Elements Of Geology By Charles Lyell Preview

STUDENT’S

ELEMENTS OF

GEOLOGY CHARLES LYELL

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Elements of Geology Charles Lyell ISBN 1 84327 115 X

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Elements of Geology Sir Charles Lyell, Bart., F.R.S. Fourth edition revised by P.Martin Duncan F.R.S. Professor of Geology in King’s College London, etc. WITH A TABLE OF BRITISH FOSSILS AND MORE THAN 600 ILLUSTRATIONS

First edition published by John Murray, London, 1838 Fourth edition, revised published by John Murray, 1885 This edition 1997

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Charles Lyell (1797-1875) Born at Kinnordy, in Forfarshire, Charles Lyell was the eldest son of a Scottish father and an English mother. The family moved to Hampshire when he was a child, and in 1816 he went up to Oxford to study Classics. There, he attended lectures on geology by William Buckland, and his burgeoning interest in the subject led him to make geological tours of England and Scotland in 1817, and mainland Europe in 1818. In 1819, Lyell began to study law, but was hindered in his perusal of legal papers by weak eyesight (at least, that was his excuse). He became increasingly active as a geologist, and was made Secretary of the Geological Society in 1826. Lyell’s great work, Principles of Geology, was published in three volumes in 1830, 1832 and 1833; it established the principle of uniformitarianism, which holds that the only forces needed to explain the present appearance of the Earth’s surface are the same forces at work today (notably earthquakes and volcanism) operating over an immense period of time. Lyell became a leading figure in Victorian science. He was the first Professor of Geology in King’s College, London, and was knighted in 1848. Ironically, he never came to terms with Charles Darwin’s theory of evolution by natural selection, which was, as Darwin acknowledged, partly inspired by Lyell’s uniformitarian ideas. John Gribbin

Editors’note: We have endeavoured here to keep to the original punctuation and spellings used by Lyell, even when they are inconsistent, as happens not infrequently, particularly with the capitalisation of people’s names and the names of genera and species. A number of illustrations are drawn as natural size or a fraction of it. Obviously, the size on screen depends on the magnification used. The true size of the original illustration can only be seen if the page is printed.

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AUTHOR’S PREFACE BETWEEN the years 1838 and 1865 I published six editions of the ‘Elements of Geology’, beginning with a small duodecimo volume, which increased with each successive edition, as new facts accumulated, until in 1865 it had become a large and somewhat expensive work. When a seventh edition was called for, I was strongly urged by my friends to attempt to bring the book back again to a size more approaching the original, so that it might be within the reach of the ordinary student. In order to do this I resolved, in the first place, to omit some theoretical discussions which belonged more properly to my ‘Principles of Geology’, and further to confine myself to examples of British rocks, wherever this could be done only seeking foreign illustrations when, as in the case of the Upper Miocene or Falunian Tertiaries, no good representatives were to be found in this country. I therefore published in 1871 what was substantially a new work under the title of ‘The Student’s Elements of Geology’, and the success of the attempt has been proved by the steady demand which has exhausted an unusually large edition in less than three years. The present work has been carefully revised and corrected, with the addition of such new matter as the plan of the volume permitted. I have also added a new and very important table illustrative of the successive appearance and development in time of the different forms of animal and vegetable life throughout the British fossiliferous rocks. This table has been compiled for me by Mr. ETHERIDGE, of the London School of Mines, from materials which he has been collecting for many years. Among the numerous scientific friends who have rendered me valuable assistance in different parts of this new edition, I should wish especially to mention Mr. SEARLES WOOD, Mr. DAVID FORBES, Mr. JUDD, and the Rev. T. G. BONNEY, of St. John’s, Cambridge. CHARLES LYELL. 73 HARLEY STREET: Fcbruary 1874.

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PREFACE to THE FOURTH EDITION. This book was written by the late SIR CHARLES LYELL in order to meet the requirements of those students who are entirely ignorant of the science of Geology. The work contained those parts of Sir CHARLES LYELL’S celebrated book, The Elements of Geology,’ which were the most indispensable to a beginner, and was published in 1871. It was subsequently revised by Sir CHARLES LYELL in 1874, and he availed himself of the assistance of Messrs. Etheridge, Searles Wood, David Forbes, Professor Judd, and the Rev. Professor Bonney. A Third Edition was published in 1878, and it was the result of careful correction and revision on the part of Mr. Leonard Lyell, assisted by Professor Judd and Mr. Etheridge, sen. The utility of the book has been proved by the necessity of producing these successive editions, and, when the work went out of print some years since, a very great want was felt by students and teachers. The present revised edition contains the results of the more important geological investigations which have taken place since the appearance of the last edition, but the original plan and character of the book are preserved. I have availed myself of the works of the distinguished geologists whose names have been mentioned above, and also of the writings of Professor A. Geikie, Director of the Geological Survey of the United Kingdom; of Professor Prestwich, of Oxford; and Professor T. MeKenny Hughes, of Cambridge. It has been a most pleasing task to follow the thoughts and to continue in the method of the great man to whom the science of Geology is so greatly indebted, and I sincerely hope that these pages may be as useful as those of the original book. P. MARTIN DUNCAN.

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CONTENTS Chapter I. Chapter II. Chapter III. Chapter IV. Chapter V. Chapter VI. Chapter VII. Chapter VIII. Chapter IX. Chapter X. Chapter XI. Chapter XII. Chapter XIII. Chapter XIV. Chapter XV. Chapter XVI. Chapter XVII. Chapter XVIII. Chapter XIX. Chapter XX. Chapter XXI. Chapter XXII.

On the Different Classes of Rocks. Aqueous Rocks—Their Composition and Forms of Stratification. Arrangement of Fossils in Strata —Freshwater and Marine. Consolidation and subsequent alterations of Strata and Petrification of Fossils. Elevation of Strata above the Sea. —Horizontal and Inclined Stratification. Denudation. Joint action of Denudation, Upheaval, and Subsidence in remodelling the earth’s crust. Chronological Classification of Rocks. Classification of Tertiary Formations. Recent and Pleistocene Periods. Pleistocene Period continued. — Glacial conditions. Pleistocene Period continued. —Glacial conditions concluded. Pliocene Period. Miocene Period. Oligocene. Eocene Formations. Eocene Formations continued. Upper Cretaceous Group. Lower Cretaceous or Neocomian Formation. Jurassic Group.—Purbeck beds and Oolites. Jurassic Group continued.—Lias. Trias, or New Red Sandstone Group.

Page 8 18 31 47 58 89 97 106 124 132 155 164 179 209 220 242 265 271 300 315 347 361

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Chapter XXIII. Permian or Magnesian Limestone Group. Chapter XXIV. The Carboniferous Formation. Chapter XXV. Carboniferous Formation. Chapter XXVI. Devonian, or Old Red Sandstone Formation. Chapter XXVII. Silurian Group. Chapter XXVIII. Cambrian, Pre-Cambrian, and Laurentian Formations. Chapter XXIX. Volcanic phenomena, and the succession of Volcanic Rocks. Chapter XXX. Age of Volcanic Rocks continued. Chapter XXXI. Plutonic Rocks. Chapter XXXII. Metamorphic Rocks. Chapter XXXIII. Metamorphic Rocks continued. Chapter XXXIV. The different ages of Metamorphic Rocks. Chapter XXXV. Mineral Veins.

381 391 415 440 462

Appendix Table of British Fossils

616 639

493 510 537 552 566 584 593 600

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STUDENT’S

ELEMENTS OF GEOLOGY. 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 imbedded fossils—Aërial or Æolian rocks—Volcanic rocks, with and without cones and craters—Plutonic rocks, and their relation to the volcanic—Metamorphic rocks, and their probable origin—Hypogene rocks. 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 γε, ge, the earth, and λογοζ, 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 charges 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, inhabited 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 have 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 conclusion, discovering proofs that the external parts of the earth

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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 can show that they have acquired their actual configuration and condition gradually, under a great variety of circumstances, and at successive periods, during each of which distinct races of living beings 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. It comprises not merely all the parts of the earth which are laid open in precipices, or in cliffs overhanging a river or the sea, or which the miner may reveal in artificial excavations; but the whole of that outer covering of the planet on which we are enabled to reason by observations made at or near the surface. These reasonings may extend to a depth of perhaps twenty miles, a very fractional part of the distance from the surface to the centre of the globe. 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 organic beings which people our globe. Referring to this standard of magnitude, the geologist may admire the ample limits of his domain, and admit, at the same time, that not only the exterior of the planet, but the entire earth, is but an atom in the midst of the countless 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 old 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 is often so insensible a passage 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 felsart in German. The beginner, however, must constantly bear in mind that the term rock by no means invariably implies that a mineral mass is in an indurated or stony condition.

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The most natural and convenient mode of classifying the various rocks which compose the earth’s crust is to refer, to a certain extent, to their origin and relative age, but mainly to their physical structure and chemical composition. A useful classification which refers to the origin of the rocks, or to the different circumstances and causes by which they have been produced, separates the rocks, firstly, into those which are the products of aqueous or watery action; secondly, those which are aërial in their method of production and accumulation; and, thirdly, those which are volcanic, and the result of igneous action near the surface of the earth. A fourth group contains plutonic rocks, or deeply-seated masses which had an igneous origin; and a fifth group contains rocks which have undergone chemical and mechanical alterations, and are called metamorphic. Aqueous rocks.—The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a larger part of the earth’s surface than any others, and they have been formed under water. Some consist of mechanical deposits (pebbles, sand, and mud), and others are of organic origin, especially the limestones. A few are of chemical origin like calc-sinter. These rocks are usually stratified, or divided into distinct layers, or strata. The term stratum means simply a bed, or anything spread out or strewed over a given surface; and we infer that these strata have been generally spread out by the action of water, from what we daily see taking place near the mouths of rivers, or on 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 frequently find a series of deposits at the bottom, disposed with considerable regularity, one above the other; the uppermost, perhaps, may be a stratum of peat, next below is 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 set of beds is met with, yet with slight variations; some, for example, of the layers of sand, clay, or marl, may be wanting, one or more of them having thinned

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out and given place to others, or sometimes one of the layers first examined is observed to increase in thickness to the exclusion of other beds. The term ‘formation,’ which I have used in the above explanation, 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-metalliferous formations. In the estuaries of large rivers, such as the Ganges and the Mississippi, 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, colour, and in the fineness or coarseness of their particles, and some of them are occasionally characterised 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 and saltwater shells are included. In Egypt, where the Nile is always adding to its delta by filling up part of the Mediterranean with mud, the newly deposited sediment 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.1 1

See Principles of Geology, by the Author, Index, ‘Nile,’ ‘Rivers,’ &c.

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 a similar origin to them; and the more we examine their characters in minute detail, the more exact do we find the resemblance. Thus, for example, at various heights and depths in the 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

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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.2 2

See p. 20, fig. 7.

If a stratified arrangement, and the rounded form of pebbles, are alone sufficient to lead us to the conclusion that certain rocks originated under water, this opinion is confirmed by the distinct and independent evidence of fossils, often very 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. Every stratum was the burial-ground of its time. Now the remains of animals, especially of aquatic species, are found almost everywhere imbedded, in stratified rocks, and sometimes, in the case 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, impressions 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 elevations of more than 8,000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above 18,000 feet in the Himalaya.3 3

Gen. Sir R. Strachey found Oolitic fossils at an altitude of 18,400 feet in the Himalayas.

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. Aërial or Æolian rocks were not much considered in the early days of Geology, but it is evident that they are forming at the present time over large surfaces of the earth, and that this was also the case in former ages. Changes take place on the surface of the earth which cannot be attributed to movements by water, and deposits accumulate which are also not referable to that agent. The vast deposits of loess in Eastern Asia are attributed to blown dust; the desert sands of rainless regions, the sand dunes of many coasts and inland areas, and of the sides of lakes, are due to removal, by air in movement, of

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substances which have often been entirely eroded by atmospheric action and sometimes by water. Soils and thick deposits, like the laterite of Hindostan, are the result of aërial changes upon the rocks. The collection of organic remains, both vegetable and animal, in masses, is often without the intervention of an aqueous agency, and coal and plant remains, and some collections of bones, were former examples. Volcanic ash is wafted far and wide by wind, and forms important deposits, many of which occurred on dry land. Frost destroys the rocks, and the relics are not aqueous in their origin. Moraine matter, the product of land glaciers, and the blocks carried by ice, or simply remaining as the relics of sub-aërial denudation, are considered under this group of aërial rocks. Many of these rocks assume the stratified form, and contain organic remains. Volcanic rocks.—The third division of rocks which we may next consider are the volcanic, or those which have been produced at or near the surface, whether in ancient or modern times, by the action of subterranean heat, by water, and pressure, and these rocks are for the most part unstratified, and are devoid of fossils. They are more partially distributed than aqueous formations, at least in respect to horizontal 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-Dôme, Haute-Loire, and Ardèche, 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. 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, Hungary, and many parts of Western America and Australia, where spent volcanos may be seen, still preserving, in many cases, a conical form, and having craters and often lava-streams connected with them. These cones are composed, moreover, 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 beneath 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 human history, their forms

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are often very perfect. Some, however, have been compared to the mere skeletons of volcanos, the rains and torrents having washed their sides, and removed all the loose sand and scoriæ, leaving only the harder and more solid materials. By this erosion, and by earthquakes, their internal structure has occasionally been laid open to view, in fissures and ravines; and we then behold not only many successive beds and masses of lava, sand, and porous scoriæ, 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. There are also other rocks in 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 recent volcanos. We find also similar basaltic and other igneous rocks associated with beds of tuff in various parts of the British Isles and also forming dikes, such as have been spoken of; and some of the strata through which they cut are occasionally altered at the point of contact, as if there had been an exposure 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 partly to be attributed to the eruptions having been sub-marine, just as a considerable proportion of volcanos in our own times burst out beneath the sea: or the eruption may have been from fissures in the earth’s surface. 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 have flowed over the land or bed of the sea, or have been injected into fissures, at many different epochs; so that the igneous as well as the aqueous and aërial rocks may be classed as a chronological series of monuments, throwing light on a succession of events in the history of the earth. Plutonic rocks.—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

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which we can neither assimilate to deposits such as are now accumulated in lakes or seas nor to those generated by ordinary volcanic action. The members of both these classes of rocks agree in being highly crystalline and destitute of organic remains. The rocks of one class have been called plutonic, comprehending all the granites, syenites, and certain porphyries, which are allied in some of their characters to volcanic rocks. The members of the other class are stratified or foliated, and often slaty. They are the crystalline schists, or metamorphic rocks, in which group are included gneiss, micaceous schist, 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 rocks 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. It may be stated as the result of careful study that the various kinds of rocks, such as granite and Syenite, which constitute the plutonic family, are of igneous or aqueo-igneous origin, and have been formed under great pressure, at a considerable depth in the earth, or sometimes, perhaps, under a certain weight of incumbent ocean. 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 producing such volcanic rocks. 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 time 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 and steam give rise in ordinary lava. Metamorphic crystalline rocks.—The last great division of rocks includes the crystalline strata and slates, or schistose, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other classes. They rarely contain any pebbles, or sand, or scoriæ, or angular pieces of imbedded stone, or traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corresponding in form and arrangement to those of sedimentary formations, and are therefore said to be stratified. The beds sometimes consist of an alternation of minerals varying in colour, composition, and thickness, precisely

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as we see in stratified fossiliferous deposits. According to the Huttonian theory, which I adopt as the most probable, and which will be afterwards more fully explained, the materials of these rocks were originally deposited from water in the form of sediment, but they were subsequently so altered by heat, chemical action, and pressure, as to assume a new texture, and often mineral composition. 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 crystalline 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. Heated water permeating stratified masses amid great pressure have no doubt played their part in producing the schistose and foliated texture and other changes, and it is clear that the transforming influence has often 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 µετα, meta, trans, and, µορπη, morphe, forma. This metamorphism may be local or regional, and was more intense in the earlier geological periods than subsequently. Hence there are five classes of rocks considered in reference to their origin—the aqueous, the aërial, the volcanic, the plutonic, and the metamorphic. In the course of this work it will be shown that portions of each of these five distinct classes have originated at successive periods of the world’s history. The aqueous and aërial classes have been produced by energies acting on the outside of the globe, and the volcanic and plutonic have been produced by internal energies. The metamorphic rocks have had a double origin, so far as their producing agents are concerned. The term plutonic applies to the crystalline rocks, like granite and syenite, which differ in degree from the volcanic rocks. The term ‘hypogene’ was proposed in the ‘Principles of Geology’ (ed. i. vol. iii.), a word derived from

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υπα, under, and γινοµαιι, or be born, to indicate that the crystalline plutonic rocks are nether formed rocks, and which have not assumed their present form and structure at the surface. They never simply repose on volcanic or sedimentary rocks, and can be traced beneath everything, and they underlie all other rocks. But metamorphic rocks of the advanced type of gneiss and mica schist do not appear to have necessarily been formed at great depths, or under the conditions which granite required for its genesis. Hence the term hypogene is hardly applicable to them. The term hypogene action has lately been aptly employed by Professor A. Geikie4 to express the changes within the earth caused by original internal heat and chemical action, of which the intrusion of granites as eruptive rocks and metamorphism on a grand scale are examples. From what has now been said, the reader will understand that each of the 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 chemical 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 of its living inhabitants. I shall accordingly proceed to treat of each class 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. 4

Text book of Geology (1882), p. 196.

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CHAPTER II. AQUEOUS ROCKS—THEIR COMPOSITION AND FORMS OF STRATIFICATION. Mineral composition of strata—Siliceous rocks— Argillaceous—Calcareous—Gypsum—Loess—Coal— Soil—Ice-borne rocks—Forms of stratification—Original horizontality—Thinning out—Diagonal arrangement— Ripple mark. IN pursuance of the arrangement explained in the last chapter, we shall begin by examining the aqueous and aërial or sedimentary rocks, which are for the most part distinctly stratified, and often 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 sedimentary formations, independently of their age, and we may afterwards consider them chronologically 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, with rare exceptions, 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 siliceous, the argillaceous, and the calcareous, which are formed respectively of flint, clay, and carbonate of lime. Of these, the siliceous are chiefly made up of sand, or flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter with certain proportions of aluminous earth; and, lastly, the calcareous rocks or limestones, of carbonic acid and lime, sometimes with carbonate of magnesia. Siliceous and arenaceous rocks.—To speak first of the sandy division beds of loose sand are frequently met with, of which the grains consist entirely of silica, which term comprehends all purely siliceous minerals, as quartz and common flint.1 The siliceous grains in sand are usually more or less 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

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are bound together by a slight quantity of siliceous or calcareous matter, or by oxide of iron, clay or felspar. 1

For a description of all usual rock-making minerals see the Appendix.

Amongst the siliceous rocks, the result of deposition of fragments of previously existing formations, are gravels, river sands, and sandstones, which may be flagstones, which are then bedded and split along the lines of stratification; freestones, which are not divided by laminæ, and grits, which have the grains large and visible to the eye; breccias, or angular masses of sandstone more or less included in a common uniting material; conglomerates, with rounded pebbles included in a calcareous or siliceous matrix; arkoses or granitic sandstones; micaceous sandstones with thin silvery plates of mica arranged in layers parallel to the planes of stratification, giving a slaty texture to the rock. Greywacke, a hard grey or dark coloured rock, is a collection of rounded or sub-angular grains of quartz felspar, slate, &c., cemented by a paste which may be siliceous, clayey, felspathic or calcareous, and even anthracitic. This rock, often ripple-marked and sun-cracked, accumulated in running water in ancient geological periods. In nature there is every intermediate gradation from perfectly loose sand to the hardest sandstone. Argillaceous rocks.—Clay, strictly speaking, is a mixture of silica or flint with a large proportion, usually about one-fourth, of alumina; but in common language, any earth which possesses sufficient 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 granite. 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 always divides into more or less regular laminæ. Flinty slate, or Lydian-stone, and clay-slate are forms of clay which have undergone changes from the infiltration of chemical solutions. Mud-stone is a sandy, clayey rock. Fullers’ earth is a greenish or brownish earth, soft and unctuous, with a shining streak, crumbling into mud in water. Brick clay is any clay, loam, or earth from which bricks can be made, and is an impure clay as a rule, with a good deal of iron. Gannister is a siliceous rock with clay found