. 31
(fig. 52.); or is not prolonged into a canal, as that seen at a in Pleurotoma (fig. 51.).
The mouths of a large proportion of the marine univalves have these notches or canals, and almost all such species are carnivorous ;
Fig. 51.
Pleurotoma J rotata. Subap. hills, Italy.
: Ancillaria subulata. London clay. whereas nearly all testacea having entire mouths, are plant-eaters ; whether the species be marine, freshwater, or terrestrial.
There is, however, one genus which affords an occasional ex- Ception to one of the above rules. The Cerithium (fig. 44.), although provided with a short canal, comprises some species which inhabit salt, others brackish, and others fresh water, and they are Said to be all plant-eaters. ~
Among the fossils very common in freshwater deposits are the shells of Cypris, a minute crustaceous animal, having a shell much resembling tha of the bivalve mollusca.* Many minute living Species of this genus swarm in lakes and stagnant pools in Great Britain ; but their shells are not, if considered separately, conclusive as to the freshwater origin of a deposit, because the majority of Species in another kindred genus of the same order, the Cytherina of Lamarck (see above, fig. 21. p. 26.), inhabit salt water; and, although the animal differs slightly, the shell is scarcely distinguishable from that of the Cypris.
The seed-vessels and stems of Chara, a genus of aquatic plants, are very frequent in freshwater strata. These seed-vessels were Called, before their true nature was known, gyrogonites, and were Supposed to be foraminiferous shells. (See fig. 53. a.)
The Chare inhabit the bottom of lakes and ponds, and flourish Mostly where the water is charged with carbonate of lime. Their — Seed-vessels are covered with a very tough integument, capable of Tesisting decomposition; to which circumstance we may attribute their abundance in a fossil state. The annexed figure (fig. 54.) represents a branch of one of many new species found by Professor Amici in the lakes of Northern Italy. The seed-vessel in this plant 18 more globular than in the British Chare, and therefore more nearly resembles in form the extinct fossil species found in England,
* For figures of fossil species of Purbeck, see below, ch. xx
an are aE EEEE TNE AE ET AS
maana aaa eanan E a E E iA AE E St ISDS 2 a RR PE a URE a a te ee
32 FRESHWATER AND MARINE FORMATIONS. [Cm. III.
France, and other countries. The stems, as well as the seed-vessels, of these plants occur both in modern shell marl and in ancient
Chara medicaginula ; Chara elastica ; recent. Italy. fossil. Upper Eocene, Isle of Wight. = a. Sessile seed vessel between the divisions of a. Seed-vessel, the leaves of the female plant. magnified 20 b. Magnified transverse section of a branch, diameters. with five seed-vessels, seen from below 6. Stem, magnified. upwards.
freshwater formations. They are generally composed of a large tube surrounded by smaller tubes ; the whole stem being divided at certain intervals by transverse partitions or joints. (See 4, fig. 53.)
It is not uncommon to meet with-layers of vegetable matter, impressions of leaves, and branches of trees, in strata containing freshwater shells; and we also find occasionally the teeth and bones of land quadrupeds, of species now unknown. The manner in which such remains are occasionally carried by rivers into lakes,
especially during floods, has been fully treated of in the “ Principles of Geology.” *
The remains of fish are occasionally useful in determining the freshwater origin of strata. Certain genera, such as carp, perch, pike, and loach (Cyprinus, Perca, Esox, and Cobitis), as also Lebias, being peculiar to freshwater. Other genera contain some freshwater and some marine species, as Cottus, Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea, as the salmon; or are exclusively characteristic of salt water. The above observa- tions respecting fossil fishes are applicable only to the more modern or tertiary deposits; for in the more ancient rocks the forms depart so widely from those of existing fishes, that it is very difficult, at least in the present state of science, to derive any positive information from icthyolites respecting the element in which strata were deposited.
The alternation of marine and freshwater formations, both on a small and large scale, are facts well ascertained in geology. When it occurs on a small scale, it may have arisen from the alternate occupation of certain spaces by river water and the sea; for in the flood season the river forces back the ocean and freshens it over a large area, depositing at the same time its sediment; after which the salt water again returns, and, on resuming its former place, brings with it sand, mud, and marine shells.
* See Index of Principles, “ Fossilization.”
Cu. IV. ] CONSOLIDATION OF STRATA. 33
There are also lagoons at the mouths of many rivers, as the Nile and Mississippi, which are divided off by bars of sand from the sea, and which are filled with salt and fresh water by turns. They often communicate exclusively with the river for months, years, or even Centuries; and then a breach being made in the bar of sand, they are for long periods filled with salt water.
The Lym-Fiord in Jutland offers an excellent illustration of
analogous changes; for, in the course of the last thousand years, the Western extremity of this long frith, which is 120 miles in length, Including its windings, has been four times fresh and four times salt, a bar of sand between it and the ocean having been as often formed and removed. The last irruption of salt water happened in 1824, when the North Sea entered, killing all the freshwater shells, fish, and plants; and from that time to the present, the sea-weed Fucus vesiculosus, together with oysters and other marine mollusca, have Succeeded the Cyclas, Lymnea, Paludina, and Chare.* _ But changes like these in the Lym-Fiord, and those before men- tioned as occurring at the mouths of great rivers, will only account or some cases of marine deposits of partial extent resting on fresh- Water strata. When we find, as in the south-east of England, a teat series of freshwater beds, 1000 feet in thickness, resting upon Marine formations and again covered by other rocks, such as the Cretaceous, more than 1000 feet thick, and of deep-sea origin, we Shall find it necessary to seek for a different explanation of the phe- nomena. f
CHAPTER IV. CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.
Chemical and mechanical deposits — Cementing together of particles— Hardening by exposure to air — Concretionary nodules — Consolidating effects of pressure— Mineralization of organic remains— Impressions and casts how formed — Fossil wood — Göppert’s experiments — Precipitation of stony matter most rapid where Putrefaction is going on— Source of lime in solution—Silex derived from de- composition of felspar— Proofs of the lapidification of some fossils soon after burial, of others when much decayed.
Having spoken in the preceding chapters of the characters of sedi- mentary formations, both as dependent on the deposition of inorganic Matter and the distribution of fossils, I may next treat of the con- Solidation of stratified rocks, and the petrifaction of imbedded or- Sanic remains,
Chemical and mechanical deposits. — A distinction has been made
* See Principles, Index, “ Lym-Fiord.” T See below, Chap. XVIIL, on the Wealden. D
34 CONSOLIDATION OF STRATA, [Cu. IV.
by geologists between deposits of a chemical, and those of a me- chanical, origin. By the latter name are designated beds of mud, sand, or pebbles produced by the action of running water, also ac- cumulations of stones and scorie thrown out by a volcano, which have fallen into their present place by the force of gravitation. But the matter which forms a chemical deposit has not been mechanically suspended in water, but in a state of solution until separated by chemical action. In this manner carbonate of lime is often precipi- tated upon the bottom of lakes and seas in a solid form, as may be well seen in many parts of Italy, where mineral springs abound, and where the calcareous stone, called travertin, is deposited. In these springs the lime is usually held in solution by an excess of carbonic acid, or by heat if it be a hot spring, until the water, on issuing from the earth, cools or loses part of its acid. The calcareous matter then falls down in a solid state, encrusting shells, fragments of wood and leaves, and binding them together.*
In coral reefs, large masses of limestone are formed by the stony skeletons of zoophytes ; and these. together with shells, become ce- mented together by carbonate of lime, part of which is probably furnished to the sea water by the decomposition of dead corals, Even shells of which the animals are still living, on these reefs, are very commonly found to be encrusted over with a hard coating of limestone.t _
If sand and pebbles are carried by a river into the sea, and these are bound together immediately by carbonate of lime, the deposit may be described as of a mixed origin, partly chemical, and partly mechanical.
Now, the remarks already made in Chapter II. on the original horizontality of strata are strictly applicable to mechanical deposits, and only partially to those of a mixed nature. Such as are purely chemical may be formed on a very steep slope, or may even encrust the vertical walls of a fissure, and be of equal thickness throughout ; but such deposits are of small extent, and for the most part confined to vein-stones.
Cementing of particles. — It is chiefly in the case of calcareous rocks that solidification takes place at the time of deposition. But there are many deposits in which a cementing process comes into operation long afterwards. We may sometimes observe, where the water of ferruginous or calcareous springs has flowed through a bed of sand or gravel, that iron or carbonate of lime has been deposited in the interstices between the grains or pebbles, so that in certain places the whole has been bound together into a stone, the same set of strata remaining in other parts loose and incoherent.
Proofs of a similar cementing action are seen in a rock at Kello- way in Wiltshire. A peculiar band of sandy strata belonging to the group called Oolite by geologists, may be traced through several
* See Principles, Index, “ Calcareous f Ibid. “Travertin,” “Coral Reefs,” Springs,” &c. ts
Cx. IV.] CONSOLIDATION OF STRATA. -85
Counties, the sand being for the most part loose and unconsolidated, but becoming stony near Kelloway. In this district there are nu- merous fossil shells which have decomposed, having for the most part left only their casts. The calcareous matter hence derived has evidently served, at some former period, as a cement to the siliceous rains of sand, and thus a solid sandstone has been produced. If we take fragments of many other argillaceous grits, retaining the casts of shells, and plunge them into dilute muriatic or other acid, we see them immediately changed into common sand and mud; the cement of lime, derived from the shells, having been dissolved by the acid,
Traces of impressions and casts are often extremely faint. In Some loose sands of recent date we meet with shells in so advanced à Stage of decomposition as to crumble into powder when touched. It is clear that water percolating such strata may soon remove the calcareous matter of the shell; and unless circumstances cause the carbonate of lime to be again deposited, the grains of sand will not
e cemented together; in which case no memorial of the fossil will remain, The absence of organic remains from many aqueous rocks may be thus explained; but we may presume that in many of them no fossils were ever imbedded, as there are extensive tracts on the bottoms of existing seas even of moderate depth on which no frag- ment of shell, coral, or other living creature can be detected by dredging. On the other hand, there are depths where the zero of animal life has been approached; as, for example, in the Mediter- ranean, at the depth of about 230 fathoms, according to the researches of Prof. E. Forbes. In the 4Egean Sea a deposit of yellowish mud of a very uniform character, and closely resembling chalk, is going on in regions below 230 fathoms, and this formation must be wholly devoid of organic remains. *
In what manner silex and carbonate of lime may become widely diffused in small quantities through the waters which permeate the earth’s crust will be spoken of presently, when the petrifaction of fossil bodies is considered ; but I may remark here that such waters are always passing in the case of thermal springs from hotter to colder parts of the interior of the earth; and, as often as the tem- perature of the solvent is lowered, mineral matter has a tendency to Separate from it and solidify. Thus a stony cement is often supplied to sand, pebbles, or any fragmentary mixture. In some conglo- merates, like the pudding-stone of Hertfordshire (a Lower Eocene deposit), pebbles of flint and grains of sand are united by a siliceous Cement so firmly, thatif a block be fractured the rent passes as readily through the pebbles as through the cement.
It is probable that many strata became solid at the time when they emerged from the waters in which they were deposited, and when they first formed a part of the dry land, A well-known fact seems to confirm this idea: by far the greater number of the stones used for building and road-making are much softer when first taken from
* Report Brit, Ass. 1843, p. 178. D 2
36 ; CONSOLIDATION OF STRATA. [Cu. IV.
the quarry than after they have been long exposed to the air; and these, when once dried, may afterwards be immersed for any length
of time in water without becoming soft again. Hence it is found ` desirable to shape the stones which are to be used in architecture while they are yet soft and wet, and while they contain their “ quarry-water,” as it is called ; also to break up stone intended for roads when soft, and then leave it to dry in the air for months that it may harden. Such induration may perhaps be accounted for by supposing the water, which penetrates the minutest pores of rocks, to deposit, on evaporation, carbonate of lime, iron, silex, and other minerals previously held in solution, and thereby to fill up the pores partially. These particles, on crystallizing, would not only be them- selves deprived of freedom of motion, but would also bind together other portions of the rock which before were loosely aggregated. On the same principle wet sand and mud become as hard as stone when frozen; because one ingredient of the mass, namely, the water, has crystallized, so as to hold firmly together all the separate particles of which the loose mud and sand were composed.
Dr. MacCulloch mentions a sandstone in Skye, which may be moulded like dough when first found; and some simple minerals, which are rigid and as hard as glass in our cabinets, are often flexible and soft in their native beds: this is the case with asbestos, sahlite, tremolite, and chalcedony, and it is reported also to happen in the case of the beryl.*
The marl recently deposited at the bottom of Lake Superior, in North America, is soft, and often filled with freshwater shells ; but if a piece be taken up and dried, it becomes so hard that it can only be broken by a smart blow of the hammer. If the lake therefore was drained, such a deposit would be found to consist of strata of marl- stone, like that observed in many ancient European formations, and like them containing freshwater shells.
It is probable that some of the heterogeneous materials which rivers transport to the sea may at once set under water, like the arti- ficial mixture called pozzolana, which consists of fine volcanic sand charged with about 20 per cent. of oxide of iron, and the addition of a small quantity of lime. This substance hardens, and becomes a solid stone in water, and was used by the Romans in constructing the foundations of buildings in the sea.
Consolidation in these cases is brought about by the action of chemical affinity on finely comminuted matter previously suspended in water. After deposition similar particles seem to exert a mutual attraction on each other, and congregate together in particular spots, forming lumps, nodules, and concretions. Thus in many argillaceous deposits there are calcareous balls, or spherical concretions, ranged in layers parallel to the general stratification ; an arrangement which took place after the shale or marl had been thrown down in succes- sive lamine; for these laminæ are often traced in the concretions,
* Dr. MacCulloch, Syst. of Geol, vol. i. p. 123,
Cu. IV.] CONCRETIONARY STRUCTURE. 37
remaining parallel to those of the surrounding unconsolidated rock. (See fig. 55.) Such nodules of lime- —~<=| stone have often a shell or other foreign
body in the centre.”
Among the most remarkable ex- amples of concretionary structure are those described by Professor Sedgwick as abounding in the magnesian limestone of the north of England. The spherical balls are of various sizes, from that of a pea to a dia- meter of several feet, and they have both a concentric and radiated. Structure, while at the same time the lamine of original deposition pass uninterruptedly through them. In some cliffs this limestone resembles a great irregular pile of cannon balls. Some of the globular masses have their centre in one stratum, while a portion of their exterior passes through to the stratum above or below. Thus the larger spheroid in the annexed section (fig. 56.) passes from the stratum 6 upwards into a. In this instance we must suppose the deposition of a series of minor layers, first forming the stra- 4 tum 6, and afterwards the incumbent stratum a; then a movement of the par-
J ticles took place, and the carbonates of
Sporo Ta E et ee ACE NR S et magnesia separated from the
more impure and mixed matter forming the still unconsolidated parts
of the stratum. Crystallization, beginning at the centre, must have
gone on forming concentric coats around the original nucleus without interfering with the laminated structure of the rock.
When the particles of rocks have been thus re-arranged by chemi- cal forces, it is sometimes difficult or impossible to ascertain whether certain lines of division are due to original deposition or to the sub- Sequent aggregation of similar particles. Thus suppose three strata
Fig. 57. of grit, A, B, C, are charged unequally with calcareous matter, and that B is the os dT e most calcareous. If consolidation takes
EBT , B fai : ;
pu place in B, the concretionary action may spread upwards into a part of A, where the carbonate of lime is more abundant than in the rest; so that a mass, d, e, f, forming a portion of the superior stratum, becomes united with B into one solid mass of stone. The original line of division d, e, being thus effaced, the line d, J, would generally be Considered as the surface of the bed B, though not strictly a true plane of stratification.
Pressure and heat.— When sand and mud sink to the bottom of a ` deep sea, the particles are not pressed down by the enormous weight of the incumbent ocean ; for the water, which becomes mingled with the sand and mud, resists pressure with a force equal to that of the Column of fluid above. The same happens in regard to organic re-
Caicareous nodules in Lias.
* De la Beche, Geol. Researches, p. 95., and Geol. Observer (1851), p. 686. D 3
38 MINERALIZATION OF [Cu. IV.
mains which are filled with water under great pressure a they sink otherwise they would be immediately crushed to pieces and flattened. Nevertheless, if the materials of a stratum remain in a yielding state, and do not set or solidify, they will be gradually squeezed down by the weight of other materials successively heaped upon them, just as soft clay or loose sand on which a house is built may give way. By such downward pressure particles of clay, sand, and marl, may be- come packed into a smaller space, and be made to cohere together permanently.
Analogous effects of condensation may arise when the solid parts of the earth’s crust are forced in various directions by those me- chanical movements afterwards to be described, by which strata have been bent, broken, and raised above the level of the sea. Rocks of more yielding materials must often have been forced against others previously consolidated, and, thus compressed, may have acquired a new structure. A recent discovery may help us to comprehend how fine sediment derived from the detritus of rocks may be solidified by mere pressure. The graphite or “black lead” of commerce having become very scarce, Mr. Brockedon contrived a method by which the dust of the purer portions of the mineral found in Borrowdale might be recomposed into a mass as dense and compact as native graphite. The powder of graphite is first carefully prepared and freed from air, and placed under a powerful press on a strong steel die, with air-tight fittings. It is then struck several blows, each of a power of 1000 tons; after which operation the powder is so perfectly solidified that it can be cut for pencils, and exhibits when broken the same texture as native graphite.
But the action of heat at various depths in the earth is probably the most powerful of all causes in hardening sedimentary strata. To this subject I shall refer again when treating of the metamorphic rocks, and of the slaty and jointed structure.
Mineralization of organie remains. — The changes which fossil organic bodies have undergone since they were first imbedded in rocks, throw much light on the consolidation of strata. Fossil shells in some modern deposits have been scarcely altered in the course of centuries, having simply lost a part of their animal matter. But in other cases the shell has disappeared, and left an impression only of its exterior, or a cast of its interior form, or thirdly, a cast of the shell itself, the original matter of which has been removed. These different forms of fossilization may easily be understood if we examine the mud recently thrown out from a pond or canal in which there are shells, If the mud be argillaceous, it acquires consistency on drying, and on breaking open a portion of it we find that each shell has left impressions of its external form. If we then remove the shell itself, we find within a solid nucleus of clay, having the form of the interior of the shell. This form is often very different from that of the outer shell. Thus a cast such as @, fig. 58., commonly called a fossil screw, would never be suspected by an inexperienced conchologist to be the internal shape of the fossil univalve, 6, fig. 58.. Nor should we
Cu. IV.] ORGANIC REMAINS. ‘ABO
have imagined at first sight that the shell æ and the cast b, fig. 59., were different parts of the same fossil. The reader will observe, in
Phasianella Heddingtonensis, Trochus Anglicus, and and cast of the same. Coral Rag. cast. Lias.
the last-mentioned figure (b, fig. 59.), that an empty space shaded dark, which the shell itself once occupied, now intervenes between the enveloping stone and the cast of the smooth interior of the whorls. In such cases the shell has been dissolved and the component par- ticles removed by water percolating the rock. If the nucleus were taken out, a hollow mould would remain, on which the external form of the shell with its tubercles and striz, as seen in a, fig. 59., would
be seen embossed. Now if the space alluded to between the nucleus and the impression, instead of being left empty, has been filled up with calcareous spar, flint, pyrites, or other mineral, we then obtain from the mould an exact cast both of the external and internal form of the original shell. In this manner silicified casts of shells have been formed ; and if the mud or sand of the nucleus happen to be incoherent, or soluble in acid, we can then procure in flint an empty Shell, which in shape is the exact counterpart of the original. This Cast may be compared to a bronze statue, representing merely the Superficial form, and not the internal organization; but there is another description of petrifaction by no means uncommon, and of a much more wonderful kind, which may be compared to certain ana- tomical models in wax, where not only the outward forms and fea- tures, but the nerves, blood-vessels, and other internal organs are also Shown. Thus we find corals, originally calcareous, in which not only the general shape, but also the minute and complicated internal or- ganization are retained in flint.
Such a process of petrifaction is still more remarkably exhibited in fossil wood, in which we often perceive not only the rings of annual growth, but all the minute vessels and medullary rays. Many of the minute pores and fibres of plants, and even those spiral vessels which in the living vegetable can only be discovered by the mi- Croscope, are preserved. Among many instances, I may mention a fossil tree, 72 feet in length, found at Gosforth near Newcastle, in Sandstone strata associated with coal. By cutting a transverse slice So thin as to transmit light, and magnifying it about fifty-five times,
D4
40 MINERALIZATION OF [Cu. LV.
the texture seen in fig. 60. is exhibited. A texture equally minute and complicated has been observed in the wood =, of large trunks of fossil trees found in the 2 Craigleith quarry near Edinburgh, where the stone was not in the slightest degree siliceous, but consisted chiefly of carbonate. of lime, with oxide of iron, alumina, and carbon. The pa- rallel rows of vessels here seen are the rings Texture of atree from the coal of annual growth, but in ong part they are im- strata, magnified. ( Witham.) perfectly preserved, the wood having probably
decayed before the mineralizing matter had penetrated to that portion of the tree.
In attempting to explain the process of petrifaction in such cases, we may first assume that strata are very generally permeated by water charged with minute portions of calcareous, siliceous, and other earths in solution. In what manner they become so impregnated will be afterwards considered. If an organic substance is exposed in the open air to the action of the sun and rain, it will in time putrefy, or be dissolved into its component elements, which consist chiefly of oxygen, hydrogen, and carbon. These will readily be absorbed by the atmosphere or be washed away by rain, so that all vestiges of the dead animal or plant disappear. But if the same substances be submerged in water, they decompose more gradually ; and if buried in earth, still more slowly, as in the familiar example of wooden piles or other buried timber. Now, if as fast as each ‘particle is set free by putrefaction in a fluid or gaseous state, a particle equally minute of carbonate of lime, flint, or other mineral, is at hand and ready to be precipitated, we may imagine this in- organic matter to take the place just before left unoccupied by the organic molecule. In this manner a cast of the interior of certain vessels may first be taken, and afterwards the more solid walls of the same may decay and suffer a like transmutation. Yet when the whole is lapidified, it may not form one homogeneous mass of stone or metal. Some of the original ligneous, osseous, or other organic elements may remain mingled in certain parts, or the lapidifying substance itself may be differently coloured at different times, or so crystallized as to reflect light differently, and thus the texture of the original body may be faithfully exhibited.
The student may perhaps ask whether, on chemical principles, we have any ground to expect that mineral matter will be thrown down precisely in those spots where organic decomposition is in progress ? The following curious experiments may serve to illustrate this point. Professor Géppert of Breslau attempted recently to imitate the na- tural process of petrifaction. For this purpose he steeped a variety of animal and vegetable substances in waters, some holding siliceous, others calcareous, others metallic matter in solution. He found that in the period of a few weeks, or even days, the organic bodies thus immersed were mineralized to a certain extent. Thus, for example, thin vertical slices of deal, taken from the Scotch fir (Pinus syl-
Fig. 60. =
=
Cu. IV.] ORGANIC REMAINS. . a
vestris), were immersed in a moderately strong solution of sulphate of iron. When they had been thoroughly soaked in the liquid for several days they were dried and exposed to a red-heat until the vegetable matter was burnt up and nothing remained but an oxide of iron, which was found to have taken the form of the deal so exactly that casts even of the dotted vessels peculiar to this family of plants were distinctly visible under the microscope.
Another accidental experiment has been recorded by Mr. Pepys in the Geological Transactions.* An earthen pitcher containing several quarts of sulphate of iron had remained undisturbed and unnoticed for about a twelvemonth in the laboratory. At the end of this time when the liquor was examined an oily appearance was observed on the surface, and a yellowish powder, which proved to be sulphur, together with a quantity of small hairs. At the bottom were dis- Covered the bones of several mice in a sediment consisting of small grains of pyrites, others of sulphur, others of crystallized green sul- phate of iron, and a black muddy oxide of iron. It was evident that Some mice had accidentally been drowned in the fluid, and by the mutual action of the animal matter and the sulphate of iron on each other, the metallic sulphate had been deprived of its oxygen; hence the pyrites and the other compounds were thrown down. Although the mice were not mineralized, or turned into pyrites, the pheno- menon shows how mineral waters, charged with sulphate of iron,
May be deoxydated on coming in contact with animal matter under- going putrefaction, so that atom after atom of pyrites may be pre- Cipitated, and ready, under favourable circumstances, to replace the oxygen, hydrogen, and carbon into which the original body would be resolved.
The late Dr. Turner observes, that when mineral matter is in a “nascent state,” that is to say, just liberated from a previous state of chemical combination, it is most ready to unite with other matter, and form a new chemical compound. Probably the particles or atoms just set free are of extreme minuteness, and therefore move more freely, and are more ready to obey any impulse of chemical affinity. Whatever be the cause, it clearly follows, as before stated, that where organic matter newly imbedded in sediment is decomposing, there will chemical changes take place most actively.
An analysis was lately made of the water which was flowing off from the rich mud deposited by the Hooghly river in the Delta of the Ganges after the annual inundation. This water was found to be highly charged with carbonic acid gas holding lime in solution. f Now if newly-deposited mud is thus proved to be permeated by Mineral matter in a state of solution, it is not difficult to perceive that decomposing organic bodies, naturally imbedded in sediment, may as readily become petrified as the substances artificially im- mersed by Professor Géppert in various fluid mixtures.
* Vol. i. p. 399. first series. = t Piddington, Asiat. Research. vol, xviii, p. 226.
42 FLINT OF SILICIFIED FOSSILS, (Cu. IV.
It is well known that the water of springs, or that which is continually percolating the earth’s crust, is rarely free from a slight admixture either of iron, carbonate of lime, sulphur, silica, potash, or some other earthy, alkaline, or metallic ingredient: Hot springs in particular are copiously charged with one or more of these elements ; and it is only in their waters that silex is found in abundance. In certain cases, therefore, especially in volcanic regions, we may imagine the flint of silicified wood and corals to have been supplied by the waters of thermal springs. In other instances, as in tripoli, it may have been derived in great part, if not wholly, from the decomposi- tion of diatomaces, sponges, and other bodies. But even if this be granted, we have still to inquire whence a lake or the ocean can be constantly replenished with the calcareous and siliceous matter so abundantly withdrawn from it by the secretions of living beings.
In regard to carbonate of lime there is no difficulty, because not only are calcareous springs very numerous, but even rain- water, when it falls on ground where vegetable matter is decom- posing, may become so charged with carbonic acid as to acquire a power of dissolving a minute portion of the calcareous rocks over which it flows. Hence marine corals and mollusca may be provided by rivers with the materials of their shells and solid supports. But pure silex, even when reduced to the finest powder and boiled, is insoluble in water, except at very high temperatures. Nevertheless, Dr. Turner has well explained, in an essay on the chemistry of geology *, how the decomposition of felspar may be a source of silex in solution. He has remarked that the siliceous earth, which con- stitutes more than half the bulk of felspar, is intimately combined with alumine, potash, and some other elements. The alkaline matter of the felspar has a chemical affinity for water, as also for the car- bonic acid which is more or less contained in the waters of most springs. The water therefore carries away alkaline matter, and leaves behind a clay consisting of alumine and silica. But this re- sidue of the decomposed mineral, which in its purest state is called porcelain clay, is found to contain a part only of the silica which existed in the original felspar. The other part, therefore, must have been dissolved and removed: and this can be accounted for in two ways ; first, because silica when combined with an alkali is soluble in water ; secondly, because silica, in what is technically called its nascent state, is also soluble in water. Hence an endless supply of silica is afforded to rivers and the waters of the sea. For the fel- spathic rocks are universally distributed, constituting, as they do, so large a proportion of the voleanic, plutonic, and metamorphic for- mations. Even where they chance to be absent in mass, they rarely fail to occur in the superficial gravel or alluvial deposits of the basin of every large river.
The disintegration of mica also, another mineral which enters largely into the composition of granite and various sandstones, may
* Jam, Ed. New Phil. Journ. No. 30. p. 246,
Cu. 1V.] PROCESS OF PETRIFACTION. 43
yield silica which may be dissolved in water, for nearly half of this mineral consists of silica, combined with alumine, potash, and about atenth part of iron. The oxidation of this iron in the air is the Principal cause of the waste of mica.
We have still, however, much to learn before the conversion of fossil bodies into stone is fully understood. Some phenomena seem to imply that the mineralization must proceed with considerable rapidity, for stems of a soft and succulent character, and of a most perishable nature, are preserved in flint; and there are instances of the complete silicification of the young leaves of a palm-tree when just about to shoot forth, and in that state which in the West Indies is called the cabbage of the palm.* It may, however, be questioned whether in such cases there may not have been some antiseptic quality in the water which retarded putrefaction, so that the soft parts of the buried substance may have remained for a long time without disin- tegration, like the flesh of bodies imbedded in peat.
Mr. Stokes has pointed out examples of petrifactions in which the more perishable, and others where the more durable, portions of wood are preserved. These variations, he suggests, must doubtless have depended on the time when the lapidifying mineral was introduced. Thus, in certain silicified stems of palm-trees, the cellular tissue, that most destructible part, is in good condition, while all signs of the hard woody fibre have disappeared, the Spaces once occupied by it being hollow or filled with agate. Here, petrifaction must have com- menced soon after the wood was exposed to the action of moisture, and the supply of mineral matter must then have failed, or the water must have become too much diluted before the woody fibre decayed. But when this fibre is alone discoverable, we must suppose that an interval of time elapsed before the commencement of lapidification, during which the cellular tissue was obliterated. When both struc- tures, namely, the cellular and the woody fibre, are preserved, the Process must have commenced at an early period, and continued without interruption till it was completed throughout.
* Stokes, Geol. Trans., vol. v. p. 212. second series. t Ibid.
LAND HAS BEEN RAISED, [Cu V.
CHAPTER YV.
ELEVATION OF STRATA ABOVE THE SEA— HORIZONTAL AND INCLINED STRATIFICATION.
Why the position of marine strata, above the level of the sea, should be referred to the rising up of the land, not to the going down of the sea— Upheaval of exten- sive masses of horizontal strata— Inclined and vertical stratification — Anticlinal and synclinal lines— Bent strata in east of Scotland — Theory of folding by lateral movement — Creeps — Dip and strike— Structure of the Jura— Various forms of outcrop—Rocks broken by flexure— Inverted position of disturbed strata— Unconformable stratification — Hutton and Playfair on the same— Fractures of strata— Polished surfaces — Faults— Appearance of repeated alter- nations produced by them— Origin of great faults.
Lanp has been raised, not the sea lowered. —It has been already stated that the aqueous rocks containing marine fossils extend over wide continental tracts, and are seen in mountain chains rising to great heights above the level of the sea (p. 4.). Hence it follows, that what is now dry land was once under water. But if we admit this conclusion, we must imagine, either that there has been a general lowering of the waters of the ocean, or that the solid rocks, once covered by water, have been raised up bodily out of the sea, and have thus become dry land. The earlier geologists, finding themselves reduced to this alternative, embraced the former opinion, assuming that the ocean was originally universal, and had gradually sunk down to its actual level, so that the present islands and continents were left dry. It seemed to them far easier to conceive that the water had gone down, than that solid land had risen upwards into its present position. It was, however, impossible to invent any satisfactory hypothesis to explain the disappearance of so enormous a body of water throughout the globe, it being necessary to infer that the ocean had once stood at whatever height marine shells might be detected. It moreover appeared clear, as the science of Geology advanced, that certain spaces on the globe had been alternately sea, then land, then estuary, then sea again, and, lastly, once more habitable land, having remained in each of these states for considerable periods. In order to account for such phenomena, without admitting any movement of the land itself, we are required to imagine several retreats and returns of the ocean ; and even then our theory applies merely to cases where the marine strata composing the dry land are horizontal, leaving unexplained those more common instances where strata are inclined, curved, or placed on their edges, and evidently not in the position in which they were first deposited. Geologists, therefore, were at last compelled to have recourse to the other alternative, namely, the doctrine that the solid land has been repeatedly moved upwards or downwards, so as permanently to change its position relatively to the sea. There are several distinct
Cu. V.J } NOT THE SEA LOWERED. 45
grounds for preferring this conclusion. First, it will account equally for the position of those elevated masses of marine origin in which the stratification remains horizontal, and for those in which the strata are disturbed, broken, inclined, or vertical. Secondly, it is consistent with human experience that land should rise gradually in some places and be depressed in others. Such changes have actually occurred in our own days, and are now in progress, having been accompanied in Some cases by violent convulsions, while in others they have pro- ceeded go insensibly, as to have been ascertainable only by the most careful scientific observations, made at considerable intervals of time. On the other hand, there is no evidence from human experience of a lowering of the sea’s level in any region, and the ocean cannot sink 1n one place without its level being depressed all over the globe. These preliminary remarks will prepare the reader to understand the great theoretical interest attached to all facts connected with the Position of strata, whether horizontal or inclined, curved or vertical. Now the first and most simple appearance is where strata of Marine origin occur above the level of the sea in horizontal position. Such are the strata which we meet with in the south of Sicily, filled With shells for the most part of the same species as those now living in the Mediterranean. Some of these rocks rise to the height of More than 2000 feet above the sea. Other mountain masses might be mentioned, composed of horizontal strata of high antiquity, which Contain fossil remains of animals wholly dissimilar from any now known to exist. In the south of Sweden, for example, near Lake
Wener, the beds of one of the oldest of the fossiliferous deposits, namely that formerly called Transition, and now Silurian, by geo- logists, occur in as level a position as if they had recently formed Part of the delta of a great river, and been left dry on the retiring of ‘the annual floods. Aqueous rocks of about the same age extend for hundreds of miles over the lake-district of North America, and exhibit
in like manner a stratification nearly undisturbed. The Table Moun- tain at the Cape of Good Hope is another example of highly elevated yet perfectly horizontal strata, no less than 3500 feet in thickness, and consisting of sandstone of very ancient date. Instead of imagining that such fossiliferous rocks were always at their present level, and that the sea was once high enough to cover em, we suppose them to have constituted the ancient bed of the ocean, and that they were gradually uplifted to their present height. his idea, however startling it may at first appear, is quite in accordance, as before stated, with the analogy of changes now going on in certain regions of the globe. Thus, in parts of Sweden, and the shores and islands of the Gulf of Bothnia, proofs have been Obtained that the land is experiencing, and has experienced for Centuries, a slow upheaving movement. Playfair argued in favour of this opinion in 1802; and in 1807, Von Buch, after his travels in : Candinavia, announced his conviction that a rising of the land was n progress. Celsius and other Swedish writers had, a century efore, declared their belief that a gradual change had, for ages,
46 RISING AND SINKING OF LAND. [Cu V.
been taking place in the relative level of land and sea. They attri- buted the change to a fall of the waters both of the ocean and the Baltic. This theory, however, has now been refuted by abundant evidence; for the alteration of relative level has neither been universal nor everywhere uniform in quantity, but has amounted, in some regions, to several feet in a century, in others to a few inches; while in the southernmost part of Sweden, or the province of Scania, there has been actually a loss instead of a gain of land, buildings having gradually sunk below the level of the sea.*
It appears, from the observations of Mr. Darwin and others, that very extensive regions of the continent of South America have been undergoing slow and gradual upheaval, by which the level plains of Patagonia, covered with recent marine shells, and the Pampas of Buenos Ayres, have been raised above the level of the seat On the other hand, the gradual sinking of the west coast of Greenland, for the space of more than 600 miles from north to south, during the last four centuries, has been established by the observations of a Danish naturalist, Dr. Pingel. And while these proofs of continental elevation and subsidence, by slow and insensible movements, have been recently brought to light, the evidence has been daily strength- ened of continued changes of level effected by violent convulsions in countries where earthquakes are frequent. There the rocks are rent from time to time, and heaved up or thrown down several feet at once, and disturbed in such a manner, that the original position of strata may, in the course of centuries, be modified to any amount.
It bas also been shown by Mr. Darwin, that, in those seas where circular coral islands and barrier reefs abound, there is a slow and continued sinking of the submarine mountains on which the masses of coral are based; while there are other areas of the South Sea, where the land is on the rise, and where coral has been upheaved far above the sea-level.
It would require a volume to explain to the reader the various facts which establish the reality of these movements of land, whether of elevation or depression, whether accompanied by earthquakes or accomplished slowly and without local disturbance, Having treated fully of these subjects in the Principles of Geology }, I shall assume, in the present work, that such changes are part of the actual course of nature; and when admitted, they will be found to afford a key to the interpretation of a variety of geological appearances, such as the elevation of horizontal, inclined, or disturbed marine strata, and the superposition of freshwater to marine deposits, afterwards to be described. It will also appear, in the sequel, how much light the
* In the first three editions of my opinion in the Phil. Trans. 1835, Part I.
Principles of Geology, I expressed many doubts as to the validity of the alleged proofs of a gradual rise of land in Sweden ; but after visiting that country, in 1834, I retracted these objections, and published a detailed statement of the observations which led me to alter my
See also the Principles, 4th and subse- quent editions. i
t See his Journal of a Naturalist in Voyage of the Beagle, and his work on Coral Reefs,
Í See chaps. xxvii, to xxxii. inclusive, - and chap. 1.
Cu. V.] INCLINED STRATIFICATION, 47
doctrine of a continued subsidence of land may throw on the manner in which a series of strata, formed in shallow water, may have accu- mulated to a great thickness. The excavation of valleys also, and other effects of denudation, of which I shall presently treat, can alone be understood when we duly appreciate the proofs, now on record, of the prolonged rising and sinking of land, throughout wide areas.
To conclude this subject, I may remind the reader, that were we to embrace the doctrine which ascribes the elevated position of marine formations, and the depression of certain freshwater strata, to oscil- lations in the level of the waters instead of the land, we should be Compelled to admit that the ocean has been sometimes every where much shallower than at present, and at others more than three miles deeper.
Inclined stratification. — The most unequivocal evidence of a change in the original position of strata is afforded by their standing Up perpendicularly on their edges, which is by no means a rare phenomenon, especially in mountainous countries. Thus we find in* Scotland, on the southern skirts of the Grampians, beds of pudding- Stone alternating with thin layers of fine sand, all placed vertically to the horizon. When Saussure first ob- Served certain conglomerates in a simi- lar position in the Swiss Alps, he re- marked that the pebbles, being for the most part of an oval shape, had their longer axes parallel to the planes of stratification (see fig. 61.). From this he inferred, that such strata must, at first, have been horizontal, each oval Vertical conglomerate and sandstone. pebble having originally settled at the bottom of the water, with its flatter side parallel to the horizon, for the same reason that an ege will not stand on either end if unsupported. Some few, indeed, of the rounded stones in a conglomerate occasionally afford an exception to the above rule, for the same reason that we see on a shingle beach Some oval or flat-sided pebbles resting on their ends or edges; these having been forced along the bottom and against each other by a Wave or current so as to settle in this position.
Vertical strata, when they can be traced continuously upwards or
Ownwards for some depth, are almost invariably seen to be parts of Sreat curves, which may have a diameter of a few yards, or of several miles. Ishall first describe two curves of considerable regularity, Which occur in Forfarshire, extending over a country twenty miles in breadth, from the foot of the Grampians to the sea near Arbroath.
The mass of strata here shown may be nearly 2000 feet in thick- ness, consisting of red and white sandstone, and various coloured Shales, the beds being distinguishable into four principal groups, namely, No. 1. red marl or shale; No. 2. red sandstone, used for building ; No. 3. conglomerate ; and No. 4. grey paving-stone, and tile-stone, with green and reddish shale, containing peculiar organic Temains. A glance at the section will show that each of the forma-
CURVED STRATA. eo fer, Vv:
A ao
tions 2, 3, 4, are repeated thrice at the surface, twice with a southerly, and once with a northerly inclination or dip, and the beds in No. 1, which are nearly horizontal, are still brought up twice by a slight curvature to the surface, once on each side of A. Beginning at the north-west extremity, the tile-stones and conglomerates No. 4. and No. 3. are ver- tical, and they generally form a ridge parallel to the southern skirts of the Grampians. The superior strata Nos. 2. and 1. become less and less inclined on descending to the valley of Strathmore, where the strata, having a concave bend, are said by geologists to lie in a “trough” or “basin.” Through the centre of this valley runs an imaginary line A, called technically a “synclinal line,” where the beds, which are tilted in opposite directions, may be supposed to meet. It is most important for the observer to mark such lines, for he will perceive by the diagram, that in travel- ling from the north to the centre of the basin, he is always passing from older to newer beds; whereas, after crossing the line A, and pursuing his course in the same southerly direction, he is con- tinually leaving the newer, and advane- ing upon older strata. All the deposits which he had before examined begin then to recur in reversed order, until he arrives at the central axis of the Sidlaw hills, where the strata are seen to form an arch or saddle, having an anticlinal line B, in the centre. On passing this line, and continuing towards the S. E., the formations 4, 3, and 2, are again repeated, in the same relative order of superposition, but with a southerly dip. At Whiteness (see diagram) it will be seen that the inclined strata are covered by a newer deposit, a, in horizontal beds. These are composed of red conglomerate and sand, and are newer than any of the groups, 1, 2, 3, 4, before described, and rest uncon- formably upon strata of the sandstone group, No. 2. : An example of curved strata, in which the bends or convolutions of the rock are sharper and far more numerous within an equal space, has been well described by Sir James Hall.* It occurs near St.
N. W.
W. Ogle.
Valley of Strathmore.
Findhayen.
Length of section twenty miles.
Sidlaw Hills. Level of sea.
Leys Mill.
Fig. 62 Section of Forfarshire, from N. W. to S. E., from foot of the Grampians to the sea at Arbroath (volcanic or trap rocks omitted).
Whiteness, Arbroa h.
m vi
\
* Edin. Trans. vol. vii. pl. 3.
“Cx. V.] EXPERIMENTS TO ILLUSTRATE CURVED STRATA, 49
Abb’s Head, on the east coast of Scotland, where the rocks consist Principally of a bluish slate, having frequently a ripple-marked sur- face. The undulations of the beds reach from the top to the bottom
On the removal of the weight, curved and folded, so as to bear ain the cliffs. We must, how- €ver, bear in mind, that in the natural section or sea-cliff we only See the foldings imperfectly, one part being invisible beneath the Sea, and the other, or upper portion, being supposed to have been Carried away by denudation, or that action of water which will be
explained in the next chapter. The dark lines in the accompanying
Plan (fig. 64.) represent what is actually seen of the strata in part of
the line of cliff alluded to; the fainter lines, that portion which i g
-50 CURVED STRATA. [Cu. V.
concealed beneath the sea level, as also that which is supposed to have once existed above the present surface.
We may still more easily illustrate the effects which a lateral thrust might produce on flexible strata, by placing several pieces of differ- ently coloured cloths upon a table, and when they are spread out hori-
Fig. 65.
zontally, cover them with a book. Then apply other books to each end, and force them towards each other. ‘The folding of the cloths will exactly imitate those of the bent strata. (See fig. 65.)
Whether the analogous flexures in stratified rocks have really been due to similar sideway movements is a question of considerable diffi- culty. It will appear when the volcanic and granitic rocks are de- scribed that:some of them have, when melted, been injected forcibly into fissures, while others, already in a solid state, have been pro- truded upwards through the incumbent crust of the earth, by which a great displacement of flexible strata must have been caused.
But we also know by the study of regions liable to earthquakes, that there.are causes at work in the interior of the earth capable of producing a sinking in of the ground, sometimes very local, but some- times extending over a wide area. The frequent repetition, or con- tinuance throughout long periods, of such downward movements seems to imply the formation and renewal of cavities at a certain depth below the surface, whether by the removal of matter by vol- canos and hot springs, or by the contraction of argillaceous rocks by heat and pressure, or any other combination of circumstances. What- ever conjectures we may indulge respecting the causes, it is certain that pliable beds may, in consequence of unequal degrees of subsi- dence, become folded to any amount, and have all the appearance of having been compressed suddenly by a lateral thrust.
The “ Creeps,” as they are called in coal-mines, afford an excellent illustration of this fact.— First, it may be stated generally, that the excavation of coal at a considerable depth causes the mass of over- lying strata to sink down bodily, even when props are left to support the roof of the mine. “In Yorkshire,” says Mr. Buddle, “three dis- tinct subsidences were perceptible at the surface, after the clearing out of three seams of coal below, and innumerable vertical cracks were caused in the incumbent mass of sandstone and shale, which thus settled down.”* The exact amount of depression in these cases
* Proceedings of Geol. Soc. vol, iii, p. 148,
Ca. V.] CREEPS IN COAL-MINES.
can only be accurately measured where water accumulates on the Surface, or a, railway traverses a coal-field.
hen a bed of coal is worked out, pillars or rectangular masses of coal are left at intervals as props to support the roof, and protect the colliers. Thus in fig. 66., representing a section at Wallsend,
Ud Call |
SM ene
i i
2 g © 2 n ec] £ Cy nm nD k] © Q © = = = n
SIL
at Wallsend, Newcastle, showing ‘ Creeps.” (J. Buddle, Esq.) The upper seam, or main coal, here worked out, was 630 feet below the surface.
Section of carboniferous strata,
Horizontal length of section 174 feet.
Main Coal 6 feet Gin. Fe Metal Coal
Newcastle, the galleries which have been excavated are represented
Y the white Spaces a 6, while the adjoining dark portions are parts
0 the original coal-seam left as props, beds of sandy clay or shale
constituting the floor of the mine. When the, props have been re- E2
52 CURVED STRATA. [Cu. V.
duced in size, they are pressed down by the weight of overlying rocks (no less than 630 feet thick) upon the shale below, which is thereby squeezed and forced up into the open spaces.
Now it might have been expected, that instead of the floor rising up, the ceiling would sink down, and this effect, called a “ Thrust,” does, in fact, take place where the pavement is more solid than the roof. But it usually happens, in coal-mines, that the roof is com- posed of hard shale, or occasionally of sandstone, more unyielding than the foundation, which often consists of clay. Even where the argillaceous substrata are hard at first, they soon become softened and reduced to a plastic state when exposed to the contact of air and water in the floor of a mine.
The first symptom of a “creep,” says Mr. Buddle, is a slight cur- vature at the bottom of each gallery, as at a, fig. 66.: then the pavement continuing to rise, begins to open with a longitudinal crack, as at b: then the points of the fractured ridge reach the roof, as at c; and, lastly, the upraised beds close up the whole gallery, and the broken portions of the ridge are re-united and flattened at the top, exhibiting the flexure seen atd. Meanwhile the coal in the props has become crushed and cracked by pressure. It is also found that below the creeps a, b, c, d, an inferior stratum, called the “ metal coal,” which is 3 feet thick, has been fractured at the points e, f, g, h, and has risen, so as to prove that the upward movement, caused by the working out of the “main coal,” has been propagated through a thickness of 54 feet of argillaceous beds, which intervene between the two coal seams. This same displacement has also been traced downwards more than 150 feet below the metal coal, but it grows continually less and less until it becomes imperceptible.
No part of the process above described is more deserving of our notice than the slowness with which the change in the arrangement of the beds is brought about. Days, months, or even years, will sometimes elapse between the first bending of the pavement and the time of its reaching the roof, Where the movement has been most rapid, the curvature of the beds is most regular, and the reunion of the fractured ends most complete; whereas the signs of displacement or violence are greatest in those creeps which have required months or years for their entire accomplishment. Hence we may conclude that similar changes may have been wrought on a larger scale in the earth’s crust by partial and gradual subsidences, especially where the ground has been undermined throughout long periods of time ; and we must be on our guard against inferring sudden violence, simply because the distortion of the beds is excessive.
Between the layers of shale, accompanying coal, we sometimes see the leaves of fossil ferns spread out as regularly as dried plants between sheets of paper in the herbarium of a botanist. These fern- leaves, or fronds, must have rested horizontally on soft mud, when first deposited. If, therefore, they and the layers of shale are now inclined, or standing on end, it is obviously the effect of subsequent derangement. The proof becomes, if possible, still more striking
Cu. V.] DIP AND STRIKE. 53
When these strata, including vegetable remains, are curved again ana
again, and even folded into the form of the letter Z, so that the same
Continuous layer of coal is cut through several times in the same
perpendicular shaft. Thus, in the coal-field near Mons, in Belgium, Fig. 67.
m ce ne M
frs
B
Y
these zigzag bendings are repeated four or five times, in the manner represented in fig. 67., the black lines representing seams of coal.* Dip and Strike. — In the above remarks, several technical terms have been used, such as dip, the wnconformable position of strata, and the anticlinal and synelinal lines, which, as well as the strike of the beds, I shall now explain. If a stratum or bed of rock, instead of being quite level, be inclined to one side, it is said to dip; the Point of the compass to which it is inclined is called the point of dip, and the degree of deviation from a level or horizontal line is called
Fig. 68. the amount of dip, or the angle 5 N
f if
Zigzag flexures of coal near Mons.
diagram (fig. 68.), a series of strata are inclined, and they dip to the north at an angle of forty- five degrees. The strike, or line of bearing, is the prolongation or extension of the strata in a direction wd right angles to the dip; and hence it is sometimes called the di- "ection of the strata. Thus, in the above instance of strata dipping to the north, their strike must necessarily be east and west. We ave borrowed the word from the German geologists, streichen sig- nifying to extend, to have a certain direction. Dip and strike may ° aptly illustrated by a row of houses running east and west, the ong ridge of the roof representing the strike of the stratum of slates, w uch dip on one side to the north, and on the other to the south. “ stratum which is horizontal, or quite level in all directions, has neither dip nor strike. It is always important for the geologist, who is endeavouring to comprehend the structure of a country, to learn how the beds dip in every part of the district; but it requires some practice to avoid
eing occasionally deceived, both as to the point of dip and the amount of it.
* See plan by M. Chevalier, Burat’s D’Aubuisson, tom. ii. p. 334. E3
of dip. Thus, in the annexed’
miat am antennia e
54 DIP AND STRIKE. [Cu.
If the upper surface of a hard stony stratum be uncovered, whether artificially in a quarry, or by the waves at the foot of a cliff, it is easy to determine towards what point of the compass the slope is steepest, or in what direction water would flow, if poured upon it. This is the true dip. But the edges of highly inclined strata may give rise to perfectly horizontal lines i in the face of a vertical cliff, if the observer see the strata in the line of their strike, the dip being inwards from the face of the cliff. If, however, we come to a break in the cliff, which exhibits a section exactly at right angles to the line of the strike, we are then able to ascertain the true dip. In the annexed drawing (fig. 69.), we may suppose a headland, one side of
; lay Uf i a MY eee ll wlll] y SS igo on ran
i gg i ae z
ii >` j
Mss
Apparent horizontality of inclined strata.
which faces to the north, where the beds would appear perfectly horizontal to a person in the boat; while in the other side facing the west, the true dip would be seen by the person on shore to be at an angle of 40°. If, therefore, our observations are confined to a vertical precipice facing in one direction, we must endeavour to find a ledge or portion of the plane of one of the beds projecting beyond the others, in order to ascertain the true dip.
It is rarely important to determine the angle of inclination with such minuteness as to require the aid of the instrument called a clinometer. We may measure the angle within a few degrees by
standing exactly opposite to a cliff where
the true dip is exhibited, holding the
hands immediately before the eyes, and
placing the fingers of one in a perpen-
dicular, and of the other in a horizontal
position, as in fig. 70. It is thus easy
to discover whether the lines of the in-
clined beds bisect the angle of 90°, formed
. by the meeting of the hands, so as to give
; an angle of 45°, or whether it would di-
vide the space into two equal or unequal
portions. The ùnpet dotted line may express a stratum dipping to the north ; but should the beds dip precisely to the opposite point of
`
Cae Ve] DIP AND STRIKE. 55 the compass as in the lower dotted line, it will be seen that the amount of inclination may still be measured by the hands with equal facility. It has been already seen, in describing the curved strata on the east coast of Scotland, in Forfarshire and Berwickshire, that a series of concave and convex bendings are occasionally repeated several times. These usually form part of a series of parallel waves of Strata, which are prolonged in the same direction throughout a con- Siderable extent of country: Thus, for example, in the Swiss Jura, that lofty chain of mountains has been proved to consist of many parallel ridges, with intervening longitudinal valleys, as in eT, the ridges being formed by curved fossiliferous strata, of which the nature and dip are occasionally displayed in deep transverse gorges, called “cluses,” caused by fraetures at right angles to the Irection of the chain.* Now let us suppose these ridges and parallel valleys to run north and south, we should then say that the strike of the beds is north and south, and the dip east and west. Lines drawn along the summits of the ridges, A, B, would be anticlinal lines, and one following the bottom of the adjoining valleys a syn- Clinal line. It will be observed that some of these ridges, A, B, are unbroken on the summit, whereas one of them, C, has been fractured along the line of strike, and a portion of it carried away by denud- ation, so that the ridges of the beds in: the: formations a, b, c, come
Fig. 71.
Oil
am = Tl oo
Section illustrating the structure of the Swiss Jura.
out to the day, or, as the miners say, crop out, on the sides of a valley. The ground plan of such a denuded ridge as C, as given in a geological map, may be ex- pressed by the diagram fig. 72., and the cross section of the same by fig. 73. The line DE, fig. 72., is the anticlinal line, on each side
Transverse section.
Ground plan of the denuded ridge C, fig. 71.
* See M. Thurmann’s work, “Essai rentruy, Paris, 1832,” with whom I ex- - Sur Jes Soulévemens Jurassiques du Por- - amined part of these mountains in 1835. E 4
56 OUTCROP OF STRATA. [Cm. V.
of which the dip is in opposite directions, as expressed by the arrows. The emergence of strata at the surface is called by miners their out-crop or basset. a
If, instead of being folded into parallel ridges, the beds form a boss or dome-shaped protuberance, and if we suppose the summit of the dome carried off, the ground plan would exhibit the edges of the strata forming a succession of circles, or ellipses, round a com- mon centre. These circles are the lines of strike, and the dip being always at right angles is inclined in the course of the circuit to every point of the compass, constituting what is termed a qua-quaversal dip — that is, turning each way.
There are endless variations in the figures described by the basset- edges of the strata, according to the different inclination of the beds, and the mode in which they happen to have been denuded. One of the simplest rules with which every geologist should be acquainted, relates to the V-like form of the beds as they crop out in an ordinary valley. First, if the strata be horizontal, the V-like form will be also on a level, and the newest strata will appear at the greatest heights.
Secondly, if the beds be inclined and intersected by a valley sloping in the same direction, and the dip of the beds be less steep than the slope of the valley, then the V’s, as they are often termed by miners, will point upwards (see fig. 74.), those formed by the
newer beds appearing in a superior position, and extending highest up the valley, as A is seen above
Thirdly, if the dip of the beds be steeper than the slope of the valley, then the V’s will point downwards (see fig. 75.), -and those formed of the older beds will now appear uppermost, as B appears above A.
Fourthly, in every case where ‘the strata dip in a ‘contrary direction to the slope of the valley, what- ever be the angle of in- clination, the newer beds will appear the highest, as in the first and second cases. This is shown by the drawing (fig. 76.), j which exhibits strata ris- Slope ot valley 20°, dip of strata 50°. ing at an angle of 20°,
SS & SS S SV À N
SS
Cx. V.] ANTICLINAL AND SYNCLINAL LINES. a7
and crossed by a valley, which declines in an oppo- site direction at 20°.* These rules may often be of great practical uti- lity ; for the different de- e grees of dip occurring in the two cases represented in figures 74 and 75. may occasionally be encoun- tered in following the same line of flexure at points a few miles distant from each other. A miner un- acquainted with the rule, who had first explored the valley (fig. 4.), may have sunk a vertical shaft below the coal seam A, until he reached the inferior bed B. He might then pass to the valley fig. 75., and discovering there also the outcrop of two coal seams, might begin his workings in the uppermost in the expectation of Coming down to the other bed A, which would be observed cropping out lower down the valley. Buta glance at the section will demon- Strate the futility of such hopes.
In the majority of cases, an anticlinal axis forms a ridge, and a synelinal axis a valley, as in A, B, fig. 62. p. 48.; but there are Fig. 77. exceptions to this rule, the beds sometimes
sloping inwards from either side of a moun- tain, as in fig. 77. | On following one of the anticlinal ridges of the Jura, before mentioned, A, B, C, fig. 71., we often discover longitudinal cracks and sometimes large fissures along the line Where the flexure was greatest. Some of these, as above stated, have been enlarged by denudation into valleys of considerable width, as at C, fig. 71., which follow the line of strike, and which we may Suppose to have been hollowed out at the time when these rocks were Still beneath the level of the sea, or perhaps at the period of their Sradual emergence from beneath the waters. The existence of such cracks at the point of the sharpest bending of solid strata of limestone is precisely what we should have expected; but the occasional’ Want of all similar signs of fracture, even where the strain has been STeatest, as ata, fig. 71., is not always easy to explain. We must imagine that many strata of limestone, chert, and other rocks which are now brittle, were pliant when bent into their present position.
Slope of valley 20°, dip of strata 20°, in opposite directions.
* I am indebted to the kindness of originals, turning them about in different E. Sopwith, Esq., for three models which ways, he would at once comprehend their ave copied in the above diagrams ; meaning as well as the import of others but the beginner may find it by no means far more complicated, which the same easy to understand such copies, although, engineer has constructed to illustrate lf he were to examine and handle the Saults.
58 REVERSED DIP OF STRATA. [Cu. V.
They may have owed their flexibility in part to the fluid matter which they contained in their minute pores, as before described (p. 85.), and in part to the permeation of sea-water while they were yet submerged.
At the western extremity of the Pyrenees, great curvatures of the strata are seen in the sea cliffs, where the rocks consist of marl, grit, and chert. At certain points, as at a, fig. 78., some of the bendings
Strata of chert, grit, and marl, near St. Jean de Luz.
of the flinty chert are so sharp, that specimens might be broken off, well fitted to serve as ridge-tiles on the roof of a house. Although this chert could not have been brittle as now, when first folded into this shape, it presents, nevertheless, here and there at the points of greatest flexure small cracks, which show that it was solid, and not wholly incapable of breaking at the period of its displacement. The numerous rents alluded to are not empty, but filled with caleedony and quartz. Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous marls occur, with here and there thin beds of Fig. 79. solid gypsum interstratified. Sometimes these solid layers have been broken into detached fragments, still preserving their Wy sharp edges (g g, fig. 79.), while the con- may, ie tinuity of the more pliable and ductile Be 2- marls, m m, has not been interrupted. I shall conclude my remarks on bent strata by stating, that, in mountainous g- gypsum. m. marl, regions like the Alps, it is often difficult for an experienced geologist to. determine correctly the relative age of beds by superposition, so often have the strata been folded back upon themselves, the upper parts. of the curve having been removed by denudation. Thus, if we met with the strata seen in the section fig. 80., we should naturally suppose that there were twélve distinct Fig. 80. beds, or sets of beds, No. 1. being the newest, and No. 12. the oldest of the series. But this section may, perhaps, DN NY 4\ 5 \2 e exhibit merely six beds, which have been folded in the manner seen in fig. 81., so that each of them is twice repeated, the position of one half being reversed, and part of No. 1., originally the uppermost, having now become the lowest of the series. These phenomena are often observable on a magnificent scale in certain regions in Switzer- land in precipices from 2000 to 8000 feet in perpendicular height.
ies S
CURVED STRATA IN THE ALPS.
Fig. 81.
Q
EAA
in the valley of the Lutschine, between Unterseen
In the Iselten Alp,
and Grindelwald, curves of calcareous shale are seen from 1000 to
1500 feet in height, in which the beds sometimes plunge down ver-
tically for a depth of 1000 feet and more, before they bend round
Fig. 82,
Curved strata of the Iselten Alp.
again. There are many flexures not inferior in dimensions in the yrenees, as those near Gavarnie, at the base of Mont Perdu. Uneonformable stratification. — Strata are said to be unconform- able, when one series is so placed over another, that the planes of the Superior repose on the edges of the inferior (see fig. 83.). In this
Uneonformabie Junction of old red sandstone and Silurian schist at the Siccar Point, near St. Abb’s Head, Berwickshire. See also Frontispiece.
Case it is evident that a period had elapsed between the production of the two. sets of strata, and that, during this interval, the older
60 UNCONFORMABLE STRATIFICATION, (Cu. V.
series had been tilted and disturbed. Afterwards the upper series was thrown down in horizontal strata upon it. If these superior beds, as d, d, fig. 83., are also inclined, it is plain that the lower strata, a, a, have been twice displaced; first, before the deposition of the newer beds, d, d, and a second time when these same strata were thrown out of the horizontal position.
Playfair has remarked * that this kind of junction which we now call unconformable had been described before the time of Hutton, but that he was the first geologist who appreciated its importance, as illustrating the high antiquity and great revolutions of the globe. He had observed that where such contacts occur, the lowest beds of the newer series very generally consist of a breccia or conglomerate consisting of angular and rounded fragments, derived from the break- ing up of the more ancient rocks. On one occasion the Scotch geologist took his two distinguished pupils, Playfair and Sir James Hall, to the cliffs on the east coast of Scotland, near the village of Eyemouth, not far from St. Abb’s Head, where the schists of the Lammermuir range are undermined and dissected by the sea. Here the curved and vertical strata, now known to be of Silurian age, and which often exhibit a ripple-marked surface, are well exposed at the headland called the Siccar Point, penetrating with their edges into the incumbent beds of slightly inclined sandstone, in which large pieces of the schist, some round and others angular, are united by an arenaceous cement. “What clearer evidence,” exclaims Playfair, “could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom of the deep? We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited in the shape of sand or mud, from the waters of a superincumbent ocean. An epoch still more remote presented itself, when even the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immea- surable force which has burst asunder the solid pavement of the globe. Revolutions still more remote appeared in the distance of this extraordinary perspective. The mind seemed to grow giddy by looking so far into the abyss of time ; and while we listened with earnestness and admiration to the philosopher who was now unfold- ing to us the order and series of these wonderful events, we became sensible how much farther reason may sometimes go than imagina- tion can venture to follow.” f
In the frontispiece of this volume the reader will see a view of this classical spot, reduced from a large picture, faithfully drawn and coloured from nature by the youngest son of the late Sir James Hall. It was impossible, however, to do justice to the original sketch, in an
* Biographical account of Dr. Hutton. Ï Playfair, ibid.; see his Works, Edin. 1822, vol. iy. p. 81,
Cu. V.] FISSURES IN STRATA. 61
engraving, as the contrast of the red sandstone and the light fawn- Coloured vertical schists could not be expressed. From the point of view here selected, the underlying beds of the perpendicular schist, a, are visible at b through a small opening in the fractured beds of the covering of red sandstone, d d, while on the vertical face of the old schist at a’ a” a conspicuous ripple-mark is displayed.
It often happens that in the interval between the deposition of two Sets of unconformable strata, the inferior rock has not only been denuded, but drilled by perforating shells. Thus, for example, at Autreppe and Gusigny, near Mons, beds of an ancient (primary or
Fig, 84,
Junction of unconformable strata near Mons, in Belgium.
paleozoic) limestone, highly inclined, and often bent, are covered with horizontal strata of greenish and whitish marls of the Cretaceous formation. The lowest and therefore the oldest bed of the horizontal Series is usually the sand and conglomerate, a, in which are rounded fragments of stone, from an inch to two feet in diameter. These frag- ments have often adhering shells attached to them, and have been bored by perforating mollusca. The solid surface of the inferior limestone has also been bored, so as to exhibit cylindrical and pear- Shaped cavities, as at c, the work of saxicavous mollusca; and many rents, as at b, which descend several feet or yards into the limestone, have been filled with sand and shells, similar to those in the stratum a.
Fractures of the strata and faults. —Numerous rents may often be Seen in rocks which appear to have been simply broken, the sepa- rated parts remaining in the same places; but we often find a fissure, Several inches or yards wide, intervening between the disunited por- tions. These fissures are usually filled with fine earth and sand, or With angular fragments of stone, evidently derived from the fracture of the contiguous rocks.
Tt is not uncommon to find the mass of rock, on one side of a fissure thrown up above or down below the mass with which it was once in contact on the other side. “This mode of displacement is Called a shift, slip, or fault. “The miner,” says Playfair, describing a fault, “is often perplexed, in his subterraneous journey, by a derange- ment in the strata, which changes at once all those lines and bearings Which had hitherto directed his course. When his mine reaches a Certain plane, which is sometimes perpendicular, as in A B, fig, 85., Sometimes oblique to the horizon (as in C D, ibid.), he finds the beds of rock broken asunder, those on the one side of the plane having changed their place, by sliding in a particular direction along the face of the others. In this motion they have sometimes preserved their parallelism, as in fig. 85., so that the strata on each side of the
FAULTS.
Fig. 85.
Faults. A B perpendicular, C D oblique to the horizon.
faults A B, C D, continue parallel to one another; in other cases, the strata on each side are inclined, as in a, b, c, d (fig. 86.), though
E F, fault or fissure filled with rubbish, on each side of which the shifted 4 strata are not parallel.
their identity is still to be recognized by their possessing the same thickness and the same internal characters.”*
In Coalbrook Dale, says Mr. Prestwich +, deposits of sandstone, shale, and coal, several thousand feet thick, and occupying an area of many miles, have been shivered into fragments, and the broken remnants have been placed in very discordant positions, often at levels differing several hundred feet from each other. The sides of the faults, when perpendicular, are commonly separated several yards, but are sometimes as much as 50 yards asunder, the interval being filled with broken débris of the strata. In following the course of the same fault it is sometimes found to produce in different places very unequal changes of level, the amount of shift being in one place 300, and in another 700 feet, which arises, in some cases, from the union of two or more faults. In other words, the disjointed strata have in certain districts been subjected to renewed movements, which they have not suffered elsewhere,
We may occasionally see exact counterparts of these slips, on a small scale, in pits of loose sand and gravel, many of which have doubtless been caused by the drying and shrinking of argillaceous and other beds, slight subsidences having taken place from failure of support. Sometimes, however, even these small slips may have been produced during earthquakes; for land has been moved, and its level, relatively to the sea, considerably altered, within the period when much of the alluvial sand and gravel now covering the surface of continents was deposited.
* Playfair, Ilust. of Hutt. Theory, lee Trans. second series, vol. v.
§ 42. p. 452,
Ca. V.] FAULTS. 63
I have already stated that a geologist must be on his guard, in a region of disturbed strata, against inferring repeated alternations of rocks, when, in fact, the same strata, once continuous, have been bent round so as to recur in the same section, and with the same dip. A similar mistake has often been occasioned by a series of faults.
If, for example, the dark line A H (fig. 87.) represent the surface of a country on which the strata abe frequently crop out, an observer,
Apparent alternations of strata caused by vertical faults.
Who is proceeding from H to A, might at first imagine that at every Step he was approaching new strata, whereas the repetition of the Same beds has been caused by vertical faults, or downthrows. Thus, Suppose the original mass, A, B, C, D, to have been a set of uniformly Inclined strata, and that the different masses under EF, FG, and GD, sank down successively, so as to leave vacant the spaces marked in the diagram by dotted lines, and to occupy those marked by the Continuous lines, then let denudation take place along the line A H, _ 80 that the protruding masses indicated by the fainter lines are swept away,—a miner, who has not discovered the faults, finding the mass % which we will suppose to be a bed of coal four times repeated, might hope to find four beds, workable to an indefinite depth, but first on arriving at the fault G he is stopped suddenly in his workings, upon reaching the strata of sandstone c, or On arriving at the line of fault F he comes partly upon the shale 6, and partly on the sandstone Sand on reaching E he is again stopped by a wall composed of the Tock d, The very different levels at which the separated parts of the same Strata are found on the different sides of the fissure, in some faults, 18 truly astonishing. One of the most celebrated in England is that Called the « ninety-fathom dike,” in the coal-field of Newcastle. This same has been given to it,</