UNIVERSITY OF CALIFORNIA 
 SAN FRANCISCO LIBRARY 
 
GEOLOGY 
 
 BY 
 
 Sir Archibald | Geikie, F. R. S. 
 
 D.SC. CAMB., DUB.; LL.D. EDIN., ST. AND. 
 
 CORRESPONDENT OF THE INSTITUTE OF FRANCE, 
 
 DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF THE UNITED KINGDOM, AND 
 
 DIRECTOR OF THE MUSEUM OF PRACTICAL GEOLOGY, LONDON} FORMERLY 
 
 MURCHISON PROFESSOR OF GEOLOGY AND MINERALOGY IN 
 
 THE UNIVERSITY OF EDINBURGH. 
 
 WITH MANY ILLUSTRATIONS 
 
 E1 
 
 c-trittoit He iiurc 
 
 CoRege 
 
 NEW YORK 
 
 J. A. HILL AND COMPANY 
 
 1904 
 
CONTENTS. 
 
 CHAPTER I. 
 
 Page 
 
 INTBODUCTOBY 1 
 
 PART I. 
 THE MATERIALS FOR THE HISTORY OF THE EARTH. 
 
 CHAPTER II. 
 
 INFLUENCE OF THE ATMOSPHEBE IN THE CHANGES OF THE EABTH'S 
 SUBFACE 10 
 
 CHAPTER III. 
 
 INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND HOW 
 IT is RECOBDED 24 
 
 CHAPTER IV. 
 MEMORIALS LEFT BY LAKES 44 
 
 CHAPTER V. 
 How SPBINGS LEAVE THEIB MABK IN GEOLOGICAL HISTORY 51 
 
 CHAPTER VI. 
 
 ICE-BECORDS 7 G3 
 
 CHAPTER VII. 
 MEMORIALS OF THE SEA 72 
 
 CHAPTER VIII. 
 RECORDS OF PLANTS AND ANIMALS 83 
 
 CHAPTER IX. 
 VOLCANOES AND EARTHQUAKES 96 
 
 PAET II. 
 ROCKS, AND HOW THEY TELL THE HISTORY OF THE EARTH. 
 
 CHAPTER X. 
 ELEMENTS AND MINERALS 119 
 
 CHAPTER XI. 
 THE MOBE IMPOBTANT ROCKS 145 
 
 4083* 
 
iv CONTENTS 
 
 PART III. 
 THE STRUCTURE OF THE CRUST OF THE EARTH. 
 
 CHAPTER XII. 
 
 Page 
 
 SEDIMENTARY ROCKS 179 
 
 CHAPTER XIII. 
 SEDIMENTARY ROCKS AFTER FORMATION 193 
 
 CHAPTER XIV. 
 ERUPTIVE ROCKS AND MINERAL VEINS 208 
 
 CHAPTER XV. 
 FOSSIL REMAINS 220 
 
 PART IV. 
 
 THE GEOLOGICAL RECORD OF THE HISTORY OF THE 
 EARTH. 
 
 CHAPTER XVI. 
 EARLIEST CONDITIONS OF THE GLOBE 234 
 
 CHAPTER XVII. 
 PALAEOZOIC PERIODS CAMBRIAN 246 
 
 CHAPTER XVIII. 
 THE SILURIAN PERIOD 258 
 
 CHAPTER XIX. 
 DEVONIAN AND OLD RED SANDSTONE 268 
 
 CHAPTER XX. 
 THE CARBONIFEROUS PERIOD 277 
 
 CHAPTER XXI. 
 THE PERMIAN ROCKS 295 
 
 CHAPTER XXII. 
 MESOZOIO PERIODS TRIASSIC 302 
 
 CHAPTER XXIII. 
 THE JURASSIC PERIOD. . . 310 
 
CONTENTS V 
 
 CHAPTER XXIV. 
 
 Page 
 THE CRETACEOUS PERIOD 324 
 
 CHAPTER XXV. 
 THE TERTIARY PERIODS 338 
 
 CHAPTER XXVI. 
 TERTIARY PERIODS CONTINUED 350 
 
 CHAPTER XXVII. 
 
 POST-TERTIARY PERIODS 363 
 
 INDEX . 383 
 
GEOLOGY. 
 
 CHAPTEE I. 
 
 INTRODUCTORY. 
 
 THE main features of the dry land on which we live seem 
 to remain unchanged from year to year. The valleys 
 and plains familiar to our forefathers are still familiar 
 to us, bearing the same meadows and woodlands, the same 
 hamlets and villages, though generation after generation of 
 men has meanwhile passed away. The hills and mountains 
 now rise along the sky-line as they did long centuries ago, 
 catching as of old the fresh rains of heaven and gathering 
 them into the brooks and rivers which, through unknown ages, 
 have never ceased to flow seawards. So steadfast do these 
 features appear to stand, and so strong a contrast do they 
 offer to the shortness and changeableness of human life, that 
 they have become typical in our minds of all that -is ancient 
 and durable. We speak of the firm earth, of the everlasting 
 hills, of the imperishable mountains, as if, where all else is 
 fleeting and mutable, these forms at least remain unchanged. 
 
 And yet attentive observation of what takes place from day 
 to day around us shows that the surface of a country is not 
 now exactly as it used to be. We notice various changes of 
 its topography going on now, which have doubtless been in 
 progress for a long time, and the accumulated effect of which 
 may ultimately transform altogether the character of a land- 
 scape. A strong gale, for instance, will level thousands of 
 trees in its pathway, turning a tract of forest or woodland into 
 a bare space, which may become a quaking morass, until per- 
 haps changed into arable ground by the farmer. A flooded 
 river will in a few hours cut away large slices from its banks, 
 and spreading over fields and meadows, will bury many acres 
 
2 GEOLOGY 
 
 of fertile land under a covering of barren sand and shingle. 
 A long-continued, heavy rain, by loosening masses of earth or 
 rock on steep slopes, causes destructive landslips. A hard frost 
 splinters the naked fronts of crags and cliffs, and breaks up 
 bare soil. In short, every shower of rain and gust of wind, 
 if we could only watch them narrowly enough, would be found 
 to have done something towards modifying the surface of the 
 land. Along the sea-margin, too, how ceaseless is the progress 
 of change! In most places, the waves are cutting away the 
 land, sometimes even at so fast a rate as two or three feet 
 in a year. Here and there, on the other hand, they cast sand 
 and silt ashore so as to increase the breadth of the dry land. 
 
 These are ordinary everyday causes of alteration, and though 
 singly insignificant enough, their united effect after long cen- 
 turies cannot but be great. Prom time to time, however, other 
 less frequent but more powerful influences come into play. In 
 most large regions of the globe, the ground is often convulsed 
 by earthquakes, many of which leave permanent scars upon 
 the surface of the land. Volcanoes, too, in many countries 
 pour forth streams of molten rock and showers of dust and 
 cinders that bury the surrounding districts and greatly alter 
 their appearance. 
 
 Turning to the pages of human history, we find there the 
 records of similar changes in bygone times. Lakes, on which 
 our rude, forefathers paddled their canoes and built their 
 wattled island-dwellings, have wholly disappeared. Bogs, over 
 whose treacherous surface these early hunters could not follow 
 the chase of red deer or Irish elk, have become meadows and 
 fields. Forests, where they hunted the wild boar, have been 
 turned into grassy pastures. Cities have been entirely destroyed 
 by earthquakes or have been entombed under the piles of ashes 
 discharged from a burning mountain. So great have been the 
 inroads of the sea that, in some instances, the sites of what a 
 few hundred years ago were farms and hamlets, now lie under 
 the sea half a mile or more from the modern shore. . Else- 
 where the land has gained upon the sea, and the harbours 
 of an earlier time are now several miles distant from the 
 coast-line. 
 
 But man has naturally kept note only of the more impressive 
 changes; in other words, of those which had most influence 
 
INTRODUCTORY 3 
 
 upon his own doings. We may be certain, however, that there 
 have been innumerable minor alterations of the surface of the 
 land within human history, of which no chronicler has made 
 mention, either because they seemed too trivial, or because 
 they took place so imperceptibly as never to be noticed. For- 
 tunately, in many cases, these mutations of the land have 
 written their own memorials, which can be as satisfactorily in- 
 terpreted as the ancient manuscripts from which our early na- 
 tional history is compiled. 
 
 In illustration of the character of these natural chronicles, 
 let us for a moment consider the subsoil beneath cities that 
 have been inhabited for many centuries. In London, for ex- 
 ample, when excavations are made for drainage, building, or 
 other purposes, there are sometimes found, many feet below 
 the level of the present streets, mosaic pavements and founda- 
 tions, together with earthen vessels, bronze implements, orna- 
 ments, coins, and other relics of Eoman time. Now, if we 
 knew nothing, from actual authentic history, of the existence 
 of such a people as the Eomans, or of their former presence 
 in England, these discoveries, deep beneath the surface of mod- 
 ern London, would prove that long before the present streets 
 were built, the site of the city was occupied by a civilised race 
 which employed bronze and iron for the useful purposes of life, 
 had a metal coinage, and showed not a little artistic skill in 
 its pottery, glass, and sculpture. But down beneath the rubbish 
 wherein the Eoman remains are embedded, lie gravels and 
 sands from which rudely-fashioned human implements of flint 
 have been obtained. Whence we further learn that, before the 
 civilised metal-using people appeared, an earlier race had been 
 there, which employed weapons and instruments of roughly 
 chipped flint. 
 
 That this was the order of appearance of the successive 
 peoples that have inhabited the site of London is, of course, 
 obvious. But let us ask ourselves why it is obvious. We observe 
 that there are, broadly speaking, three layers or deposits from 
 which the evidence is derived. The upper layer is that which 
 contains the foundations and rubbish of modern London. Next 
 comes that which encloses the relics of the Eoman occupation. 
 At the bottom lies the layer that preserves the scanty traces of 
 the early flint-folk. The upper deposit is necessarily the newest,. 
 
4 GEOLOGY 
 
 for it could not be laid down until after the accumulation of 
 those below it, which must, of course, be progressively older, 
 as they are traced deeper from the surface. By the mere fact 
 that the layers lie one above another, we are furnished with a 
 simple clue which enables us to determine their relative time 
 of formation. We may know nothing whatever as to how old 
 they are measured by years or centuries. But we can be abso- 
 lutely certain of what is termed their " order of superposition/' 
 or chronological sequence; in other words, we can be confident 
 that the bottom layer came first and the top layer last. 
 
 This kind of observation and reasoning will enable us to 
 detect almost everywhere proofs that the surface of the land 
 has not always been what it is to-day. In some districts, for 
 example, when the dark layer of vegetable soil is turned up 
 which supports the plants that keep the land so green, there 
 may be found below it sand and gravel, full of smooth well- 
 rounded stones. Such materials are to be seen in the course 
 of formation where water keeps them moving to and fro, as 
 on the beds of rivers, the margins of lakes, or the shores of 
 the sea. Wherever smoothed rolled pebbles occur, they point 
 to the influence of moving water; so that we conclude, even 
 though the site is now dry land, that the sand and gravel 
 underneath it prove it to have been formerly under water. 
 Again, below the soil in other regions, lie layers of oysters and 
 other sea-shells. These remains, spread out like similar shells 
 on the beach or bed of the sea at the present day, enable us to 
 infer that where they lie the sea once rolled. 
 
 Pits, quarries, or other excavations that lay open still deeper 
 layers of material, bring before us interesting and impressive 
 testimony regarding the ancient mutations of the land. Suppose, 
 by way of further illustration, that underneath a bed of sand 
 full of oyster-shells, there lies a dark brown band of peat. This 
 substance, composed of mosses and other water-loving plants, 
 is formed in boggy places by the growth of marshy vegetation. 
 Below the peat there might occur a layer of soft white marl 
 full of lake-shells, such as may be observed on the bottoms of 
 many lakes at the present time (compare Fig. 30). These three 
 layers oyster-bed, peat, and marl would present a perfectly 
 clear and intelligible record of a curious series of changes in 
 the site of the locality. The bottom layer of white marl with 
 
INTRODUCTORY 5 
 
 its peculiar shells would show that at one time the place was 
 occupied by a lake. The next layer of peat would indicate that, 
 by the growth of marshy vegetation, the lake was gradually 
 changed into a morass. The upper layer of oyster-shells would 
 prove that the ground was then submerged beneath the sea. The 
 present condition of the ground shows that subsequently the 
 sea retired and the locality passed into dry land as it is to-day. 
 
 It is evident that by this method of examination information 
 may be gathered regarding early conditions of the earth's sur- 
 face, long before the authentic dates of human history. Such 
 inquiries form the subject of Geology, which is the science that 
 investigates the History of the Earth. The records in which 
 this history is chronicled are the soils and rocks under our feet. 
 It is the task of the geologist so to arrange and interpret these 
 records as to show through what successive changes the globe 
 has passed, and how the dry land has come to wear the aspect 
 which it presents at the present time. 
 
 Just as the historian would be wholly unable to decipher the 
 inscriptions of an ancient race of people unless he had first 
 discovered a key to the language in which they are written, so 
 the geologist would find himself baffled in his efforts to trace 
 backward the history of the earth if he were not provided with 
 a clue to the interpretation of the records in which that history 
 is contained. Such a clue is furnished to him by a study of 
 the operations of nature now in progress upon the earth's surface. 
 Only in so far as he makes himself acquainted with these modern 
 changes, can he hope to follow intelligently and successfully 
 the story of earlier phases in the earth's progress. It will be 
 seen that this truth has already been illustrated in the instances 
 above given of the evidence that the surface of the land has 
 not been always as it is now. The beds of sand and gravel, of 
 oyster-shells, of peat and of marl, would have told us nothing 
 as to ancient geography had we not been able to ascertain their 
 origin and history by finding corresponding materials now in 
 course of accumulation. To one ignorant of the peculiarities 
 of fresh-water shells, the layer of marl would have conveyed 
 no intelligible meaning. But knowing and recognising these 
 peculiarities, we feel sure that the marl marks the site of a 
 former lake. Thus the study of the Present supplies a key that 
 unlocks the secrets of the Past. 
 
6 GEOLOGY 
 
 In order, therefore, to trace back the history of the Earth, 
 the geologist must begin by carefully watching the changes 
 that now take place, and by observing how nature elaborates the 
 materials that preserve more or less completely the record of 
 these changes. In the following pages, I propose to follow 
 this method of inquiry, and, as far as the subject will permit, to 
 start with no assumptions which the learner cannot easily verify 
 for himself. We shall begin with the familiar everyday opera- 
 tions of the air, rain, frost, and other natural agents. It will not 
 be needful here to consider them in detail. We shall rather 
 pass on to inquire in what various ways they are engaged in 
 contributing to the formation of new mineral accumulations, 
 and in thereby providing fresh materials for the preservation 
 of the facts on which geological history is founded. Having 
 thus traced how new rocks are formed, we may then proceed 
 to arrange the similar rocks of older time, marking what are 
 the peculiarities of each and how they may best be classified. 
 
 If the labours of the geologist were concerned merely with 
 the former mutations of the earth's surface, how sea and land 
 have changed places, how rivers have altered their courses, how 
 lakes have been filled up, how valleys have been excavated, how 
 mountains, peaks, and precipices have been carved, how plains 
 have been spread out, and how the story of these revolutions 
 has been written in enduring characters upon the very frame- 
 work of the land, he would feel the want of one of the great 
 sources of interest in the study of the present face of nature. 
 We naturally connect all modern changes of the earth's surface 
 with the life of the plants and animals that flourish there, and 
 more especially with their influence on the progress of Man 
 himself. If there were no similar connection of the ancient 
 changes with once living things if the history of the earth 
 were merely one of dead inert matter it would lose much of 
 its interest for us. But happily that history includes the records 
 of successive generations of plants and animals which, from 
 early times, have peopled land and sea. The remains of these 
 organisms have been preserved in the deposits of different ages, 
 and can be compared and contrasted with those of the modern 
 world. 
 
 To realise how such preservation has been possible, and how 
 far the forms so retained afford an adequate picture of the life 
 
INTRODUCTORY 7 
 
 of the time to which they belonged, we must turn once more 
 to watch how nature deals with this matter at the present time. 
 Of the millions of flowers, shrubs, and trees which year after 
 year clothe the land with beauty, how many relics are preserved ? 
 Where are the successive generations of insect, bird, and beast 
 which have appeared in this country since man first set foot 
 upon its soil? They have utterly vanished. If all their living 
 descendants could suddenly be swept away, how could we tell 
 that such plants and animals ever lived at all? It must be 
 confessed that the vast majority of them leave no trace behind. 
 Nevertheless we should be able to recover relics of some of 
 them by searching in the comparatively few places where, at 
 the present day, dead plants and animals are entombed and 
 preserved. From the alluvial terraces of rivers, from the silt of 
 lake-bottoms, from the depths of peat-mosses, from the floors 
 of subterranean caverns, from the incrustations left by springs, 
 we might recover traces of some at least of the living things 
 that people the land. And from these fragmentary and incom- 
 plete records we might conjecture what may have been the gen- 
 eral character of the life of the time. By searching the similar 
 records of earlier ages the geologist has brought to light many 
 profoundly interesting vestiges of vegetation and of animal life 
 belonging to types that have long since passed away. 
 
 It must be evident, however, that were we to confine our 
 inquiries merely to its surface, we should necessarily gain a 
 most imperfect view of the general history of the Earth. Be- 
 neath that surface, as volcanoes show, there lies a hot interior, 
 which must have profoundly influenced the changes of the outer 
 parts or crust of the planet. The study of volcanoes enables us 
 to penetrate, as it were, a little way into that interior, and to 
 understand some of the processes in progress there. But our 
 knowledge of the inside of the Earth can obviously be based 
 only to a very limited extent on direct observation, for man 
 cannot penetrate far below the surface. The deepest mines do 
 not go deep enough to reach materials differing in any essential 
 respect from those visible above ground. Nevertheless, by infer- 
 ence from such observations as can be made, and by repeated 
 and varied experiments in laboratories, imitating as closely as 
 can be devised what may be supposed to be the conditions that 
 exist deep within the globe, some probable conclusions can be 
 
8 GEOLOGY 
 
 drawn even as to the changes that take place in those deeper 
 recesses that lie for ever concealed from our eyes. These con- 
 clusions will be stated in later chapters of this book, and the 
 rocks will be described, on the origin of which they appear to 
 throw light. 
 
 I have compared the soils and rocks with which geology deals 
 to the records out of which the historian writes the chronicles 
 of a nation. We might vary the simile by likening them to the 
 materials employed in the construction of a great building. It 
 is of course interesting enough to know what kinds of marble, 
 granite, mortar, wood, brass, or iron, have been chosen by an 
 architect. But much more important is it to inquire how these 
 various substances have been grouped together so as to form 
 such a building. In like manner, besides the nature and mode 
 of origin of the various rocks of which the visible and accessible 
 part of the earth consists, we ought to know how these varied 
 substances have been arranged so as to build up what we can 
 see of the outer part or crust of our globe. In short, we should 
 try to trace what may be called the architecture of the planet, 
 noting how each variety of rock occupies its own characteristic 
 place, and how they are all grouped and braced together in the 
 solid framework of the land. This then will be the next subject 
 for consideration in this volume. 
 
 ' But in a great historical edifice, like one of the Gothic min- 
 sters of Europe, for example, there are often several different 
 styles. A student of architecture can detect these distinctions, 
 and by their means can show that a cathedral has not been 
 completed in one age; that it may even have been partially 
 destroyed and rebuilt during successive centuries, only finally 
 taking its present form after many political vicissitudes and 
 many changes of architectural taste. Each edifice has thus a 
 separate history, which is recorded by the way the materials 
 have been shaped and put together in the various parts of the 
 masonry. So it is with the architecture of the Earth. We 
 have evidence of many demolitions and rebuildings, and the 
 story of their general progress can still be deciphered among 
 the rocks. It is the business of Geology to trace out that story, 
 to put all the scattered materials together, and to make known 
 by what a long succession of changes the Earth has reached 
 its present state. An outline of what science has accomplished 
 
INTRODUCTORY 9 
 
 in this task will form the last and concluding part of this book. 
 In the following chapters I wish two principles to be kept 
 steadily in View. In the first place, looking upon Geology as 
 the study of the Earth's history, we need not at first concern 
 ourselves with any details, save those that may be needed to 
 enable us clearly to understand what the general character and 
 progress of this history have been. In a science which em- 
 braces so vast a range as Geology, the multiplicity of facts to 
 be examined and remembered may seem at first to be almost 
 overwhelming. But a selection of the essential facts is sufficient 
 to give the learner a clear view of the general principles and 
 conclusions of the science, and to enable him to enter with 
 intelligence and interest into more detailed treatises. In the 
 second place, Geology is essentially a science of observation. 
 The facts with which it deals should, as far as possible, be 
 verified by our own personal examination. We should lose no 
 opportunity of seeing with our own eyes the actual progress of 
 the changes which it investigates, and the proofs which it adduces 
 of similar changes in the far past. To do this will lead us 
 into the fields and hills, to the banks of rivers and lakes, and 
 to the shores of the sea. We can hardly take any country walk, 
 indeed, in which with duly observant eye we may not detect 
 either some geological operation in actual progress, or the evi- 
 dence of one which was completed long ago. Having learnt 
 what to look for and how to interpret it when seen, we are 
 as it were gifted with a new sense. Every landscape comes to 
 possess a fresh interest and charm, for we carry about with us 
 everywhere an added power of enjoyment, whether the scenery 
 has long been familiar or presents itself for the first time. I 
 would therefore seek at the outset to impress upon those who 
 propose to read the following pages, that one of the main 
 objects with which this book is written is to foster a habit of 
 observation, and to serve as a guide to what they are them- 
 selves to look for, rather than merely to relate what has been 
 seen and determined by others. If they will so learn these 
 lessons, I feel sure that they will never regret the time and 
 labour they may spend over the task. 
 
10 GEOLOGY 
 
 PART I. 
 
 THE MATEKIALS FOR THE HISTOEY OF THE EAETH. 
 CHAPTER II. 
 
 INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE 
 EARTH'S SURFACE. 
 
 IN the history of mankind no sharp line can be drawn between 
 the events that are happening now or have happened within 
 the last few generations, and those that took place long 
 ago, and which are sometimes, though inaccurately, spoken of 
 as historical. Every people is enacting its history today just 
 as fully as it did many centuries ago. The historian recognises 
 this continuity in human progress. He knows that the feelings 
 and aspirations which guided mankind in old times were essen- 
 tially the same influences that impel them now, and therefore 
 that the wider his knowledge of his fellowmen of the present 
 day, the broader will be his grasp in dealing with the transac- 
 tions of former generations. So too is it with the history of 
 the Earth. That history is in progress now as really as it has 
 ever been, and its events are being recorded in the same way 
 and by the same agents as in the far past. Its continuity has 
 never been broken. Obviously, therefore, if we would explore 
 its records fe in the dark backward and abysm of time," we 
 should first make ourselves familiar with the manner in which 
 these records are being written from day to day before our 
 eyes. 
 
 In this first Part, attention will accordingly be given to the 
 changes in progress upon the Earth at the present time, and 
 to the various ways in which the passing of these changes is 
 chronicled in natural records. We shall watch the actual trans- 
 action of geological histor}^, and mark in what way its incidents 
 
INFLUENCE OF THE ATMOSPHERE 11 
 
 inscribe themselves on the page of the earth's surface. Every- 
 day and hour witness the enacting of some geological event, 
 trifiing and transient or stupendous and durable. Sometimes 
 the event leaves behind it only an imperceptible trace of its 
 passage, at other times it graves itself almost imperishably in 
 the annals of tne globe. In tracing the origin and development 
 of these geological annals of the present time, we shall best 
 qualify ourselves for deciphering the records of the early revo- 
 lutions of the planet. We are thereby led to study the various 
 chronicles compiled respectively by the air, rain, rivers, springs, 
 glaciers, the sea, plants and animals, volcanoes and earthquakes 
 in other words, all the deposits left by the operations of 
 these agents, the scars or other features made by them upon 
 the earth's surface, and all other memorials of geological change. 
 Having learnt how modern deposits are produced, and how they 
 preserve the story of their origin, we shall then be able to group 
 with them the corresponding deposits of earlier times, and to 
 embrace all the geological records, ancient as well as modern, 
 in one general scheme of classification. Such a scheme will 
 enable us to see the continuity of the materials of geological 
 history, and will fix definitely for us the character and relative 
 position of all the chief rocks out of which the visible part of 
 the globe is composed. 
 
 WEATHERING. The gradual change that overtakes everything 
 on the face of the earth is expressed in all languages by familiar 
 phrases which imply that the mere passing of time is the cause 
 of the change. As Sir Thomas Browne quaintly said more than 
 two hundred years ago, "time antiquates antiquities, and hath 
 an art to make dust of all things." We speak of the dust of 
 antiquity and the gnawing tooth of time. We say that things 
 are time-eaten, worn with age, crumbling under a weight of 
 years. Nothing suggests such epithets so strikingly as an old 
 building. We know that the masonry at first was smooth and 
 fresh; but now we describe it as weather-beaten, decayed, cor- 
 roded. So distinctive is this appearance that it is always looked 
 for in an ancient piece of stone-work; and if not seen, its 
 absence at once suggests a doubt whether the masonry can really 
 be old. No matter of what varieties of stone the edifice may 
 have been built, a few generations may be enough to give them 
 this look of venerable antiquity. The surface that was left 
 
12 GEOLOGY 
 
 smoothly polished by the builders grows rough and uneven, with 
 scars and holes eaten into it. Portions of the original polish 
 that may here and there have escaped, serve as a measure of 
 how much has actually been removed from the rest of the 
 surface. 
 
 Now, if in the lapse of time, stone which has been artificially 
 dressed is wasted away, we may be quite certain that the same 
 stone in its natural position on the slope of a hill or valley, 
 or by the edge of a river or of the sea, must decay in a similar 
 
 way. Indeed, an examination of 
 any crumbling building will show 
 that, in proportion as the chiselled 
 surface disappears, the stone puts 
 on the ordinary look which it wears 
 where it has never been cut by man, 
 and where only the finger of time 
 has touched it. Could we remove 
 some of the decayed stones from 
 the building and insert them into a 
 natural crag or cliff of the same 
 
 Fig. 1. Weathering of rock, as u-^j o f cfrmp fhpir Twnliar tirnp- 
 
 Kin Q6 l 
 
 shown by old masonry. (The 
 
 " false-bedding " and other worn aspect would be found to be 
 STare "reveal 'by "weatt so exactly that of the rest of the cliff 
 ering.) that probably no one would ever 
 
 suspect that a mason's tools had once been upon them. 
 
 From this identity of surface between the time-worn stones 
 of an old building and the stone of a cliff we may confidently 
 infer that the decay so characteristic of ancient masonry is as 
 marked upon natural faces of rock. The gradual disappearance 
 of the artificial smoothness given by the mason, and its replace- 
 ment by the ordinary natural rough surface of the stone, shows 
 that this natural surface must also be the result of decay. And 
 as the peculiar crumbling character is universal, we may be 
 sure that the decay with which it is connected must be general 
 over the globe. 
 
 But the mere passing of time obviously cannot change any- 
 thing, and to say that it does is only a convenient figure of 
 speech. It is not time^ but the natural processes which require 
 time for their work, that produce the widespread decay over 
 the surface of the earth. Of these natural processes, there are 
 
INFLUENCE OF THE ATMOSPHERE 13 
 
 four that specially deserve consideration changes of tempera- 
 ture,, saturation and desiccation, frost, and rain. 
 
 (1) Changes of Temperature. In countries where the 
 days are excessively hot, with nights correspondingly cool, the 
 surfaces of rocks heated sometimes, as in parts of Africa, up to 
 more than 130 Fahr. by a tropical sun, undergo considerable 
 expansion in consequence of this increase of temperature. At 
 night, on the other hand, the rapid radiation quickly chills the 
 stone and causes it to contract. Hence the superficial parts, 
 being in a perpetual state of strain, gradually crack up or peel 
 off. The face of a cliff is thus worn slowly backward, and the 
 prostrate blocks that fall from it are reduced to smaller frag- 
 ments and finally to dust. Where, as in Europe and the settled 
 parts of North America, the contrasts of temperature are not 
 so marked, the same kind of waste takes place in a less striking 
 manner. 
 
 (2) Saturation and Desiccation. Another cause of the decay 
 of the exposed surfaces of rocks is to be sought in the alter- 
 nate soaking of them with rain and drying of them in sunshine, 
 whereby the component particles of the stone are loosened and 
 fall to powder. Some kinds of stone freshly quarried and left 
 to this kind of action are rapidly disintegrated. The rock 
 called shale is peculiarly liable to decay from this cause. The 
 cliffs into which it sometimes rises show at their base long trails 
 of rubbish entirely derived from its waste. 
 
 (3) Frost. A third and familiar source of decay in stone 
 exposed to the atmosphere is to be found in the action of Frost. 
 The water that falls from the air upon the surface of the land 
 soaks into the soil and into the pores of rocks. When the 
 temperature of the air falls belong the freezing point, the im- 
 prisoned moisture expands as it passes into ice, and in expanding 
 pushes aside the particles between which it is entangled. Where 
 this takes place in soil, the pebbles and the grains of sand and 
 earth are separated from each other by the ice that shoots be- 
 tween them. They are all frozen into a solid mass that rings 
 like stone under our feet; but, as soon as a thaw sets in, the 
 ice that formed the binding cement passes into water which 
 converts the soil into soft earth or mud. This process, repeated 
 winter after winter, breaks up the materials of the soil, and 
 enables them to be more easily made use of by plants and more 
 
14 GEOLOGY 
 
 readily blown away by wind or washed of? by rain. Where 
 the action of frost affects the surface of a rock, the particles 
 separated from each other are eventually blown or washed away, 
 or the rock peels off in thin crusts or breaks up into angular 
 pieces, which are gradually disintegrated and removed. 
 
 (4) Rain. One further cause of decay may be sought in 
 the remarkable power possessed by Rain of chemically corrod- 
 ing stones. In falling through the atmosphere, rain absorbs 
 the gases of the air, and with their aid attacks surfaces of 
 rock. With the oxygen thus acquired, it oxidises those substances 
 which can still take more of this gas, causing them to rust 
 (See Metalloids). As a consequence of this alteration, the co- 
 hesion of the particles is usually weakened, and the stone 
 crumbles down. With the carbon-dioxide, or carbonic acid, it 
 dissolves and removes some of the more soluble ingredients in 
 the form of carbonates, thereby also usually loosening the com- 
 ponent particles of the stone. In general, the influence of rain 
 is to cause the exposed parts of rocks to rot from the surface 
 inward. Where the ground is protected with vegetation, the 
 decay is no doubt retarded; but in the absence of vegetation, 
 the outer crust of the decayed layer is apt to be washed off by 
 rain, or when dried to powder may be blown away and scattered 
 by wind. As fast as it is removed from the surface, however, 
 it is renewed underneath by the continued soaking of rain into 
 the stone. 
 
 Effects of Weathering. Hence one of the first lessons to 
 be learnt when from the common evidence around us we seek 
 to know what has been the history of the ground on which we 
 live is one of ceaseless decay. All over the land, in all kinds 
 of climates, and from various causes, bare surfaces of soil and 
 rock yield to the influences of the atmosphere or weather. The 
 decay thus set in motion is commonly called " weathering." 
 That it may often be comparatively rapid is familiarly and in- 
 structively shown in buildings or open-air monuments of which 
 the dates are precisely known. Marble tombstones in the grave- 
 yards of large towns, for example, hardly keep their inscriptions 
 legible for even so long as a century. Before that time, the 
 surface of the stone has crumbled away into a kind of sand. 
 Everywhere the weather-eaten surfaces, the crumbling crust of 
 
INFLUENCE OF THE ATMOSPHERE 
 
 15 
 
 decayed stone, and the scattered blocks and trains of rubbish, 
 tell their tale of universal waste. 
 
 It is well to take numerous opportunities of observing the 
 process of this decay in different situations and on various kinds 
 of materials. We can thus best realise the important part which 
 weathering must play in the changes of the earth's surface, and 
 we prepare ourselves for the consideration of the next question 
 that arises, What becomes of all the rotted material? a ques- 
 tion to answer which leads us into the very foundations of 
 geological history. 
 
 Openings from the soil down into the rock underneath often 
 afford instructive lessons regarding the decay of the surface 
 of the land. Fig. 2, for instance, is a 
 drawing of one of these sections, in which 
 a gradual passage may be traced from solid 
 sandstone (a) underneath up into broken- 
 up sandstone (&), and thence into the 
 earthy layer (c) that supports the vegeta- 
 tion of the surface. Traced from below 
 upwards, the rock is found to become more 
 and more broken and crumbling, with an 
 increasing number of rootlets that strike 
 freely through it in all directions, until 
 
 it passes insensibly into the uppermost Fig 2 Passage of 
 
 dark layer of vegetable soil or humus, sandstone upwards 
 This dark layer owes its characteristic 
 brown or black colour to the decaying remains of vegetation 
 diffused through it. Again, granite 
 in its unweathered state is a hard, 
 compact, crystalline rock that may 
 be quarried out in large solid blocks 
 (a in Fig. 3), yet when traced up- 
 ward to within a few feet from the 
 surface it may be seen to have been 
 split by innumerable rents into 
 fragments which are nevertheless 
 still lying in their original position. 
 As these fragments are attacked by 
 percolating moisture, their surfaces 
 decay, leaving the still unweathered 
 
16 GEOLOGY 
 
 parts as rounded blocks (&), which might at first be mistaken for 
 transported boulders. They are, however, part? of the rock bro- 
 ken up in place, and not fragments that have been carried from a 
 distance. The little quartz veins that traverse the solid granite 
 can be recognised running through the decayed and fresh parts 
 alike. But, besides being broken into pieces, the granite rots 
 away and loses its cohesion. Some of the smaller pieces can 
 be crumbled down between the fingers, and this decay increases 
 upwards, until the rock becomes a mere sand or sandy clay in 
 which a few harder kernels are still left. Into this soft layer 
 roots may descend from the surface, and, like the sandstone, 
 the granite merges above into the overlying soil (c). 
 
 Soil and Subsoil. In such sections as the foregoing, three 
 distinct layers can be recognised which pass into each other. 
 At the bottom lies the rock, either undecayed or at least still 
 fresh enough to show its true nature. Next comes the broken- 
 up crumbling layer through which stray roots descend, and 
 which is known as the subsoil. At the top lies the dark band, 
 crowded with rootlets and forming the true soil. These three 
 layers obviously represent successive stages in the decay of the 
 surface of the land. The soil is the layer of most complete 
 decay. The subsoil is an intermediate band where the progress 
 of decomposition has not advanced so far, while the shattered 
 rock underneath shows the earlier stages of disintegration. 
 Vegetation sends its roots and rootlets through the rotted rock. 
 As the plants die, they are succeeded by others, and the rotted 
 remains of their successive generations gradually darken the 
 uppermost decomposed layer. Worms, insects, and larger ani- 
 mals that may die on the surface, likewise add their mouldering 
 remains to this uppermost deposit. And thus from animals 
 and plants there is furnished to the soil that organic matter on 
 which its fertility so much depends. The very decay of the 
 vegetation helps to promote that of the underlying rock, for 
 it supplies various organic acids ready to be absorbed by per- 
 colating rain-water, the power of which to decompose rocks is 
 thereby increased. 
 
 It is obvious, then, that in answer to the question, What be- 
 comes of the rotted material produced by weathering? we may 
 confidently assert that, over surfaces of land protected by a 
 cover of vegetation, this material in large measure accumulates 
 
INFLUENCE OF THE ATMOSPHERE 17 
 
 where it is formed. Such accumulation will naturally take 
 place chiefly on flat or gently inclined ground. Where the slope 
 is steep, the decomposed layer will tend to travel down-hill by 
 mere gravitation, and to be further impelled downward by 
 descending rain-water. 
 
 If there is so intimate a connection between the soil at the 
 surface and. the rock underneath, we can readily understand 
 that soils should vary from one district to another, according 
 to the nature of the underlying rocks. Clays will produce clayey 
 soil, sandstones, sandy soil, or, where these two kinds of rock 
 occur together, they may give rise to sandy clay or loam. Hence, 
 knowing what the underlying rock is, we may usually infer 
 what must be the character of the overlying soil, or, from the 
 nature of the soil, we may form an opinion respecting the quality 
 of the rock that lies below. 
 
 But it will probably occur to the thoughtful observer that 
 when once a covering of soil and subsoil has been formed over 
 a level piece of ground, especially where there is also an over- 
 lying carpet of verdure, the process of decay should cease the 
 very layer of rotted material coming eventually to protect the 
 rock from further disintegration. Undoubtedly, under these 
 circumstances, weathering is reduced to its feeblest condition. 
 But that it still continues will be evident from 'some considera- 
 tions, the force of which will be better understood a few pages 
 further on. If the process were wholly arrested, then in course 
 of time plants growing on the surface would extract from the 
 soil all the nutriment they could get out of it, and with the 
 increasing impoverishment of the soil, they would dwindle away 
 and finally die out, until perhaps only the simpler forms of 
 vegetation would grow on the site. Something of this kind 
 not improbably takes place where forests decay and are replaced 
 by scrub and grass. But the long-continued vigorous growth of 
 the same kind of plants upon a tract of land doubtless indicates 
 that in some way the process of weathering is not entirely ar- 
 rested, but that, as generation succeeds generation, the plants 
 are still able to draw nutriment from fresh portions of decom- 
 posed rock. A cutting made through the soil and subsoil shows 
 that roots force their way downward into the rock, which splits 
 up and allows percolating water to soak downwards through it. 
 The subsoil thus gradually eats its way into the solid rock 
 
18 GEOLOGY 
 
 below. Influences are at work also, whereby there is an imper- 
 ceptible removal of material from the surface of the soil. 
 Notable among these influences are Rain, Wind, and Earth- 
 worms. 
 
 Wherever soil is bare of vegetation it is directly exposed to 
 removal by RAIN. Ground is seldom so flat that rain may not 
 flow a little way along the surface before sinking underneath. 
 In its flow, it carries off the finer particles of the soil. These 
 may travel each time only a short way, but as the operation is 
 repeated, they are in the course of years gradually moved down 
 to lower ground or to some runnel or brook that sweeps them 
 away seaward. Both on gentle and on steep slopes, this trans- 
 porting power of rain is continually removing the upper layer 
 of bared soil. 
 
 Where soil is exposed to the sun, it is liable to be dried into 
 mere dust, which is borne off by WIND. How readily this may 
 happen is often strikingly seen after dry weather in spring-time. 
 The earth of ploughed fields becomes loose and powdery, and 
 clouds of its finer particles are carried up into the air and trans- 
 ported to other farms, as gusts of wind sweep across. "March 
 dust," which is a proverbial expression, may be remembered as 
 an illustration of one way in which the upper parts of the soil 
 are removed. 
 
 Even where a grassy turf protects the general surface, bare 
 places may always be found whence this covering has been 
 removed. Rabbits, moles, and other animals throw out soil 
 from their burrows. Mice sometimes lay it bare by eating the 
 pasture down to the roots. The common EARTHWORMS bring 
 up to daylight in the course of a year an almost incredible 
 quantity of it in their castings. Mr. Darwin estimated that 
 this quantity is in some places not less than 10 tons per annum 
 over an acre of ground. Only the finest particles of mould 
 are swallowed by worms and conveyed by them to the surface, 
 and it is precisely these which are most apt to be washed off 
 by rain or to be dried and blown away as dust by the wind. 
 Where it remains on the ground, the soil brought up by worms 
 covers over stones and other objects lying there, which con- 
 sequently seem to sink into the earth. The operation of these 
 animals causes the materials of the soil to be thoroughly mixed. 
 In tropical countries, the termite or "white ant" conveys a 
 
INFLUENCE OF THE ATMOSPHERE 19 
 
 prodigious amount of fine earth up into the open air. With 
 this material it builds hills sometimes 60 feet high and visible 
 for a distance of several miles; likewise tunnels and chambers, 
 which it plasters all over the stems and branches of trees, often 
 so continuously that hardly any bark can be seen. The fine 
 soil thus exposed is liable to be blown away by the wind or 
 washed off by the fierce tropical rains. 
 
 Although, therefore, the layer of vegetable soil which covers 
 the land appears to be a permanent protection, it does not really 
 prevent a large amount of material from being removed even 
 from grassy ground. It forms the record of the slow and 
 almost imperceptible geological changes that affect the regions 
 where it accumulates, the quiet fall of rain, the gradual rot- 
 ting away of the upper part of the underlying rock, the growth 
 and decay of a long succession of generations of plants, the 
 
 ^:..^K- . 
 
 
 Fig. 4. Talus-slopes at the foot of a line of cliffs. 
 
 ceaseless labours of the earthworm, the scarcely appreciable 
 removal of material from the surface by the action of rain and 
 wind, and the equally insensible descent of the crumbling subsoil 
 farther and farther into the solid stone below. Having learnt 
 how all this is told by the soil beneath our feet, we should be 
 ready to recognise in the soil of former ages a similar chronicle 
 of quiet atmospheric disintegration. 
 
 Talus. Besides soil and subsoil, there are other forms in 
 which decomposed rock accumulates on the surface of the land. 
 Where a large mass of bare rock rises up as a steep bank or 
 cliff, it is liable to constant degradation, and the materials 
 detached from its surface accumulate down the slopes, forming 
 what is known as a Talus (Fig. 4). In mountainous or hilly 
 regions, where rocky precipices rise high into the air, there 
 
20 GEOLOGY 
 
 gather at their feet and down their clefts long trails or screes 
 of loose blocks that have been split off from them by the weather. 
 Such slopes, especially where they are not too steep, and where 
 the rubbish that forms them is not too coarse, may be more 
 or less covered with vegetation, which in some measure arrests 
 the descent of the debris. But from time to time, during heavy 
 rains, deep gullies are torn out of them by rapidly formed 
 torrents, which sweep down their materials to lower levels 
 (Fig. 10). The sections laid bare in these gullies show that 
 the rubbish is arranged in more or less distinct layers which 
 lie generally parallel with the surface of the slope; in other 
 words, it is rudely stratified, and its layers or strata are inclined 
 at the angle of the declivity which seldom exceeds 35. 
 
 Rain-wash, Brick-earth. On more gen- 
 tle slopes, even where no bare rock projects 
 into the air, the fall of rain gradually 
 washes down the upper parts of the soil 
 to lower levels. Hence arise thick accum- 
 ulations of what is known as rain-wasli 
 soil mixed often with angular fragments 
 of still undecomposed rock, and not in- 
 frequently forming a kind of brick-earth 
 (Fig. 5). Deposits of this nature are still 
 
 _,. .. gathering now, though their lower portions 
 
 Fig. 5. Section of g to 
 
 rain-wash or brick- may be oi great antiquity. In the soutn- 
 
 Ioih h '6 7 Br7ck g -fa a rth e east of En g land > for instance, the brick- 
 
 5. White sand. 4. earths contain the bones of animals that 
 
 W hYtV e sand. I. nave ^ on since passed away. 
 
 Brick-earth. 1. Dust. By the action of wind, above 
 
 Gravel with seams /? n , * n ^ n 
 
 of sand. referred to, a vast amount oi fine dust and 
 
 sand is carried up into the air and 
 strewn far and wide over the land. In dry countries, such as 
 large tracts of Central Asia, the air is often thick with a fine 
 yellow dust which may entirely obscure the sun at mid-day, 
 and which settles over everything. After many centuries, a 
 deposit, which may be hundreds of feet deep, is thus accumulated 
 on the surface of the land. Some of the ancient cities of the 
 Old World, Nineveh and Babylon for example, after being long 
 abandoned by man, have gradually been buried under the fine 
 soil drifted over them by the wind and intercepted and protected 
 
INFLUENCE OF THE ATMOSPHERE 21 
 
 by the weeds that grew up over the ruins. Even in regions 
 where, as in Britain, there is a large annual rainfall, seasons 
 of drought occur, during which there may be a considerable 
 drifting of the finer particles of soil by the wind. We probably 
 hardly realise how much the soil may be removed here and 
 heightened there from this cause. 
 
 Sand-dunes. Some of the most striking and familiar ex- 
 amples of the accumulation of loose deposits by the wind are 
 those to which the name of Dunes is given. On sandy shores, 
 exposed to winds that blow landwards, the sand is dried and 
 then carried away from the beach, gathering into long mounds 
 or ridges which run parallel to the coast-line. These ridges are 
 often 50 or 60 feet, sometimes even more than 250 feet high, 
 with deep troughs and irregular circular hollows between them, 
 and they occasionally form a strip several miles broad, bor- 
 dering the sea. The particles of sand are driven inland by 
 
 Fig. 6. Sand-dunes. 
 
 the wind, and the dunes gradually bury fields, roads, and villages, 
 unless their progress is arrested by the growth of vegetation 
 over their shifting surfaces. On many parts of the west coast 
 of Europe, the dunes are marching into the interior at the rate 
 of 20 feet in a year. Hence large tracts of land have within 
 historic times been entirely lost under them. In the north of 
 Scotland, for example, an ancient and extensive barony, so 
 noted for its fertility that it was called " the granary of Moray/' 
 
22 GEOLOGY 
 
 I 
 
 was devastated about the middle of the seventeenth century by 
 the moving sands, which now rise in barren ridges more than 
 100 feet above the site of the buried land. In the interior of 
 continents also, where with great dryness of climate there is a 
 continual disintegration of the surface of rocks, wide wastes 
 of sand accumulate, as in the deserts of Libya, Arabia, and 
 Gobi, in the heart of Australia, and in many of the western 
 parts of the United States. 
 
 There can be no doubt, however, that though the layer of 
 vegetable soil, the heaps of rubbish that gather on slopes and 
 at the base of rocky banks and precipices, and the widespread 
 drifting of dust and sand over the land, afford evidence that 
 much of the material arising from the general decay of the 
 surface of the land accumulates under various forms upon that 
 surface, nevertheless its stay there is not permanent. Wind 
 and rain are continually removing it, sometimes in vast quan- 
 tities, into the sea. Every brook, made muddy by heavy rain, 
 is an example of this transport, for the mud that discolours 
 the water is simply the finer material of the soil washed off 
 by rain. When we reflect upon the multitude of streams, large 
 and small, in all parts of the globe, and consider that they 
 are all busy carrying their freights of mud to the sea, we can 
 in some measure appreciate how great must be the total annual 
 amount of material so removed. What becomes of this material 
 will form the subject of succeeding chapters. 
 
 SUMMARY. The first lesson to be learnt from an examination 
 of the surface of the land is, that everywhere decay is in progress 
 upon it. Wherever the solid rock rises into the air, it breaks 
 up and crumbles away under the various influences combined 
 in the process of Weathering. The wasted materials caused by 
 this universal disintegration partly accumulate where they are 
 formed, and make soil. But in large measure, also, they are 
 blown away by wind and washed off by rain. Even where they 
 appear to be securely protected by a covering of vegetation, the 
 common earth-worm brings the finer parts of them up to the 
 surface, where they come within reach of rain and wind, so that 
 on tracts permanently grassed over, there may be a continuous 
 and not inconsiderable removal of fine soil from the surface. 
 In proportion as the upper layers of soil are removed, roots and 
 percolating water are enabled to reach down farther into the 
 
INFLUENCE OF THE ATMOSPHERE 23 
 
 solid rock which is broken up into subsoil, and thus the general 
 surface of the land is insensibly lowered. 
 
 Besides accumulating in situ as subsoil and soil, the debris 
 of decomposed rock forms talus-slopes and screes at the foot of 
 crags, and a layer of rain-wash or brick-earth over gentler slopes. 
 Where the action of wind comes markedly into play, tracts of 
 sand-dunes may be piled up along the borders of the sea and 
 of lakes, or in the arid interior of continents ; and wide regions 
 have been in course of time buried under the fine dust which 
 is sometimes so thick in the air as to obscure the noonday sun. 
 But in none of these forms can the accumulation of decomposed 
 material be regarded as permanent. So long as it is exposed 
 to the influences of the atmosphere, this material is still liable 
 to be swept away from the surface of the land and borne out- 
 wards into the sea. 
 
24 GEOLOGY 
 
 CHAPTER III. 
 
 INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND 
 HOW IT IS RECORDED. 
 
 IT appears, then, that from various causes all over the globe, 
 there is a continual decay of the surface of the land; that 
 the decomposed material partly accumulates as soil, subsoil, 
 and sheets or heaps of loose earth or sand, but that much of it 
 is washed off the land by rain or blown into the rivers or into 
 the sea by wind. We have now to consider the part taken by 
 Running Water in this transport. From the single rain-drop 
 up to the mighty river, every portion of the water that flows 
 over the land is busy with its own share of the work. When 
 we reflect on the amount of rain that falls annually over the 
 land, and on the number of streams, large, and small, that are 
 ceaselessly at work, we realise how difficult it must be to form 
 any fit notion of the entire amount of change which, even in 
 a single year, these agents work upon the surface of the earth. 
 
 The influence of rain in the decay of the surface of the land 
 was briefly alluded to in the last chapter. As soon as a drop 
 of rain reaches the ground, it begins its appointed geological 
 task, dissolving what it can carry off in solution, and pushing 
 forward and downward whatever it has power to move. As the 
 rain-drops gather into runnels, the same duty, but on a larger 
 scale, is performed by them; and as the runnels unite into 
 large streams, and these into yet mightier rivers, the operations, 
 though becoming colossal in magnitude, remain essentially the 
 same in kind. In the operations of the nearest brook, we see 
 before us in miniature a sample of the changes produced by 
 the thousands of rivers which, in all quarters of the globe, are 
 flowing from the mountains to the sea. Watching these opera- 
 tions from day to day, we discover that they may all be classed 
 under two heads. In the first place, the brook hollows out the 
 
INFLUENCE OF RUNNING WATER 25 
 
 channel in which it flows and thus aids in the general waste 
 of the surface of the land; and in the second place, it carries 
 away fine silt and other material resulting from that waste, 
 and either deposits it again on the land or carries it out to sea. 
 Rivers are thus at once agents that themselves directly degrade 
 the land, and that sweep the loosened detritus towards the 
 ocean. An acquaintance with each of these kinds of work is 
 needful to enable us to understand the nature of the records 
 which river-action leaves behind it. 
 
 i. EROSIVE AND TRANSPORTING POWER OF RUNNING WATER. 
 
 CHEMICAL ACTION. We have seen that rain in its descent 
 from the clouds absorbs air, and that with the oxygen and 
 carbonic acid which it thus obtains it proceeds to corrode the 
 surfaces of rock on which it falls. When it reaches the ground 
 and absorbs the acids termed "humous," which are supplied 
 by the decomposing vegetation of the soil, it acquires increased 
 power of eating into the stones over which it flows. When it 
 rolls along as a runnel, brook, or river, it no doubt still attacks 
 the rocks of its channel, though its action in this respect is not 
 so easily detected. In some circumstances, however, the solvent 
 influence of river-water upon solid rocks is strikingly displayed. 
 Where the water contains a large proportion of the acids of 
 the soil, and flows over a kind of rock specially liable to be 
 eaten away by these acids, the most favourable conditions are 
 presented for observing the change. Thus, a stream which 
 issues from a peat-bog is usually dark brown in colour, from 
 the vegetable solutions which it extracts from the moss. Among 
 these solutions are some of the organic acids referred to, ready 
 to eat into the surface of the rocks or loose stones which the 
 stream may encounter in its descent. No kind of rock is more 
 liable than limestone to corrosion under such circumstances. 
 Peaty water flowing over it eats it away with comparative 
 rapidity, while those portions of the rock that rise above the 
 stream escape solution, except in so far as they are attacked 
 by rain. Hence arise some curious features in the scenery of 
 limestone districts. The walls of limestone above the water, 
 being attacked only by the atmosphere, are not eaten away so 
 fast as their base, over which the stream flows. They are con- 
 
26 GEOLOGY 
 
 sequently undermined, and are sometimes cut into dark tunnels 
 and passages (Fig. 7). Even where the solvent action of the 
 water of rivers is otherwise inappreciable, it can be detected by 
 means of chemical analysis. Thus rivers, partly by the action 
 of their water upon the loose stones and solid rocks of their 
 channels, and parti} b} th' contributions they receive from 
 Spring!: (which will be afterward; described), convey a vast 
 amount of dissolved material into the sea. The mineral sub- 
 stance thus invisibly transported consists of various salts. One 
 of the most abundant of these carbonate of lime is the 
 substance that form limestone, and furnishes the mineral mat- 
 ter required for the hard parts of a large proportion of the 
 lower animals. It is a matter of some interest to know that 
 
 Fig. 7. Erosion of limestone by the solvent action of a peaty stream, 
 Durness, Sutherlandshire. 
 
 this substance, so indispensable for the formation of the shells 
 of so large a number of sea-creatures, is constantly supplied 
 to the sea by the streams that flow into it. 1 The rivers of 
 Western Europe, for instance, ha.ve been ascertained to convey 
 about 1 part of dissolved mineral matter in every 5000 parts 
 of water, and of this mineral matter about a half consists of 
 carbonate of lime. It has been estimated that the Ehine bears 
 
 1 There is now reason, however, to suspect that the carbonate of lime 
 in marine organisms is not derived so much from the comparatively 
 minute proportion of that substance present in solution in sea-water, 
 as from the much more abundant sulphate of lime which undergoes 
 apparently a process of chemical transformation into carbonate within 
 the living animals. 
 
INFLUENCE OF RUNNING WATER 27 
 
 enough carbonate of lime into the sea every year to make three 
 hundred and thirty-two thousand millions of oysters of the 
 usual size. Another abundant ingredient of river-water is 
 gypsum or sulphate of lime, of which the Thames is computed 
 to carry annually past London not less than 180,000 tons. The 
 total quantity of carbonate of lime, removed from the limestones 
 of its basin by this river in a year, amounts, on an average, 
 to 140 tons from every square mile, which is estimated to be 
 equal to the lowering of the general surface to the extent 
 of dfo of an inch from each square mile in a century, or one foot 
 in 13,200 years. 
 
 MECHANICAL ACTION. (1) Transport. The dissolved ma- 
 terial forms but a small proportion of the total amount of 
 mineral substances conveyed by rivers from land to sea. A 
 single shower of rain washes off fine dust and soil from the 
 surface of the ground into the nearest brook which then rolls 
 along with a discoloured current. An increase in the volume 
 of the water enables a stream to sweep along sand, gravel, and 
 blocks of stone lying in its channel, and to keep these materials 
 moving until, as the declivity lessens and the rain ceases, the 
 current becomes too feeble to do more than lazily carry onward 
 the fine silt that discolours it. Every stream, large or small, is 
 ceaselessly busy transporting mud, sand, or gravel. And as the 
 ultimate destination of all this sediment is the bottom of the 
 sea, it is evident that if there be no compensating influences 
 at work to repair the constant loss, the land must in the end 
 be all worn away. 
 
 Some of the most instructive lessons regarding the work of 
 running water on land are afforded by the beds of mountain- 
 torrents. Huge blocks, detached from the crags and cliffs on 
 either side, may there be seen cumbering the pathway of the 
 water, which seems quite powerless to move such masses and 
 can only sweep round them or find a passage beneath them. 
 But visit such a torrent when it is swollen with heavy rains 
 or rapidly melted snow, and you will hear the stones knocking 
 against each other or on the rocky bottom, as they are driven 
 downwards by the flood. Or when the stream is at its lowest, 
 in dry summer weather, follow its course a little way down hill, 
 and you will see that by degrees the blocks, losing their sharp 
 edges, have become rounded boulders, and that these are gradu- 
 
28 GEOLOGY 
 
 ally replaced by coarse shingle and then by finer gravel. In 
 the quieter reaches of the water, sheets of sand begin to make 
 their appearance, and at last when the stream reaches the plains, 
 no sediment of coarser grain than mere silt may be seen in 
 its channel. It is thus obvious that in the constant transport 
 maintained by watercourses, the carried materials, by being rolled 
 along rocky channels and continually ground against each other, 
 diminish in size as they descend. A river flowing from a range 
 of mountains to the distant ocean may be likened to a mill, 
 into which large angular masses of rock are cast at the upper 
 end, and out of which only fine sand and silt are discharged at 
 the lower. 
 
 Partly, therefore, owing to the fine dust and soil swept into 
 them by wind and rain from the slowly decomposing surface 
 of the land, and partly to the friction of the detritus which 
 they sweep along their channels, rivers always contain more 
 or less mineral matter suspended in their water or travelling 
 with the current on the bottom. The amount of material thus 
 transported varies greatly in different rivers, and at successive 
 seasons even in the same river. In some cases, the rainfall is 
 spread so equably through the year that the rivers flow onward 
 with a quiet monotony, never rising much above nor sinking 
 much below their average level. In such circumstances, the 
 amount of sediment they carry downward is proportionately 
 small. On the other hand, where either from heavy periodical 
 rains or from rapid melting of snow, rivers are liable to floods, 
 they acquire an enormously increased power of transport, and 
 their burden of sediment is proportionately augmented. In a 
 few days or weeks of high water, they may convey to the sea a 
 hundredfold the amount of mineral matter which they could 
 carry in a whole year of their quieter mood. 
 
 Measurements have been made of the proportions of sediment 
 in the waters of different rivers at various seasons of the year. 
 The results, as might be expected, show great variations. Thus 
 the Garonne, rising among the higher peaks of the Pyrenees, 
 drains a large area of the south of France, and is subject to floods 
 by which an enormous quantity of sediment is swept down from 
 the mountains to the plains. Its proportion of mud has been 
 estimated to be as much as 1 part in 100 parts of water. The 
 Durance, which takes its source high on the western flank of the 
 
INFLUENCE OF RUNNING WATER 29 
 
 Cottian Alps, is one of the rapidest and muddiest rivers in 
 Europe. Its angle of slope varies from 1 in 208 to 1 in 467, the 
 average declivity of the great rivers of the- globe being probably 
 not more than 1 in 2600, while that of a navigable stream ought 
 not to exceed 10 inches per mile or 1 in 6336. The Durance is, 
 therefore, rather a torrent than a river. With this rapidity of 
 descent is conjoined an excessive capacity for transporting sedi- 
 ment. In floods of exceptional severity, the proportion of mud 
 in the stream has been estimated at one-tenth by weight of the 
 water, while the average proportion for nine years from 1867 
 to 1875 was about -sh. Probably the best general average is 
 to be obtained from a river which drains a wide region exhibiting 
 considerable diversities of climate, topography, rocks, and soils. 
 The Mississippi presents a good illustration of these diversities, 
 and has accordingly been taken as a kind of typical river, fur- 
 nishing, so to speak, a standard by which the operations of other 
 rivers may be compared, and which may perhaps be assumed as 
 a fair average for all the rivers of the globe. Numerous meas- 
 urements have been made of the proportion of sediment carried 
 into the Gulf of Mexico by this vast river, with the result of 
 showing that the average amount of sediment is by weight 1 part 
 in every 1500 parts of water, or little more than one-third of the 
 proportion in the water of the Durance. 
 
 If now we assume that, all over the world, the general average 
 proportion of sediment floating in the water of rivers is 1 part in 
 every 1500 of water, we can readily understand how seriously in 
 the course of time must the land be lowered by the constant 
 removal of so much decomposed rock from its surface. Knowing 
 the area of the basin drained by a river, and also the proportion 
 of sediment in its water, we can easily calculate the general loss 
 from the surface of the basin. The ratio of the weight or 
 " specific gravity " of the silt to that of solid rock may be taken 
 to be as 19 is to 25. Accordingly the Mississippi conveys annually 
 from its drainage basin an amount of sediment equivalent to the 
 removal of FoW part of a foot of rock from the general sur- 
 face of the basin. At this rate, one foot of rock will be worn 
 away every 6000 years. If we take the general height of the 
 land of the whole globe to be 2120 feet, and suppose it to be con- 
 tinuously wasted at the same rate at which the Mississippi basin 
 is now suffering, then the whole dry land would be carried into 
 
30 GEOLOGY 
 
 the sea in 12,720,000 years. Or if we assume the mean height of 
 Europe to be 973 feet and that this continent is degraded at the 
 Mississippi rate of waste until the last vestige of it disappears, 
 the process of destruction would be completed in rather less than 
 6,000,000 years. Such estimates are not intended to be close 
 approximations to the truth. As the land is lowered, the rate 
 of decay will gradually diminish, so that the later stages of decay 
 will be enormously protracted. But by taking the rate of opera- 
 tion now ascertained to be in progress in such a river basin as the 
 Mississippi, we obtain a valuable standard of comparison, and 
 learn that the degradation of the land is much greater and more 
 rapid than might have been supposed. 
 
 (2) Erosion. But rivers are not merely carriers of the mud, 
 sand, and gravel swept into their channels by other agencies. By 
 keeping these materials in motion, the currents reduce them in 
 size, and at the same time employ them to hollow out the chan- 
 nels wherein they move. The mutual friction that grinds down 
 large blocks of rock into sand and mud, tells also upon the rocky 
 beds along which the material is driven. The most solid rocks 
 are worn down; deep long gorges are dug out, and the water- 
 courses, when they have once chosen their sites, remain on them 
 and sink gradually deeper and deeper beneath the general level 
 of the country. The surfaces of stone exposed to this attrition 
 assume the familiar smoothed and rounded appearance which is 
 known as water-worn. The loose stones lying in the channel of 
 a stream, and the solid rocks as high up as floods can scour them, 
 present this characteristic aspect. Here and there, where a few 
 stones have been caught in an eddy of the current, and are kept 
 in constant gyration, they reduce each other in dimensions, and 
 at the same time grind out a hollow in the underlying rock. The 
 sand and mud produced by the friction are swept off by the 
 current, and the stones when sufficiently reduced in size are also 
 carried away. But their places are eventually taken by other 
 blocks brought down by floods, so that the supply of grinding 
 material is kept up until the original hollow is enlarged into a 
 wide deep caldron, at the bottom of which the stones can only 
 be stirred by the heaviest floods. Cavities of this kind, known as 
 pot-holes, are of frequent occurrence in rocky watercourses as 
 well as on rocky shores, in short, wherever eddies of water can 
 keep shingle rotating upon solid rock. As they often coalesce 
 
INFLUENCE OF RUNNING WATER 
 
 31 
 
 by the wearing away of the intervening wall of rock, they greatly 
 aid in the deepening of a watercourse. In most rocky gorges, a 
 succession of old pot-holes may be traced far above the present 
 level of the stream (Fig. 8). 
 
 That it is by means of the gravel and other detritus pushed 
 along the bottom by the current, rather than by the mere friction 
 of the water on its bed, that a river excavates its channel, is most 
 strikingly shown immediately below a lake. In traversing a lake^ 
 
 Fig. 8. Pot-holes worn out by the gyration of stones in the bed of a 
 
 stream. 
 
 the tributary streams deposit their sediment on its bottom, be- 
 cause the still water checks their current and, by depriving the 
 water of its more rapid movement, compels it to drop its burden 
 of gravel, sand, and silt (see Lakes). Filtered in this way, the 
 various streams united in the lake escape at its lower end as a 
 clear transparent river. The Khone, for instance, flows into the 
 Lake of Geneva as a turbid stream; it issues from that great 
 reservoir at Geneva as a rushing current of the bluest, most 
 translucent water which, though it sweeps over ledges of rock, 
 has not yet been able to grind them down into a deep gorge. The 
 
32 GEOLOGY 
 
 Niagara, also, filtered by Lake Erie, has not acquired sediment 
 enough to enable it to cut deeply into the rocks over which it 
 foams in its rapids before throwing itself over the great Falls. 
 
 One of the most characteristic features of streams is the 
 singularly sinuous courses they follow. As a rule, too, the natter 
 the ground over which they flow, the more do they wind. Not 
 uncommonly they form loops, the nearest bends of which in the 
 end unite, and as the current passes along the now straightened 
 channel, the old one is left to become by degrees a lake or pond 
 of stagnant water, then a marsh, and lastly, dry ground. We 
 might suppose that in flowing off the land, water would take the 
 shortest and most direct road to the sea. But this is far from 
 being the case. The slightest inequalities of level have originally 
 determined sinuosities of the channels, while trifling differences 
 in the hardness of the banks, in the accumulation of sediment, 
 and in the direction of the currents and eddies, have been enough 
 to turn a stream now to one side now to another, until it has 
 assumed its present meandering course. How easily this may be 
 done can be instructively observed on a roadway or other bare 
 surface of ground. When quite dry and smooth, hardly any de- 
 pressions in which water would flow might be detected on such 
 a surface. But after a heavy shower of rain, runnels of muddy 
 water will be seen coursing down the slope in serpentine chan- 
 nels that at once recall the winding rivers of a great drainage- 
 system. The slightest differences of level have been enough to 
 turn the water from side to side. A mere pebble or projecting 
 heap of earth or tuft of grass has sufficed to cause a bend. The 
 water, though always descending, has only been able to reach 
 the bottom by keeping the lowest levels, and turning from right 
 to left as these guided it. 
 
 When a river has once taken its course and has begun to ex- 
 cavate its channel, only some great disturbance, such as an 
 earthquake or volcanic eruption, can turn it out of that course. 
 If its original pathway has been a winding one, it goes on dig- 
 ging out its bed which, with all its bends, gradually sinks below 
 the level of the surrounding country. The deep and pictur- 
 esque gorge in which the Moselle winds from Treves to Coblenz 
 has in this way been slowly eroded out of the undulating table- 
 land across which the river originally flowed. 
 
 In another and most characteristic way, the shape of the 
 
INFLUENCE OF RUNNING WATER 
 
 33 
 
34 GEOLOGY 
 
 ground and the nature and arrangement of the rocks over which 
 they flow, materially influence rivers in the forms into which 
 they carve their channels. The Rhone and the Niagara, for 
 instance, though filtered by the lakes through which they flow, 
 do not run far before plunging into deep ravines. Obviously 
 such ravines cannot have been dug out by the same process of 
 mechanical attrition whereby river-channels in general are 
 eroded. Yet the frequency of gorges in river scenery shows that 
 they cannot be due to any exceptional operation. They may 
 generally be accounted for by some arrangement of rocks wherein 
 a bed of harder material is underlain by one more easily re- 
 movable. Where a stream, after flowing over the upper bed, 
 encounters the decomposable bed below, it eats away the latter 
 more rapidly. The overlying hard rock is thus undermined, 
 and, as its support is destroyed, slice after slice is cut away 
 from it. The waterfall which this kind of structure produces 
 continues to eat its way backward or up the course of the stream, 
 so long as the necessary conditions are maintained of hard rocks 
 lying upon soft. Any change of structure which would bring 
 the hard rocks down to the bed of the channel, and remove the 
 soft rocks from the action of the current and the dash of the 
 spray, would gradually destroy the waterfall. It is obvious, 
 that, by cutting its way backward, a waterfall excavates a ravine. 
 The renowned Falls of Niagara supply a striking illustration 
 of the process now described. The vast body of water which 
 issues from Lake Erie, after flowing through a level country 
 for a few miles, rushes down its rapids and then plunges over a 
 precipice of solid limestone. Beneath this hard rock lies a 
 band of comparatively easily eroded shale. As the water loosens 
 and removes the lower rock, large portions of the face of the 
 precipice behind the Falls are from time to time precipitated 
 into the boiling flood below. The waterfall is thus slowly pro- 
 longing the ravine below the Falls. The magnificent gorge in 
 which the Niagara, after its tumultuous descent, flows sullenly to 
 Lake Ontario is not less than 7 miles long, from 200 to 400 
 yards wide, and from 200 to 300 feet deep. There is no reason 
 to doubt that this chasm has been entirely dug out by the grad- 
 ual recession of the Falls from the cliffs at Queenston, over 
 which the river at first poured. We may form some conception 
 of the amount of rock thus removed from the estimate that it 
 
INFLUENCE OF RUNNING WATER 35 
 
 would make a rampart about 12 feet high and 6 feet thick ex- 
 tending right round the whole globe at the equator. Still more 
 gigantic are the gorges or canons of the Colorado and its tribu- 
 taries in Western America. The Grand Canon of the Colorado 
 is 300 miles long, and in some places more than 6000 feet deep 
 (Fig. 9). The country traversed by it is a network of profound 
 ravines, at the bottom of which the streams flow that have eroded 
 them out- of the table-land. 
 
 ii. DEPOSITION OF MATERIALS BY RUNNING WATER. 
 
 Permanent Records of River- Action. If, then, all the 
 streams on the surface of the globe are engaged in the double 
 task of digging out their channels and carrying away the loose 
 materials that arise from the decomposition of the surface of 
 the land, let us ask ourselves what memorials of these operations 
 they leave behind them. In what form do the running waters 
 of the land inscribe their annals in geological history ? If these 
 waters could suddenly be dried up all over the earth, how could 
 we tell what changes they had once worked upon the surface of 
 the land ? Can we detect the traces of ancient rivers where there 
 are no rivers now? 
 
 From what has been said in this lesson it will be evident that 
 in answer to such questions as these, we may affirm that one un- 
 mistakable evidence of the former presence of rivers is to be 
 found in the channels which they have eroded. The gorges, 
 rocky denies, pot-holes, and water-worn rocks which mark the 
 pathway of a stream would long remain as striking memorials 
 of the work of running water. In districts, now dry and bar- 
 ren, such as large regions in the Levant, there are abundant 
 channels (wadies) now seldom or never occupied by a stream, 
 but which were evidently at one time the beds of active torrents. 
 
 Alluvium. But more universal testimony to the work of 
 running water is to be found in the deposits which it has accumu- 
 lated. To these deposits the general name of alluvium has been 
 given. Spreading out on either side, sometimes far beyond the 
 limits of the ordinary or modern channels, these deposits, even 
 when worn into fragmentary patches, retain their clear record of 
 the operations of the river. Let us in imagination follow the 
 course of a river from the mountains to the sea, and mark as 
 
36 GEOLOGY 
 
 we go the circumstances under which the accumulation of sedi- 
 ment takes place. 
 
 The power possessed by running water to carry forward sedi- 
 ment depends mainly upon the velocity of the current. The 
 more rapidly a stream flows, the more sediment can it transport, 
 and the larger are the blocks which it can move. The velocity is 
 regulated chiefly by the angle of slope ; the greater the declivity, 
 the higher the velocity and the larger the capacity of the stream 
 
 Fig. 10. Gullies torn out of the side of a mountain by descending tor- 
 rents, with cones of detritus at their base. 
 
 to carry down debris. Any cause, therefore, which lessens the 
 velocity of a current diminishes its carrying power. If water, 
 bearing along gravel, sand, or mud, is checked in its flow, some 
 of these materials will drop and remain at rest on the bottom. 
 In the course of every stream, various conditions arise whereby 
 the velocity of the current is reduced. One of the most obvious 
 of these is a diminution in the slope of the channel. Another is 
 the union of a rapid tributary with a more gently flowing stream. 
 
INFLUENCE OP RUNNING WATER 37 
 
 A third is the junction of a stream with the still waters of a lake 
 (see Lakes) or with the sea. In these circumstances, the flow of 
 the water being checked, the sediment at once begins to fall to 
 the bottom. 
 
 Tracing now the progress of a river, for illustrations of this 
 law of deposition, we find that among the mountains where the 
 river takes its rise, the torrents that rush down the declivities 
 have torn out of them such vast quantities of soil and rock as to 
 seam them with deep clefts and gullies. Where each of these 
 rapid streamlets reaches the valley below, its rapidity of motion 
 is at once lessened, and with this slackening of speed and conse- 
 quent loss of carrying power, there is an accompanying deposit of 
 detritus. Blocks of rock, angular rubbish, rounded shingle, 
 sand, and earth are thrown down in the form of a cone of which 
 the apex starts from the bottom of the gully and the base spreads 
 out over the plain (Fig. 10) . Such cones vary in dimensions ac- 
 cording to the size of the torrent and the comparative ease with 
 which the rocks of the mountain-side can be loosened and re- 
 moved. Some of them, thrown down by the transient runnels 
 of the last sudden rain-storm, may not be more than a few cubic 
 yards in bulk. But on the skirts of mountainous regions they 
 may grow into masses hundreds of feet thick and many miles in 
 diameter. The valleys in a range of mountains afford many 
 striking examples of these alluvial cones or fans, as they are 
 called. Where the tributary torrents are numerous, a succession 
 of such cones or fans, nearly or quite touching each other, spreads 
 over the floor of a valley. From this cause, so large an amount 
 of detritus has within historic times been swept down into some 
 of the valleys of the Tyrol that churches and other buildings are 
 now half -buried in the accumulation. 
 
 Looking more closely at the materials brought down by the 
 torrents, we find them arranged in rude irregular layers, sloping 
 downwards into the plain, the coarsest and most angular detritus 
 lying nearest to the mountains, while more rounded and water- 
 worn shingle or sand extends to the outer margin of the cone. 
 This grouping of irregular layers of angular and half-rounded 
 detritus is characteristic of the action of torrents. Hence, where 
 it occurs, even though no water may run there at the present day, 
 it may be regarded as indicating that at some former time a 
 torrent swept down detritus over that site. 
 
38 GEOLOGY 
 
 Quitting the more abrupt declivities, and augmented by numer- 
 ous tributaries from either side, the stream whose course we 
 are tracing loses the character of a torrent and assumes that of a 
 river. It still flows with velocity enough to carry along not only 
 mud and sand, but even somewhat coarse gravel. The large 
 angular blocks of the torrential part of its course, however, are 
 no longer to be seen, and all the detritus becomes more and 
 more rounded and smoothed as we follow it towards the plains. 
 At many places, deposits of gravel or sand take place, more 
 especially at the inner side of the curves which the stream makes 
 as it winds down the valley. Sweeping with a more rapid flow 
 round the outer side of each curve, the current lingers in eddies 
 on the inner side and drops there a quantity of sediment. 
 When the water is low, strips of bare sand and shingle on the 
 concave side of each bend of the stream form a distinctive feat- 
 ure in river scenery. It is interesting to walk along one of these 
 strips and to mark how the current has left its record there. 
 The stones are well smoothed and rounded, showing that they 
 have been rolled far enough along the bottom of the channel 
 to lose their original sharp edges, and to pass from the state of 
 rough angular detritus into that of thoroughly water-worn 
 gravel. Further, they will be found not to lie entirely at random, 
 as might at first sight be imagined. A little examination will 
 show that, where the stones are oblong, they are generally placed 
 with their longer axis pointing across the stream. This would 
 naturally be the position which they would assume where the 
 current kept rolling them forward along the channel. Those 
 which are flat in shape will be observed usually to slope up 
 stream. That the sloping face must look in the direction from 
 which the current moves will be evident from Fig. 11, where 
 
 a current, moving in the 
 direction of the arrow 
 
 and gradually dimmish- 
 Fig. 11. Flat stones in a bank of river- j? T T 
 shingle, showing the direction of the ln g m or ce, would no 
 current (indicated by the arrow) that longer be able to overturn 
 transported and left them. 
 
 the stones which it had 
 
 so placed as to offer the least obstacle to its passage. Had the 
 current flowed from the opposite quarter, it would have found 
 the upturned edges of the stones exposed to it, and would have 
 readily overturned them until they found a position in which 
 
INFLUENCE OF RUNNING WATER 
 
 39 
 
 they again presented least resistance to the water. In a section 
 of gravel, it is thus often quite possible to tell from what quarter 
 the current flowed that deposited the pebbles. 
 
 Yet another feature in the arrangement of the materials is 
 well seen where a digging has been made in one of the alluvial 
 banks, but better still in a section of one of the terraces to be im- 
 mediately referred to. The layers of gravel or sand in some 
 bands may be observed to be inclined at a steeper angle than in 
 others, as shown in the accompanying figure (Fig. 12). In 
 such cases, it will be noticed 
 that the slope of the more in- 
 clined layers is down the stream, 
 and hence that their direction 
 gives a clue to that of the cur- 
 rent which arranged them. We 
 may watch similar layers in the 
 act of deposition among shallow 
 pools into which currents are 
 discharging sediment. The 
 
 gravel or sand may be observed 
 
 , ,, J , ,, , Fig. 12. Section of alluvium 
 
 moving along the bottom, and showing direction of currents. 
 
 then falling over the edge of 
 
 the bank into the bottom of the pool. As the sediment 
 advances by successive additions to its steep slope in front, 
 it gradually fills the pool up. Its progress may be com- 
 pared to that of a railway embankment formed by the discharge 
 of waggon-loads of rubbish down its end. A section through such 
 an embankment would reveal a series of bands of variously col- 
 oured materials inclined steeply towards the direction in which 
 the waggon-loads were thrown down. Yet the top of the em- 
 bankment may be kept quite level for the permanent way. The 
 nearly level bands (b, c) in Fig. 12 represent the general bottom 
 on which the sediment accumulated, while the steeper lines in 
 the lower gravel (a) point to the existence and direction of the 
 currents by which sediment was pushed forward along that bot- 
 tom. 
 
 As the river flows onward through a gradually expanding 
 valley, another characteristic feature becomes prominent. Flank- 
 ing each side of the flat land through which the stream pursues 
 its winding course, there runs a steep slope or bank a few feet or 
 
40 GEOLOGY 
 
 yards in height, terminating above in a second or higher plain, 
 which again may be bordered with another similar bank, above 
 which there may lie a third plain. These slopes and plains form 
 a group of terraces, rising step by step above and away from the 
 river, sometimes to a height of several hundred feet, and occa- 
 sionally to the number of 6 or 8 or even more (Fig. 13). Here 
 and there, by the narrowing of the intervening strip of plain, 
 two terraces merge into one, and at some places the river in 
 winding down the valley has cut away great slices from the 
 terraces, perhaps even entirely removing them and eating back 
 
 Fig. 13. River-terraces. 
 
 into the rock out of which the valley has been excavated. Sec- 
 tions are thus exposed showing a succession of gravels, sands, and 
 loams like those of the present river. From the line of the up- 
 permost terrace down to the spits of shingle now forming in the 
 channel, we have evidently a chronologically arranged series of 
 river-deposits, the oldest being at the top and the youngest at 
 the bottom. But how could the river have flowed at the level of 
 these high gravels, so far above its present limits? An exam- 
 ination of the behaviour of the stream during floods will help 
 towards an answer to this questiton. 
 
 When from heavy rains or melted snows the river overflows 
 its banks, it spreads out over the level ground on either side. 
 The tract liable to be thus submerged during inundations is 
 called the flood-plain. As the river rises in flood, it becomes 
 more and more turbid from the quantity of mud and silt poured 
 into it by its tributaries on either side. Its increase in volume 
 likewise augments its velocity, and consequently its power of 
 
INFLUENCE OF RUNNING WATER 41 
 
 scouring its bed and of transporting the coarser detritus resting 
 there. Large quantites of shingle >may thus be swept out of the 
 ordinary channel and be strewn across the nearer parts of the 
 flood-plain. As the current spreads over this plain, its velocity 
 and transporting capacity diminish, and consequently sediment 
 begins to be thrown down. Grass, bushes, and trees, standing 
 on the flood-plain, filter some of the sediment out of the water. 
 Fine mud and sand, for instance, adhere to the leaves and stems, 
 whence they are eventually washed off by rain into the soil under- 
 neath. In this way, the flood-plain is gradually heightened by 
 the river itself. At the same time, the bed of the river is deep- 
 ened by the scour of the current, until, in the end, even the 
 highest floods are no longer able to inundate the flood-plain. The 
 difference of level between that plain and the surface of the 
 river gradually increases; by degrees the river begins to cut 
 away the edges of the terrace which it cannot now overflow, and 
 to form a new flood-plain at a lower level. In this manner, it 
 slowly lowers its bed, and leaves on either side a set of alluvial 
 terraces to mark successive stages in the process of excavation. 
 If during this process the level of the land should be raised, the 
 slope of the rivers, and consequently their scour, would be aug- 
 mented, and they would thereby acquire greater capacity for the 
 formation of terraces. There is reason to believe that this has 
 taken place both in Europe and North America. 
 
 While it is obvious that the highest terraces must be the 
 oldest, and that the series is progressively younger down to the 
 terrace that is being formed at the present time, nevertheless, in 
 the materials comprising any one terrace, those lying at the top 
 must be the youngest. This apparent contradiction arises from 
 the double action of the river in eroding its bed and depositing its 
 sediment. If there were no lowering of the channel, then the 
 deposits would follow the usual order of sequence, the oldest be- 
 ing below and the youngest above. This order is maintained in 
 the constituents of each single terrace, for the lowermost layers 
 of gravel must evidently have been accumulated before the de- 
 posit of those that overlie them. But when the level of the water 
 is lowered, the next set of deposits must, though younger, lie 
 at a lower level than those that preceded them. In no case, 
 however, will the older beds, though higher in position, be found 
 
42 GEOLOGY 
 
 really to overlie the younger. They have been formed at differ- 
 ent levels. 
 
 The gravel, sand, and loam laid down by a river are marked, 
 as we have seen, by an arrangement in layers, beds, or strata 
 lying one upon another. This stratified disposition indeed is 
 characteristic of all sedimentary accumulations, and is best 
 developed where currents have been most active in transporting 
 and assorting the various materials. It is the feature that first 
 catches the eye in any river-bank, where a section of the older 
 deposits or " alluvium " is exposed. Beds of coarser and finer 
 detritus alternate with each other, but the coarsest are generally 
 to be observed below and the finest above. The " deltas " accu- 
 mulated by rivers in lakes and in the sea will be noticed in 
 Chapters IV and VII. 
 
 But besides the inorganic detritus carried forward by a river, 
 we have also to consider the fate of the remains of plants and 
 the carcases of animals that are swept down, especially during 
 floods. Swollen by sudden and heavy rains, a river will rise 
 above its ordinary level and uproot trees and shrubs. On sucli 
 occasions, too, moles and rabbits are drowned and buried in their 
 burrows on the alluvial flood-plain. Birds, insects, and even 
 some of the larger mammals are from time to time drowned, 
 swept away by floods and buried in the sediment, and their re- 
 mains, where of a durable kind or where sufficiently covered 
 over, may be preserved for an indefinite period. The shells and 
 fishes living in the river itself may also be killed during the 
 flood, and may be entombed with the other organisms in the 
 sediment. 
 
 SUMMARY. The material produced by the universal decay of 
 the surface of the land is washed off by rain and swept seawards 
 by brooks and rivers. The rate at which the general level of the 
 land is being lowered by the operation of running water may be 
 approximately ascertained by measuring or estimating the amount 
 of mineral matter carried seaward every year from a definite 
 region, such as a river-basin. Taking merely the matter in 
 mechanical suspension, and assuming that the proportion of it 
 transported annually in the water of the Mississippi may be re- 
 garded as an average proportion for the rivers of Europe, we find 
 that this continent, at the Mississippi rate of degradation, might 
 be reduced to the sea-level in rather less than 6,000,000 years. 
 
INFLUENCE OF RUNNING WATER 43 
 
 In pursuing their course over the land, running waters gradu- 
 ally deepen and widen the channels in which they flow, partly 
 by chemically dissolving the rocks and partly by. rubbing them 
 down by the friction of. the transported sand, gravel, and stones. 
 When they have once chosen their channels, they usually keep to 
 them, and the sinuous windings, at first determined by trifling 
 inequalities on the surface of country across which the streams 
 began to flow, are gradually deepened into picturesque gorges. 
 In the excavation of such ravines, waterfalls play an important 
 part by gradually receding up stream. River-channels, espe- 
 cially if cut deeply into the solid rock, remain as enduring monu- 
 ments of the work of running water. 
 
 But still more important as geological records, because more 
 frequent and covering a larger area, are the deposits which rivers 
 leave as their memorials. Whatever checks the velocity of a 
 current weakens its transporting power, and causes it to drop 
 some of its sediment to the bottom. Accordingly, accumulations 
 of sediment occur at the foot of torrent slopes, along the lower 
 and more level ground, especially on the inner or concave side of 
 the loops, over the flood-plains, and finally in the deltas formed 
 where rivers enter lakes or the sea. In these various situations, 
 thick stratified beds of silt, sand, and gravel may be formed, 
 enclosing the remains of the plants and animals living on the 
 land at the time. As a river deepens its channel, it leaves on 
 either side alluvial terraces that mark successive flood-plains 
 over which it has flowed. 
 
44 GEOLOGY 
 
 CHAPTER IV. 
 
 MEMORIALS LEFT BY LAKES. 
 
 FKESH-WATER LAKES. According to the law stated in 
 last chapter, that when water is checked in its flow, it must 
 drop some of its sediment, lakes are pre-eminently places 
 for the deposition and accumulation of mineral matter. In 
 their quiet depths, the debris worn away from the surface of the 
 land is filtered out of the water and allowed to gather undisturbed 
 upon the bottom. The tributary streams may enter a large lake 
 swollen and muddy, but the escaping river is transparent. It 
 is evident, therefore, that lakes must be continually silting up, 
 and that when this process is complete, the site of a lake will be 
 occupied by a series of deposits comprising a record of how the 
 water was made to disappear. 
 
 To those who know the aspect of lakes only in fine weather, 
 they may seem places where geological operations are at their 
 very minimum of activity. The placid surface of the water rip- 
 ples upon beaches of gravel or spits of sand; reeds and marshy 
 plants grow out into the shallows ; the few streamlets that tumble 
 down from the surrounding hills furnish perhaps the only sounds 
 that break the stillness, but their music and motion are at once 
 hushed when they lose themselves in the lake. The scene might 
 serve as the very emblem of perfectly undisturbed conditions of 
 repose. But come back to this same scene during an autumn 
 storm, when the mists have gathered all round the hills, and the 
 rain, after pouring down for hours, has turned every gully into 
 the track of a roaring torrent. Each tributary brook, hardly 
 visible perhaps in drought, now rushes foaming and muddy from 
 its dell and sweeps out into the lake. The large streams bear 
 along on their swift brown currents trunks of trees, leaves, twigs, 
 with now and then the carcase of some animal that has been 
 drowned by the rising flood. Hour after hour, from every side, 
 
MEMORIALS LEFT BY LAKES 45 
 
 these innumerable swollen waters bear their freights of gravel, 
 sand, and mud into the lake. Hundreds or thousands of tons of 
 sediment must thus be swept down during a single storm. When 
 we multiply this result by the number of storms in a year and 
 by the number of years in an ordinary human life, we need not 
 be surprised to be told that even within the memory of the pres- 
 ent generation, and still more within historic times, conspicuous 
 changes have taken place in many lakes. 
 
 Filling up of Lakes. In the Lake of Lucerne, for exam- 
 ple, the River Reuss, which bears down the drainage of the huge 
 mountains round the St. Gothard, deposits about 7,000,000 cubic 
 feet of sediment every year. Since the year 1714 the Kander, 
 
 Fig. 14. Alluvial terraces on the side of an emptied reservoir. 
 
 which drains the northern flanks of the centre of the Bernese 
 Oberland, is said to have thrown into the lower end of the Lake 
 of Thun such an amount of sediment as to form an area of 230 
 acres, now partly woodland, partly meadow and marsh. Since 
 the time of the Romans, the Rhone has filled up the upper end 
 of the Lake of Geneva to such an extent that a Roman harbour, 
 still called Port Valais, is now nearly two miles from the edge of 
 the lake, the intervening ground having been converted first into 
 marshes and then into meadows and farms. 
 
 It is at the mouths of streams pouring into a lake that the 
 process of filling up is most rapid and striking. But it may be 
 detected at many other places round the margin. Instructive 
 lessons on this subject may be learned at a reservoir formed by 
 damming back the waters of a steep-sided valley, and liable to 
 be sometimes nearly dry (Fig. 14). In such a situation, when 
 the water is low, it may be noticed that a series of parallel lines 
 runs all round the sides of the reservoir, and that these lines 
 consist of gravel, sand, or earth. Each of them marks a former 
 
46 GEOLOGY 
 
 level of the water, and they show that the reservoir was not 
 drained off at once but intermittently, each pause in the diminu- 
 tion of level being marked by a line of sediment. It is easy to 
 watch how these lines are formed along the present margin of 
 the water. The loose debris from the bare slope above, partly by 
 its own gravitation, partly by the wash of rain, slides down into 
 the water. But as soon as it gets there, its further downward 
 movement is arrested. By the ripple of the water it is gently 
 moved up and down, but keeps on the whole just below the line 
 to which the water reaches. So long as it is concealed under the 
 water, its position and extent can hardly be realised. But as 
 
 Fig. 15. Parallel roads of Glen Roy. 
 
 soon as the level of the reservoir sinks, the sediment is left as a 
 marked shelf or terrace. In natural lakes, the same process is 
 going on, though hardly recognisable, because concealed under 
 the water. But if by any means a lake could be rapidly emptied, 
 its former level would be marked by a shelf or alluvial terrace. 
 In some cases, the barrier of a lake has been removed, and the 
 sinking of the water has revealed the terrace. The famous 
 " parallel roads " of Glen Roy, in the west of Scotland, are nota- 
 ble examples (Fig. 15) . The valleys in that region were ancient- 
 ly dammed up by large glaciers. The drainage accumulated be- 
 hind the ice, filled up the valleys and converted them into a series 
 
MEMORIALS LEFT BY LAKES 47 
 
 of lakes or fresh-water " fjords/' The former levels of these 
 sheets of water and the successive stages of their diminution and 
 disappearance are shown by the series of alluvial shelves known 
 as " parallel roads." The highest of these is 1,140 feet, the mid- 
 dle 1,059 feet, and the lowest 847 feet above the level of the sea. 
 Thus, partly by the washing of detritus down from the ad- 
 joining slopes by rain, partly by the sediment carried into them 
 by streams, and partly by the growth of marshy vegetation along 
 their margins, lakes are visibly diminishing in size. In moun- 
 tainous countries, every stage of this appearance may be ob- 
 served (Fig. 16). Where the lakes are deep, the tongues of 
 
 are 
 
 A B 
 
 Fig. 16. Stages in the filling up of a lake. In A two streamlets . 
 represented as pouring their " deltas " into a lake. In B they have 
 filled the lake up, converting it into a meadow across which they wind 
 on their way down the valley. 
 
 sediment or " deltas " which the streams push in front of them 
 have not yet been able to advance far from the shore. In other 
 cases, every tributary has built up an alluvial plain which grows 
 outwards and along the coast, until it unites with those of its 
 neighbours to form a nearly continuous belt of flat meadow and 
 marsh round the lake. By degrees, as this belt increases in 
 width, the lake narrows, until the whole tract is finally converted 
 into an alluvial plain, through which the river and its tribu- 
 taries wind on their way to lower levels. The successive flat 
 meadow-like expansions of valleys among hills and mountains 
 were probably in most cases originally lakes which have in this 
 manner been gradually filled up. 
 
 Lake Deposits. The bottoms of lakes must evidently con- 
 tain many interesting relics. Dispersed through the shingle, 
 sand, and mud that gather there, are the remains of plants and 
 animals that lived on the surrounding land. Leaves, fruits, 
 twigs, branches, and trunks embedded in the silt may preserve 
 for an indefinite period their record of the vegetation of the time. 
 
48 GEOLOGY 
 
 The wings or wing-cases of insects, the shells of land-snails, the 
 bones of birds and mammals, carried down into the depths of a 
 lake and entombed in the silt there, will remain as a chronicle 
 of the kind of animals that haunted the surrounding hills and 
 valleys. 
 
 The layers of gravel, sand, and silt laid down on the floor of a 
 lake differ in some respects from those deposited in the terraces 
 of a river, being generally finer in grain, and including a larger 
 proportion of silt, mud, or clay among them, especially away 
 from the margin of the lake. They are, no doubt, further dis- 
 tinguished by the greater abundance of the remains of plants and 
 animals preserved in them. 
 
 But lakes likewise serve as receptacles for a series of deposits 
 which are peculiar to them, and which consequently have much 
 interest and importance as they furnish a ready means of detect- 
 ing the sites of lakes that have long disappeared. The molluscs 
 that live in lacustrine waters are distinct from the snails of the 
 adjoining shores. Their dead shells gather on the bottoms of 
 some lakes in such numbers as to form there a deposit of the 
 white crumbling marl, already referred to. In the course of 
 time this deposit may grow to be many feet or yards in thick- 
 ness. The shells in the upper parts may be quite fresh, some 
 of the animals having only recently died ; but they become more 
 and more decayed below until, towards the bottom of the deposit, 
 the marl passes into a more compact chalk-like substance in which 
 
 few or no shells may 
 be recognisable (Fig. 17). 
 On the sites of lakes 
 that have been naturally 
 filled up or artificially 
 drained, such marl has 
 been extensively dug as 
 a manure for land. Be- 
 sides the shells from the 
 decay of which it is 
 Fig. 17. Piece of shell-marl containing chiefly formed, it some- 
 shells of Ltmnwa peregra. . .11 , 
 
 times yields the bones of 
 
 deer, oxen, and other animals, whose carcases must originally 
 have sunk to the bottom of the lake and been there gradually 
 covered up in the growing mass of marl. Many examples of 
 
MEMORIALS LEFT BY LAKES 49 
 
 , 
 
 these marl-deposits are to be found among the drained lakes of 
 Scotland and Ireland. 
 
 Yet another peculiar accumulation is met with on the bottom 
 of some lakes, particularly in Sweden. In the neighbourhood 
 of banks of reeds and on the sloping shallows of the larger lakes, 
 a deposit of hydrated peroxide of iron takes place, in the form 
 of concretions varying in size from small grains like gunpowder 
 up to cakes measuring six inches across. The iron is no doubt 
 dissolved out of the rocks of the neighbourhood by water con- 
 taining organic acids or carbonic acid. In this condition, it is 
 liable to be oxidised on exposure. As after oxidation it can no 
 longer be retained in solution, it is precipitated to the bottom 
 where it collects in grains which by successive additions to their 
 surface become pellets, balls, or cakes. Possibly some of the 
 microscopic plants (diatoms) which abound on the bottoms of 
 the lakes may facilitate the accumulation of the iron by abstract- 
 ing this substance from the water and depositing it inside their 
 siliceous coverings. Beds of concretionary brown ironstone are 
 formed in Sweden from 10 to 200 yards long, 5 to 15 yards 
 broad, and from 8 to 30 inches thick. During winter when 
 the lakes are frozen over, the iron is raked up from the bottom 
 through holes made for the purpose in the ice, and is largely 
 used for the manufacture of iron in the Swedish furnaces. 
 When the iron has been removed, it begins to form again, and 
 instances are known where, after the supply had been completely 
 exhausted, beds several inches in thickness were formed again in 
 twenty-six years. 
 
 SALT-LAKES. The salt-lakes of desert regions present a 
 wholly peculiar series of deposits. These sheets of water have no 
 outlet ; yet there is reason to believe that most of them were at 
 first fresh, and discharged their outflow like ordinary lakes. 
 Owing to geological changes of level and of climate, they have 
 long ceased to overflow. The water that runs into them, instead 
 of escaping by a river, is evaporated back into the air. But the 
 various mineral salts carried by it in solution from rocks and 
 soils are not evaporated also. They remain behind in the lakes, 
 which are consequently becoming gradually salter. Among the 
 salts thus introduced, common salt (sodium-chloride) and gyp- 
 sum (calcium-sulphate) are two of the most important. These 
 substances, as the water evaporates in the shallows, bays, and 
 
50 GEOLOGY 
 
 pools, are precipitated to the bottom where they form solid 
 layers of salt and gypsum. The latter substance begins to be 
 thrown down when 37 per cent of the water containing it has 
 been evaporated. The sodium-chloride does not appear until 93 
 per cent of the water has disappeared. In the order of deposit, 
 therefore, gypsum comes before the salt (see chap. 10). Some 
 bitter lakes contain sodium-carbonate, in others magnesium- 
 chloride is abundant. The Dead Sea, the Great Salt Lake of 
 Utah, and many other salt lakes and inland seas furnish inter- 
 esting evidence of the way in which they have gradually changed. 
 In the upper terraces of the Great Salt Lake, 1,000 feet or more 
 above the present level of the water, fresh-water shells occur, 
 showing that the basin was at first fresh. The valley-bottoms 
 around saline lakes are now crusted with gypsum, salt, or other 
 efflorescence, and their waters are almost wholly devoid of life. 
 Such conditions as these help us to understand how great de- 
 posits of gypsum and rock-salt were formed in England, Ger- 
 many, and many other regions where the climate would not now 
 permit of any such condensation of the water (Chapter XXII). 
 
 SUMMARY. The records inscribed by lakes in geological his- 
 tory consist of layers of various kinds of sediment. These de- 
 posits may form mere shelves or terraces along the margin of 
 the water which, if drained off, will leave them as evidence of its 
 former levels. By the long-continued operations of rain, brooks, 
 and rivers, continually bringing down sediment, lakes are gradu- 
 ally filled up with alluvium, and finally become flat meadow-land 
 with tributary streams winding through it. The deposits that 
 thus replace the lacustrine water consist mainly of sand or gravel 
 near shore, while finer silt occupies the site of the deeper water. 
 They may also include beds of marl formed of fresh-water shells, 
 and sheets of brown iron ore. Throughout them all, remains of 
 the plants and animals of the surrounding land are likely to be 
 entombed and preserved. 
 
 Salt lakes leave, as their enduring memorial, beds of rock-salt 
 and gypsum, sometimes carbonate of soda and other salts. Many 
 of them were at first fresh, as is shown by the presence of ordi- 
 nary fresh-water shells in their upper terraces. But by change 
 of climate and long-continued excess of evaporation over precipi- 
 tation, the water has gradually become more and more saline, and 
 has sometimes disappeared altogether, leaving behind it de- 
 posits of common salt, gypsum, and other chemical precipitates. 
 
HOW SPRINGS LEAVE THEIR MARK 51 
 
 CHAPTER V. 
 
 HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HISTORY. 
 
 THE changes made by running water upon the land are not 
 confined to that portion of the rainfall which courses 
 along the surface. Even when it sinks underground and 
 seems to have passed out of the general circulation, the subter- 
 ranean moisture does not remain inactive. After travelling for 
 a longer or shorter distance through the pores of rocks, or along 
 their joints and other divisional planes, it finds its way once more 
 to daylight and reappears in Springs. In this underground 
 journey, it corrodes rocks, somewhat in the same way as rain at- 
 tacks those that are exposed to the outer air, and it works some 
 curious changes upon the face of the land. Subterranean water 
 thus leaves distinct and characteristic memorials as its contribu- 
 tion to geological history. 
 
 There are two aspects in which the work of underground water 
 may be considered here. In the first place, portions of the sub- 
 stance of subterranean rocks are removed by the percolating water 
 and in large measure carried up above ground; in the second 
 place, some of these materials are laid down again in a new form 
 and take a conspicuous place among the geological monuments of 
 their time. 
 
 In the removal of mineral substance, water percolating through 
 rocks acts in two distinct ways, mechanical and chemical, each of 
 which shows itself in its own peculiar effects upon the surface. 
 
 (1) MECHANICAL ACTION. While slowly filtering through 
 porous materials, water tends to remove loose particles and thus 
 to lessen the support of overlying rocks. But even where there 
 is no transport, the water itself, by saturating a porous layer that 
 rests upon a more or less impervious one, loosens the cohesion of 
 that porous layer. The overlying mass of rock is thus made to 
 rest upon a watery and weakened platform, and if from its posi- 
 
52 GEOLOGY 
 
 tion it should have a tendency to gravitate in any given direction, 
 it may at last yield to this tendency and slide downwards. Along 
 the sides of sea-cliffs, on the precipitous slopes of valleys or river- 
 gorges, or on the declivities of hills and mountains, the conditions 
 are often extremely favourable for the descent of large masses 
 of rock from higher to lower levels. 
 
 Remarkable illustrations of such Landslips, as they are called, 
 have been observed along the south coast of England, where cer- 
 tain porous sandy rocks underlying a thick sheet of chalk rest 
 upon more or less impervious clays, which, by arresting the 
 water in its descent, throw it out along the base of the slopes. 
 After much wet weather, the upper surface of these clays be- 
 comes, as it were, lubricated by the accumulation of water, and 
 large slices of the overlying rocks, having their support thereby 
 weakened, break off from the solid cliffs behind and slide down 
 towards the sea. The most memorable example occurred at 
 Christmas time, in the year 1839, on the coast of Devonshire not 
 far from Axmouth. At that locality, the chalk-downs end off in 
 a line of broken cliff some 500 feet above the sea. From the 
 edge of the downs flanked by this cliff a tract about 800 yards 
 long, containing not less than 30 acres of arable land, sank down 
 with all its fields, hedgerows, and pathways. This sunken mass, 
 where it broke away from the upland, left behind it a new cliff, 
 showing along the crest the truncated ends of the fields, of which 
 the continuation was to be found in a chasm more than 200 feet 
 deep. While the ground sank into this defile and was tilted 
 steeply towards the base of the cliff, it was torn up by a long 
 rent running on the whole in the line of the cliff, and by many 
 parallel and transverse fissures. Half a century has passed away 
 since this landslip occurred. The cliff remains much as it was at 
 first, and the sunken fields with their bits of hedgerow still 
 slope steeply down to the bottom of the declivity (Fig. 18) . But 
 the lapse of time has allowed the influence of the atmosphere to 
 come into play. The outstanding dislocated fragments with 
 their vertical walls and flat tops, showing segments of fields, have 
 been gradually worn into tower-like masses with sloping de- 
 clivities of debris. The long parallel rent has been widened by 
 rain into a defile with shelving sides. Everywhere the rawness 
 of the original fissures has been softened by the rich tapestry of 
 verdure which the genial climate of that southern coast fosters 
 
HOW SPRINGS LEAVE THEIR MARK 
 
 53 
 
54 GEOLOGY 
 
 in every sheltered nook. But the scars have not been healed, 
 and they will no doubt remain still visible for many a year to 
 come. 
 
 Along the south coast of England, many landslips, of which 
 there is no historical record, have produced some of the most 
 picturesque scenery of that region. Masses that have slipped 
 away from the main cliff have so grouped themselves down the 
 slopes that hillocks and hollows succeed each other in endless 
 confusion, as in the well-known Undercliff of the Isle of Wight. 
 Some of the tumbled rocks are still fresh enough to show that 
 they have fallen at no very remote period, or even that the slip- 
 ping still continues; others, again, have yielded so much to the 
 weather that their date doubtless goes far back into the past, and 
 some of them are crowned with what are now venerable ruins. 
 
 The most stupendous landslips on record have occurred in 
 mountainous countries. Upwards of 150 destructive examples 
 have been chronicled in Switzerland. Of these, one of the most 
 memorable was that of the Kossberg, a mountain lying behind the 
 Eigi, and composed of thick masses of hard red sandstone and 
 conglomerate so arranged as to slope down into the valley of 
 Goldau. The summer of the year 1806 having been particularly 
 wet, so large an amount of water had collected in the more porous 
 layers of rock as to weaken the support of the overlying mass ; 
 consequently a large part of the side of the mountain suddenly 
 gave way and rushed down into the valley, burying under the 
 debris about a square German mile of fertile land, four villages 
 containing 330 cottages and outhouses, and 457 inhabitants. To 
 this day, huge angular blocks of sandstone lying on the farther 
 side of the valley bear witness to the destruction caused by this 
 landslip, and the scar on the mountain-slope whence the fallen 
 masses descended is still fresh. 
 
 (2) CHEMICAL ACTION (a) Solution. But it is by its 
 chemical action on the rocks through which it flows that sub- 
 terranean water removes by far the largest amount of mineral 
 matter, and produces the greatest geological change. Even pure 
 water will dissolve a minute quantity of the substance of many 
 rocks. But rain is far from being chemically pure water. In 
 previous chapters it has been described as taking oxygen and car- 
 bonic acid out of the air in its descent, and abstracting organic 
 acids and carbonic acid from the soil through which it sinks. 
 
HOW SPRINGS LEAVE THEIR MARK 55 
 
 By help of these Ingredients, it is enabled to attack even the most 
 durable rocks, and to carry some of their dissolved substance up 
 to the surface of the ground. 
 
 One of the substances most readily attacked and removed even 
 by pure water is the mineral known as carbonate of lime. Among 
 other impurities, natural waters generally contain carbonic acid, 
 which may be derived from the air or from the soil ; occasionally 
 from some deeper subterranean source. The presence of this acid 
 gives the water greatly increased solvent power, enabling it 
 readily to attack carbonate of lime, whether in the form of lime- 
 stone, or diffused through rocks composed mainly of other sub- 
 stances. Even lime, which is not in the form of carbonate, but 
 is united with silica in various crystalline minerals (silicates, Ch. 
 X), may by this means be decomposed and combined with car- 
 bonic acid. It is then removed in solution as carbonate. So 
 long as the water retains enough of free carbonic acid, it can 
 keep the carbonate of lime in solution and carry it onward. 
 
 Limestone is a rock almost entirely composed of carbonate of 
 lime. It occurs in most parts of the world, covering sometimes 
 tracts of hundreds or thousands of square miles, and often rising 
 into groups of hills, or even into ranges of mountains (see Lime- 
 stone, Ch. XI). The abundance of this rock affords ample oppor- 
 tunity for the display of the solvent action of subterranean 
 water. Trickling down the vertical joints and along the planes 
 between the limestone beds, the water dissolves and removes the 
 stone, until in the course of centuries these passages are gradu- 
 ally enlarged into clefts, tunnels, and caverns. The ground be- 
 comes honey-combed with openings into dark subterranean- 
 chambers, and running streams fall into these openings and con- 
 tinue their course underground. 
 
 Every country which possesses large limestone tracts furnishes 
 examples of the way in which such labyrinthine tunnels and 
 systems of caverns are excavated. In England, for example, the 
 Peak Cavern of Derbyshire is believed to be 2,300 feet long, and 
 in some places 120 feet high. On a much more magnificent 
 scale are the caverns of Adelsberg near Trieste, which have been 
 explored to a distance of between four and five miles, but are 
 probably still more extensive. The river Poik has broken into 
 one part of the labyrinth of chambers, through which it rushes 
 before emerging again to the light. Narrow tunnels expand into 
 
56 GEOLOGY 
 
 spacious halls, beyond which egress is again afforded by low 
 passages into other lofty recesses. The most stupendous cham- 
 ber measures 669 feet in length, 630 feet in breadth, and 111 feet 
 in height. From the roofs hang pendent white stalactites, 
 which, in uniting with the floor, form pillars of endless vari- 
 eties of form and size. Still more gigantic is the system of sub- 
 terranean passages in the Mammoth Cave of Kentucky, the acces- 
 sible parts of which are believed to have a combined length of 
 about 150 miles. The largest cavern in this vast labyrinth has 
 an. area of two acres, and is covered by a vault 125 feet high. 
 
 Of the mineral matter dissolved by permeating water out of 
 the rocks underground, by far the larger part is discharged by 
 springs into rivers, and ultimately finds its way to the sea. The 
 total amount of material thus supplied to the sea every year 
 must be enormous. Much of it, indeed, is abstracted from ocean- 
 
 Fig. 19. Section of cavern with stalactites and stalagmite. 
 
 water by the numerous tribes of marine plants and animals. In 
 particular, the lime, silica, and organic matter are readily seized 
 upon to build up the framework and furnish the food of these 
 creatures. But probably more mineral matter is supplied in solu- 
 tion than is required by the organisms of the sea, in which case 
 the water of the sea must be gradually growing heavier and salter. 
 (5) Deposition. But it is the smaller proportion of the 
 material not conveyed into the sea that specially demands atten- 
 tion. Every spring, even the purest and most transparent, con- 
 tains mineral solutions in sufficient quantity to be detected by 
 chemical analysis. Hence all plants and animals that drink the 
 water of springs and rivers necessarily imbibe these solutions 
 which, indeed, supply some of the mineral salts whereof the 
 
HOW SPRINGS LEAVE THEIR MARK 57 
 
 harder parts both of plants and animals are constructed. Many 
 springs, however, contain so large a proportion of mineral matter, 
 that when they reach the surface and begin to evaporate^ they 
 drop their solutions as a precipitate, which settles down upon 
 the bottom or on objects within reach of the water. After years 
 of undisturbed continuance, extensive sheets of mineral material 
 may in this manner be accumulated, which remain as enduring 
 monuments of the work of underground water, even long after 
 the springs that formed them have ceased to flow. 
 
 Calcareous Springs. Among the accumulations of this nature 
 by far the most frequent and important are those formed by what 
 are called Calcareous Springs. In regions abounding in lime- 
 stone or rocks containing much carbonate of lime, the subter- 
 ranean waters which, as we have seen, gradually erode such vast 
 systems of tunnels, clefts, and caverns, carry 
 away the dissolved rock, and retain it in solu- 
 tion only so long as they can keep their car- 
 bonic acid. As soon as they begin to evapo- 
 rate and to lose some of this acid, they lose 
 also the power of retaining so much carbonate 
 of lime in solution. This substance is accord- 
 ingly dropped as a fine white precipitate, 
 which gathers on the surfaces over which the 
 water trickles or flows. 
 
 The most familiar example of this process 
 is to be seen under the arches of bridges and 
 vaults. Long pendent white stalks or stalac- 
 tites hang from between the joints of the 
 masonry, while wavy ribs of the same sub- 
 stance run down the piers or walls, and even 
 collect upon the ground (stalagmite') . A few 
 years may suffice to drape an archway with a 
 kind of fringe of these pencil-like icicles of 
 stone. Percolating from above through the 
 joints between the stones of the masonry, the Fig. 20. Section 
 rain-water, armed with its minute proportion |frj in s * cce s- 
 of carbonic acid at once attacks the lime of growth in a stal- 
 the mortar and forms carbonate of lime, which actlte - 
 is carried downward in solution. Arriving at the surface of the 
 arch, the water gathers into a drop, which remains hanging there 
 
58 GEOLOGY 
 
 for a brief interval before it falls to the ground. That interval 
 suffices to allow some of the carbonic acid to escape, and some of 
 the water to evaporate. Consequently, round the outer rim of 
 the drop a slight precipitation of white chalky carbonate of lime 
 takes place. This circular pellicle, after the drop falls, is in- 
 creased by a similar deposit from the next drop, and thus drop by 
 drop the original rim or ring is gradually lengthened into a tube 
 which may eventually be filled up inside, and may be thickened 
 irregularly outside by the trickle of calcareous water (Fig. 20). 
 But the deposition on the. roof does not exhaust the stock of 
 dissolved carbonate. When the drops reach the ground the same 
 process of evaporation and precipitation continues. Little 
 mounds of the same white substance are built up on the floor, 
 and, if the place remain undisturbed, may grow until they unite 
 with the stalactites from the roof, forming white pillars that 
 reach from floor to ceiling (Fig. 19). 
 
 It is in limestone caverns that stalactitic growth is seen on the 
 most colossal scale. These quiet recesses having remained un- 
 disturbed for many ages, the process of solution and precipitation 
 has advanced without interruption until, in many cases, vast 
 caverns have been transformed into grottoes of the most marvel- 
 lous beauty. While glistening fringes and curtains of crystalline 
 carbonate of lime, or spar, as it is popularly called, hang in end- 
 less variety and beauty of form from the roof. Pillars of every 
 dimension, from slender wands up to thick-ribbed columns like 
 those of a cathedral, connect the roof and the pavement. The 
 walls, projecting in massive buttresses and retiring into alcoves, 
 are everywhere festooned with a grotesque drapery of stone. The 
 floor is crowded with mounds and bosses of strangely imitative 
 forms which recall some of the oddest shapes above ground. 
 Wandering through such a scene, the visitor somehow feels him- 
 self to be in another world, where much of the architecture and 
 ornament belongs to styles utterly unlike those which can be 
 seen anywhere else. 
 
 The material composing stalactite and stalagmite is at first, 
 as already stated, a fine white chalky pulp-like substance which 
 dries into a white powder. But as the deposition continues, the 
 older layers, being impregnated with calcareous water, receive a 
 precipitation of carbonate of lime between their minute pores 
 and crevices, and assume a crystalline structure. Solidifying 
 

 HOW SPRINGS LEAVE THEIR MARK 59 
 
 and hardening by degrees, they end by becoming a compact crys- 
 talline stone (spar) which rings under the hammer. 
 
 The numerous caverns of limestone districts have offered ready 
 shelter to various kinds of wild animals and to man himself. 
 Some of them (Bone-Caves) have been hyaena-dens, and from 
 under their hard floor of stalagmite the bones of hyaenas and 
 of the creatures they fed upon are disinterred in abundance. 
 Eude human implements have likewise been obtained from the 
 same deposits, showing that man was contemporary with animals 
 which have long been extinct. The solvent action of under- 
 ground water has thus been of the utmost service in geological 
 history, first, in forming caverns that could be used as retreats, 
 and then in providing a hard incrustation which should effectu- 
 ally seal up and preserve the relics of the denizens left upon the 
 cavern-floors. 
 
 Calcareous springs, issuing from limestone or other rock 
 abounding in lime, deposit carbonate of lime as a white pre- 
 cipitate. So large is the proportion of mineral contained by 
 some waters that thick and extensive accumulations of it have 
 been formed. The substance thus deposited is known by the 
 name of Calcareous Tufa,, Calc-sinter, or Travertine. It varies 
 in texture, some kinds being loose and crumbling, others hard 
 and crystalline. In many places it is composed of thin layers or 
 laminae, of which sixty may be counted in the thickness of an 
 inch, but bound together into a solid stone. These laminae mark 
 the successive layers of deposit. They are formed parallel to the 
 surface over which the water flows or trickles, hence they may be 
 observed not only on the flat bottoms of the pools, but irregularly 
 enveloping the walls of the channel as far up as the dash of 
 water or spray can reach. Bounded bosses may thus be formed 
 above the level of the stream, and the recesses may be hung with 
 stalactites. 
 
 The calcareous springs of Northern and Central Italy have 
 long been noted for the large amount of their dissolved lime, the 
 rapidity with which it is deposited, and the extensive masses in 
 which it has accumulated. Thus at San Filippo in Tuscany, it 
 is deposited in places at the rate of one foot in four months, and 
 it has been piled up to a depth of at least 250 feet, forming a 
 hill a mile and a quarter long, and a third of a mile broad. So 
 compact are many of the Italian travertines that they have from 
 
60 GEOLOGY 
 
 time immemorial been extensively used as a building stone, 
 which can be dressed and is remarkably durable. Many of the 
 finest buildings of ancient and modern Eome have been con- 
 structed of travertine. 
 
 A familiar feature of many calcareous springs deserves notice 
 here. The precipitation of calc-sinter is not always due merely 
 to evaporation. In many cases, where the proportion of car- 
 bonate of lime in solution is so small that under ordinary cir- 
 cumstances no precipitation of it would take place, large masses 
 of it have been deposited in a peculiar fibrous form. On exam- 
 ination, this precipitation will be found to be caused by the ac- 
 tion of plants, particularly bog-mosses which, decomposing the 
 carbonic acid in the water, cause the lime-carbonate to be de- 
 
 Fig. 21. Travertine with impressions of leaves. 
 
 posited along their stems and leaflets. The plants are thus in- 
 crusted with sinter which, preserving their forms, looks as if it 
 were composed of heaps of moss turned into stone. Hence the 
 name of petrifying springs often given to waters where this proc- 
 ess is to be seen. There is, however, no true petrifaction or con- 
 version of the actual substance of the plants into stone. The 
 fibres are merely incrusted with travertine, inside of which they 
 eventually die and decay. But as the plants continue to grow 
 outward, they increase the sinter by fresh layers, while the in- 
 ner and dead parts of the mass are filled up and solidified by the 
 deposit of the precipitate within their cavities. 
 
HOW SPRINGS LEAVE THEIR MARK 61 
 
 A growing accumulation of travertine presents a special 
 interest to the geologist from the fact that it offers exceptional 
 facilities for the preservation of remains of the plants and ani- 
 mals of the neighborhood. Leaves from the surrounding trees 
 and shrubs are blown into pools or fall upon moist surfaces where 
 the precipitation of lime is actively going on (Fig. 21). Dead 
 insects, snail-shells, birds, small mammals, and other denizens of 
 the district may fall or be carried into similar positions. These 
 remains may be rapidly enclosed within the stony substance be- 
 fore they have time to decay, and even if they should afterwards 
 moulder into dust, the sinter enclosing them retains the mould 
 of their forms, and thus preserves for an indefinite period the 
 record of their former existence. 
 
 Chalybeate Springs. A second but less abundant deposit from 
 springs is found in regions where the rocks below ground contain 
 decomposing sulphide of iron (Ch. X). Water percolating 
 through such rocks and oxidising the sulphur of that mineral, 
 forms sulphate of iron (ferrous sulphate) which it removes in 
 solution. The presence of any notable quantity of this sulphate 
 is at once revealed by the marked inky taste of the water and 
 by the yellowish-brown precipitate on the sides and bottom of 
 the channel. Such water is termed Chalybeate. When it mixes 
 with other water containing dissolved carbonates (which are so 
 generally present in running water), the sulphate is decomposed, 
 the sulphuric acid passing over to the lime or alkali of the car- 
 bonate, while the iron takes up oxygen and falls to the bottom as 
 a yellowish-brown precipitate (limonite, Ch. X). This inter- 
 change of combinations, with the consequent precipitation of 
 iron-oxide, may continue for a considerable distance from the 
 outflow of the chalybeate water. Nearest the source the deposit 
 of hydrated ferric oxide or ochre is thickest. It encloses leaves, 
 stems, and other organic remains, and preserves moulds or casts 
 of their forms. It also cements the loose sand and shingle of a 
 river-bottom into solid rock. 
 
 Siliceous Springs. One other deposit from spring-water may 
 be enumerated here. In volcanic regions, hot springs (geysers) 
 rise to the surface which, besides other mineral ingredients, con- 
 tain a considerable proportion of silica (Ch. X). This substance 
 is deposited as Siliceous Sinter round the vents whence the water 
 is discharged, where it forms a white stone rising into mounds 
 
62 GEOLOGY 
 
 and terraces with fringes and bunches of coral-like growth. 
 [Where many springs have risen in the same district, their re- 
 spective sheets of sinter may unite, and thus extensive tracts are 
 buried under the deposit. In Iceland, for example, one of the 
 sheets is said to be two leagues long, a quarter of a league wide, 
 and a hundred feet thick. In the Yellowstone Park of North 
 America, many valleys are floored over with heaps of sinter, and 
 in New Zealand other extensive accumulations of the same ma- 
 terial are to be found. It is obvious that, like travertine, siliceous 
 sinter may readily entomb and preserve a record of the plants 
 and animals that lived at the time of its deposition. 
 
 SUMMARY. The underground circulation of water produces 
 changes that leave durable records in geological history. These 
 changes are of two kinds. (1) Landslips are caused, by which 
 the forms of cliffs, hills, and mountains are permanently altered. 
 Vast labyrinths of subterranean tunnels, galleries, and caverns 
 are dissolved out of calcareous rocks, and openings are made 
 from these passages up to the surface whereby rivers are en- 
 gulfed. Many of the caves thus hollowed out have served as 
 dens of wild beasts and dwelling-places for man, and the relics 
 of these inhabitants have been preserved under the stalagmite of 
 the floors. (2) An enormous quantity of mineral matter is 
 brought up to the surface by springs. Most of the solutions are 
 conveyed ultimately to the sea where they partly supply the sub- 
 stances required by the teeming population of marine plants and 
 animals. But, under favourable circumstances, considerable de- 
 posits of mineral matter are made by springs, more especially in 
 the form of travertine, siliceous sinter, and ochre. In these de- 
 posits the remains of terrestrial vegetation, also of insects, birds, 
 mammals, and other animals, are not infrequently preserved, and 
 remain as permanent memorials of the life of the time when they 
 flourished. 
 
ICE-RECORDS 
 
 CHAPTER VI. 
 
 ICE-RECORDS. 
 
 ICE In various ways alters the surface of the land. By disin- 
 tegrating and eroding even the most durable rocks, and by 
 removing loose materials and piling them up elsewhere, it 
 greatly modifies the details of a landscape. As it assumes various 
 forms, so it accomplishes its work with considerable diversity. 
 The action of frost upon soil and upon bare surfaces of rock has 
 already been described. We have now to consider the 
 action of frozen rivers and lakes, snow and glaciers, which have 
 each their own characteristic style of operation, and leave behind 
 them their distinctive contribution to the geological history of 
 the earth. 
 
 FROZEN RIVERS AND LAKES. In countries with a severe win- 
 ter climate, the rivers and lakes are frozen over, and the cake of 
 ice that covers them may be more than two feet thick. When 
 this cake is broken up in early summer, large masses of it are 
 driven ashore, tearing up the littoral boulders, gravel, sand, or 
 mud, and pushing them to a height of many feet above the ordi- 
 nary level of the water. When the ice melts, huge heaps of detri- 
 tus are found to have been piled up by it, which remain as en- 
 during monuments of its power. Not only so, but large frag- 
 ments of the ice that has been formed along shore and has en- 
 closed blocks of stone, gravel, and sand, are driven away and 
 may travel many miles before they melt and drop their freight 
 of stones. On the St. Lawrence and on the coast of Labrador, 
 there is a constant transportation of boulders by this means. 
 Further, besides freezing over the surface, the water not in- 
 frequently forms a loose spongy kind of ice on the bottom 
 (Anchor-ice, Ground-ice) which encloses stones and gravel, and 
 carries them up to the surface where it joins the cake of ice 
 there. This bottom-ice is formed abundantly on some parts of 
 
64 GEOLOGY 
 
 the Canadian rivers. Swept down by the current, it accumulates 
 against the bars or banks, or is pushed over the upper ice, and 
 from time to time gathers into temporary barriers, the bursting 
 of which may cause destructive floods. In the river St. Law- 
 rence, banks and islets have been to a large extent worn down by 
 the grating of successive ice-rafts upon them. 
 
 SNOW. On level or gently inclined ground, whence snow dis- 
 appears merely by melting or evaporation, it exercises, while it 
 remains, a protective influence upon the soil and vegetation, 
 shielding them from the action of frost. On slopes of suffi- 
 cient declivity, however, the sheet of snow acquires a tendency to 
 descend by gravitation, as we may often see on house-roofs in 
 winter. In many cases, it creeps or slides down the side of a hill 
 or valley, and in so doing pushes forward bare soil, loose stones, 
 or other objects lying on the surface. By this means, the debris 
 of weathered rock in exposed situations is gradually thrust down- 
 hill and the rock is bared for further disintegration. But where 
 the declivities are steep enough to allow the snow to break off in 
 large sheets and to rush rapidly down, the most striking changes 
 are observable. Such descending masses are known as Ava- 
 lanches. Varying from 10 to 50 feet or more in thickness and 
 several hundred yards broad and long, they sweep down the 
 mountain sides with terrific force, carrying away trees, soil, 
 houses, and even large blocks of rock. The winter of 1884-85 
 was especially remarkable for the number of avalanches in the 
 valleys of the Alps, and for the enormous loss of life and proper- 
 ty which they caused. In such mountain ground, not only are 
 declivities bared of their trees, soil, and boulders, but huge 
 mounds of debris are piled up in the valleys below. Frequently, 
 also, such a quantity of snow, ice, and rubbish is thrown across 
 the course of a stream as to dam back the water, which accumu- 
 lates until it overflows or sweeps away the barrier. In another 
 but indirect way, snow may powerfully affect the surface of a 
 district where, by rapid melting, it so swells the rivers as to give 
 rise to destructive floods. 
 
 While, therefore, the influence of snow is on the whole to pro- 
 tect the surface of the land, it shows itself in mountainous 
 regions singularly destructive, and leaves as chief memorials of 
 this destructiveness the mounds and rough heaps of earth and 
 
ICE-RECORDS 
 
 05 
 
 stones that mark where the down-rushing avalanches have come 
 to rest. 
 
 GLACIERS AND ICE-SHEETS leave their record in characters so 
 distinct that they cannot usually be confounded with those of any 
 other kind of geological agent. The changes which they produce 
 on the surface of the land may be divided into two parts: (1) 
 the transport of materials from the high grounds to lower levels, 
 and (2) the erosion of their beds. 
 
 (1) Transport. As a glacier descends its valley, it re- 
 ceives upon its surface the earth, sand, mud, gravel, boulders, and 
 blocks of rock that roll or are washed down from the slopes on 
 either side. Most of this rubbish accumulates on the edges of 
 
 Fig. 22. Glacier with medial and lateral moraines, 
 the glacier, where it is slowly borne to lower levels as the ice 
 creeps downwards. But some of it falls into the crevasses or 
 rents by which the ice is split, and may either be imprisoned with- 
 in the glacier, or may reach the rocky floor over which the ice is 
 sliding. The rubbish borne onward upon the surface of the 
 glacier is known as moraine-stuff. The mounds of it running 
 along each side of the glacier form lateral moraines,, those on the 
 right-hand side as we look down the length of the valley being the 
 right lateral moraine, those on the other side the left lateral 
 moraine. Where two glaciers unite, the left lateral moraine of 
 the one joins the right lateral moraine of the other, forming what 
 
66 GEOLOGY 
 
 is called a medial moraine that runs down the middle of the 
 united glacier. Where a glacier has many tributaries bearing 
 much moraine-stuff, its surface may be like a bare plain covered 
 with earth and stones, so that, except where a yawning crevasse 
 reveals the clear blue gleam of the ice below, nothing but earth 
 and stones meets the eye. [When the glacier melts, the detritus is 
 thrown in heaps upon the valley, forming there the terminal 
 moraine. 
 
 Glaciers, like rivers, are subject to variations of level. Even 
 from year to year they slowly sink below their previous limit or 
 rise above it. The glacier of La Brenva, for example, on the 
 Italian side of Mont Blanc, subsided no less than 300 feet in the 
 first half of the present century. One notable consequence of 
 
 Fig. 23. Perched blocks scattered over ice-worn surface of rock. 
 
 such diminution is that the blocks of rock lying on the edges of 
 a glacier are stranded on the side of the valley, as the ice shrinks 
 away from them. Such Perched Blocks or Erratics (Fig. 23), 
 as they are called, afford an excellent means of noting how much 
 higher and longer a glacier has once been than it is now. Their 
 great size (some of them are as large as good-sized cottages) and 
 their peculiar positions make it quite certain that they could not 
 have been transported by any current of water. They are often 
 poised on the tops of crags, on the very edges of precipices, or 
 on steep slopes where they could never have been left by any 
 flood, even had the flood been capable of moving them. The 
 agent that deposited them in such positions must have been one 
 that acted very quietly and slowly, letting the blocks gently sink 
 into the sites they now occupy. The only agent known to us 
 
ICE-RECORDS 67 
 
 that could have done this Is glacier-ice. We can actually see 
 similar blocks on the glaciers now, and others which have only 
 recently been stranded on the side of a valley from which the ice 
 has sunk. In the Swiss valleys, the scattered ice-borne boulders 
 may be observed by hundreds far above the- existing level of the 
 glaciers and many miles beyond where these now end. If the 
 origin of the dispersed erratics is self-evident in a valley where a 
 glacier is still busy transporting them, those that occur in valleys 
 which are now destitute of glaciers can offer no difficulty; they 
 become, indeed, striking monuments that glaciers once existed 
 there. 
 
 Scattered erratic blocks offer much interesting evidence of the 
 movements of the ice by which they were transported. In a 
 glacier-valley, the blocks that fall upon the ice remain on the side 
 from which they have descended. Hence, if there is any notable 
 difference between the rocks of the two sides, this difference will 
 be recognisable in the composition of the moraines, and will 
 remain distinct even to the end of the glacier. If, therefore, in a 
 district from which the glaciers have disappeared, we can trace 
 up the scattered blocks to their sources among the mountains, we 
 thereby obtain evidence of the actual track followed by the van- 
 ished glaciers. The limits to which these blocks are traceable do 
 not, of course, absolutely fix the limits of the ice that transported 
 them. They prove, however, that the ice extended at least as far 
 as they occur, but it may obviously have risen higher and ad- 
 vanced farther than the space within which the blocks are now 
 confined. In Europe, some striking examples occur of the use of 
 this kind of evidence. Thus the peculiar blocks of the Valais can 
 be traced all the way to the site of the modern city of Lyons. 
 There can therefore be no doubt that the glacier of the Ehone 
 once extended over all that intervening country and reached at 
 least as far as Lyons, a distance of not less than 170 miles from 
 where it now ends. Again, from the occurrence of blocks of some 
 of the characteristic rocks of Southern Scandinavia, in Northern 
 Germany, Belgium, and the east of England, we learn that a great 
 sheet of ice once filled up the bed of the Baltic and the North 
 Sea, carrying with it immense numbers of northern erratics. In 
 Britain, where there are now neither glaciers nor snow-fields, the 
 abundant dispersion of boulders from the chief tracts of high 
 ground shows that this country was once in large part buried 
 
68 GEOLOGY 
 
 under ice, like modern Greenland. The evidence for these state- 
 ments will be more fully given in a later part of this volume 
 (Chapter XXVII). 
 
 Besides the moraine-stuff carried along on the surface, loose 
 detritus and blocks of rock are pushed onwards under the ice. 
 When a glacier retires, this earthy and stony debris, where not 
 swept away by the escaping river, is left on the floor of the valley. 
 One remarkable feature of the stones in it is that a large pro- 
 portion of them are smoothed, polished, and covered with fine 
 scratches or ruts, such as would be made by hard sharp-pointed 
 fragments of stone or grains of sand. These markings run for 
 the most part along the length of each oblong stone, but not 
 infrequently cross each other, and sometimes an older may be 
 noticed partially effaced by a newer set. This peculiar striation 
 
 Fig. 24. Stone smoothed and striated by glacier-ice. 
 
 is a most characteristic mark of the action of glaciers. The 
 stones under the ice are fixed in the line of least resistance that 
 is, end on. In this position, under the weight of hundreds pf feet 
 of ice, they are pressed upon the floor over which the glacier is 
 travelling. Every sharp edge of stone or grain of sand, pressed 
 along the surface of a block, or over which the block itself is 
 slowly drawn, engraves a fine scratch or a deeper rut. As the 
 block moves onward, it is more and more scratched, losing its 
 corners and edges, and becoming smaller and smoother till, if it 
 travel far enough, it may be entirely ground into sand or mud 
 (Fig. 24). 
 
 (2) Erosion. The same process of erosion is carried on upon 
 the solid rocks over which the ice moves. These are smoothed^ 
 
ICE-RECORDS 
 
 69 
 
 striated, and polished by the friction of the grains of sand, peb- 
 bles, and blocks of stone crushed against them by the slowly 
 creeping mass of ice. Every boss of rock that looks toward the 
 quarter from which the overlying ice is moving is ground away, 
 while those that face to the opposite side are more or less sharp 
 and unworn. The striation is especially noteworthy. From the 
 fine scratches, such as are made by grains of sand, up to deep 
 ruts like those of cart-wheels in unmended roadways, or to still 
 wider and deeper hollows, all the friction-markings run in a 
 general uniform direction, which is that of the motion of the 
 glacier. Such striated surfaces could only be produced by some 
 
 Fig. 25. Ice-striation on the floor and side of a valley. 
 
 agent with rigidity enough to hold the sand-grains and stones 
 in position, and press them steadily onward upon the rocks. A 
 river polishes the rocks of its channel by driving shingle and sand 
 across them; but the currents are perpetually tossing these 
 materials now to one side now to another so that smoothed and 
 polished surfaces are produced; but with nothing at all resem- 
 bling striation. A glacier, however, by keeping its grinding mate- 
 rials fixed in the bottom of the ice, engraves its characteristic 
 parallel strias and groovings, as it slowly creeps down the valley. 
 All the surfaces of rock within reach of the ice are smoothed, pol- 
 
70 GEOLOGY 
 
 ished, and striated. Such surfaces present the most unmistakable 
 evidence of glacier-action, for they can be produced by no other 
 known natural agency. Hence, where they occur in glacier 
 valleys, far above and beyond the present limits of the ice, they 
 prove how greatly the ice has sunk. In regions also where there 
 are now no glaciers, these rock-markings remain as almost 
 imperishable witnesses that glaciers once existed. By means of 
 their evidence, for example, we can trace the march of great ice- 
 sheets which once enveloped the whole of Scandinavia and lay 
 deep upon nearly the whole of Britain. 
 
 The river that escapes from the end of a glacier is always 
 milky or muddy. The fine sand and mud that discolour the 
 water are not supplied by the thawing of the clear ice, nor by the 
 sparkling brooks that gush out of the mountain-slopes, nor by the 
 melting of the snows among the peaks that rise on either side. 
 This material can only come from the rocky floor of the glacier 
 itself. It is the fine sediment ground away from the rocks and 
 loose stones by their mutual friction under the pressure of the 
 overlying ice. It serves thus as a kind of index or measure of 
 the amount of material worn off the rocky bed by the grinding 
 action of the glacier. We can readily see that as this erosion 
 and transport are continually in progress, the amount of material 
 removed in the course of time must be very great. It has been 
 estimated, for example, that the Justedal glacier in Norway 
 removes annually from its bed 2,427,000 cubic feet of sediment. 
 At this rate the amount removed in a century would be enough to 
 fill up a valley or ravine 10 miles long, 100 feet broad, and 40 
 feet deep. 
 
 In arctic and antarctic latitudes, where the land is buried under 
 a vast ice-sheet, which is continually creeping seaward and break- 
 ing off into huge masses that float away as icebergs, there must 
 be a constant erosion of the terrestrial surface. Were the ice to 
 retire from these regions, the ground would be found to wear 
 what is called a glaciated surface; that is to say, all the bare 
 rocks would present a characteristic ice-worn aspect, rising into 
 smooth rounded bosses like dolphins' backs (roches moutonnees), 
 and sinking into hollows that would become lake-basins. Every- 
 where these bare rocks would show the striae and groovings graven 
 upon them by the ice, radiating generally from the central high 
 grounds, and thus indicating the direction of flow of the main 
 
ICE-RECORDS 71 
 
 streams of the ice-sheet. Piles of earth, ice-polished stones, and 
 blocks of rock would be found strewn over the country, especially 
 in the valleys and over the plains. These materials would still 
 further illustrate the movements of the ice, for they would be 
 found to be singularly local in character, each district having 
 supplied its own contribution of detritus. Thus from a region of 
 red sandstone, the rubbish would be red and sandy; from one of 
 black slate, it would be black and clayey (see Chapter XXVI). 
 
 SUMMARY. In this chapter we have seen that Ice in various 
 ways affects the surface of the land and leaves its mark there. 
 Frost, as already explained in Chapter II, pulverises soil, dis- 
 integrates exposed surfaces of stone, and splits open bare rocks 
 along their lines of natural joint. On rivers and lakes, the 
 disrupted ice wears down banks and pushes up mounds of sand, 
 gravel, and boulders along the shores. Snow lying on the surface 
 of the land protects that surface from the action of frost and air. 
 In the condition of avalanches, snow causes large quantities of 
 earth, soil, and blocks of rock to be removed from the mountain- 
 slopes and piled up on the valleys. In the form of glaciers, ice 
 transports the debris of the mountains to lower levels, bear- 
 ing along and sometimes stranding masses of rock as large as 
 cottages, which no other known natural agent could transport. 
 Moving down a valley, a glacier wears away the rocks, giving 
 them a peculiar smoothed and striated surface which is thoroughly 
 characteristic. By this grinding action, it erodes its bed and 
 produces a large amount of fine sediment, which is carried away 
 by the river that escapes at the end of the ice-stream. Land-ice 
 thus leaves thoroughly distinctive and enduring memorials of its 
 presence in polished and grooved rocks, in masses of earth, clay, 
 or gravel, with striated stones, and in the dispersal of erratic 
 blocks from principal masses of high ground. These memorials 
 may remain for ages after the ice itself has vanished. By their 
 evidence we know that the present glaciers of the Alps are only a 
 shrunk remnant of the great ice-fields which once covered that 
 region ; that the Scandinavian glaciers swept across what is now 
 the bed of the North Sea as far as the mouth of the Thames ; and 
 that Scotland, Ireland, Wales, and the greater part of England 
 were buried under great sheets of ice which crept downwards 
 into the North Sea on the one side, and into the Atlantic on the 
 other (Chapter XXVII). 
 
72 GEOLOGY 
 
 CHAPTER VII. 
 
 MEMORIALS OF THE SEA. 
 
 WE HAVE now to inquire how the work of the Sea is 
 registered in geological history. This work is broadly 
 of two kinds. In the first place,, the sea is engaged in 
 wearing away the edges of the land, and in the second place, 
 being the great receptacle into which all the materials, worn away 
 from the land, are transported, it arranges these materials over 
 its floor, ready to be raised again into land at some future time. 
 
 i. DEMOLITION OF THE LAND. In its work of destruction 
 along the coasts of the land, the sea acts to some extent (though 
 we do not yet know how far) by chemically dissolving the rocks 
 and sediments which it covers. Cast-iron bars, for example, have 
 been found to be so corroded by sea-water as to lose nearly half 
 their strength in fifty years. Doubtless many minerals and rocks 
 are liable to similar attacks. 
 
 But it is by its mechanical effects that the sea accomplishes 
 most of its erosion. The mere weight with which ocean-waves fall 
 upon exposed coasts breaks off fragments of rock from cliffs. 
 Masses, 13 tons in weight, have been known to be quarried out of 
 the solid rock by the force of the breakers in Shetland, at a height 
 of 70 feet above sea-level. As a wave may fall with a blow equal 
 to a pressure of 3 tons on the square foot, it compresses the air in 
 every cleft and cranny of a cliff, and when it drops it allows the 
 air instantly to expand again. By this alternate compression and 
 expansion, portions of the cliff are loosened and removed. Where 
 there is any weaker part in the rock, a long tunnel may be exca- 
 vated, which may even be drilled through to the daylight above, 
 forming an opening at some distance inland from the edge of the 
 cliff. During storms, the breakers rush through such a tunnel, 
 and spout forth from the opening (or blow-hole) in clouds of 
 spray (Fig. 26). 
 
MEMORIALS OF THE SEA 73 
 
 Probably the most effective part of the destructive action of the 
 sea is to be found in the battery of gravel shingle, and loose blocks 
 of stone which the waves discharge against cliffs exposed to their 
 fury. These loose materials, caught up by the advancing breakers 
 and thrown with great force upon the rocks of a coast-line, are 
 dragged back in the recoil of the water, but only to be again lifted 
 and swung forward. In this loud turmoil, the loose stones are 
 reduced in size and are ground smooth by friction against each 
 other and upon the solid cliff. The well-rounded and polished 
 aspect of the gravel on such storm-beaten shores is an eloquent 
 testimony to the work of the waves. But still more striking, 
 
 Fig. 26. Duller of Buchan a caldron-shaped cavity or blow-hole worn 
 out of granite by the sea on the coast of Aberdeenshire. 
 
 because more measurable, is the proof that the very cliffs them- 
 selve cannot resist the blows dealt upon them by the wave-borne 
 stones. Above the ordinary limit reached by the tides, the rocks 
 rise with a rough ragged face, bearing the scars inflicted on it by 
 the ceaseless attacks of the air, rain, frost, and the other agencies 
 that waste the surface of the land. But all along the base of the 
 cliff, within reach of the waves, the rocks have been smoothed and 
 polished by the ceaseless grinding of the shingle upon them, while 
 arches, tunnels, solitary pillars, half-tide skerries, creeks, and 
 
74 GEOLOGY 
 
 caves attest the steady advance of the sea and the gradual demoli- 
 tion of the shore. 
 
 Every rocky coast-line exposed to a tempestuous sea affords 
 illustrations of these features of the work of waves. Even where 
 the rocks are of the most durable kind, they cannot resist the 
 ceaseless artillery of the ocean. They are slowly battered down, 
 and every stage in their demolition may be witnessed, from the 
 sunken reef, which at some distance from the shore marks where 
 the coast-line once ran, up to the tunnelled cliff from which a 
 huge mass was detached during the storms of last winter. But 
 where the materials composing the cliffs are more easily removed, 
 the progress of the waves may be comparatively rapid. Thus on 
 the east coast of Yorkshire between Spurn Point and Flam- 
 borough Head, the cliffs consist of boulder-clay, and vary up to 
 more than 100 feet in height. At high water, the tide rises 
 against the base of these cliffs, and easily scours away the loose 
 debris which would otherwise gather there and protect them. 
 Hence, within historic times, a large tract of land, with its 
 parishes, farms, villages, and seaports, has been washed away, the 
 rate of loss being estimated at not less than 2% yards in a year. 
 Since the Roman occupation a strip of land between 2 and 3 
 miles broad is believed to have disappeared. 
 
 It is evident that to carry on effectively this mechanical erosion, 
 the sea-water must be in rapid motion. But in the deeper re- 
 cesses of the ocean, where there is probably no appreciable move- 
 ment of the water, there can hardly be any sensible erosion. In 
 truth it is only in the upper parts of the sea, which are liable to 
 be affected by wind, that the conditions for marine erosion can 
 be said to exist. The space within which these conditions are to 
 be looked for is that comprised between the lowest depth to which 
 the influence of waves and marine currents extends, and the great 
 est height to which breakers are thrown upon the land. These 
 limits, no doubt, vary considerably in different regions. In some 
 parts of the open sea, as off the coast of Florida, the disturbing 
 action of the waves has been supposed to reach to a depth of 600 
 feet, though the average limit is probably greatly less. On ex- 
 posed promontories in stormy seas, such as those .of the north of 
 Scotland, breakers have been known to hurl up stones to a height 
 of 300 feet above sea-level. But probably the zone, within which 
 
MEMORIALS OF THE SEA 
 
 75 
 
76 GEOLOGY 
 
 the erosive work of the sea is mainly carried on, does not as a rule 
 exceed 300 feet in vertical range. 
 
 Within some such limits as these, the sea is engaged in gnaw- 
 ing away the edges of the land. A little reflection will show us 
 that, if no counteracting operation should come into play, the 
 prolonged erosive action of the waves would reduce the land be- 
 low the sea-level. If we suppose the average rate of demolition to 
 be 10 feet in a century, then it would take not less than 52,800 
 years to cut away a strip one mile broad from the edge of the 
 land. But while the sea is slowly eating away the coast-line, 
 the whole surface of the land is at the same time crumbling down, 
 and the wasted materials are being carried away by rivers into the 
 sea at such a rate that, long before the sea could pare away more 
 than a mere narrow selvage, the whole land might be worn down 
 to the sea-level by air, rain, and rivers. 
 
 But there are counteracting influences in nature that would 
 probably prevent the complete demolition of the land. What 
 these influences are will be more fully considered in a later chap- 
 ter. In the meantime, it will be enough to bear in mind that 
 while the land is constantly worn down by the forces that are 
 acting upon its surface, it is liable from time to time to be up- 
 lifted by other forces acting from below. And the existing rela- 
 tion between the amount and height of land, and the extent of 
 sea, on the face of the globe, must be "looked upon as the balance 
 between the working of both these antagonistic classes of agencies. 
 
 Fig. 28. Section of submarine plain. I, Land cut into caves, tunnels, 
 sea-stacks, reefs, and skerries by the waves, and reduced to a platform 
 below the level of the sea (s s) on which the gravel, sand, and mud (d) 
 produced by the waste of the coast may accumulate. 
 
 But without considering for the present whether the results of 
 the erosion performed by the sea will be interrupted or arrested, 
 we can readily perceive that their tendency is toward the reduc- 
 tion of the level of the land to a submarine plain (Fig. 28). As 
 the waves cut away slice after slice from a coast-line, the portion 
 of land which they thus overflow, and over which they drive the 
 
MEMORIALS OF THE SEA 77 
 
 shingle to and fro, is worn down until it comes below the lower 
 limit of breaker-action, where it may be covered up with sand or 
 mud. When the abraded land has been reduced to this level, it 
 reaches a limit where erosion ceases, and where the sea, no longer 
 able to wear it down further, protects it from injury by other 
 agents of demolition. This lower limit of destruction on the sur- 
 face of the earth has been termed " the base-level of erosion." 
 
 We see, then, that the goal toward which all the wear and tear 
 of a coast-line tends, is the formation of a more or less level plat- 
 form cut out of the land. Yet an attentive study of the process 
 will convince us that in the production of such a platform the sea 
 has really had less to do than the atmospheric agents of destruc- 
 tion. An ordinary sea-cliff is not a vertical wall. In the great 
 majority of cases it slopes seaward at a steep angle ; but if it had 
 been formed, and were now being cut away, mainly by the sea, 
 it ought obviously to have receded fastest where the waves attack 
 it that is, at its base. In other words, if sea-cliffs retired chief- 
 ly because they are demolished by the sea, they ought to be most 
 eroded at the bottom, and should therefore be usually overhanging 
 precipices. That this is not the case shows that some other 
 agency is concerned which causes the higher parts of a cliff to 
 recede faster than those below. This agency can be no other 
 than that of the atmospheric forces air, frost, rain, and 
 springs. These cause the face of the cliff to crumble down, 
 detaching mass after mass, which, piled up below, serve as a 
 breakwater, and must be broken up and removed by the waves 
 before the solid cliff behind them can be attacked. 
 
 ii. ACCUMULATIONS FORMED BY THE SEA. It is not its erosive 
 action that constitutes the most important claim of the sea to 
 the careful study of the geologist. After all, the mere marginal 
 belt or fringe within which this action is confined forms such a 
 small fraction of the whole terrestrial area of the globe, that its 
 importance dwindles down whjen we compare it with the enor- 
 mously vaster surface over which the operations of the air, rain, 
 rivers, springs, and glaciers are displayed. But when we regard 
 the sea as the receptacle into which all the materials worn off 
 the land ultimately find their way, we see what a large part it 
 must play in geological history. 
 
 During the last fifteen years great additions have been made 
 to our knowledge of the sea-bottom all over the world. Portions 
 
78 GEOLOGY 
 
 of the deposits accumulating there have been dredged up even 
 from the deepest abysses, so that it is now possible to construct 
 charts, showing the general distribution of materials over the 
 floor of the ocean. 
 
 Beginning at the shore, let us trace the various types of marine 
 deposits outward to the floors of the great ocean-abysses. In 
 many places, the sea is more or less barred back by the accumu- 
 lation of sediment worn away from the land. In estuaries, for 
 example, there is often such an amount of mud in the water that 
 the bottom on either side is gradually raised above the level of tide 
 mark, and forms eventually a series of meadows which the sea 
 can no longer overflow. At the mouths of rivers with a consider- 
 able current, a check is given to the flow of the water when it 
 reaches the sea, and there is a consequent arrest of its detritus. 
 Hence, a bar is formed across the outflow of a river, which during 
 floods is swept seawards, and during on-shore gales is driven 
 again inland. Even where there is no large river, the smaller 
 streams flowing off the surface of a country may carry down 
 sediment enough to be arrested by the sea, and to be thrown up 
 as a long bank or bar running parallel with the coast. Behind 
 this bar, the drainage of the interior accumulates in long lagoons, 
 which find an outflow through some breach in the bar, or by soak- 
 ing through the porous materials of the bar itself. A large part 
 of the eastern coast of the United States is fringed with such 
 bars and lagoons. A space several hundred miles long on the 
 east coast of India is similarly bordered. 
 
 But the most remarkable kind of accumulation of terrestrial 
 detritus in the sea is undoubtedly that of river-deltas. Where 
 the tidal scour is not too great, the sediment brought down by a 
 large river into a marine bay or gulf gradually sinks to the bot- 
 tom as the fresh spreads over and mingles with the salt water. 
 During floods, coarse sediment is swept along, while during low 
 states of the river nothing but fine mud may be transported. Al- 
 ternating sheets of different kinds of sediment are thus laid down 
 one upon another on the sea-floor, until by degrees they reacli 
 the surface, and thus gradually increase the breadth of the land. 
 Some deltas are of enormous size and depth. That of the Gan- 
 ges and Brahmaputra covers an area of between 50,000 and 
 60,000 square miles that is, about as large as England and 
 Wales. It has been bored through to a depth of 481 feet, and 
 
MEMORIALS OF THE SEA 79 
 
 has been found to consist of numerous alternations of fine clays, 
 marls, and sands or sandstones, with occasional layers of gravel. 
 In all this great thickness of sediment, no trace of marine organ- 
 isms was found, but land-plants and bones of terrestrial and 
 fluviatile animals occurred. Lower Egypt has been formed by 
 the growth of the delta of the Nile, whereby a wide tract of allu- 
 vial land has not only filled up the bottom of the valley, but has 
 advanced into the Mediterranean. 
 
 Turning now to the deposits that are more distinctively those 
 of the sea itself, we find that ridges of coarse shingle, gravel, and 
 sand are piled up along the extreme upper limit reached by the 
 waves. The coarsest materials are for the most part thrown 
 highest, especially in bays and narrow creeks where the breakers 
 are confined within converging shores. In such situations, during 
 heavy gales, storm-beaches of coarse rounded shingle are formed 
 sometimes several yards above ordinary high-tide mark 
 (Fig. 29). 
 
 Fig. 29. Storm-beach ponding back a stream and forming a lake ; west 
 coast of Sutherlandshire. 
 
 Where a barrier of this kind is thrown across the mouth of a 
 brook, the fresh water may be ponded back to form a small lake, 
 of which the outflow usually escapes by percolation through the 
 shingle. In sheltered bays, behind headlands, or on parts of a 
 coast-line where tidal currents meet, detritus may accumulate in 
 spits or bars. Islands have in this way been gradually united to 
 each other or to the mainland, while the mainland itself has 
 
80 GEOLOGI 
 
 gained considerably in breadth. At Eomney Marsh, on the 
 south-east coast of England, for instance, a tract of more than 
 80 square miles, which in lloman times was in great part cov- 
 ered by the sea at high water, is now dry land, having been 
 gained partly by the natural increase of shingle thrown up by 
 the waves and partly by the barriers artificially erected to exclude 
 the sea. 
 
 While the coarsest shingle usually accumulates towards the 
 upper part of the beach, the materials generally arrange them- 
 selves according to size and weight, becoming on the whole finer 
 as they are traced towards low-water mark. But patches of 
 coarse gravel may be noticed on any part of a beach, and large 
 boulders may be seen even below the limits of the lowest 
 tides. As a rule, the deposits formed along a beach, and in 
 the sea immediately beyond, include the coarsest kinds of marine 
 sediment. They are also marked by frequent alternations of 
 coarse and fine detritus, these rapid interchanges, pointing to 
 the varying action of the waves and strong shore-currents. To- 
 wards the lower limit of breaker-action, fine gravel and sand 
 are allowed to settle down, and beyond these, in quiet depths 
 where the bottom is not disturbed, fine sand and mud washed 
 away from the land slowly accumulate. 
 
 The distance to which the finer detritus of the land is carried 
 by ocean-currents before it finds its way to the bottom, varies 
 up to about 200 miles or more. Within this belt of sea, the 
 land-derived materials are distributed over the ocean-floor. 
 Coarse and fine gravel and sand are the most common materials 
 in the areas nearest the land. Beyond these lie tracts of fine 
 sand and silt with occasional patches of gravel. Still farther 
 from the land, at depths of 600 feet and upwards, fine blue and 
 green muds are found, composed of minute particles of such 
 minerals as form the ordinary rocks of the land. But traced 
 out into the open ocean, these various deposits of recognisable 
 terrestrial origin give place to thoroughly oceanic accumula- 
 tions, especially to widespread sheets of exceedingly fine red 
 and brown clay. This clay, the most generally diffused deposit 
 of the deeper or abysmal parts of the sea, appears to be derived 
 from the decomposition of volcanic fragments either washed 
 away from volcanic islands or supplied by submarine eruptions. 
 That it is accumulated with extreme slowness is shown by two 
 
MEMORIALS OF THE SEA 81 
 
 curious and interesting kinds of evidence. Where it occurs 
 farthest removed from land, great numbers of sharks' teeth, 
 with ear-bones and other bones of whales, have been dredged 
 up from it, some of these relics being quite fresh, others partially 
 coated with a crust of brown peroxide of manganese, some wholly 
 and thickly enveloped in this substance. The same haul of the 
 dredge has brought up bones in all these conditions, so that 
 they must be lying side by side on the red clay floor of the 
 ocean abysses. The deposition of manganese is no doubt an 
 exceedingly slow process, but it is evidently faster than the 
 deposition of the red clay. The bones dredged up probably 
 represent a long succession of generations of animals. Yet so 
 tardily does the red clay gather over them, that the older ones 
 are not yet covered up by it, though they have had time to be 
 deeply encased in oxide of manganese. The second kind of 
 evidence of the extreme slowness of deposit in the ocean abysses 
 is supplied by minute spherules of metallic iron, which occurring 
 in numbers dispersed through the red clay, have been identified 
 as portions of meteorites or falling stars. These particles no 
 doubt fall all over the ocean, but it is only where the rate of 
 deposition of sediment is exceedingly slow that they may be 
 expected to be detected. 
 
 Besides the sediments now enumerated, the bottom of the sea 
 receives abundant accumulations of the remains of shells, corals, 
 foraminifera and other marine creatures; but these will be de- 
 scribed in the next chapter, where an account is given of the 
 various ways in which plants and animals, both upon the land 
 and in the sea, inscribe their records in geological history. It 
 must also be borne in mind that throughout all the sediments 
 of the sea-floor, from the upper part of the beach down to the 
 bottom of the deepest and remotest abyss, the remains of the 
 plants, sponges, corals, shells, fishes and other organisms of the 
 ocean may be entombed and preserved. It will suffice here to 
 remember that various depths and regions of the sea have their 
 own characteristic forms of life the remains of which are pre- 
 served in the sediments accumulating there, and that although 
 gravel, sand, and mud laid down beneath the sea may not differ 
 in any recognisable detail from similar materials deposited in 
 a lake or river, yet the presence of marine organisms in them 
 would be enough to prove that they had been formed in the sea. 
 
82 GEOLOGY 
 
 It is evident, also, that if the sea-floor over a wide area were 
 raised into land, the extent of the deposits would show that 
 they could not have been accumulated in any mere river or 
 lake, but must bear witness to the former presence of the sea 
 itself. 
 
 SUMMARY. The sea records its work upon the surface of the 
 earth in a twofold way. In the first place, in co-operation with 
 the atmospheric agents of disintegration, it eats away the margin 
 of the land and planes it down. The final result of this process 
 if uninterrupted would be to reduce the level of the land to 
 that of a submarine platform, the position of the surface of 
 which would be determined by the lower limit of effective 
 breaker-action. In the second place, the sea gathers over its 
 floor all the detritus worn by every agency from the surface 
 of the land. This material is not distributed at random; it is 
 assorted and arranged by the waves and currents, the coarsest 
 portions being laid down nearest the land, and the finest in 
 stiller and deeper water. The belt of sea-floor within which 
 this deposition takes place probably does not much exceed a 
 breadth of 200 miles. Beyond that belt, the bottom of the 
 ocean is covered to a large extent with deposits of red clay 
 derived from the decomposition of volcanic material and laid 
 down with extreme slowness. These and the widespread deposits 
 of dead sea-organisms (to be described in next chapter) 
 are truly oceanic accumulations, recognisably distinct from those 
 derived from terrestrial sources within the narrow zone of de- 
 position near the land. 
 
RECORDS OF PLANTS AND ANIMALS 
 
 CHAPTER VIII. 
 
 RECORDS OF PLANTS AND ANIMALS. 
 
 BROADLY considered, there are two distinct ways in which 
 Plants and Animals leave their mark upon the surface 
 of the earth. In the first place, they act directly by 
 promoting or arresting the decay of the land, and by forming 
 out of their own remains deposits which are sometimes thick 
 and extensive. In the second place, their remains are trans- 
 ported and entombed in sedimentary accumulations of many 
 different kinds, and furnish important evidence as to the con- 
 ditions under which these accumulations were formed. Each 
 of these two forms of memorial deserves our careful attention, 
 for, taken together, they comprise the most generally interesting 
 departments of geology, and those in which the history of the 
 earth is principally discussed. 
 
 i. DIRECT ACTION OF LIVING THINGS UPON THE SURFACE OF 
 THE GLOBE. This action is often of a destructive kind, both 
 plants and animals taking their part in promoting the general 
 disintegration of rocks and soils. Thus, by their decay they 
 furnish to the soil those organic acids which have been referred to 
 as so important in increasing the solvent power of water, and 
 thereby promoting the waste of rocks. By thrusting their roots 
 into crevices of cliffs, plants loosen and gradually wedge off 
 pieces of rock, and by sending their roots and rootlets through 
 the soil, they open up the subsoil to be attacked by the air and 
 the descending moisture. The action of the common earth- 
 worm in bringing up fine soil to be exposed to the influences 
 of wind and rain has been referred to. Many burrowing 
 animals also, such as the mole and rabbit, throw up large quan- 
 tities of soil and subsoil which are liable to be blown or washed 
 away. 
 
84 GEOLOGY 
 
 On the other hand, the action may be conservative, as, for 
 instance, where, by forming a covering of turf, vegetation 
 protects the soil underneath from being rapidly removed, or 
 where sand-loving plants bind together the surface of dunes, 
 and thereby arrest the progress of the sand, or where forests 
 shield a mountain-side from the effects of heavy rains and 
 descending avalanches. 
 
 (1) Deposits formed of the remains of Plants. But it is 
 chiefly by the aggregation of their own remains into more or 
 less extensive deposits that plants and animals leave their most 
 prominent and enduring memorials. As examples of the way in 
 which this is done by plants, reference may be made to peat- 
 bogs, mangrove-swamps, infusorial earth, and calcareous sea- 
 weeds. 
 
 Peat-bogs. In temperate and arctic countries, marshy vege- 
 tation accumulates in peat-bogs over areas from an acre or two 
 to many square miles, and to a depth of sometimes 50 feet. 
 These deposits are largely due to the growth of bog-mosses and 
 other aquatic plants which, dying in their lower parts, continue 
 to grow upward on the same spot. On flat or gently-inclined 
 moors, in hollows between hills, on valley-bottoms, and in shallow 
 lakes, this marshy vegetation accumulates as a wet spongy fibrous 
 mass, the lower portions of which by de- 
 grees become a more or less compact 
 dark brown or black pulpy substance, 
 wherein the fibrous texture, so well seen 
 in the upper or younger parts, in large 
 measure disappears. In a thick bed of 
 peat, it is not infrequently possible to 
 detect a succession of plant remains, 
 showing that one kind of vegetation has 
 Fig. 30. Section of a given place to another during the accum- 
 ulation of the mass. In Europe, as has 
 
 been already mentioned, peat-bogs often rest directly upon fresh- 
 water marl containing remains of lacustrine shells (1 in Fig. 30). 
 In every such case, it is evident that the peat has accumulated 
 on the site of a shallow lake which has been filled up, and con- 
 verted into a morass by the growth of marsh-plants along its 
 edges and over its floor. The lowest parts of the peat may 
 
RECORDS OF PLANTS AND ANIMALS 85 
 
 contain remains of the reeds, sedges, and other aquatic plants 
 which choked up the lake (2, 3). Higher up, the peat consists 
 almost entirely of the matted fibres of different mosses, espe- 
 cially of the kind known as Bog-moss or Sphagnum (4). The 
 uppermost layers (5, 6) may be full of roots of different heaths 
 which spread over the surface of the bog. 
 
 The rate of growth of peat has been observed in different 
 situations in Central Europe to vary from less than a foot to 
 about two feet in ten years; but in more northern latitudes the 
 growth is probably slower. Many thousand square miles of 
 Europe and North America are covered with peat-bogs, those 
 of Ireland being computed to occupy a seventh part of the sur- 
 face of the island, or upwards of 4000 square miles. 
 
 As the aquatic plants grow from the sides toward the centre 
 of a shallow lake, they gradually cover over the surface of the 
 water with a spongy layer of matted vegetation. Animals, and 
 man himself, venturing on this treacherous surface sink through 
 it, and may be drowned in the black peaty mire underneath. 
 Long afterwards, when the morass has become firm ground, and 
 openings are made in it for digging out the peat to be used 
 as fuel, their bodies may be found in an excellent state of preser- 
 vation. The peaty water so protects them from decay that the 
 very skin and hair sometimes remain. In Ireland, numerous 
 skeletons of the great Irish elk have been obtained from the 
 bogs, though the animal itself has been extinct since before the 
 beginning of the authentic history of the country. 
 
 Mangrove-swamps. Along the flat shores of tropical lands, 
 the mangrove trees grow out into the salt water, forming a belt 
 of jungle which runs up or completely fills the creeks and bays. 
 So dense is the vegetation that the sand and mud, washed into 
 the sea from the land, are arrested among the roots and radicles 
 of the trees, and thus the sea is gradually replaced by firm 
 ground. The coast of Florida, is fringed with such mangrove- 
 swamps for a breadth of from 5 to 20 miles. In such regions, 
 not only does the growth of these swamps add to the breadth 
 of the land, but the sea is barred back, and prevented from 
 attacking the newly-formed ground inside. 
 
 Infusorial earth. A third kind of vegetable deposit to be 
 referred to here is that known by the names of infusorial earth, 
 
86 
 
 GEOLOGY 
 
 diatom-earth, and tripoli-powder. It consists almost entirely of 
 the minute frustules of microscopic plants called diatoms, which 
 are found abundantly in lakes and likewise in some region? of 
 the ocean (Fig. 31). These lowly organisms are remarkable 
 for secreting silica in their structure. As they die, their singu- 
 larly durable siliceous remains fall like a fine dust on the bottom 
 of the water, and accumulate there as a pale gray or straw- 
 coloured deposit, which, when dry, is like flour, and in its pure 
 varieties is made almost entirely of silica (90 to 97 per cent). 
 Underneath the peat-bogs of Britain a layer of this material is 
 sometimes met with. One of the most famous examples is that 
 of Kichmond, Virginia, where a bed of it occurs 30 feet thick. 
 At Bilin in Bohemia also an important bed has long been known. 
 The bottom of some parts of the Southern Ocean is covered 
 
 Fig. 31. Diatom-earth from floor of Antarctic Ocean, magnified 300 
 diameters (Challenger Expedition). 
 
 with a diatom-ooze made up mainly of siliceous diatoms, but 
 containing also other siliceous organisms (radiolarians) and cal- 
 careous foraminifera (Fig. 31). 
 
 Accumulations of sea-weeds. Yet one further illustration 
 of plant-action in the building up of solid rock may be given. 
 As a rule the plants of the sea form no permanent accumulations, 
 though here and there under favourable conditions, such as in 
 bays and estuaries, they may be thrown up and buried under 
 sand so as eventually to be compressed into a kind of peat. Some 
 sea-weeds, however, abstract from sea-water carbonate of lime, 
 which they secrete to such an extent as to form a hard stony 
 structure, as in the case of the common nullipore. When the 
 plants die, their remains are thrown ashore and pounded up 
 by the waves, and being durable they form a white calcareous 
 
RECORDS OF PLANTS AND ANIMALS 87 
 
 sand. By the action of the wind, this sand is blown inland and 
 may accumulate into dunes. But unlike ordinary sand, it is 
 liable to be slightly dissolved by rain-water, and as the portion 
 so dissolved is soon redeposited by the evaporation of the mois- 
 ture, the little sand-grains are cemented together, and a hard 
 crust is formed which protects the sand underneath from being 
 blown away. Meanwhile rain-water percolating through the 
 mounds gradually solidifies them by cementing the particles of 
 sand to each other, and thick masses of solid white stone are 
 thus produced. Changes of this kind have taken place on a 
 great scale at Bermuda, where all the dry land consists of lime- 
 stone formed of compacted calcareous sand, mainly the detritus 
 of sea-weeds. 
 
 
 Fig. 32. Recent limestone (Common Cockle, etc., cemented in a matrix 
 of broken shells). 
 
 (2) Deposits formed of the remains of Animals. Animals 
 are, on the whole, far more successful than plants in leaving 
 enduring memorials of their life and work. They secrete hard 
 outer shells and internal skeletons endowed with great durabil- 
 ity, and capable of being piled up into thick and extensive 
 deposits which may be solidified into compact and enduring 
 stone. On land, we have an example of this kind of accumula- 
 tion in the lacustrine marl already described as formed of the 
 congregated remains of various shells. But it is in the sea that 
 animals, secreting carbonate of lime, build up thick masses of 
 rock, such as shell-banks, ooze, and coral reefs (see Chapter XI). 
 
 Shell-banks. Some molluscs, such as the o} 7 ster, live in 
 populous communities upon submarine banks. In the course of 
 generations, thick accumulations of their shells are formed on 
 these banks. By the action of currents also large quantities 
 of broken shells are drifted to various parts of the sea-bottom 
 
88 GEOLOGY 
 
 not far from land. Such deposits of shells, in situ or trans- 
 ported, may be more or less mixed with or buried under sand 
 and silt, according as the currents vary in direction and force. 
 On the other hand, they may be gradually cemented into a 
 solid calcareous mass, as has been observed off the coast of 
 Florida, where they form on the sea-bottom a sheet of lime- 
 stone, made up of their remains. 
 
 Ooze. From observations made during the great expedition 
 of the Challenger, it has been estimated that in a square mile 
 of the tropical ocean down to a depth of 100 fathoms there 
 are more than 16 tons of carbonate of lime in the form of 
 living animals. A continual rain of dead calcareous organisms 
 is falling to the bottom, where their remains accumulate as a 
 soft chalky ooze. Wide tracts of the ocean-floor are covered with 
 a pale gray ooze of this nature, composed mainly of the remains 
 of the shells of the foraminifer Globigerina (Fig. 33). In the 
 
 Fig. 33. Globigerina ooze dredged up by Challenger Expedition from a 
 depth of 1900 fathoms in the North Atlantic 
 
 north Atlantic this deposit probably extends not less than 1300 
 miles from east to west, and several hundred miles from north 
 to south. 
 
 Here and there, especially among volcanic islands, portions 
 of the sea-bed have been raised up into land, and masses of 
 modern limestone have thereby been exposed to view. Though 
 they are full of the same kind of shells as are still living in 
 the neighbouring sea, they have been cemented into compact 
 and even somewhat crystalline rock, which has been eaten into 
 caverns by percolating water, like limestones of much older date. 
 This cementation, as above remarked, is due to water permeat- 
 ing the stone, dissolving from its outer parts the calcareous 
 
RECORDS OF PLANTS AND ANIMALS 89 
 
 matter of shells, corallines, and other organic remains, and 
 redepositing it again lower down, so as to cement the organic 
 detritus into a compact stone. 
 
 Coral-reefs offer an impressive example of how extensive 
 masses of solid rock may be built up entirely of the aggregated 
 remains of animals. In some of the warmer seas of the globe, 
 and notably in the track of the great ocean-currents, where 
 marine life is so abundant, various kinds of coral take root 
 upon the edges and summits of submerged ridges and peaks, 
 as well as on the shelving sea-bottom facing continents or 
 encircling islands (1 in Fig. 34). These creatures do not appear 
 to nourish at a greater depth than 15 or 20 fathoms, and they 
 are killed by exposure to sun and air. The vertical space within 
 which they live may therefore be stated broadly as about 100 
 feet. They grow in colonies, each composed of many indivi- 
 duals, but all united into one mass, which at first may be 
 merely a little solitary clump on the sea-floor, but which, as 
 it grows, joins other similar clumps to form what is known as 
 
 Fig. 34. Section of a coral-reef. 1. Top of the submarine ridge or 
 bank on which the corals begin to build. 2. Coral-reef. 3. Talus of 
 large blocks of coral-rock on which the reef is built outward. 4. Fine 
 coral sand and mud produced by the grinding action of the breakers 
 on the edge of the reef. 5. Coral sand thrown up by the waves 
 and gradually accumulating above their reach to form dry ground. 
 
 a reef. Each individual secretes from the sea-water a hard 
 calcareous skeleton inside its transparent jelly-like body, and 
 when it dies, this skeleton forms part of the platform upon 
 which the next generation starts. Thus the reef is gradually 
 built upward as a mass of calcareous rock (2), though only its 
 upper surface is covered with living corals. These creatures 
 continue to work upward until they reach low-water mark, and 
 then their further upward progress is checked. But they are 
 still able to grow outward. On the outer edges of the reef they 
 flourish most vigorously, for there, amid the play of the breakers, 
 they find the food that is brought to them by the ocean-currents. 
 
90 GEOLOGY 
 
 From time to time fragments are torn off by breakers from 
 the reef and roll down its steep front (3). There, partly by 
 the chemical action of the sea-water, and partly by the fine 
 calcareous mud and sand (4), produced by the grinding action 
 of the waves and washed into their crevices, these loose blocks 
 are cemented into a firm, steep slope, on the top of which the 
 reef continues to grow outwards. Blocks of coral and quan- 
 tities of coral-sand are also thrown up on the surface of the 
 reef, where by degrees they form a belt of low land above the 
 reach of the waves (5). On the inside of the reef, where the 
 corals cannot find the abundant food-supply afforded by the 
 open water outside, they dwindle and die. Thus the tendency 
 of all reefs must be to grow seawards, and to increase in breadth. 
 Perhaps their breadth may afford some indication of their rela- 
 tive age. 
 
 Where a reef has started on a shelving sea-bottom near the 
 coast of a continent, or round a volcanic island, the space of 
 water inside is termed the Lagoon Channel. Where the reef 
 has been built up on some submarine ridge or peak, and there 
 is consequently no land inside, the enclosed space of water is 
 called a Lagoon, and the circular reef of coral is known as an 
 Atoll. If no subsidence of the sea-bottom takes place, the maxi- 
 mum thickness of a reef must be limited by the space within 
 which the corals can thrive that is, a vertical depth of about 
 100 feet from the surface of the sea. But the effect of the 
 destruction of the ocean-front of the reef, and the piling up 
 of a slope of its fragments on the sea-bottom outside, will be 
 to furnish a platform of the same materials on which the reef 
 itself may grow outward, so that the united mass of calcareous 
 rock may attain a very much greater thickness than 100 feet. 
 On the other hand, if the sea-bottom were to sink at so slow 
 a rate that the reef-building corals could keep pace with the 
 subsidence, a mass of calcareous rock many thousand feet thick 
 might obviously be formed by them. It is a disputed question 
 in which of these two ways atolls have been formed. 
 
 It is remarkable how rapidly and completely the structure 
 of the coral-skeleton is effaced from the coral-rock, and a more 
 or less crystalline and compact texture is put in its place. The 
 change is brought about partly by the action of both sea-water 
 and rain-water in dissolving and redepositing carbonate of lime 
 
RECORDS OF PLANTS AND ANIMALS 91 
 
 among the minute interstices of the rock, and partly also by 
 the abundant mud and sand produced by the pounding action 
 of the breakers on the reef, and washed into the crevices. On 
 the portion of a reef laid dry at low water, the coral-rock looks 
 in many places as solid and old as some of the ancient white 
 limestones and marbles of the land. There, in pools where a 
 current or ripple of water keeps the grains of coral-sand in 
 motion, each grain may be seen to have taken a spherical form 
 unlike that of the ordinary irregularly rounded or angular 
 particles. This arises because carbonate of lime in solution 
 in the water is deposited round each grain as it moves along. 
 A mass of such grains aggregated together is called oolite, from 
 its resemblance to fish-roe. In many limestones, now forming 
 wide tracts of richly cultivated country, this oolitic structure 
 is strikingly exhibited. There can be no doubt that in these 
 cases it was produced in a similar way to that now in progress 
 on coral-reefs. 
 
 In the coral tracts of the Pacific Ocean there are nearly 300 
 coral islands, besides extensive reefs round volcanic islands. 
 Others occur in the Indian Ocean. Coral-reefs abound in the 
 West Indian Seas, where, on many of the islands, they have 
 been upraised into dry land, in Cuba to a height of 1100 feet 
 above sea-level. The Great Barrier Eeef that fronts the north- 
 eastern coast of Australia is 1250 miles long, and from 10 to 
 90 miles broad. 
 
 There are other ways in which the aggregation of animal 
 remains forms more or less extensive and durable rocks. To 
 some of these reference will be made in later chapters. Enough 
 has been said here to show that by the accumulation of their 
 hard parts animals leave permanent records of their presence 
 both on land and in the sea. 
 
 ii. PRESERVATION OF REMAINS or PLANTS AND ANIMALS IN 
 SEDIMENTARY DEPOSITS. But it is not only in rocks formed 
 out of their remains that living things leave their enduring 
 records. These remains may be preserved in almost every kind 
 of deposit, under the most wonderful variety of conditions. And 
 as it is in large measure from their occurrence in such deposits 
 that the geologist derives the evidence that successive tribes 
 of plants and animals have peopled the globe, and that the 
 climate and geography of the earth have greatly varied at 
 
92 GEOLOGY 
 
 different periods, we shall find it useful to observe the different 
 ways in which the remains both of plants and animals are at 
 this moment being entombed and preserved upon the land and 
 in the sea. With the knowledge thus gained, it will be easier 
 to understand the lessons taught by the organic remains that 
 lie among the various solid rocks around us. 
 
 It is evident that in the vast majority of cases, the plants 
 and animals of the land leave no perceptible trace of their pres- 
 ence. Of the forests that once covered so much of Central and 
 Northern Europe, which is now bare ground, most have disap- 
 peared, and unless authentic history told that they had once 
 flourished, we should never have known anything about them. 
 There were also herds of wild oxen, bears, wolves, and other 
 denizens contemporaneous with the vanished forests. But they 
 too have passed away, and we might ransack the soil in vain 
 for any trace of them. 
 
 If the remains of terrestrial vegetation and animals are any- 
 where preserved it must obviously be only locally, but the favour- 
 able circumstances for their preservation, although not every- 
 where to be found, do present themselves in many places if 
 we seek for them. The fundamental condition is that the relics 
 should, as soon as possible after death, be so covered up as 
 to be protected from the air and from too rapid decomposition. 
 Where this condition is fulfilled, the more durable of them may 
 be preserved for an indefinite series of ages. 
 
 (a) On the Land there are various places where the remains 
 both of plants and animals are buried and shielded from decay. 
 To some of these reference has already been made. Thus amid 
 the fine silt, mud, and marl gathering on the floors of lakes, 
 leaves, fruits, and branches, or tree-trunks, washed from the 
 neighbouring shores, may be imbedded, together with insects, 
 birds, fishes, lizards, frogs, field-mice, rabbits, and other in- 
 habitants. These remains may of course often decay on the 
 lake-bottom, but where they sink into or are quickly covered 
 up by the sediment, they may be effectually preserved from 
 obliteration. They undergo a change, indeed, being gradually 
 turned into stone, as will be described in Chapter XV. But 
 this conversion may be effected so gently as to retain the finest 
 microscopic textures of the original organisms. 
 
 In peat-bogs also, as stated elsewhere, wild animals are often 
 
RECORDS OF PLANTS AND ANIMALS 93 
 
 engulfed, and their soft parts are occasionally preserved as 
 well as their skeletons. The deltas of river-mouths must re- 
 ceive abundantly the remains of animals swept off by floods. 
 As the carcases float seawards, they begin to fall to pieces and 
 the separate bones sink to the bottom, where they are soon 
 buried in the silt. Among the first bones to separate from 
 the rest of the skeleton are the lower jaws (see Ch. XXIII) . We 
 should therefore expect that in excavations made in a delta these 
 bones would occur most frequently. The rest of the skeleton 
 is apt to be carried farther out to sea before it can find its way 
 to the bottom. The stalagmite floor of caverns has already been 
 referred to (Ch. V) as an admirable material for enclosing and 
 preserving organic remains. The animals that fell into these 
 recesses, or used them as dens in which they lived or into which 
 they dragged their prey, have left their bones on the floors, 
 where encased in or covered by solid stalagmite, these relics 
 have remained for ages. Most of our knowledge of the animals 
 which inhabited Europe at the time when man appeared, is 
 derived from the materials disinterred from these Bone-caves. 
 Allusion has also been made to the travertine formed by mineral- 
 springs and to the facility with which leaves, shells, insects, and 
 small birds, reptiles, or mammals may be enclosed and pre- 
 served in travertine. Thus, while the plants and animals of 
 the land for the most part die and decay into mere mould, there 
 are here and there localities where their remains axe covered 
 up from decay and preserved as memorials of the life of the 
 time. 
 
 (&) On the bottom of the Sea the conditions for the pres- 
 ervation of organic remains are more general and favourable 
 than on land. Among the sands and gravels of the shore, some 
 of the stronger shells that live in the shallower waters near 
 land may be covered up and preserved, though often only in 
 rolled fragments. It is below tide-mark, however, and more 
 especially beneath the limit to which the disturbing action of 
 breakers descends, that the remains of the denizens of the sea 
 are most likely to be buried in sediment and to be preserved 
 there as memorials of the life of the sea. It is evident that 
 hard and therefore durable relics have the best chance of escap- 
 ing destruction. Shells, corals, corallines, spicules of sponges, 
 teeth, vertebra, and ear-bones of fishes may be securely entombed 
 
94 GEOLOGY 
 
 in successive layers of silt or mud. But the vast crowds of 
 marine creatures that have no hard parts must almost always 
 perish without leaving any trace whatever of their existence. 
 And even in the case of those which possess hard shells or skele- 
 tons, it will be easily understood that the great majority of them 
 must be decomposed upon the sea-bottom, their component ele- 
 ments passing back again into the sea-water from which they 
 were originally derived. It is only where sediment is deposited 
 fast enough to cover them up and protect them before they have 
 time to decay, that they may be expected to be preserved. 
 
 In the most favourable circumstances, therefore, only a very 
 small proportion of the creatures living in the sea at any time 
 leave a tangible record of their presence in the deposits of the sea- 
 bottom. It is in the upper waters of the ocean, and especially 
 in the neighbourhood of land, that life is most abundant. The 
 same region also is that in which the sediment derived from the 
 waste of the land is chiefly distributed. Hence it is in these 
 marginal parts of the ocean that the conditions for preserving 
 memorials of the animals that inhabit the sea are best developed. 
 
 As we recede from the land, the rate of deposit of sediment on 
 the sea-floor gradually diminishes, until in the central abysses it 
 reaches that feeble stage so strikingly brought before us by the 
 evidence of the manganese nodules (Ch. VII). The larger and 
 thinner calcareous organisms are attacked by the sea-water and 
 dissolved, apparently before they can sink to the bottom; at 
 least their remains are comparatively rarely found there. It is 
 such indestructible objects as sharks' teeth and vertebrae and ear- 
 bones of whales that form the most conspicuous organic relics 
 in these abysmal deposits. 
 
 SUMMARY. Plants and animals leave their records in geo- 
 logical history, partly by forming distinct accumulations of their 
 remains, partly by contributing their remains to be imbedded in 
 different kinds of deposits both on land and in the sea. As 
 examples of the first mode of chronicling their existence, we 
 may take the growth of marsh-plants in peat-bogs, the spread 
 of mangrove-swamps along tropical shores, and the deposition 
 of infusorial earth on the bottom of lakes and of the sea; the 
 accumulation of nullipore sand into solid stone, the formation 
 of extensive shell-banks in many seas, the wide diffusion of 
 organic ooze over the floor of the sea, and the growth of coral 
 
RECORDS OF PLANTS AND ANIMALS 95 
 
 reefs. As illustrations of the second method, we may cite the 
 manner in which the remains of terrestrial plants and animals 
 are preserved in peat-bogs, in the deltas of rivers, in the stalag- 
 mite of caverns, and in the travertine of springs ; and the way in 
 which the hard parts of marine creatures are entombed in the 
 sediments of the sea-floor, more especially along that belt fring- 
 ing the continents and islands, where the chief deposit of sedi- 
 ment from the disintegration of the land takes place. Never- 
 theless, alike on land and sea, the proportion of organic remains 
 thus sealed up and preserved is probably always but an insignifi- 
 cant part of the total population of plants and animals living at 
 any given moment. 
 
 How the remains of plants and animals when once entombed 
 in sediment are then hardened and petrified, so as to retain their 
 minute structures, and to be capable of enduring for untold 
 ages, will be treated of in Chapter XV. 
 
96 GEOLOGY 
 
 CHAPTER IX. 
 
 VOLCANOES AND EARTHQUAKES. 
 
 THE geological changes described in the foregoing chapters 
 affect only the surface of the earth. A little reflection 
 will convince us that they may all be referred to one com- 
 mon source of energy the sun. It is chiefly to the daily influ- 
 ence of that great centre of heat and light that we must ascribe 
 the ceaseless movements of the atmosphere, the phenomena 
 of evaporation and condensation, the circulation of water over 
 the land, the waves and currents of the sea, in short the whole 
 complex s}>-stem which constitutes what has been called the Life 
 of the Earth. Could this influence be conceivably withdrawn, 
 the planet would become cold, dark, silent, lifeless. 
 
 But besides the continual transformations of its surface due 
 to solar energy, our globe possesses distinct energy of its own. 
 Its movements of rotation and revolution, for example, provide 
 a vast store of force, whereby many of the most important geo- 
 logical processes are initiated or modified, as in the phenomena 
 of day and night and the seasons, with the innumerable me- 
 teorological and other effects that flow therefrom. These move- 
 ments, though slowly growing feebler, bear witness to the won- 
 derful vigour of the earlier phases of the earth's existence. In- 
 side the globe too lies a vast magazine of planetary energy in 
 the form of an interior of intensely hot material. The cool 
 outer shell is but an insignificant part of the total bulk of the 
 globe. To this cool part the name of " crust " was given at a 
 time when the earth was believed to consist of an inner molten 
 nucleus enclosed within an outer solid shell or crust. The term 
 is now used merely to denote the cool solid external part of the 
 globe, without implying any theory as to the nature of the in- 
 terior. 
 
VOLCANOES AND EARTHQUAKES 97 
 
 CONDITION OF THE EARTH'S INTERIOR.: It is obvious that we 
 are not likely ever to learn by direct observation what may be the 
 condition of the interior of our planet. The cool solid outer 
 shell is far too thick to be pierced through by human efforts; 
 but by various kinds of observations, more or less probable con- 
 clusions may be drawn with regard to this problem. In the first 
 place, it has been ascertained that all over the world, wherever 
 borings are made for water or in mining operations, the temper- 
 ature increases in proportion to the depth pierced, and that the 
 average rate of increase amounts to about one degree Fahrenheit 
 for every 64 feet of descent. If the rise of temperature con- 
 tinues inward at this rate, or at any rate at all approaching it, 
 then at a distance from the surface, which in proportion to the 
 bulk of the whole globe is comparatively trifling, the heat must 
 be as great as that at which the ordinary materials of the crust 
 would melt at the surface. In the second place, thermal springs 
 in all quarters of the globe, rising sometimes with the temper- 
 ature of boiling water, and occasionally even still hotter, prove 
 that the interior of the planet must be very much warmer than 
 its exterior. In the third place, volcanoes widely distributed 
 over the earth's surface throw out steam and heated vapours, 
 red-hot stones, and streams of molten rock. 
 
 It is quite certain therefore that the interior of the globe 
 must be intensely hot ; but whether it is actually molten or solid 
 has been the subject of prolonged discussion. Three opinions 
 have found stout defenders. (1) The older geologists main- 
 tained that the phenomena of volcanoes and earthquakes could 
 not be explained, except on the supposition of a crust only a 
 few miles thick, enclosing a vast central ocean of molten ma- 
 terial. (2) This view has been opposed by physicists who have 
 shown that the globe, if this were actually its structure, could not 
 resist the attraction of sun and moon, but would be drawn 
 out of shape, as the ocean is in the phenomenon of the tides, 
 and that the absence of any appreciable tidal deformation in the 
 crust shows that the earth must be practically solid and as rigid 
 as a ball of glass, or of steel. (3) A third opinion has been 
 advanced by geologists who, while admitting that the earth 
 behaves on the whole as a solid rigid body, yet believe that many 
 geological phenomena can only be explained by the existence 
 of some liquid mass beneath the crust. Accordingly they sup- 
 
98 GEOLOGY 
 
 pose that while the nucleus is retained in the solid state by the 
 enormous superincumbent pressure under which it lies, and the 
 crust has become solid by cooling, there is an intermediate liquid 
 or viscous layer which has not yet cooled sufficiently to pass into 
 the solid crust above, and does not lie under sufficient pressure 
 to form part of the solid nucleus below. At present, the balance 
 of evidence and argument seems to be in favour of the practical 
 rigidity and solidity of the globe as a whole. But the materials 
 of its interior must possess temperatures far higher than those 
 at which they would melt at the surface. They are no doubt 
 kept solid by the vast overlying pressure, and any change which 
 could relieve them of this pressure would allow them to pass into 
 the liquid form. This subject will be again alluded to in Chap- 
 ter XVI. Meanwhile, let us consider how the intensely hot 
 nucleus of the planet reacts upon its surface. 
 
 Eocks are bad conductors of heat. So slowly is the heat of 
 the interior conducted upwards by them that the temperature 
 of the surface of the crust is not appreciably affected by that of 
 the intensely hot nucleus. But the fact that the surface is not 
 warmed from this source shows that the heat of the interior 
 must pass off into space as fast as it arrives at the surface, and 
 proves that our planet is gradually cooling. For many millions 
 of years the earth has been radiating heat into space, and has 
 consequently been losing energy. Its present store of planetary 
 vitality therefore must be regarded as greatly less than it once 
 was. 
 
 VOLCANOES. 
 
 Of all the manifestations of this planetary vitality, by far the 
 most impressive are those furnished by volcanoes. The general 
 characters of these vents of communication between the hot 
 interior and cool surface of the planet are doubtless already 
 familiar to the reader of these chapters the volcano itself, a 
 conical hill or mountain, formed mainly or entirely of materials 
 ejected from below, having on its truncated summit the basin- 
 shaped crater, at the bottom of which lies the vent or funnel 
 from which, as well as from rents on the flanks of the cone, 
 hot vapours, cinders, ashes, and streams of molten lava are 
 discharged, till they gradually pile up the volcanic cone round 
 the vent whence they escape. 
 
VOLCANOES AND EARTHQUAKES 99 
 
 A volcanic cone, so long as it remains, bears eloquent testi- 
 mony to the nature of the causes that produced it. Even many 
 centuries after it has ceased to be active, when no vapours rise 
 from any part of its cold, silent, and motionless surface, its 
 conical form, its cup-shaped crater, its slopes of loose ashes, and 
 its black bristling lava-currents remain as unimpeachable wit- 
 nesses that the volcanic fires, now quenched, once blazed forth 
 fiercely. The wonderful groups of volcanoes in Auvergne and 
 the Eifel are as fresh as if they had not yet ceased to be active, 
 and might break forth again at any moment ; yet they have been 
 quiescent ever since the beginning of authentic human history. 
 
 But in the progress of the degradation which everywhere 
 slowly changes the face of the land, it is impossible that vol- 
 canic hills should escape the waste which befalls every other 
 kind of eminence. We can picture a time when the volcanic 
 cones of Auvergne will have been worn away, and when the 
 lava-streams that descend from them will be cut into ravines 
 and isolated into separate masses by the streams that have even 
 already deeply trenched them. Where all the ordinary and 
 familiar signs of a volcano have been removed, how can we tell 
 that any volcano ever existed? What enduring record do vol- 
 canoes inscribe in geological history? 
 
 Now, it must be obvipus that among the operations of an 
 active volcano, many of the most striking phenomena have 
 hardly any importance as aids in recognising the traces of long- 
 extinct volcanic action. The earthquakes and tremors that ac- 
 company volcanic outbursts, the constant and prodigious out- 
 rushing of steam, the abundant discharge of gases and acid 
 vapours, though singularly impressive at the time, leave little 
 or no lasting mark of their occurrence. It is not in phenomena, 
 so to speak, transient in their effects, that we must seek for a 
 guide in exploring the records of ancient volcanoes, but in those 
 which fracture or otherwise affect the rocks below ground, and 
 pile up heaps of material above. 
 
 Keeping this aim before us, we may obtain from an examina- 
 tion of what takes place at an active volcano such durable proofs 
 of volcanic energy as will enable us to recognise the former ex- 
 istence of volcanoes over many tracts of the globe where human 
 eye has never witnessed an eruption, and where, indeed, all trace 
 of what could be called a volcano has utterly vanished. A 
 
100 GEOLOGY 
 
 method of observation and reasoning has been established, from 
 the use of which we learn that in some countries, Britain for ex- 
 ample, though there is now no sign of volcanic activity, there has 
 been a succession of volcanoes during many protracted and 
 widely separated periods, and that probably the interval that has 
 passed away since the last eruptions is not so vast as that which 
 separated these from those that preceded them. A similar 
 story has been made out in many parts of the continent of Eu- 
 rope, in the United States, India, and New Zealand, and, in- 
 deed, in most countries where the subject has been fully investi- 
 gated. 
 
 A little reflection on this question will convince us that the 
 permanent records of volcanic action must be of two kinds : first 
 and most obvious are the piles of volcanic materials which have 
 been spread out upon the surface of the earth, not only round 
 
 Fig. 35. Cellular Lava with a few of the cells filled up with infiltrated 
 mineral matter (Amygdules). 
 
 the immediate vents of eruption, but often to great distances 
 from them; secondly, the rents and other openings in the solid 
 crust of the earth caused by the volcanic explosions, and some of 
 which have served as channels by which the volcanic materials 
 have been expelled to the surface. 
 
 VOLCANIC PRODUCTS. We shall first consider those materials 
 which are erupted from volcanic vents and are heaped up on the 
 surface as volcanic cones or spread out as sheets. They may be 
 conveniently divided into two groups: 1st, Lava, and 2d, Frag- 
 mentary materials. 
 
 (1) Lava. Under this name are comprised all the molten 
 rocks of volcanoes. These rocks present many varieties in com- 
 
VOLCANOES AND EARTHQUAKES 
 
 101 
 
 position and texture, some of the more important of which will 
 be described in Chapter XI. Most of them are crystalline 
 that is, are made up wholly or in greater part of crystals of two 
 or more minerals interlocked and felted together into a coherent 
 mass. Some are chiefly composed of a dark brown or black 
 glass, while others consist of a compact stony substance with 
 abundant crystals imbedded in it. Probably most of them 
 when in completest fusion within the earth's crust existed in 
 the condition of thoroughly molten glass, the transition from 
 that state to a stony or lithoid one being due to a process of 
 "devitrification" (Ch. XI) consequent on cooling. During the 
 process some of the component ingredients of the glass crystallise 
 out as separate minerals, and this crystallisation sometimes pro- 
 ceeds so far as to use up all the glass and to transform it into 
 a completely crystalline substance. 
 
 In many cases lavas are strikingly cellular that is to say, 
 they contain a large number of spherical or almond-shaped cavi- 
 ties somewhat like those of a sponge or of bread, formed by the 
 expansion of the steam absorbed in the molten rock (Figs. 35 
 and 36 and Ch. XI). Lavas vary much in weight and in colour. 
 The heavier kinds are more than three times the weight of water; 
 or, in other words, they have 
 a specific gravity ranging up to 
 3.3; and are commonly dark 
 gray to black. The lighter va- 
 rieties, on the other hand, are 
 little more than twice the 
 weight of water, or have a spe- 
 cific gravity which may be as 
 low as 2.3, while their colours 
 are usually paler, sometimes 
 almost white. 
 
 Fig ' 36 '- Section ot a 
 
 When lava is poured out at the surface it issues at a white heat 
 that is, at a temperature sometimes above that of melting 
 copper, or more than 2204 Fahr. ; but its surface rapidly dark- 
 ens, cools, and hardens into a solid crust which varies in aspect 
 according to the liquidity of the mass. Some lavas are remark- 
 ably fluid, flowing along swiftly like melted iron; others move 
 sluggishly in a stiff viscous stream. In many pasty lavas, the 
 surface breaks up into rough cindery blocks or scoriae like the 
 
102 GEOLOGY 
 
 slags of a foundry, which grind upon each other as the still 
 molten stream underneath creeps forward (Ch. XI). In gen- 
 eral, the upper part of a lava-stream is more cellular than the 
 central portions, no doubt because the imprisoned steam can 
 there more easily expand. The bottom, too, is often rough and 
 slaggy, as the lava is cooled by contact with the ground, and 
 portions of the chilled bottom-crust are pushed along or broken 
 up and involved in the still fluid portion above. 
 
 There are thus three more or less well-defined zones in a 
 solidified lava-current a cellular or slaggy upper part (c in 
 Fig. 36), a more solid and jointed centre (&), embracing usually 
 by much the largest proportion of the whole, and a cellular or 
 slaggy bottom (a). A rock presenting these characters tells its 
 
 Fig. 37. Elongation of cells in direction of flow of a lava-stream. 
 
 story of volcanic action in quite unmistakable language. It re- 
 mains as evidence of the existence of some neighbouring volcanic 
 vent, now perhaps entirely covered up, whence it flowed. We 
 may even be able to detect the direction in which the lava 
 moved. The cells opened by the segregation and expansion of 
 the steam entangled in the interstices of a mass of lava which 
 is at rest are, on the whole, spherical. But if the rock is still 
 moving, the cells will be drawn out and flattened into almond- 
 shaped (amygdaloidal) vesicles, with their flat sides parallel to 
 the surface of the lava, and their longer axes ranged in one gen- 
 eral direction, which is that of the motion of the molten stream 
 (Fig. 37). 
 
 At a volcanic vent, the mass of erupted lava is generally 
 thickest, and it thins away as its successive streams terminate 
 
VOLCANOES AND EARTHQUAKES 103 
 
 on the lower grounds surrounding the cone. But sometimes a 
 lava-current may flow for 40 miles or more from its source, and 
 may here and there attain locally a great thickness by rolling 
 into a valley and filling it up, as has been witnessed among the 
 Icelandic eruptions. As a rule, where ancient lava-streams are 
 found to thicken in a certain direction, we may reasonably infer 
 that in that direction lay the vent from which they flowed. 
 
 Again, sheets of lava that solidify on the slopes of a volcanic 
 cone are inclined ; they may congeal on declivities of as much as 
 30 or 40. If a series of ancient lavas were observed to slope 
 upward to a common centre, we might search there for some 
 trace of the funnel from which they were discharged. But, of 
 course, in proportion to their antiquity, lava-streams, like every 
 other kind of rock, have suffered from geological revolutions, 
 among which those that involve upheaval and dislocation are 
 especially important, so that the inclination of an ancient lava- 
 bed must not be too hastily assumed as an indication of the 
 slope of the cone of a volcano. It must be taken in connection 
 with the rest of the evidence supplied by the whole district. 
 
 Where lavas reach the lower grounds beyond the foot of a 
 volcanic cone, they may spread out in wide nearly horizontal 
 sheets. As current succeeds current, the original features of the 
 plain may be entirely buried under a mass of lava many feet 
 thick. If a section could be cut through such an accumulation, 
 it might be possible to determine the thickness of each succes- 
 sive lava-stream by means of the slaggy upper and lower surfaces. 
 Here and there, too, where two eruptions were separated by an 
 interval long enough to allow the surface of the older mass par- 
 tially to crumble into soil and support some vegetation, the 
 layer of burnt soil between the two sheets of lava would remain 
 as a witness of this interval. 
 
 In other instances, we can understand that in the larger hol- 
 lows of a lava-plain, ponds or lakes might gather, on the floor 
 of which there might be deposited layers of fine silt full of ter- 
 restrial leaves, insect remains, land and fresh-water shells, and 
 other organic relics of a land-surface. If, now, a lacustrine ac- 
 cumulation of this kind were to be buried under a new outburst 
 of lava, it would be sealed up and might preserve its record 
 intact for vast ages. In any section cut through such a series 
 of lava-beds by a river or the sea, or by man, the layers of silt 
 
104 GEOLOGY 
 
 with their organic remains intercalated between the lava-streams 
 would prove the eruptions to have taken place on land, and to 
 have been separated by a long interval, during which a lake was 
 formed on the cold and decomposing surface of the earlier 
 lava. 
 
 The conditions under which the volcanic outbursts occurred 
 may be thus inferred, not so much from the nature of the vol- 
 canic materials themselves, as from that of the layers of sedi- 
 ment that may happen to have been preserved among them. 
 Seams of red baked soil, with charred remains of terrestrial 
 vegetation interposed between the upper and under sides of suc- 
 cessive lavas would point to subaerial eruptions. Bands of hard- 
 ened clay or marl, with leaves and fresh-water shells, would show 
 that the lavas had invaded a lake. Beds of limestone or other 
 rock, containing corals, sponges, marine shells, and other traces 
 of the life of the sea, would demonstrate that the eruptions were 
 submarine. Examples of each of these varieties of evidence 
 occur abundantly among the old volcanic tracts of Britain. 
 
 (2) Fragmentary Products. These supply some of the most 
 striking proofs of volcanic energy. They vary in size from 
 huge blocks of stone, weighing many tons, down to the finest 
 dust. The coarsest materials naturally accumulate round the 
 vent, while the finest may be borne away by wind to distances 
 of many hundreds of miles. On the volcano itself, the stones, 
 ashes, and dust form beds of coarse and fine texture which, on 
 the outside of the cone, have the usual slope of the declivity. By 
 degrees, they become more or less consolidated, and are then 
 known by the general name of Tuffs. 
 
 The materials composing a tuff are generally derived from 
 lavas. The fine dust, discharged from a large volcano in such 
 prodigious quantities as to make the sky dark ap midnight for 
 days together, is simply lava that has been blown into this finely 
 divided condition by the explosion of the vapours and gases 
 which exist absorbed in it while still deep down within the 
 earth's crust. The cinder-like fragments, and scoria? or slags, 
 that are ejected in such numbers and fall back into the crater 
 and upon the outer slopes of the cone are pieces of lava frothed 
 up by the expansion of the imprisoned steam, torn off from the 
 column of lava in the vent, and shot whirling up into the air. 
 Large blocks of lava, or of the rocks through which the volcanic 
 
VOLCANOES AND EARTHQUAKES 105 
 
 funnel has been opened, are often broken off by the force of the 
 explosions and discharged with the other volcanic detritus from 
 the vent. These materials, descending to the ground, form suc- 
 cessive beds that vary in dimensions according to the vigour of 
 eruption and their distance from the vent. Around the focus 
 of activity there may be thick accumulations of blocks, bombs, 
 and pieces of scoriae mixed with fine ashes and sand. A notable 
 feature is the generally cellular character of these stones a 
 peculiarity which marks them as made of truly volcanic materials. 
 An examination of the finest dust likewise discloses the presence 
 of the glass and crystals that constitute the lava from the ex- 
 plosion of which the dust was derived. 
 
 From the wide area over which the fragmentary materials 
 ejected by volcanoes are dispersed in the atmosphere before they 
 fall to the earth, they are more likely than lavas, to be preserved 
 among contemporaneous sedimentary accumulations. They 
 often descend upon lakes, and must there be interstratified with 
 the mud, marl, or other deposit in progress at the time. They 
 are also widely diffused over the sea-floor. Eecent dredgings of 
 the ocean-basins have shown that traces of fine volcanic detritus 
 may be detected even at remote distances from land. As active 
 volcanoes almost always rise near the sea, as the oceans are 
 dotted over with volcanic islands, and as, doubtless, many erup- 
 tions take place on the sea-bottom, there are obvious reasons why 
 volcanic particles should be universally diffused over the sea-bot- 
 tom. Geologists can also understand why, in the records of 
 volcanic history in bygone ages, so large a proportion of the 
 evidence should be of submarine eruptions. 
 
 Beds of tuff often contain traces of the plants or animals that 
 lived on the surfaces on which the volcanic materials fell 
 sometimes remains of terrestrial or lacustrine vegetation and 
 animals ; but in the great majority of instances, shells and other 
 relics of the inhabitants of the sea. 
 
 In a series of layers of tuff round a volcanic orifice, the 
 memorials of the earliest discharges are of course preserved in 
 the layers at the bottom. Accordingly, in such situation?, 
 abundant fragments of the rocks of the surrounding country 
 may be noticed. We could hardly ask for more convincing evi- 
 dence of the blowing out of the vent and the ejection of the rock- 
 
106 
 
 GEOLOGY 
 
 fragments from' it, before the volcano began to discharge only 
 volcanic materials. 
 
 Large blocks of lava ejected obliquely from the crater may fall 
 beyond the limits of the cone. If a block thus discharged should 
 fall into a lake-basin, it would be covered up in the silt ac- 
 cumulating there, and might be the only remaining record of 
 the eruption to which it belonged. In after times, were the 
 lake-floor laid dry, the stone might be found in its place, the 
 
 layers of sediment into which 
 it fell pressed down by the 
 force with which it landed on 
 them and by its weight, the 
 later layers mounting over 
 and covering it. Examples 
 of this kind of evidence may 
 
 3 - 
 
 Fig. 38. Volcanic block ejected 
 during the deposition of strata 
 in water. 
 
 be gathered in many old vol- 
 canic districts. One taken 
 from the coast of Fife is given in Fig. 38. The lowest 
 bed there shown (1) is a brown shaly fire-clay, about 5 inches 
 thick, which was once a vegetable soil, for abundant rootlets can 
 be seen branching through it. It is overlain by a seam of coal 
 (2), 5 or 6 inches thick, representing the dense growth of 
 vegetation that flourished upon the soil; the next layer (3) is a 
 green crumbling fire-clay about a foot thick, covered by a dark 
 shale with remains of plants (4). The feature of special in- 
 terest in this section is the angular block of lava (diabase) 
 weighing about six or eight pounds, which is stuck vertically 
 in bed No. 3. There can be no doubt that this was ejected by 
 an explosion, of which there is here no other record. It proba- 
 bly descended with some force, for, as shown in the drawing, the 
 lower layers of the fire-clay are pressed down by it, and the 
 coal itself is compressed. We see that the stone fell before the 
 upper half of the fire-clay was formed, for the layers of that part 
 of the bed are heaped around the stone and finally spread over 
 it. There are layers of lava and tuff above and below the strata 
 depicted in this section, so that there is abundant other evidence 
 of preceding and subsequent volcanic action here (see Fig. 109). 
 As the fine volcanic dust may be transported by wind for 
 hundreds of miles before reaching the surface of the earth, its 
 presence does not necessarily show that a volcano existed in the 
 
VOLCANOES AND EARTHQUAKES 107 
 
 neighbourhood in which it fell. The fine ashes from the Ice- 
 landic volcanoes, for example, have been found abundantly even 
 as far as Sweden and the Orkney Islands. But where the frag- 
 mentary materials are of coarse grain, and more especially where 
 they contain large slags, scoriaB, and bombs, and where they are 
 interstratified with sheets of lava, they unquestionably indicate 
 the proximity of some volcanic vent from which the whole pro- 
 ceeded. 
 
 VOLCANIC VENTS AND FISSURES. The various materials 
 ejected from a volcano to the surface may conceivably be in 
 course of time entirely swept away. Nevertheless, though every 
 sheet of lava and every bed of tuff is removed, there will still re- 
 main the filled up vent, funnel, or fissure, up which these ma- 
 terials rose, and out of which they were ejected. This open- 
 ing in the solid crust of the earth must evidently be one of the 
 most durable, as it is certainly one of the fundamental features 
 in a volcano. Let us first consider the vent of an ordinary vol- 
 cano. 
 
 In many instances, there is reason to believe that volcanic 
 vents are opened along lines of fracture in the earth's crust. 
 This seems especially to be the case where a group of volcanoes 
 runs along one definite line, as is represented in Fig. 39. Along 
 
 Fig. 39. Volcanoes on lines of fissure. 
 
 such a fissure, either of older date or due to the energy of the 
 volcanic explosions themselves, there must be weaker places 
 where the overlying mass is less able to bear the strain of the 
 pent-up vapours underneath. At these places, after successive 
 shocks, openings may at length be made to the surface, whence 
 the lavas and ashes will be emitted, and each such opening will 
 be marked by a cone of the erupted material. 
 
 But in innumerable examples, it is found that a fissure is not 
 necessary for the formation of a volcano. The effect of the 
 
108 GEOLOGY 
 
 volcanic explosions is such as to drill a pipe or funnel even 
 through the solid unfractured crust of the earth. The volcanic 
 energy, so far from requiring a line of fracture for its assistance, 
 seems often to have avoided making use of such a line, even when 
 it existed. In the volcanic plateaux of Utah, which are dotted 
 over with little volcanic cones, and are also traversed by great 
 dislocations, it is noticeable that the vents are not clustered 
 along the lines of fracture. In the same region, and also along 
 the courses of the Ehine and Moselle, volcanic vents have been 
 opened near the brink of deep ravines rather than at the bottom. 
 These are features of volcanic action, for which no satisfactory 
 explanation has yet been found. 
 
 The crater of an active volcano is a hideous yawning cauldron, 
 with rough red and black walls, at the bottom of which lie steam- 
 ing pools of molten lava. Every now and then a sharp explosion 
 tears the lava open, sending up a shower of glowing fragments 
 and hot ashes. These pools of liquid lava lie evidently on the 
 top of a column of melted rock which descends in the volcanic 
 chimney to an unknown depth into the earth's interior. If the 
 volcano were to become extinct, this lava column would cool and 
 solidify, and even after the entire destruction and removal of the 
 cone and crater, would remain as a stump to tell where the site 
 of the volcano had been. Layer after layer might be stripped off 
 the surface of the land; hundreds or thousands of feet of rock 
 might in this manner be removed, yet, so far as we know, the 
 stump of the volcano would still be there. No probable amount 
 of waste of the surface of the earth's crust could remove a 
 vertical column of rock which descends to an unknown depth 
 into the interior. The site of a volcanic vent can never be effaced 
 except by being buried under masses of younger rock. 
 
 Volcanic vents, affording as they do so durable a testimony to 
 volcanic action, deserve careful attention. At an active volcano, 
 or even at one which, though extinct, still retains its cone of 
 erupted materials, we cannot, of course, learn much regarding 
 the shape and size of the funnel, for only the crater, and at most 
 merely the upper part of the vent, are accessible. But among 
 volcanic tracts of older date, where the cones have been destroyed, 
 and where the filled-up funnels are laid bare, the subterranean 
 architecture of volcanoes is revealed to us. At such places we 
 are allowed, as it were, to descend the chimney of a volcano, and 
 
VOLCANOES AND EARTHQUAKES 109 
 
 to make observations altogether impossible at a modern volcanic 
 cone. 
 
 From observations made at such favourable localities, it has 
 been ascertained that the funnels of volcanoes are in general 
 rudely circular or elliptical, though liable to many modifications 
 of outline. They vary indefinitely in diameter, according to the 
 vigour of the volcanic outbursts that produced them. The 
 smaller vents are not more than a few yards in width ; but those 
 of the larger volcanoes which, as in Sumatra and Java, have 
 sometimes craters comprising an area of 40 square miles, must 
 have enormously larger funnels. 
 
 The materials that fill up a vent are sometimes only fragments 
 of the surrounding rocks. In such cases, we may suppose that 
 when the volcanic explosions had spent their force and had blown 
 out an opening to the surface of the ground, they were not 
 succeeded by the uprise of any solid volcanic materials ; that, in 
 short, only the first stage in the establishment of a volcano 
 
 Fig. 40. Outline of a Volcanic Neck. 
 
 was reached, when, owing to some failure of the subterranean 
 energy at the place, the operations came to an end. But though 
 the upper part of the vent might remain open, surrounded with a 
 crater formed of the fragments into which the rocks were blown 
 by the explosions, the lower parts would undoubtedly be filled up 
 by the fall of fragments back again into the vent. And if all 
 the material ejected to the surface were removed, the top of this 
 column of fragmentary materials would remain as an unmistak- 
 able evidence of the explosions that had originated it. 
 
 But in the vast majority of cases, the operations at a volcanic 
 vent do not end with the first explosions. Clouds of ashes and 
 stones are ejected, and streams of molten lava are poured forth. 
 In some instances, the chimney may be finally choked with vol- 
 canic blocks, scoriae, cinders, and ashes, in others with consoli- 
 dated lava. Examples of both kinds of infilling are found, and 
 
110 
 
 GEOLOGY 
 
 also others where the two forms of volcanic material occur to- 
 gether in the same vent. 
 
 A volcanic chimney filled up in this way with volcanic ma- 
 terials and exposed by the removal of the lava or ashes thrown 
 out to the surface is known as a Neck (see Figs. 40, 41, 42, 111, 
 and Ch. XIV). As these materials are usually harder and more 
 durable than the surrounding rocks, they project above the gen- 
 eral surface of the ground. The stump of the volcano is left as 
 a hill, the form and prominence of which will chiefly depend 
 upon the nature of the material; hard tough lava will rise 
 abruptly, as a crag or hill, above the surrounding country, while 
 consolidated ashes, scoriae, and other fragmentary stuff will give 
 a smoother and less marked outline. 
 
 These features will be best understood from a series of dia- 
 grams. We may take, by way of illustration, a neck composed 
 mainly of fragmentary ejections, but with a plug of lava reach- 
 ing its summit. The usual outlines of such a neck are repre- 
 sented in Fig. 40. There is nothing in the general form of this 
 
 hill to suggest a volcanic origin; 
 yet, if we examine its structure 
 and that of the ground around 
 it, we may find them to be as 
 represented in Fig. 41, where 
 the surrounding rocks are sup- 
 posed to consist of various sand- 
 stones, clays, limestones, and 
 other sedimentary deposits (a), 
 through which the volcanic vent 
 (b, c) has been drilled. The neck is represented as elliptical 
 in cross section, composed mainly of consolidated volcanic ashes 
 and blocks (&), but with a mass of lava (c) in the centre. The 
 structure of the hill is explained in the vertical section, Fig 42 
 (see also Fig. 111). We there see that the vent has been blown 
 through the surrounding strata (a, a), and has been filled up 
 mainly with fragmentary materials (&, b) ; but that through its 
 centre there has risen a column or plug of lava (c), which not 
 improbably marks the last effort of the volcano to force solid 
 ejections to the surface. The line s, s indicates the present sur- 
 face of the ground, after the prolonged waste during which all 
 the volcanic cone has been removed. But we can in imagina- 
 
 Fig. 41. Ground-plan of the 
 structure of the Neck shown 
 in Fig. 40. 
 
VOLCANOES AND EARTHQUAKES 
 
 111 
 
 tion restore the original surface, which may have been some- 
 what as shown by the dotted lines, the position of the crater be- 
 ing indicated at e, and its crest on either side at d, d. No trace 
 is here left of the original volcanic cone. The present form of 
 the ground is due to denudation, which has left the more durable 
 volcanic rocks projecting above the surrounding strata. The 
 continued progress of superficial degradation will remove still 
 more of the neck,. but the downward continuation of the volcanic 
 column must always remain, and will probably always project as 
 a hill. A volcanic neck is thus one of the most enduring and 
 unmistakable evidences of the site of a volcano. 
 
 Besides vents or funnels, other openings are made by vol- 
 canic explosions in the crust, which serve as receptacles of lava 
 
 Fig. 42. Section through the same Neck as in Figs. 40 and 41. 
 
 and ashes, and remain as durable memorials of volcanic action. 
 Of these the most important are Fissures, which are formed in 
 large numbers in and around a volcanic cone, but which may 
 also arise at a distance from any actual volcano. During the 
 convulsions of an eruption, the cone and the surrounding 
 country are sometimes split by lines of fissure, which tend to 
 radiate from the centre of disturbance, somewhat as cracks do in 
 a pane of glass through which a stone is thrown. Sometimes 
 the two sides of a fissure close together again, leaving no super- 
 ficial trace of the dislocation. More frequently steam and vari- 
 ous volcanic vapours escape from the chasm, and may deposit 
 along the walls sublimates of different minerals, such as common 
 salt, chloride of iron, specular iron, sulphur, and sal-ammoniac. 
 These deposited substances may even continue to grow there 
 until they entirely fill up the space between. In such cases, the 
 line of fissure is marked by a vertical or steeply inclined band 
 
112 
 
 GEOLOGY 
 
 of minerals interposed between the ends of the rocks that have 
 been ruptured and separated. But in most instances, the open- 
 ing is filled up by the rise of lava from below. At night, the 
 vents opened on the outside of an active volcano may be traced 
 from afar by the glow of the white-hot lava that rises in them 
 to within a short distance from the surface. When the lava 
 cools and solidifies in these fissures, it forms wall-like masses, 
 known as Dykes (Fig. 43). Inside many volcanic craters, the 
 walls are traversed with dykes which, though on the whole tend- 
 
 Fig. 43. Volcanic dykes rising through the bedded tuff of a crater. 
 
 ing to keep a vertical direction, may curve about irregularly ac- 
 cording to the form of the vents into which the lava rose. Like 
 the necks above described, dykes form enduring records of vol- 
 canic action. The superficial cones and craters may disappear, 
 but the subterranean lava-filled fissures will still remain as 
 records of volcanic action. 
 
 In some volcanic regions, where enormous floods of lava have 
 been poured forth, no great central cones have existed. Such 
 regions extend as vast black plains of naked rock, mottled with 
 shifting sand-hills, or as undulating tablelands carved by run- 
 ning water into valleys and ravines, between which the successive 
 
VOLCANOES AND EARTHQUAKES 113 
 
 sheets of lava are exposed in terraced hills. Beyond the limits 
 over which the lava-sheets are spread, dykes of the same kinds 
 of lava rise in abundance to the surface. There can be no doubt 
 that the dykes do not terminate at the edge of the lava-fields, 
 but pass underneath them. Indeed, as they increase in number 
 in that direction, they are probably more abundant underneath 
 the lava than outside of the lava-fields. Sometimes sections are 
 exposed showing how, after rising in a fissure, the lava has spread 
 out on either side as a sheet. In these vast lava-plateaux or 
 deserts, the molten rock, instead of issuing from one main central 
 Etna or Vesuvius, appears to have risen in thousands of fissures 
 opened in the shattered crust, and to have welled forth from 
 numerous vents on these fissures, spreading out sheet after sheet 
 till, like a rising lake, it has not only overflowed the lower 
 grounds, but even buried all the minor hills. Such appears to 
 have been the history of vast tracts in Western North America. 
 The area which has there been flooded with lava has been esti- 
 mated to be larger than that of France and Great Britain to- 
 gether, and the depth of the total mass of lava erupted reaches 
 in some places as much as 3700 feet. Some rivers have cut 
 gorges in this plain of lava, laying bare its component rocks to 
 a depth of 700 feet or more. Along the walls of these ravines 
 we see that the lava is arranged in parallel beds or sheets often 
 not more than 10 or 20 feet thick, each of which, of course, repre- 
 sents a separate outpouring of molten rock. 
 
 Except where such deep sections have been cut through them 
 by rivers, recent lava-floods can only be examined along their 
 surface, and we are consequently left chiefly to inference re- 
 garding their probable connection with fissures and dykes under- 
 neath. But in various parts of the world, lava-plains of much 
 older date have been so deeply eroded as to expose not only the 
 successive sheets of lava but the floor over which they were 
 poured, and the abundant dykes which no doubt served as the 
 channels wherein the lava rose towards the surface, till it could 
 escape at the lowest levels, or at weaker or wider parts of the 
 fissures. In Western Europe important examples of this struc- 
 ture occur, from the north of Ireland through the Inner Hebrides 
 and the Faroe Islands to Iceland. This volcanic belt presents a 
 succession of lava-fields which even yet, in spite of enormous 
 waste, are in some places more than 3000 feet thick. The sheets 
 
114 GEOLOGY 
 
 of lava are nearly flat, and rise in terraces one over another into 
 green grassy hills, or into the dark fronts of lofty sea-washed 
 precipices. Where this thick cake of lava has been stripped off 
 during the degradation of the land, thousands of dykes are ex- 
 posed, and many of these traverse at least the lower parts of the 
 sheets of lava. They form, as it were, the subterranean roots 
 of which these sheets were the subaerial branches; and even 
 where the whole of the material that reached the surface, more 
 than 3000 feet thick, has been worn away, the dykes still remain 
 as evidence of the reality and vigour of the volcanic forces. 
 
 EARTHQUAKES. 
 
 The rise of hot springs and the explosions of volcanoes furnish 
 impressive testimony to the internal heat of our planet ; but they 
 are by no means the only proofs that the pent-up energy of the 
 interior of the globe reacts upon the outer surface. By means 
 of delicate instruments, it can be shown that the ground beneath 
 our feet is subject to continual tremors which are too feeble 
 to be perceived by the unaided senses. From these minuter 
 vibrations, movements of increasing intensity can be detected up 
 to the calamitous earthquake, whereby a country is shaken to its 
 foundations, and thousands of human lives, together with much 
 valuable property, are destroyed. We do not yet know by what 
 different causes these various disturbances are produced. Some 
 of the fainter tremors may arise from such influences as changes 
 of temperature and atmospheric pressure, and the rise and fall 
 of the tides. But the more violent must be assigned to causes 
 working within the earth itself. The collapse of the roofs of 
 underground caverns, the sudden condensation of steam or ex- 
 plosion of volcanic vapours, the snap of rocks that can no longer 
 resist the strain to which, by the cooling and consequent con- 
 traction of the inner hot nucleus, they have been subjected with- 
 in the earth's crust these and other influences may at different 
 times come into play to determine convulsive earthquake shocks. 
 Without, however, entering into the difficult question of the 
 causes of the movements, we may inquire into their effects in so 
 far as these register their passing in the annals of geological his- 
 tory. 
 
 Their awful suddenness and devastation have invested earth- 
 quakes with a high importance in the popular estimate of the 
 
VOLCANOES AND EARTHQUAKES 115 
 
 forces by which the surface of the globe is modified. Yet if we 
 judge of them by their permanent effects, we must give them a 
 comparatively subordinate place among these forces. After 
 some of the most destructive earthquakes recorded in human 
 history, hardly any trace of the calamity is to be seen, save in 
 shattered and prostrate houses. But when these buildings have 
 been repaired or rebuilt, no one visiting the ground might be 
 able to detect any trace of the earthquake that shattered or 
 overthrew them. 
 
 Yet severe earthquakes do not pass without their self -written 
 chronicle which, though often evanescent on the face of nature, 
 is at the time conspicuous enough. Landslips are caused, large 
 masses of earth and blocks of rock being shaken down from 
 higher to lower levels; the ground is rent, and the fissures are 
 sometimes subsequently widened and deepened by rain and run- 
 nels into ravines. But more important are the marked changes 
 of level that occasionally accompany earthquake-shocks. In 
 some cases, the ground is raised for several feet, so that along 
 maritime tracts there is a gain of land from the sea; in others, 
 the ground sinks, and the sea flows in upon the land. Yet it is 
 evident that unless these changes are actually witnessed as the 
 accompaniments of the earthquakes, they may take place without 
 retaining any evidence that they were produced by such a cause. 
 The convulsion of an earthquake, notwithstanding the havoc it 
 may bring to the human population of a country, does not always 
 record itself in distinctive and enduring characters in geological 
 history. Some of its most noticeable effects also are not due 
 directly to its own action, but to the operations of the waters of 
 the land and of the sea which, when disturbed by the shock, not 
 infrequently acquire increased vigour in their own peculiar forms 
 of activity. The great waves set in motion by an earthquake roll 
 over the low lands bordering the sea, and may cause vastly more 
 destruction than is done by the mere shock of the earthquake 
 itself. 
 
 UPHEAVAL ANB SUBSIDENCE. 
 
 It is perhaps not so much by earthquakes, as by quiet, hardly 
 perceptible movements, that the relative positions of sea and land 
 are undergoing change at the present time. In some parts of 
 
116 GEOLOGY 
 
 the world the land is gradually rising, in others it is slowly sink- 
 ing. Proofs of elevation are supplied by lines of barnacles or 
 rock-boring shells, now standing above the reach of the highest 
 tides; by caves that have obviously been scooped out by the sea, 
 but now stand at a higher level than the waves can reach ; and by 
 deposits of sand, gravel, and shells which were evidently accumu- 
 lated on a beach, but which now rise above the level where 
 similar materials are now being accumulated (Raised Beaches). 
 Evidences of subsidence are furnished by traces of old land- 
 surfaces trees with roots in situ, and beds of peat, lying below 
 the limits of the tides (Submerged Forests). But it must be 
 more difficult to prove subsidence than elevation, for as the 
 land sinks, its surface is carried below the waves, which soon 
 efface the evidence of terrestrial characters. 
 
 The time within which man has been observing and recording 
 the changes of the earth's surface forms but an insignificant 
 fraction of the ages through which geological history has been in 
 progress. We cannot suppose that during this brief period he 
 has had experience of every kind of geological process by which 
 the outlines of land and sea. are modified. There may be great 
 terrestrial revolutions which happen so rarely that none has 
 occurred since man began to take note of such things. Among 
 these revolutions, of which he has had as yet no experience, the 
 most gigantic is the formation of a mountain-chain. That the 
 various mountain-chains of the globe are of very different ages, 
 and that some of the most gigantic of them are, compared with 
 others, of recent date, are facts in the history of the globe which 
 will be more fully referred' to in later pages ; but so far as human 
 history or tradition goes, man has never witnessed the uprise of a 
 range of mountains. The crust of the earth has been folded and 
 crumpled on the most colossal scale, some parts having been 
 pushed for miles away from their original position; it has been 
 rent by profound fissures, on each side of which the rocks have 
 been displaced for many thousand feet; and it has been so 
 broken, crushed, and sheared, that its component rocks have in 
 some places assumed a structure entirely different from what they 
 originally possessed. But of all these colossal mutations there is 
 no human experience. We are driven to reason regarding them 
 from the record of them preserved among the rocks, and from 
 the analogies that can be suggested by experiments devised to 
 
VOLCANOES AND EARTHQUAKES 117 
 
 imitate as far as possible the processes of nature. To this sub- 
 ject we shall return in Chapter XIII. 
 
 SUMMARY. The enduring records left by volcanoes, whence 
 their former existence in almost all regions of the world may be 
 demonstrated, are to be sought partly in the materials which 
 they have brought up to the surface, and partly in the vents and 
 fissures by which they have discharged these materials. Of the 
 former kind of evidence lava furnishes a conspicuous example; 
 its internal crystalline or glassy structure, its steam-cavities, and 
 the cellular slaggy upper and under parts of the sheets in which 
 it lies, are all proofs of its former molten condition. A succes- 
 sion of lava-beds, piled one above another, marks a series of vol- 
 canic eruptions, and the nature of the layers of non-volcanic ma- 
 terial intercalated between them may indicate the conditions 
 under which the eruptions took place, whether on land, in lakes, 
 or in the sea. The fragmentary products consolidated into beds 
 of tuff are likewise characteristic of volcanoes; they consist 
 mainly of lava-dust with cindery scoriae, slags, and blocks ; they 
 accumulate most deeply and in coarsest material at and im- 
 mediately around the volcanic vents, but their finer particles 
 may be carried to enormous distances ; they are especially liable 
 to be intercalated with contemporaneous sedimentary deposits 
 in lakes and on the sea-floor. 
 
 The vents, through which lava and ashes are ejected to the 
 surface form the most permanent record, of volcanoes, for, being 
 filled up with volcanic rocks to unknown depths, they cannot be 
 destroyed by the mere denudation of the surface, and can only 
 disappear by being buried under later accumulations. Such 
 "necks" consist sometimes of lava, sometimes of consolidated 
 volcanic debris, or of both kinds of material together, and remain 
 as the stumps of volcanoes, where every other trace of volcanic 
 action may have passed away. Not less enduring are the dykes 
 or wall-like masses of lava which have risen and solidified in open 
 fissures. Enormous sheets of lava appear to have flowed out 
 from such fissures in regions where the volcanic energy never 
 produced any great central cone. 
 
 Earthquakes do not impress their mark upon geological his- 
 tory so indelibly as might be supposed. In spite of the destruc- 
 tion which they cause to human life and property, it is by such 
 direct changes as landslips, rents of the ground, and the up- 
 
118 GEOLOGY 
 
 heaval or depression of land, and by such indirect changes as 
 may be produced by derangements of rivers, lakes, and the sea, 
 that earthquakes leave their chief record behind them. 
 
 Some of the most important changes of level now going on are 
 effected quietly and almost imperceptibly, some regions being 
 slowly elevated, and others gradually depressed. But the time 
 within which man has been an observer and recorder of nature 
 is too brief to have supplied him with experience of all the ways 
 in which the internal energy of the globe affects its surface. In 
 particular, he has never witnessed the production of a mountain- 
 chain, nor any of the plications, fractures, and displacements 
 which the crust of the earth has undergone. Eegarding these 
 revolutions we can only reason from the records of them in the 
 rocks, and from such laboratory experiments as may seem most 
 closely to imitate the processes of nature that were concerned in 
 their production. 
 
ELEMENTS AND MINERALS 119 
 
 PAET II. 
 
 BOOKS, AND HOW THEY TELL THE HISTOKY OF 
 THE EARTH. 
 
 CHAPTER X. 
 
 ELEMENTS AND MINERALS. 
 
 IN" the foregoing Part of this volume we have been engaged in 
 considering the working of various processes by which the 
 surface of the earth is modified at the present time, and 
 some of the more striking ways in which the record of these 
 changes is preserved. We have seen that, on the whole, it is by 
 deposits of some kind, laid down in situations where they can 
 escape destruction, that the story of geological revolution is 
 chronicled. In one place it is the stalagmite of a cavern, in 
 another the silt of a lake-bottom, in a third the sand and mud 
 of the sea-floor, in a fourth the lava and ashes of a volcano. In 
 these and countless other examples, materials are removed from 
 one place and set down in another, and in their new position, 
 while acquiring novel characters, they retain more or less dis- 
 tinctly the record of their source and of the conditions under 
 which their transference was effected. 
 
 In these chapters, reference has intentionally been avoided as 
 far as possible to details that required some knowledge of min- 
 erals and rocks, in order that the broad principles of geology, 
 for which such knowledge is not absolutely essential, might be 
 clearly enforced. It is obvious, however, that as minerals and 
 rocks form the records in which the history of the earth has been 
 preserved, this history cannot be followed into detail until some 
 acquaintance with these materials has been made. What now 
 lies before the reader, therefore, in order that he may be able to 
 apply the knowledge he has gained of geological processes to the 
 elucidation of former geological periods, is to make himself 
 familiar with at least the more common and important minerals 
 and rocks. This he can only do satisfactorily by handling the 
 
120 GEOLOGY 
 
 objects themselves, until he acquires such an acquaintance with 
 them as to be able to* recognise them where he meets with them 
 in nature. 
 
 At first the number and variety of these objects may appear 
 to be almost endless, and the learner may be apt to despair of 
 ever mastering more than an insignificant portion of the wide 
 circle of inquiry and observation which they present. But 
 though the detailed study of this subject is more than enough to 
 tax the whole powers of the most indefatigable student, it is not 
 by any means an arduous labour, and assuredly a most interest- 
 ing one, to acquire so much knowledge of the subject as to be 
 able to follow intelligently the progress of geological investiga- 
 tion, and even to take personal part in it. This accordingly is 
 the task to which he is invited in the present and following 
 chapters. 
 
 SIMPLE ELEMENTS COMPOSING THE EARTH'S CRUST. 
 
 Before considering the characters presented by the various 
 rocks that form the visible part of the earth's crust, we may find 
 it of advantage to inquire into the general chemical composition 
 of rocks, for by so doing we learn that though the chemist has 
 detected more than sixty substances which he has been unable to 
 decompose, and which therefore he calls elements, only a small 
 proportion of these enter largely into the composition of the 
 outer part of the globe. In fact, there are only about sixteen 
 elements that play an important part as constituents of rocks; 
 these together constitute about ninety-nine parts of the terres- 
 trial crust. Half of them are metals; and the other half are 
 metalloids or non-metals, as in the two subjoined lists, the most 
 abundant being in each case placed first. 
 
 METALLOIDS OB NON-METALS. METALS. 
 
 Oxvcen , 
 
 Symbol. 
 O 
 
 Atomic 
 Weight. 
 1596 
 
 Aluminium 
 
 Symbol. 
 Al 
 
 Atomic 
 Weight. 
 27 30 
 
 Silicon 
 
 .... Si 
 
 2800 
 
 Calcium . . 
 
 Ca 
 
 3990 
 
 Carbon 
 
 C 
 
 11 97 
 
 Magnesium . 
 
 Me 
 
 2394 
 
 Sulphur 
 
 , .. . . s 
 
 31 98 
 
 Potassium 
 
 K 
 
 3904 
 
 Hydrogen . . 
 
 H 
 
 1 00 
 
 Sodium 
 
 Na 
 
 2299 
 
 Chlorine 
 
 Cl 
 
 35.37 
 
 Iron 
 
 Fe 
 
 5590 
 
 Phosphorus 
 
 p 
 
 30 96 
 
 
 Mn 
 
 54 SO 
 
 Fluorine . 
 
 F 
 
 19.10 
 
 Barium . 
 
 Ba 
 
 136.80 
 
ELEMENTS AND MINERALS 121 
 
 Some of those elements occur in the free state, that is, not 
 combined with any other element. Carbon, for instance, is 
 found pure in the form of the diamond, and also as graphite. 
 But in the great majority of cases, they assume various combina- 
 tions. Most abundant are oxides, or compounds of oxygen with 
 another element. Compounds of sulphur and a metal are 
 known as sulphides; and similar compounds with chlorine are 
 chlorides. Some of the compounds form further combinations 
 with one or more elements. Thus the acid-forming oxides unite 
 with water to form what are called adds, which, combining with 
 metallic oxides or bases, form with them compounds termed salts. 
 Sulphur and oxygen, for example, uniting in certain proportions 
 with water, constitute sulphuric acid (EbSCh) which, parting 
 with its displaceable hydrogen and combining with the metal 
 calcium, forms the salt known as calcium-sulphate, or sulphate 
 of lime (CaSo4). 
 
 METALLOIDS. Of the non-metallic elements, by far the most 
 abundant and important is OXYGEN. In its free state, it exists 
 as a gas which has been occasionally detected at active volcanic 
 vents. But with this rare exception, it is always found mixed 
 or combined with one or more elements. Thus, mixed with 
 nitrogen, it constitutes the atmosphere, of which it forms not 
 less than 23 per cent by weight. It takes a still larger share in 
 the composition of water, which consists of 88.88 per cent of 
 oxygen and 11.12 of hydrogen. There is a continual removal of 
 oxygen from air and water in the processes of weathering de- 
 scribed in Chapter II. Substances which can take more of this 
 element abstract it especially from damp air or from water. A 
 knife or any other piece of iron, for example, will remain un- 
 changed for an indefinite length of time if kept in dry air ; but 
 as soon as it is exposed to moisture, in which there is always 
 some dissolved air, it begins to rust. The familiar brown rust 
 which slowly eats into the very centre of the iron, is due to a 
 chemical union of oxygen with the iron, forming what is called 
 an Oxide of iron with water (see Oxides). Among the rocks of 
 the earth's crust, a large proportion are liable to undergo a 
 similar change, and so enormous has been the extent of this 
 change in the past history of our globe, that somewhere about 
 one-half of the outer and accessible part of the crust consists of 
 oxygen, which was probably at first in the atmosphere. 
 
122 GEOLOGY 
 
 Next in importance to oxygen among the metalloids is SILICON, 
 which is never met with in the free state. It has been artificial- 
 ly obtained, however, in the form of a dull brown powder. In 
 nature, it always occurs united with oxygen, forming the familiar 
 substance known as Silica or Silicic Acid (SiO), which consti- 
 tutes more than a half of all the known part of the earth's crust. 
 Silica is indeed the fundamental compound of the crust, forming 
 by itself entire masses of rock, and entering as a principal con- 
 stituent into the majority of rocks. It occurs abundantly as 
 the mineral Quartz, the colourless transparent forms of which are 
 known as rock-crystal (Fig. 44), and also in combination with 
 various metallic bases as the important family of Silicates (which 
 see). It is present in solution in most natural waters, both 
 
 Fig. 44. Group of Quartz-crystals (Rock-crystals). 
 
 those of the land and of the sea, whence it is secreted by plants 
 (diatoms, grasses) and animals (radiolarians and sponges). 
 It is thus carried by percolating water into the heart of rocks, 
 and may be deposited in their interstices and cavities. Its hard- 
 ness and durability eminently fit it for the important part, it 
 plays in binding the materials of rocks together, and enabling 
 them better to resist the decomposing effects of air and water. 
 
 CARBON, though found in a nearly pure state in the clear gem 
 called Diamond, and also in the black opaque mineral Graphite, 
 more usually occurs mixed with various impurities, as in the 
 different kinds of coal. This element has a high importance in 
 nature, because it is the fundamental substance made use of by 
 both plants ajid animals to build up their structures, and be- 
 
ELEMENTS AND MINERALS 123 
 
 cause it serves as a bond of connection between the organic and 
 the inorganic worlds. In union with oxygen, Carbon forms the 
 widely-diffused gaseous compound known as Carbon-dioxide 
 (CO), which occurs in the proportion of about four parts in 
 every ten thousand parts of ordinary atmospheric air. From 
 the air it is abstracted and decomposed by living plants in pres- 
 ence of sunshine, the oxygen being in great measure sent back 
 into the atmosphere, while the carbon with some oxygen, nitro- 
 gen, and hydrogen is built up into the various vegetable cells 
 and tissues. When we look at a verdant landscape or a bound- 
 less forest, it is a striking thought that all this vegetation has 
 been chiefly constructed out of the small proportion of invisible 
 carbon-dioxide present in the atmosphere. The vast numbers 
 of beds of coal imbedded in the earth's crust have, in like man- 
 ner, been derived from the atmosphere through the agency of 
 former tribes of plants. Not only in beds of coal, but still more 
 prevalently in masses of limestone, carbon enters into the com- 
 position of rocks. Carbon-dioxide, as was pointed out in Chapter 
 II, is abstracted by rain in passing from the clouds to the earth, 
 and is also supplied by decomposing plants and animals in the 
 soil. It is readily dissolved, in water, and forms with it car- 
 bonic acid, CO (OH) 2, which has been referred to as so powerful 
 a solvent of the substance of many rocks. This acid unites with 
 a number of alkaline and earthy bases to form the important 
 family of Carbonates. Of these the most abundant is calcium- 
 carbonate, or carbonate of lime (CaCOs), which consists of 44 
 per cent carbon-dioxide, and 56 per cent lime. This carbonate 
 not only occurs abundantly diffused through many rocks, but in 
 the form of limestone builds up by itself thick mountainous 
 masses of rock many hundreds of square miles in extent. It is 
 abstracted by plants to form calcareous tufa (Chapter V), but 
 far more abundantly by animals, especially by the invertebrata, 
 as- exemplified by the familiar urchins, corallines, and shells of 
 the sea-shore. The limestones of the earth's crust appear to 
 have been mainly formed of the calcareous remains of animals. 
 Hence we perceive that the two forms in which carbon has been 
 most abundantly stored up in the earth's crust have been princi- 
 pally due to the action of organised life ; coal being chiefly carbon 
 that has been taken out of the atmosphere by plants, and lime- 
 stone consisting of carbon-dioxide, to the extent of nearly one- 
 
124 GEOLOGY 
 
 half, which has been secreted from water by the agency of 
 animals. 
 
 SULPHUR is found in the free state, more particularly at vol- 
 canic vents, in pale yellow crystals or in shapeless masses and 
 grains ; but it chiefly occurs in combination. Some of its com- 
 pounds are widely diffused among plants and animals. The 
 blackening of a silver spoon by a boiled egg is an illustration 
 of this diffusion, for it arises from the union of the sulphur in 
 the egg with the metal. Combinations with a metal (Sulphides), 
 and combinations with a metal and oxygen (Sulphates) are the 
 conditions in which sulphur chiefly exists. 
 
 HYDROGEN is a gas which has been detected in the free state 
 at active volcanic vents; but otherwise it occurs chiefly in com- 
 bination with oxygen as the oxide water (EkO), of which it 
 constitutes about one-ninth, or 11.12 per cent by weight. It also 
 enters into the composition of plant and animal substances, and 
 forms with carbon the important group of bodies known as Hy- 
 drocarbons, of which mineral oil and coal-gas are examples. In 
 smaller quantity, it is found united with sulphur (sulphuretted 
 hydrogen, H^S), with chlorine (hydrochloric acid, HC1), and a 
 few other elements. 
 
 CHLORINE is a transparent gas of a greenish-yellow colour, but 
 except possibly at active volcanic vents it does not occur in the 
 free state. United with the alkali metals (potassium, sodium, 
 and magnesium), it forms the chief salts of sea-water. The most 
 important of these salts, Sodium-chloride, or common salt 
 (NaCl) contains 60.64 per cent of chlorine, and forms 2.64 per 
 cent by weight of sea-water. This salt is found diffused in mi- 
 croscopic particles in the air, especially near the sea, and beds 
 of it hundreds of feet thick occur in many parts of the world 
 among the sedimentary rocks that constitute most of the dry 
 land. 
 
 PHOSPHORUS does not occur free; it has so strong an affinity 
 for oxygen that it rapidly oxidises on exposure to the air, and 
 even melts and takes fire. Its most frequent combination is with 
 oxygen and calcium, as Calcium-phosphate or as phosphate of 
 lime (Ca3(PO*)2. Though for the most part present in mi- 
 nute proportions, it is widely diffused in nature. It occurs in 
 fresh and sea-water, in soil and in plants, especially in their 
 fruits and seeds; it is supplied by plants to animals for the 
 
ELEMENTS AND MINERALS 125 
 
 formation of bones, which when burnt consist almost entirely 
 of phosphate of lime. 
 
 FLUORINE also is never met with uncombined ; it never unites 
 with oxygen, forming in this respect the sole exception among 
 the elements. Its most frequent combination as a rock constit- 
 uent is with calcium, when it forms the mineral Fluor-spar 
 (CaFa). Like phosphorus, fluorine is widely diffused in minute 
 proportions in the waters of some springs, rivers, and the sea, 
 and in the bones of animals. 
 
 To these metalloids we may add the colourless, tasteless gas 
 NITROGEN, which, though not largely present in the earth's crust, 
 constitutes four-fifths by volume or 77 per cent by weight of the 
 atmosphere. It does not enter into combination so readily as the 
 other elements above enumerated, but it is always found in the 
 composition of plants and is a constituent of many animal tissues. 
 It is the principal ingredient of the substance called Ammonia, 
 which is produced when moist organic matter is decomposed in 
 the air. In many rocks composed wholly or in great part of 
 organic remains, such, for instance, as peat and coal, nitrogen is 
 a constant constituent. 
 
 Metals. Though so large a proportion of the known terres- 
 trial elements are metals, these are much less abundant in the 
 earth's crust than the metalloids. The most frequent are Alumin- 
 ium, Calcium, and Magnesium. The substances most familiar 
 to us as metals occupy an altogether subordinate part among 
 rocks, the most abundant of them being Iron. 
 
 ALUMINIUM never occurs in the free state, but can be artifi- 
 cially separated from its compounds, when it is seen to be a white 
 light, malleable metal. It is almost always united with oxygen 
 as the oxide of Alumina (AbOa), which occurs crystallised as the 
 ruby and sapphire, but is for the most part united with silica, 
 and in this form constitutes the basis of the great family of 
 minerals known as the Silicates of Alumina, or Aluminous Sili- 
 cates. These silicates generally contain some other ingredient 
 which is more liable to decomposition, and when they decay and 
 their more soluble parts are removed, they pass into clay, which 
 consists chiefly of hydrated silicate of alumina. 
 
 CALCIUM is not met with uncombined, but has been artificially 
 isolated and found to be a light yellowish metal, between gold 
 and lead in hardness. It occurs in nature chiefly combined with 
 
126 GEOLOGY 
 
 carbonic acid as a carbonate, and also with sulphuric acid as 
 a sulphate, to both of which substances reference has already 
 been made; and it is also present in many silicates. So 
 abundant is Calcium-carbonate or carbonate of lime in nature 
 that it may be found in most natural waters, which dissolve it 
 and carry it in solution into the sea. Its presence in rocks may 
 be detected by a drop of any mineral acid, when the liberated 
 carbon-dioxide escapes as a gas with brisk effervescence. Calcium- 
 sulphate is likewise a common constituent of terrestrial waters, 
 especially of those which in household management are called 
 hard; it constitutes not less than 3.6 per cent of the salts in 
 ordinary sea-water, and when sea-water is evaporated this sul- 
 phate (gypsum), being least soluble, is the first to be precipi- 
 tated in minute crystals resembling in shape those shown in 
 Fig. 62. 
 
 MAGNESIUM is likewise only isolated artificially, when it ap- 
 pears as a soft, silver-white, malleable and ductile metal. It 
 occurs in sea-water combined with chlorine as Magnesium-chlo- 
 ride, which constitutes 10.8 per cent of the total proportion of 
 salts. It unites with carbonic acid as a carbonate, which with 
 carbonate of lime forms the widely diffused rock called magne- 
 sian limestone or Dolomite; it also enters into the composition 
 of the Magnesian Silicates which are only second in importance 
 to those of alumina. 
 
 POTASSIUM and SODIUM (alkali metals) are only obtainable in 
 the free state by chemical processes, when they are found to be 
 white brittle metals that float on water, and rapidly oxidise if 
 exposed to the air. Combined with chlorine, Sodium forms the 
 familiar chloride known as common salt, which constitutes 77.7 
 per cent of the salts of sea-water, is abundantly present in salt 
 lakes, and occurs also in extensive beds among the rocks of the 
 dry land. Potassium-chloride likewise occurs in the sea and 
 may be obtained from the ashes of burnt sea-weed. Enor- 
 mous deposits of it, combined with chlorides of sodium and 
 magnesium, have been met with in Germany (Stassfurt). Pot- 
 assium also exists in the sea in combination with sulphuric acid 
 as potassium-sulphate or sulphate of potash, which amounts to 
 about 2.4 of the total salts of sea-water. Sulphates of potassium, 
 sodium, magnesium, and calcium form thick masses of rock in 
 the Stassfurt deposits. Potassium and sodium in combination 
 
ELEMENTS AND MINERALS 127 
 
 with silica form silicates which enter largely into the composi- 
 tion of many rocks. They are readily attacked by water contain- 
 ing carbonic acid, giving rise to what are called carbonates of the 
 alkalies, or alkaline carbonates, which are removed in solution. 
 By this means, carbonate of potash is introduced into soil, where 
 it is taken up by plants into their leaves and succulent parts. 
 When wood is burnt, this carbonate in considerable quantity may 
 be dissolved with water out of the ash. 
 
 IRON" is found in the free or native state in minute grains 
 (rarely in large blocks) in some volcanic rocks, also in granules 
 of " cosmic dust," probably of meteoric origin, and in fragments 
 of various size which have undoubtedly fallen upon the earth's 
 surface from the regions of space. There is reason to believe 
 that much of the solid interior of the globe may consist of na- 
 tive iron and other metals. But it is in combination that iron 
 is chiefly of importance in the earth's crust. It has united with 
 oxygen to form several abundant oxides. The protoxide or 
 ferrous oxide (FeO) contains the lowest proportion of oxygen, 
 and being, therefore, prone to take up more, gives rise 
 to many of the processes of decay included under the general 
 name of Weathering. It is readily dissolved by organic 
 and other acids, and is then removed in solution, but on exposure 
 rapidly oxidises and passes into the highest oxide, known as the 
 peroxide or sesquioxide of iron or ferric oxide (FeaOs), which, 
 being the permanent insoluble form, is found abundantly among 
 the rocks of the earth's crust. Iron is the great colouring matter 
 of nature; its protoxide compounds give greenish hues to many 
 rocks, while its peroxide colours them various shades of red 
 which, when the peroxide is combined with water, pass into many 
 tints of brown, orange, and yellow. 
 
 MANGANESE is commonly associated with iron in minute pro- 
 portion in many lavas and other crystalline rocks; its oxides 
 resemble those of iron in their modes of occurrence. 
 
 BARIUM and Calcium are called metals of the alkaline earths. 
 The former can only be obtained in a free state by artificial 
 means, when it appears as a pale yellow very heavy metal which 
 rapidly tarnishes. In nature it chiefly occurs as the sulphate, 
 Barytes, or heavy spar (BaSCh), a mineral of frequent occur- 
 rence in veins associated with metallic ores. 
 
128 GEOLOGY 
 
 MINERALS OF CHIEF IMPORTANCE IN THE EARTH'S CRUST. 
 
 Passing now from the simple elements, we have next to note 
 the mineral forms in which they appear as constituents of 
 the earth's crust. A mineral may be defined as an inorganic 
 substance, having theoretically a definite chemical composition,, 
 and in most cases also a certain geometrical form. It may 
 consist of only one element, for example, the diamond, sulphur, 
 and the native metals, gold, silver, copper, etc. But in the 
 vast majority of cases, minerals consist of at least two, usually 
 more, elements in definite chemical proportions. In the follow- 
 ing short list of the more important minerals of the earth's 
 crust they are arranged chemically, according to the predom- 
 inant element in them, or the manner in which the combinations 
 of the elements have taken place, so that their leading features 
 of composition may be at once perceived. The two elements, 
 Carbon and Sulphur, in their native or uncombined state, 
 sometimes form considerable masses of rock. Some of the native 
 metals also may be enumerated as rock-constituents when they 
 occur in sufficient quantities to be commercially important. 
 Gold, for example, is found in grains and strings, in veins of 
 
 quartz, and in irregular pieces 
 or nuggets dispersed through the 
 gravel deposits of regions where 
 gold-bearing quartz-veins traverse 
 the solid rocks. Omitting, how- 
 ever, the minerals formed of a 
 single element, we may pass on to 
 combinations of two or more ele- 
 ments, and consider first those in 
 which oxygen is combined with 
 some other element, forming what 
 
 Fig. 45.- Ca1cite< Iceland spar), are commonly grouped together 
 showing its characteristic rhom- as Oxides. Then Will come the 
 
 bohedral cleavage. Silicates, or combinations of silica 
 
 with one or more bases, followed by the Carbonates, or com- 
 binations of carbon-dioxide with some base; the Sulphates, or 
 compounds of sulphuric acid and a base; the Fluorides, or 
 compounds of fluorine and a metal ; the Chlorides, or compounds 
 
ELEMENTS AND MINERALS 129 
 
 of chlorine and a metal; and the Sulphides, or compounds of 
 sulphur and a metal. 
 
 It cannot be too strongly impressed upon the mind of the 
 learner that no mere description in books will suffice to make 
 him familiar with minerals and rocks. He ought to handle 
 actual specimens of these objects and identify for himself the 
 several characters which he finds assigned to them in books. 
 
 One of the most obvious features in a crystal of any mineral 
 is the regular and sharply-defined edges and corners which it 
 presents. Take a piece of rock-crystal or quartz, for example 
 (Fig. 44), and you will find it to consist of six sides or faces, 
 forming what is called a prism, and bevelled off at the end into 
 a six-sided cone, called a pyramid. If you examine a large 
 collection of similar crystals you may find no two of them 
 exactly alike, yet they agree in presenting a six-sided figure. 
 Again, procure a piece of the common mineral calcite, either 
 a whole crystal (Figs. 59, 60), or a portion of a crystalline 
 mass (Fig. 45) ; break it and you will find each fragment to 
 possess the same form, that x of a rhombohedron ; crush one 
 of these fragments and you will observe that each little grain 
 of the powder preserv.es the same shape. The rhombohedron, 
 therefore, is called the fundamental crystalline form of the 
 mineral. The property so strikingly shown in calcite, of break- 
 ing along definite crystalline planes, is termed cleavage. So 
 perfect is the cleavage of calcite, that the crystallised mineral can 
 hardly be broken, except along the planes that define the rhom- 
 bohedron. Many minerals cleave more or less easily in one or 
 more directions, and break irregularly in others. The cleavage 
 affords a guide to the proper crystalline form of a mineral. 
 
 Though there are many hundreds of varieties of crystalline 
 form, they may all be reduced to six primary types or systems. 
 These are distinguished from each other by the number and 
 position of their axes, which are mathematical straight lines, 
 intersecting each other in the interior of a crystal, and connect- 
 ing the centre of opposite flat faces of the crystal, or opposite 
 angles or corners. The six systems, with their axes, are enumer- 
 ated in the subjoined list. 
 
 I. Isometric (monometric, cubical, tesseral, regular). In this sys- 
 tem there are three axes which are of the same length, and 
 intersect each other at a right angle. The cube, octahedron, 
 
130 
 
 GEOLOGY 
 
 and dodecahedron are examples (Fig. 46). Crystals of this 
 system are distinguished by their symmetry, their length, 
 breadth, and thickness being equal. Common salt, fluor-spar 
 (Fig. 04), and magnetite (Fig. 54) are illustrations. 
 
 Fig. 46. Cube (a), octahedron (&), dodecahedron (c). 
 
 II. Tetragonal (dimetric). The axes are three in number, and inter- 
 sect each other at a right angle, but one of them, called the 
 vertical axis, is longer or shorter than the other two, which 
 are lateral axes. Hence a crystal belonging to this system may 
 either be oblong or squat (Fig. 47). 
 
 Fig. 47. Tetragonal prism (6) 
 and pyramid (a). 
 
 Fig. 48. Orthorhombic 
 prism. 
 
 III. Orthorhombic (trimetric) has the three axes intersecting each 
 other at a right angle, but all of unequal lengths. The rec- 
 tangular and rhombic prisms and the rhombic octahedron be- 
 long to this system (Fig. 48). 
 
 Fig. 49. Hexagonal prism (a), rhombohedron (6), and 
 scalenohedron (c). 
 
 IV. Hexagonal. This is the only system with four axes (Fig. 49). 
 The lateral axes are all equal, intersect at right angles the ver- 
 
ELEMENTS AND MINERALS 131 
 
 tical axis (which is longer or shorter than they are), and form 
 with each other angles of 60. Water, for instance, crystallises 
 in this system, and the six-rayed star of a snow-flake is an 
 illustration of the way in which the lateral axes are placed. 
 Quartz is an example (Fig. 44), also calcite (Figs. 59, 60). 
 
 Fig. 50. Monoclinic prism. Fig. 51. Triclinic prism. 
 
 Crystal of Augite. Crystal of Albite felspar. 
 
 V. Monoclinic, with all the axes of unequal length. One of the 
 lateral axes cuts the vertical axis at a right angle, the other 
 intersects the vertical axis obliquely. Augite (Fig. 50), Horn- 
 blende (Fig. 57), and Gypsum (Fig. 62) are examples. 
 VI. Triclinic, the most unsymmetrical of all the systems, all the axes 
 being unequal and placed obliquely to each other (Fig. 51). 
 
 Every mineral that takes a crystalline form belongs to one or 
 other of these six systems, and through all its varieties of ex- 
 ternal form the fundamental relations of the a'xes remain un- 
 changed. 
 
 Some minerals have crystallised out of solutions in water. 
 How this may take place can be profitably studied by dissolving 
 salt, sugar, or alum in water, and watching how the crystals of 
 these substances gradually shape themselves out of the con- 
 centrated solution, each according to its own crystalline pat- 
 tern. Other minerals have crystallised from hot vapours (sub- 
 limation), as may be observed at the fissures of an active vol- 
 cano. Other minerals have crystallised out of molten solu- 
 tions, as in the case of lava. Thoroughly fused lava is a glassy 
 or vitreous solution of all the mineral substances that enter into 
 the composition of the rock, and when the lava cools, the 
 various minerals crystallise out of it, those that are least fusible 
 taking form first, the most fusible appearing last ; but a residue 
 of non-crystalline glass sometimes remaining even when the rock 
 has solidified. 
 
 It is evident that minerals can only form perfect crystals 
 
132 GEOLOGY 
 
 where they have room and time to crystallise. But where they 
 are crowded together, and where the solution in which they are 
 dissolved dries or cools too rapidly, their regular and sym- 
 metrical growth is arrested. They then form only imperfect 
 crystals, but their internal structure ft crystalline, and if ex- 
 amined carefully will be found to show that in the attempt to 
 form definite crystals each mineral has followed its own crystal- 
 line type. These characters are of much importance in the 
 study of rocks, for rocks are only large aggregates of minerals, 
 wherein definite crystals are exceptional, though the structure 
 of the whole mass may still be quite crystalline. 
 
 But minerals also occur in various indefinite or non-crystal- 
 line shapes. Sometimes they are -fibrous or disposed in minute 
 fibre-like threads (Fig. 56) ; or concretionary when they have 
 been aggregated into various irregular concretions of globular, 
 kidney-shaped, grape-like, or other imitative shapes (Figs. 61, 
 64, 65, 75) ; or stalactitic (Fig. 20) when they have been de- 
 posited in pendent forms like stalactites; or amorphous when 
 they have no definite shape of any kind, as, for instance, in 
 massive ironstone. 
 
 Oxides occur abundantly as minerals. The most important 
 are those of Silicon (Quartz) and Iron (Hematite, Limonite, 
 Magnetite, Titanic Iron. 
 
 QUARTZ (Silica, Silicic Acid, SiO), already alluded to, is the 
 most abundant mineral in the earth's crust. It occurs crystal- 
 lised, also in various crystalline and non-crystalline varieties. 
 In the crystallised form as common quartz it is, when pure, clear 
 and glassy, but is often coloured yellow, red, green, brown, or 
 black, from various impurities. It crystallises in the six-sided 
 prisms and pyramids above referred to, the clear colourless vari- 
 eties being rock-crystal (Fig. 44). When purple it is called 
 amethyst; yellow and smoke-colour varieties, found among the 
 Grampian Mountains, are popularly known as Cairngorm stones. 
 In many places, silica has been deposited as chalcedony, in 
 translucent masses with a waxy lustre, and pale grey, blue, 
 brown, red, or black colours. Deposits of this kind are not in- 
 frequent among the cavities of rocks. The common pebbles and 
 agates with concentric bands of different colours are examples 
 of chalcedony, and show how the successive layers have been de- 
 posited from the walls of the cavity inwards to the centre which 
 
ELEMENTS AND MINERALS 133 
 
 is often filled with crystalline quartz (Fig. 52). The dark 
 opaque varieties are called jasper. 
 
 Quartz can be usually recognised by its vitreous lustre and 
 hardness; it cannot be scratched with a knife, but easily 
 scratches glass, and it is not soluble in the ordinary acids. 
 It is an essential constituent of many rocks, such as granite and 
 sandstone. Silica being dissolved by natural waters, especially 
 where organic acids or alkaline carbonates are present, is intro- 
 duced by permeating water into the heart of even the most solid 
 rocks. Hence it is found abundantly in strings and veins 
 traversing rocks, also in cavities and replacing the forms of 
 plants and animals imbedded in sedimentary deposits. Soluble 
 silica is abstracted by some plants and animals and built up into 
 their organic structures (diatoms, radiolarians, sponges). 
 
 Fig. 52. Section of a pebble of chalcedony. The outer banded layers 
 are chalcedony, the interior being nearly filled up with crystalline 
 quartz. 
 
 Four minerals composed of oxides of Iron occur abundantly 
 among rocks. The peroxide is found in two frequent forms, one 
 without water (Haematite), the other with water (Limonite). 
 The peroxide and protoxide combine to form Magnetite, and a 
 mixture of the peroxide with the peroxide of the metal titanium 
 gives Titanic Iron. 
 
 HEMATITE or Specular Iron (Fe^O^ Fe70030) occurs in 
 rhombohedral crystals that can with difficulty be scratched with 
 a knife ; but is more usually found in a massive condition with a 
 compact, fibrous, or granular texture, and dark steel-gray or iron- 
 black colour, which becomes bright red when the mineral is 
 scratched or powdered. The earthy kinds are red in colour, and 
 
134 
 
 GEOLOGY 
 
 it is in this earthy form that haematite plays so important a part 
 as a colouring material in nature. Red sandstone, for example, 
 owes its red colour to a deposit of earthy peroxide of iron round 
 
 the grains of sand. Hema- 
 tite occurs crystallised in fis- 
 sures of lavas as a product of 
 the hot vapours that escape at 
 these places; but is more 
 abundant in beds and concre- 
 tionary masses (Fig. 53) 
 among various rocks. 
 
 LIMONITE or Brown Iron- 
 re differs from Hematite in 
 
 and the internal crystalline being rather softer, in con- 
 taining more than 14 per cent 
 
 of water which is combined with the iron to form the hydrated 
 peroxide, in being usually massive or earthy, in presenting a dark 
 brown to yellow colour (ochre), and in giving a yellowish-brown 
 to dull yellow powder when scratched or bruised. It may be seen 
 in the course of being deposited at the present time through the 
 action of vegetation in bogs and also in lakes, hence its name 
 of Bog-iron ore, likewise in springs and streams where the water 
 carries much sulphate of iron. The common yellow and brown 
 colours of sandstones and many other rocks are generally due to 
 the presence of this mineral. 
 
 Fig. 54. Octahedral crystals of magnetite in chlorite schist. 
 
 MAGNETITE (FeoO*) occurs crystallised in octahedrons and 
 dodecahedrons of an iron-black colour, giving a black powder 
 when scratched. It is found abundantly in many rocks (schists, 
 lavas, etc.), sometimes in large crystals (Fig. 54), sometimes in 
 such minute form as can only be detected with the microscope. 
 
ELEMENTS AND MINERALS 135 
 
 It also forms extensive beds of a massive structure. Its presence 
 in rocks may be detected by its influence on a magnetised needle. 
 By pounding basalt and some other rocks down to powder, 
 minute crystals and grains of magnetite may be extracted with 
 a magnet. 
 
 TITANIC IRON (FeTi^Oa) occurs in iron-black crystals like 
 those of haematite from which they may be distinguished by the 
 dark colour and metallic lustre of its surface when scratched. 
 Though it occurs in beds and veins in certain kinds of rock 
 (schists, serpentine, syenite), its most generally diffused condi- 
 tion is in minute crystals and grains scattered through many 
 crystalline rocks (basalt, diabase, etc.). 
 
 MANGANESE OXIDES are commonly associated with those of 
 iron in rocks. They are liable to be deposited in the form of 
 bog-manganese, under condi- 
 tions similar to those in 
 which bog-iron is thrown ^ 
 down. Earthy manganese Jj 
 oxide (wad) not infrequent- 
 ly appears between the joints 
 of fine-grained rocks in arbor- 
 escent forms that look so like 
 plants as to have been often 
 mistaken for vegetable re- 
 mains. These plant-like de- 
 posits are called Dendrites or 
 dendritic markings (Fig. 55). 
 
 SILICATES. Compounds 
 of Silica with various bases 
 form by far the most numer- 
 ous and abundant series of 
 minerals in the earth's crust. 
 They may be grouped accord- 
 ing to the chief metallic base Fig 55 ._ Dendritic markings due to 
 in their composition, the most arborescent deposit of earthy man- 
 important are the Silicates 
 
 of Alumina, and the Silicates of Magnesia. Of the 
 aluminous silicates we need consider here only the Felspars, 
 Zeolites, and Mica. Among the magnesian silicates it will be 
 enough to note the leading characters of Hornblende, Augite, 
 
136 GEOLOGY 
 
 Olivine, Talc, Chlorite, and Serpentine. When the learner has 
 made himself so familiar with these as to be able readily to 
 recognise them, he may proceed to the examination of others, of 
 which he will find descriptions in treatises on Mineralogy and in 
 more advanced text-books of Geology. 
 
 FELSPARS. This family of minerals plays an important part 
 in the construction of the earth's crust, for it constitutes the 
 largest part of the crystalline rocks, which, like lava, have been 
 erupted from below; is found abundantly in the great series of 
 schists ; and by decomposition has given rise to the clays, out of 
 which so many sedimentary rocks have been formed. The 
 felspars are divided into two series, according to crystalline form. 
 
 Orthoclase or potash-felspar contains about 16.89 per cent 
 of potash, crystallises in monoclinic or oblique rhombic prisms, 
 but also occurs massive; is. white, gray, or pink in colour; has 
 a glassy lustre ; can with difficulty be scratched with a knife, but 
 easily with quartz. Associated with quartz, it is an abundant 
 ingredient of many ancient crystalline rocks (granite, felsite, 
 gneiss, etc.) In the form of sanidine it is an essential constitu- 
 ent of many modern volcanic rocks. 
 
 Plagioclase. Under this name are grouped several species 
 of felspar which, differing much from each other in chemical 
 composition, agree in crystallising in the same type or system, 
 which is that of a triclinic or oblique rhomboidal prism. As 
 abundant ingredients of rocks they commonly appear as clear, 
 colourless, or white glassy strips, on the flat faces of which 
 a fine minute parallel ruling may be detected with the naked eye, 
 or with a lens. This striation or lamellation is a distinctive 
 character, which proves the crystals in which it occurs not to be 
 orthoclase. The plagioclase felspars occur as essential constitu- 
 ents of many volcanic rocks, and also among ancient eruptive 
 masses and schists. Among them are Microcline (a potash- 
 felspar), with 15 per cent of potash; Albite or Soda-felspar, con- 
 taining nearly 12 per cent of soda (Fig. 51) ; Anorthite or Lime- 
 felspar, with 20.10 per cent of lime; Soda-lime felspar, 
 Lime-soda felspar a group of felspars containing variable 
 proportions of soda, lime, and sometimes potash ; the chief varie- 
 ties are Oligoclase (Silica, 62-65 per cent), Andesine (Silica, 
 58-61 per cent), Labradorite (Silica, 50-56 per cent). 
 
 ZEOLITES, a characteristic family of minerals, composed essen- 
 
ELEMENTS AND MINERALS 137 
 
 tially of silicate of alumina and some alkali with water; often 
 marked by a peculiar pearly lustre, especially on certain planes 
 of cleavage ; usually found fillin'g up cavities in rocks where they 
 have been deposited from solution in water. Some of the species 
 commonly crystallise in fine needles or silky tufts. The zeolites 
 have obviously been formed from the decomposition of other 
 minerals, particularly felspars. They are especially abundant in 
 the steam-cells of old lavas in which plagioclase felspars prevail, 
 either lining the walls of the cavities, and shooting out in crystals 
 or fibres towards the centre (Fig. 56), or filling the cavities up 
 entirely. 
 
 MICA, a group of minerals (monoclinic) specially distin- 
 guished by their ready cleavage into thin, parallel, usually elastic 
 
 Fig. 56. Cavity in a lava, filled with zeolite which has crystallised in 
 long slender needles. 
 
 silvery Iamina3. They are aluminous silicates with potash 
 (soda), or with magnesia and ferrous oxide, and always with 
 water. They occur as essential constituents of granite, gneiss, 
 and many other eruptive and schistose rocks, also in worn 
 spangles in many sedimentary strata- (micaceous sandstone). 
 Among their varieties the two most important are Muscovite 
 (white mica, potash-mica), and Black mica (magnesia-mica,. 
 Biotite) . 
 
 HORNBLENDE or AMPHIBOLE, a silicate of magnesia, with lime, 
 iron-oxides, and sometimes alumina, occurs in monoclinic 
 (oblique rhombic) prisms, also columnar, fibrous, and massive. 
 It is divisible into (1) a group of pale-coloured varieties, con- 
 taining little or no alumina, white or pale green in colour, often 
 fibrous (Tremolite., Actinolite, Asbestus), found more particu- 
 
138 
 
 GEOLOGY 
 
 Fig. 57. Horn- 
 blende crystal. 
 
 larly among gneisses, marbles, and associated rocks, and (2) a 
 dark group containing 5 to 18 per cent of 
 alumina, which replaces the other bases ; dark 
 green to black in colour, in stout, dumpy 
 prisms (Fig. 57), and in columnar or bladed 
 aggregates (Common hornblende). Abun- 
 dant in many eruptive rocks, and also form- 
 ing almost entire beds of rock among the 
 crystalline schists. 
 
 AUGITE (PYROXENE), in composition re- 
 sembles hornblende; indeed, they are only 
 different forms of the same substance; differing slightly 
 in crystalline form, hornblende being the result of slow and 
 augite of rapid crystallisation. Many rocks in which the 
 dark silicate was originally augite have that mineral now 
 replaced by hornblende, as the result of a gradual internal 
 alteration. Like hornblende, augite occurs in two groups: (1) 
 pale non-aluminous, found more especially among gneisses, mar- 
 bles, and associated rocks; and (2) dark green or black (Fig. 
 50), occurring abundantly in many eruptive rocks, such as black 
 heavy lavas (basalts, etc.). 
 
 OLIVINE (PERIDOT) (SKM1.01, MgO 49.16, FeO 9.83) oc- 
 curs in small orthorhombic prisms and glassy grains in basalts 
 and other lavas ; of a pale yellowish-green or olive-green colour, 
 whence its name. These grains can often be 
 readily detected on the black ground of the 
 rock, through which they are abundantly 
 dispersed. Olivine is liable to alteration, 
 and especially to conversion into serpentine 
 by the influence of percolating water 
 (Fig. 58). 
 
 CHLORITE (SiO* 25-28, AkO 19-23, FeO 
 15-29, MgO 13-25, H*0 9-12) is a dark olive- 
 green hydrated magnesian silicate. It is so 
 soft as to be easily scratched with the nail, 
 and occurs in small six-sided tables, also in 
 various scaly and tufted aggregations dif- 
 fused through certain rocks. It appears 
 generally to be the result of the alteration of 
 some previous anhydrous magnesian silicate, such as hornblende. 
 
 Fig. 58 Olivine 
 crystal; the light 
 portions repre- 
 sent the unde- 
 composed miner- 
 al, the shaded 
 parts show the 
 conversion of the 
 olivine into ser- 
 pentine. 
 
ELEMENTS AND MINERALS 139 
 
 SERPENTINE (Mg^O-fglLO) is another hydrated mag- 
 nesian silicate, containing a little protoxide of iron and alumina, 
 usually massive, dark green but often mottled with red. It 
 occurs in thick beds among schists, is often associated with 
 limestones, and may be looked for in all rocks that contain 
 olivine, of the alteration of which it is often the result. In 
 many serpentines, traces of the original olivine crystals can be 
 detected. 
 
 CARBONATES. Though these are abundant in nature, only 
 three of them require notice here as important constituents of 
 the earth's crust, those of lime, magnesia and lime, and iron. 
 
 CALCITE (calcium-carbonate, carbonate of lime, CaCO) crys- 
 tallises in the hexagonal system, and has for its fundamental 
 
 Fig. 59. Calcite in the form of " nail-head spar." 
 
 crystalline form the rhombohedron, as already mentioned. 
 When it is quite pure it is transparent (Iceland spar, Fig. 
 45), with the lustre of glass: but more usually is translucent 
 or opaque and white. Its crystals, where the chief axis is shorter 
 than the others, sometimes take the form of flat rhombohedrons 
 (nail-head spar, Fig. 59) ; where, on the other hand, that axis 
 is elongated, they present pointed pyramids (scalenohedrons, 
 dog-tooth spar, Fig. 60). The mineral occurs also in fibrous, 
 granular, and compact forms. The decomposition of silicates 
 containing lime by permeating water gives rise to calcium-car- 
 bonate, which is removed in solution. Being readily soluble in 
 water containing carbonic acid, this carbonate is found in almost 
 all natural waters, by which it is introduced into the cavities of 
 rocks. Some plants and many animals secrete large quantities of 
 carbonate of lime, and their remains are aggregated into beds of 
 limestone, which is a massive and more or less impure form of 
 
140 GEOLOGY 
 
 calcite. Calcite is easily scratched with a knife, and is character- 
 ised by its abundant effervescence when acid is dropped upon it. 
 
 A less frequent and stable form of calcium-carbonate is 
 ARAGONITE which crystallises in orthorhombic forms, but is more 
 usually found in globular, dendritic, coral-like, or other irregular 
 shapes, and is rather harder and heavier than calcite. 
 
 DOLOMITE assumes a rhombohedral crystallisation, and is a 
 compound of 54.4 of magnesium-carbonate, with 45.6 of calcium- 
 carbonate. It is rather harder than calcite, and does not effer- 
 vesce so freely with acid. It occurs in strings and veins like 
 calcite, but also in massive beds having a prevalent pale yellow 
 or brown colour (owing to hydrated peroxide of iron), a granu- 
 lar and often cavernous texture, and a tendency to crumble down 
 on exposure. 
 
 Fig. 60. Calcite in the form of dog-tooth spar. 
 
 SIDERITE (chalybite, spathic iron, ferrous carbonate, FeCO), 
 another rhombohedral carbonate, contains 62 per cent of ferrous 
 oxide or protoxide of iron. In its crystalline form it is gray or 
 brown, becoming much darker on exposure as the protoxide 
 passes into peroxide. It also occurs mixed with clay in concre- 
 tions and beds, frequently associated with remains of plants and 
 animals (Sphcerosiderite, Clay-ironstone, Figs. 61, 65). 
 
 SULPHATES. Two sulphates deserve notice for their impor- 
 tance among rock-masses those of lime and baryta, 
 
 GYPSUM (hydrous calcium-sulphate, CaSO^+SHOz) occurs in 
 monoclinic crystals, commonly with the form of right rhomboidal 
 prisms (Fig. 62, a), which not infrequently appear as macles or 
 twin-crystals (Fig. 62, b). When pure it is clear and colour- 
 less, with a peculiar pearly lustre (Selenite) ; it is found fibrous 
 
ELEMENTS AND MINERALS 
 
 141 
 
 with a silky sheen (Satin-spar), also white and granular (Ala- 
 baster). It is so soft as to be easily cut with a knife or even 
 scratched with the finger-nails. It is readily distinguished from 
 calcite by its crystalline form, softness,, and non-effervescence 
 with acid. When burnt it be- 
 comes an opaque white powder 
 (plaster of Paris). Gypsum 
 occurs in beds associated with 
 sheets of rock-salt and also with 
 dolomite; it is soluble in water, 
 and is found in many springs and 
 rivers, as well as in the sea. One 
 thousand parts of water at 32 
 Fahr. dissolve 2.05 parts of sul- 
 phate of lime; but the solubility 
 of the substance is increased in 
 the presence of common salt, a 
 thousand parts of a saturated 
 solution of common salt taking up 
 
 as much as 8.2 parts of the sul- pig. 61. Sphserosiderite 
 phate. 
 
 Anhydrous calcium-sulphate or 
 ANHYDRITE is harder and heavier than gypsum, and is found 
 extensively in beds associated with rock-salt deposits. By 
 absorbing water, it increases in bulk and passes into gypsum. 
 
 BARYTES (Heavy spar, barium-sulphate, BaSO*), the usual 
 form in which the metal barium is distributed over the globe, 
 crystallises in orthorhombic prisms which are generally tabular; 
 but most frequently it occurs in various massive forms. The 
 purer varieties are transparent or translucent, but in general the 
 mineral is dull yellowish or pinkish white, with a vitreous lustre, 
 and is readily recognisable from other similar substances by its 
 great weight; it does not effervesce with acids. Barytes is 
 usually met with in veins traversing rocks, especially in associa- 
 tion with metallic ores. 
 
 PHOSPHATES. Only one of these requires -to be enumerated 
 in the present list of minerals the phosphate of lime or 
 Apatite. 
 
 APATITE (tricalcic phosphate, phosphate of lime) crystallises 
 in hexagonal prisms which, as minute colourless needles, are 
 
 or 
 
 Clay-ironstone concretion en- 
 closing portion of a fern. 
 
142 
 
 GEOLOGY 
 
 abundant in many crystalline rocks; it also occurs in large 
 crystals and in amorphous beds associated with gneiss. It is 
 soluble in water containing carbonic acid, ammoniacal salts, com- 
 mon salt, and other salts. Hence its introduction into the soil, 
 and its absorption by plants, as already mentioned. 
 
 a 6 
 
 Fig. 62. Gypsum crystals. 
 
 FLUORIDES. The only member of this family occurring con- 
 spicuously in the mineral kingdom is calcium fluoride or FLUOR- 
 SPAR (Fluorite, CaF*), which, in the form of colourless, but 
 more commonly light green, purple, or yellow cubes, is found in 
 
 Fig. 63. Group of fluor-spar crystals. 
 
 mineral veins not infrequently accompanying lead-ores 
 (Fig. 63). 
 
 CHLORIDES. Reference has already been made to the only 
 chloride which occurs plentifully as a rock-mass, the chloride of 
 
ELEMENTS AND MINERALS 
 
 143 
 
 sodium, known as HALITE or Hock-salt (NaCl, chlorine 60.64, 
 sodium 39.36). It crystallises in cubical forms, and is also 
 found massive in beds that mark the evaporation of former salt- 
 lakes or inland seas. 
 
 SULPHIDES. Many combinations of sulphur with the metals 
 occur, some of them of great commercial value ; but the only one 
 that need be mentioned here for its wide diffusion as a rock- 
 constituent is the iron-disulphide (FeS?), in which the elements 
 are combined in the proportion of 46.7 iron and 53.3 sulphur. 
 
 Fig. 64. Concretions, 
 o, 6, " Fairy stones ; " c, Pyrite, showing internal radiated structure. 
 
 This substance assumes two crystalline forms: (1) PYRITE 
 which occurs in cubes and other forms of the first or monometric 
 system, of a bronze-yellow colour and metallic lustre, so hard as 
 to strike fire with steel, and giving a brownish-black powder 
 when scratched. This mineral is abundantly diffused in minute 
 grains, strings, veins, concretions (Fig. 64, c), and crystals in 
 many different kinds of rocks ; it is usually recognisable by its 
 colour, lustre, and hardness; (2) MARCASITE (white pyrite) 
 
144 GEOLOGY 
 
 crystallises in the tetragonal system, has a paler colour than 
 ordinary pyrite, and is much more liable to decomposition. 
 This form, rather than pyrite, is usually associated with the 
 remains of plants and animals imbedded among rocks. The 
 sulphide has no doubt often been precipitated round decaying 
 organisms by their effect in reducing sulphate of iron. By its 
 ready decomposition, marcasite gives rise to the production of 
 sulphuric acid and the consequent formation of sulphates. One 
 of the most frequent indications of this decomposition is the 
 rise of chalybeate springs. 
 
THE MORE IMPORTANT ROCKS 145 
 
 CHAPTEE XI. 
 
 THE MORE IMPORTANT ROCKS. 
 
 FEOM the distribution of the more important elements in 
 the earth's crust and the mineral forms which they 
 assume,, we have now to advance a stage farther and in- 
 quire how the minerals are combined and distributed so as to 
 build up the crust. As a rule, simple minerals do not occur 
 alone in large masses ; more usually they are combined in various 
 proportions to form what are known as Eocks. A rock may be 
 denned as a mass of inorganic matter, composed of one or more 
 minerals, having for the most part a variable chemical composi- 
 tion, with no necessarily symmetrical external form, and rang- 
 ing in cohesion from loose or feebly aggregated debris up to 
 the most solid stone. Blown sand, peat, coal, sandstone, lime- 
 stone, lava, granite, though so unlike each other, are all included 
 under the general name of Eocks. 
 
 In entering upon the study of rocks, or the division of geology 
 known as Petrography, it is desirable to be provided with such 
 helps as are needed for determining leading external charac- 
 ters; in particular, a hammer to detach fresh splinters of rock, 
 a pocket-knife for trying the hardness of minerals, a small phial 
 of dilute hydrochloric acid for detecting carbonate of lime, and 
 a pocket lens. The learner, however, must bear in mind that 
 the thorough investigation of rocks is a laborious pursuit, requir- 
 ing qualifications in chemistry and mineralogy. He must not 
 expect to be able to recognise rocks from description until he 
 has made good progress in the study. As already stated on a 
 previous page, he must examine the objects themselves, and for 
 this purpose he will find much advantage in procuring a set of 
 named specimens, and making himself familiar with such of 
 their characters as he can himself readily observe. 
 
 Great light has in recent years been thrown upon the struc- 
 ture and history of rocks by examining them with the micro- 
 
146 GEOLOGY 
 
 scope. For this purpose, a thin chip or slice of the rock to be 
 studied is ground smooth with emery and water, and after being 
 polished with flour-emery upon plate-glass, the polished side is 
 cemented with Canada balsam to a piece of glass, and the 
 other side is then ground down until the specimen is so thin 
 as to be transparent. Thin sections of rock thus prepared 
 (which can now be obtained from any good mineral-dealer) reveal 
 under the microscope the minutest kinds of rock-structure. Not 
 only can the component minerals be detected, but it is often 
 possible to tell the order in which they have appeared, and what 
 has been the probable origin and history of the rock. Some 
 illustrations of this method of investigation will be given in a 
 later part of the present chapter. It will be of advantage to 
 begin by taking note of some of the more important characters 
 of rocks,, and of the names which geologists apply to them. 
 
 Fig. 65. Section of a septarian nodule, with coprolite of a fish as a 
 
 nucleus. 
 
 Some important Terms applied to Rocks. 
 
 Sedimentary composed of sediment which may be either a 
 mechanically suspended detritus, such as mud, sand, shells, or 
 gravel; or a chemical precipitate, as rock-salt and calcareous 
 tufa. The various deposits which are accumulated on the floors 
 of lakes, in river-courses, and on the bed of the sea, axe ex- 
 amples of sedimentary rocks. 
 
 Fragmental, Clastic composed of fragments derived from 
 some previous rock. All ordinary detritus is of this nature. 
 
THE MORE IMPORTANT ROCKS 147 
 
 Concretionary composed of mineral matter which has been 
 aggregated round some centre so as to form rounded or irregu- 
 larly-shaped lumps. Some minerals, particularly pyrite (Fig. 
 64 c), marcasite, siderite, and calcite, are frequently found in 
 
 Fig. 66. Piece of oolite. 
 
 concretionary forms, especially round some organic relic, such as 
 a shell or plant (Figs. 61, 65). In alluvial clay, calcareous con- 
 cretions which often take curious imitative shapes, are known as 
 "fairy stones " (Fig. 64, a, I). 
 
 Fig. 67. Piece of pisolite. 
 
 When nodules of limestone, ironstone, or cement-stone are 
 marked internally by cracks which radiate towards, but do not 
 reach, the outside, and are filled up with calcite or other mineral, 
 they are known as Septaria or septarian nodules (Fig. 65, and 
 layer 13 in Fig. 80). 
 
148 GEOLOGY 
 
 Oolitic made up of spherical grains, each of which has 
 been formed by the deposition of successive coatings of mineral 
 matter round some grain of sand, fragment of shell, or other 
 foreign particle (Fig. 66). A rock with this structure looks 
 like fish-roe, hence the name oolite or roe-stone; but when the 
 granules are like peas, the rock becomes pisolitic (pea-stone, 
 Fig. 67). This peculiar structure is produced in water (springs, 
 lakes, or enclosed parts of the sea), wherein dissolved mineral 
 matter (usually carbonate of lime) is so abundant as to be de- 
 posited in thin pellicles round the grains of sediment that are 
 kept in motion by the current. 
 
 Stratified, Bedded arranged in layers, strata, or beds lying 
 generally parallel to each other, as in ordinary sedimentary 
 deposits (Fig. 79). 
 
 Aqueous laid down in water, comprising nearly the whole 
 of the sedimentary and stratified rocks. 
 
 Unstratified,, Massive having no arrangement in definite 
 layers or strata. Lavas and the other eruptive rocks are exam- 
 ples (Chapter XIV). 
 
 Eruptive, Igneous forced upwards in a molten or plastic 
 condition into or through the earth's crust. All lavas are Erup- 
 tive or Igneous rocks, also called Volcanic because erupted to the 
 surface by volcanoes. In the same division must be classed 
 granite and allied masses, which have been thrust through rocks 
 at some depth within the earth's crust and may not have been 
 directly connected with any volcanic eruption; such rocks are 
 sometimes called Plutonic or Hypogene. 
 
 Crystalline consisting wholly or chiefly of crystals or crys- 
 talline grains. Eocks of this nature may have arisen from (a) 
 igneous fusion, as in the case of lavas, where the minerals have 
 separated out of a molten glass, or what is called a Magma; (b) 
 aqueous solution, as where crystalline calcite forms stalactite 
 and stalagmite in a cavern; (c) sublimation, where the materials 
 have crystallised out of hot vapours, as in the vents and clefts of 
 volcanoes. 
 
 By the aid of the microscope many rocks which to the naked 
 eye show no definite structure can be shown to be wholly or 
 partially crystalline. Moreover, it can often be ascertained that 
 the crystals or crystalline grains in a rock, as they were crystal- 
 lising out of their solution, have enclosed various foreign bodies. 
 
THE MORE IMPORTANT ROCKS 
 
 149 
 
 Among the objects thus taken up are minute globules of gas, 
 which are prodigiously abundant in certain minerals in some 
 lavas ; liquids, usually water, enclosed in cavities of the crystals, 
 but not quite filling them, and leaving a minute freely-moving 
 bubble (Fig. 68) ; glass, filling globular 
 spaces, probably part of the original glassy 
 magma of the rock; crystals and crystal- 
 lites (rudimentary crystalline forms, Fig. 
 69) of other minerals. Thus a crystal, 
 which to the eye may appear quite free 
 from impurities, may be found to be full 
 of various kinds of enclosures. Obvious- 
 ly the study of these enclosures cannot 
 Pig. 68. Cavities in but throw light on the conditions under 
 quartz containing liq- which the rocks enclosing them were pro- 
 uids (magnified). 
 
 There are various types of crystalline structure which can best 
 be examined under the microscope, as Holo crystalline, composed 
 entirely of crystalline elements without any interstitial glass 
 one of the most characteristic types of this structure is found in 
 granite, hence it is sometimes termed the granitic or granitoid 
 structure; Semi-crystalline, consisting partly of crystals, but 
 with a ground mass or base which may be partly glassy or vari- 
 
 Fig. 69. Various forms of crystallites (highly magnified). 
 
 ously devitrified; Felsitic or micro felsitic composed of in- 
 definite half -effaced granules and filaments. 
 
 Glassy, Vitreous having a structure and aspect like that of 
 artificial glass. Some lavas, obsidian for example, have solidified 
 as natural glasses, and look not unlike masses of dark bottle- 
 glass. In almost all cases, however, they contain dispersed 
 crystals, crystallites, or other enclosures. These substances have 
 generally multiplied to such an extent in most lavas as to leave 
 
150 GEOLOGY 
 
 only small interstitial portions of the original glass, while in 
 many cases the glass has entirely disappeared. When a glass has 
 thus been converted into a dull, opaque, stony, or lithoid mass, or 
 into a completely crystalline substance, it is said to be devitri- 
 fied. The microscope enables us to prove many crystalline 
 eruptive rocks to have been once molten glass which by a process 
 of devitrification have been brought into their present more or 
 less crystalline condition. 
 
 Porphyritic composed of a compact or crystalline base or 
 matrix, through which are scattered conspicuous crystals much 
 larger than those of the base, and generally of some felspar. Many 
 eruptive rocks have this structure and are sometimes spoken 
 of as " porphyries." The large crystals existed in the rock while 
 
 Fig. 70. Porphyritic structure. 
 
 still in a mobile state within the earth's crust, while the minuter 
 crystals of the base were developed by a later process of crystal- 
 lisation during the consolidation of the rock. In the successive 
 zones of growth which porphyritic crystals often present, we may 
 note by the enclosed minerals some of the successive stages of 
 consolidation. 
 
 Spherulitic composed of or containing small pea-like globu- 
 lar bodies (Spfierulites) which show a minutely fibrous internal 
 structure radiating from the centre (Fig. 71, A). This struc- 
 ture is particularly observable in vitreous rocks, where it appears 
 to be one of the stages of devitrification. 
 
 Perlitic. Many vitreous rocks show a minute fissured struc- 
 ture as one of the accompaniments of devitrification. In the 
 
THE MORE IMPORTANT ROCKS 151 
 
 structure Termed perlitic the original glass has had a series of 
 reticulated and globular or spiral cracks developed in it, some- 
 times giving rise to globules composed of successive thin shells. 
 Vesicular, Cellular containing spheroidal or irregularly 
 shaped cavities. In many eruptive rocks (as in modern lavas) 
 the expansion of interstitial steam, while the mass was still in a 
 molten condition, has produced this cellular structure (Fig. 35), 
 the vesicles have usually remarkably smooth walls; they may 
 form a comparatively small part of the whole mass,, or they may 
 so increase as to make pieces of the rock capable of floating on 
 water. Where the vesicular structure is conjoined with more 
 solid parts, as in the irregular slags of an iron furnace, it may be 
 called slaggy. Where, as in the scoria? of a volcano, the cellular 
 and solid parts are in about equal proportions, and the vesicles 
 
 A 
 
 Fig. 71. Spherulites and fluxion-structure. A, Spherulites, as seen 
 under the microscope (with polarised light). B, Fluxion-structure of 
 Obsidian, as seen under the microscope. 
 
 vary greatly in numbers and size within short distances, the 
 structure may be termed scoriaceous. The lighter and more 
 froth-like varieties that can float on water are said to be pumi- 
 ceous, or to have the characters of pumice. When exposed to 
 the influence of percolating water, vesicular rocks have had their 
 vesicles filled up by the deposition of various minerals from 
 solution, especially quartz, calcite, and zeolites. These substances 
 first begin to encrust the walls of the cells, and as layer succeeds 
 layer they gradually fill the cells up (Fig. 52) ; as the cells have 
 not infrequently been elongated in one direction by the motion of 
 the rock before consolidation was completed (Fig. 37), the 
 mineral deposits in them, taking their exact moulds, appear as 
 oval or almond-shaped bodies. Hence rocks which have been 
 
152 GEOLOGY 
 
 treated in this way are called Amygdaloids, and the kernels 
 filling up the cells are known as Amygdules (Fig. 35). An 
 amygdaloidal rock, therefore, was originally a molten lava, ren- 
 dered cellular by the expansion of its absorbed steam and gases, 
 its vesicles having been subsequently filled up by the deposit in 
 them of mineral matter, often derived out of the surrounding 
 rock by the decomposing and rearranging action of percolating 
 water. 
 
 Flow-structure, Fluxion-structure an arrangement of the 
 crystallites, crystals, or particles of a rock in streaky lines, the 
 minuter forms being grouped round the larger, indicative of the 
 internal movement of the mass previous to its consolidation. 
 The lines are those in which the particles flowed past each other, 
 
 Fig. 72. Schistose structure. 
 
 the larger crystals giving rise to obstructions and eddies in the 
 movement of the smaller objects past them. This structure is 
 characteristic of many once molten rocks; it is well seen in 
 obsidian (Fig. 71, B). But it is also found in rocks which, by 
 enormous stresses within the earth's crust, have been crushed and 
 made to undergo an interstitial movement like that of the flow of 
 liquids. The most solid gneisses and granites have in this way 
 been so sheared and squeezed that their component minerals 
 have been crushed into a fine compact mass, through which the 
 streaking lines of flow are sometimes displayed with singular 
 clearness. 
 
 Mylonitic a name sometimes applied to rocks which by 
 terrestrial movement have had their original structure entirely 
 
CLASSIFICATION OF ROCKS 153 
 
 obliterated, and which now present only a dull, crushed felsitic 
 mass, sometimes partially or completely recrystallised. 
 
 Schistose, Foliated consisting of minerals that have crystal- 
 lised in approximately parallel, wavy, and irregular laminae, lay- 
 ers or folia (Fig. 72). Such rocks are called generally schists. 
 They have, in large measure, been formed by the alteration or 
 metamorphism of other rocks of various kinds by the vast ter- 
 restrial movements referred to in the foregoing paragraphs (see 
 Chapter XIII). 
 
 CLASSIFICATION OF ROCKS. 
 
 Various schemes of classification of rocks are in use among 
 geologists, some based on mode of origin, others on mineral 
 composition or structure. For the purpose of the learner, per- 
 haps the most instructive and useful arrangement is one which 
 as far as possible combines the advantages of both these systems. 
 Accordingly, in the following account of the more important 
 rocks which enter into the structure of the earth's crust, a three- 
 fold subdivision will be adopted into: (i) sedimentary rocks; 
 (ii) eruptive rocks; (iii) schistose rocks. 
 
 I. SEDIMENTARY BOCKS. 
 
 This division includes the largest number, and to the geologist 
 the most important of the rocks accessible to our notice. It 
 comprises the various deposits that arise from the decay of the 
 surface of the land and are laid down on the land or over the 
 bed of the sea, together with all those directly or indirectly due 
 to the growth of plants and animals. It thus embraces those 
 which constitute the main mass of the earth's crust so far as 
 known to us, and which contain the evidence whence the geo- 
 logical history of the earth is chiefly worked out. It is, therefore, 
 worthy of the earliest and closest "attention of the student. 
 
 Sedimentary rocks, being due to the deposition of some kind 
 of sediment or detritus, are obviously not original or primitive 
 rocks. They have all been derived from some source, the nature 
 of which, if not its actual site, can usually be determined. In 
 no case, therefore, can sedimentary rocks carry us back to the 
 beginning of things; they are themselves derivative and pre- 
 
154 GEOLOGY 
 
 suppose the existence of some older rock or material from which 
 they could be derived. 
 
 One of their most obvious characters is that, as a rule, they 
 are stratified. They have been deposited, usually in water, some- 
 times in air, layer above layer, and bed above bed, each of these 
 strata marking a particular interval in the progress of deposition 
 (Chapter XII). As regards their mode of origin, they may be 
 subdivided into three great sections: (1) fragmental of clastic, 
 composed of fragments of pre-existing rocks; (2) chemically pre- 
 cipitated, as in the deposits from mineral springs; and (3) 
 formed of the remains of organisms, as in peat and coral-rock. 
 
 Fig. 73. Brecciated structure volcanic breccia, a rock composed of 
 angular fragments' of lava, in a paste of finer volcanic debris. 
 
 (1) Fragmental or Clastic Rocks. 
 
 These are masses of mechanically-formed sediment, derived 
 from the destruction of older rocks; they vary in coherence 
 from loose sand or mud up to the most compact sandstone or 
 conglomerate; they are accumulating abundantly at the present 
 time in the beds of rivers and lakes, and on the floor of the 
 sea, and they have been formed in a similar way all over the 
 globe from the earliest periods of known geological history. 
 Some of the more frequent kinds are the following: 
 
 CLIFF-DEBRIS coarse angular rubbish, including large 
 blocks of stone, disengaged by the weather from cliffs and 
 other bare faces of rock. This kind of detritus is formed abun- 
 dantly in rugged and mountainous regions, especially where the 
 action of frost is severe ; it slides down the slopes and accumu- 
 
CLASSIFICATION OF ROCKS 155 
 
 lates at their foot, unless washed away by torrents. In 
 glacier- valleys it descends to, the ice, where, gathering into 
 moraines (Chapter VI), it is transported to lower levels. The 
 perched blocks of such valleys are some of the larger fragments 
 of this cliff-debris left stranded by the ice, and from around 
 which the smaller detritus has been washed away (Fig. 23). 
 
 SOIL, SUBSOIL, described in Chapter II, represent the result 
 of the subaerial decomposition of the surface of the land. 
 
 BRECCIA a rock composed of angular fragments. . Such a 
 rock shows that its materials have not travelled far; otherwise, 
 they would have lost their edges, and would have been more 
 or less rounded. Ordinary cliff-debris may consolidate into a 
 breccia, more especially where it falls into water and is allowed 
 
 Fig. 74. Conglomerate. 
 
 to gather on the bottom. The angular fragments shot out of a 
 volcano often accumulate into volcanic breccia (Fig. 73). A 
 rock with abundant angular fragments is said to be brecciated. 
 GRAVEL loose rounded water-worn detritus, in which the 
 pebbles range in average size between that of a small pea and 
 that of a walnut; where they are larger they form Shingle. 
 They may consist of fragments of any kind of rock, though 
 having resulted from more or less violent water-action, as a 
 rule, pieces of only the more durable stones are found in them. 
 Quartz and other siliceous materials, from their great hardness, 
 are better able to withstand the grinding to which the detritus 
 on an exposed sea-shore, or in the bed of a rapid stream, is 
 subjected. Hence quartzose and siliceous pebbles are the most 
 frequent constituents of gravel and shingle. 
 
156 GEOLOGY 
 
 CONGLOMERATE a name given to gravel and shingle when 
 they have been consolidated into stone, the pebbles being bound 
 together by some kind of paste or cementing material, which 
 may be fine hardened sand, clay, or some calcareous, siliceous, 
 or ferruginous cement (Fig. 74). As above remarked with 
 regard to gravel, the component materials of conglomerate may 
 have been derived from any kind of rock, but siliceous pebbles 
 are of most common occurrence. Different names are given to 
 conglomerates, according to the nature of the pebbles, as quartz- 
 conglomerate, flint-conglomerate, limestone-conglomerate. 
 
 SAND a name given to fine kinds of detritus, the grains of 
 which may vary from the size of a small pea 'down to minute 
 particles that can only be detected with a lens. In general, for 
 the reason already assigned in the case of gravel, the component 
 grains of sand are of quartz or of some other durable material. 
 Examined with a good magnifying glass, they are seen to be 
 usually rounded, water-worn, but sometimes angular, unworn 
 particles of indefinite shapes which, except in their smaller size, 
 resemble those of gravel-stones. Sand may be formed by the 
 disintegration of the surface of rocks exposed to the weather, 
 more especially in dry climates, where there is a great difference 
 between the temperature of the day and the night. The 
 loosened particles are blown away by the wind, and may be 
 heaped up into great sand-wastes, as in the tracts known as 
 Deserts. On a sea-coast, where a sandy beach is liable to be 
 laid bare and exposed to be dried between tides by breezes blow- 
 ing from the sea, the upper particles of sand are lifted up by 
 the wind and borne away landward, to be piled up into dunes. 
 In some places the materials of sand are derived mainly 
 from the remains of calcareous sea-weeds, shells, corallines, and 
 other calcareous organisms exposed to the pounding action of 
 the" surf. A sand composed of such materials speedily hardens 
 into a more or less coherent and even compact limestone, for 
 rain falling on it dissolves some carbonate of lime which, being 
 immediately deposited again, as the moisture evaporates, coats 
 the grains of sand and cements them together. At Bermuda, 
 as already stated, all the rock above sea-level has been formed 
 in this way, and some of it is hard enough to make a good 
 stone for building. The ordinary siliceous or quartzose sand 
 
CLASSIFICATION OF ROCKS 157 
 
 remains loose, unless its grains are made to cohere by some 
 kind of cement, when it becomes sandstone. 
 
 SANDSTONE consolidated sand. The grains are chiefly 
 quartz, but may include particles of any other mineral or rock; 
 they are bound together by some kind of cement which has 
 either been laid down with them at the time of their deposi- 
 tion, or has subsequently been introduced by water permeating 
 the sand. The cementing material may be argillaceous that 
 is, some kind of clay; or calcareous , consisting of carbonate of 
 lime; or ferruginous, composed mainly of peroxide of iron; or 
 siliceous, where silica has been deposited in the interstices of 
 the mass. The colours of sandstone vary chiefly with the 
 nature of this cementing material. The hydrous peroxide of iron 
 colours them shades of yellow and brown ; the anhydrous peroxide 
 of iron gives them different hues of red; the mineral glauconite 
 tints them a greenish hue. Some varieties of sandstone are 
 named after a conspicuous component or structure; thus 
 micaceous sandstone is distinguished by abundant spangles of 
 mica deposited along the bedding planes, whereby the rock can 
 be split up into thin layers; freestone a thick-bedded sand- 
 stone that does not tend to split up in any one direction, and 
 can therefore be cut into blocks of any size and form; glau- 
 conitic sandstone (green sand), containing green grains and 
 kernels of glauconite; quartzose sandstone, conspicuously com- 
 posed of quartz-grains; grit a sandstone formed of coarse or 
 sharp, somewhat angular grains of quartz. 
 
 GRAYWACKE a grayish, compact, granular rock, composed 
 of rounded or subangular grains of quartz and other minerals 
 or rocks, cemented together in a compact paste; it differs from 
 sandstone chiefly in its darker colour, in the proportion of 
 other grains than those of quartz, and in the presence of a 
 tough cement. 
 
 The rocks above enumerated represent the coarser and more 
 durable kinds of detritus derived from the weathering of the 
 surface of the land ; but during the progress of the decomposi- 
 tion from which these materials are derived some of the com- 
 ponent ingredients of the rocks decay into clay, or what is 
 called argillaceous sediment. This more particularly occurs in 
 the case of felspars and other aluminous silicates, the decom- 
 position of which produces minute particles capable of being 
 
158 GEOLOGY 
 
 lifted up and carried a great distance by running water. Hence 
 argillaceous sediment, being commonly finer in grain, travels 
 farther, on the whole, than quartzose sediment; and beds of 
 clay denote, generally, deeper and stiller water than beds of 
 sand. 
 
 CLAY a fine-grained argillaceous substance, derived from 
 the decay and hydration of aluminous silicates, white when 
 pure, but usually mixed with impurities, which impart to it 
 various shades of gray, green, brown, red, purple, or blue; it 
 usually contains interstitial water, and when wet can be kneaded 
 between the fingers ; when dry it is soft and friable, and adheres 
 to the tongue. Shaken with water it becomes Mud; even a 
 small quantity will make a glass of water turbid, so fine are 
 the particles of which it is composed. 
 
 KAOLIN the name given to the white purer forms of clay, 
 resulting from the decomposition of the felspars of granite or 
 similar rocks; it is sometimes called China-clay, from its use 
 in the manufacture of porcelain. 
 
 FIRE-CLAY a white, gray, yellow, or black clay, nearly free 
 from alkalies and iron, and capable of standing a great heat 
 without fusing; it is abundantly found underneath coal-seams, 
 where it represents the ancient soil on which the plants grew 
 that have been converted into coal. 
 
 BRICK-CLAY a name commonly applied to any clay, loam, 
 or earth from which bricks can be made. Such deposits are 
 always more or less sandy and impure clays; in the south of 
 England they have largely arisen from the prolonged sub aerial 
 waste of the Cretaceous and Tertiary formations. 
 
 MUDSTONE a compact solidified clay or clay-rock, having 
 little or no tendency to split into thin laminae. 
 
 SHALE clay that has become hard and splits into thin 
 laminae which lie parallel with the planes of deposit (Fig. 79). 
 A thoroughly fissile shale can be subdivided into leaves as thin 
 as fine cardboard. This is the common form which the clays 
 of the older geological formations have assumed. Gradations 
 can be traced from shale into other sedimentary rocks; thus, 
 by additions of sand into fissile sandstones, of calcareous cement 
 into limestone, of carbonate of iron into ironstone, of car- 
 bonaceous matter into coal. These passages are interesting as 
 indications of the conditions under which the rocks were formed. 
 
CLASSIFICATION OF ROCKS 159 
 
 Where, for example, shale shades off into coral-limestone, we 
 see that mud gathered over one part of the sea-floor, while 
 not far off, probably in clearer water, corals flourished and 
 built up a limestone out of their remains. 
 
 LOESS a pale somewhat calcareous and sandy clay, found 
 in regions where it has probably been accumulated by the drift- 
 ing action of the wind. It is sufficiently coherent to be capable 
 of excavation into tunnels and passages, and in China is even 
 dug out into houses and subterranean villages. It occupies parts 
 of the valleys of the Ehine, Danube, Mississippi, and other 
 large rivers, but also crosses watersheds. 
 
 Fragmental rocks of volcanic origin may be enumerated 
 here. They consist partly of materials ejected in fragmentary 
 form from volcanic vents, and partly of the detritus derived 
 from the disintegration of volcanic rocks already erupted to 
 the surface. They are comprised under the general name of 
 Tuff. 
 
 BOMBS round elliptical or discoidal pieces of lava which 
 have been ejected in a molten state from an active vent, and 
 have acquired their form from rapid rotation in the air during 
 ascent and descent. They are often very cellular or even quite 
 empty inside. Where the large ejected stones are of irregular 
 forms, and appear to have been thrown out in an already solidi- 
 fied condition, as from the consolidated crust of the lava-plug, 
 or from the sides of the funnel or crater, they are called 
 Volcanic Blocks. 
 
 LAPILLI ejected pieces of lava, usually vesicular or porous, 
 from the size of a pea to a walnut (Fig. 73). 
 
 VOLCANIC ASH ; the fine dust produced by the explosion of 
 the superheated steam absorbed in molten lava. Under the 
 microscope, it is often found to consist of minute grains of 
 glass, and in such cases, shows that the lava from which it was 
 derived, rose from below in the condition of a liquid glassy 
 magma. In other instances, it is made up of the crystallites 
 and crystals that arose during the devitrification of the glass. 
 It consolidates into a more or less coherent mass, which is 
 known as Tuff, and which may receive some distinctive name 
 according to the nature of the lava that has supplied it, as 
 Basalt-tuff and Trachyte-tuff. Most tuffs contain angular and 
 
160 GEOLOGY 
 
 vesicular pieces of lava, and sometimes pass into coarse breccias 
 (Volcanic Breccia). In many cases, they enclose the remains 
 of plants and animals which, if of terrestrial kinds, indicate 
 that the eruptions took place on land; if of marine species, 
 that the volcanoes were probably submarine. 
 
 AGGLOMERATE a coarse, usually unstratified accumulation 
 of blocks of lava and other rocks, not infrequently filling up 
 the chimney or neck of a volcanic vent. 
 
 (2) Rocks formed by Chemical Precipitation. 
 
 In Chapter V it was pointed out that all natural waters 
 contain in solution invisible mineral matter which they have 
 dissolved out of the rocks of the earth's crust, and that the 
 quantity of this material is sometimes so great that it is pre- 
 cipitated into visible form as the water evaporates. The sub- 
 stance most abundantly dissolved and deposited is Carbonate 
 of lime. Others of frequent occurrence are Sulphate of lime. 
 Chloride of sodium, Silica, Carbonate of magnesia, and various 
 salts of iron. Among the rocks of the earth's crust, considerable 
 masses of these substances have been piled up by chemical 
 precipitation. 
 
 LIMESTONE compact or crystalline calcium-carbonate (car- 
 bonate of lime) which may be nearly pure, or may contain 
 sand, clay, or other impurity, and may consequently pass into 
 sandstone, shale, or other sedimentary rock. Probably the great 
 majority of the limestones in the earth's crust have been formed 
 by the agency of animals, as more particularly referred to at 
 p. 164. We are here concerned only with those which have 
 been deposited from chemical solution. The most familiar 
 example of this kind of limestone is afforded by stalactites 
 and stalagmite, which have already been described (Chapter V 
 and Fig. 20). Large masses of it have been deposited by 
 calcareous springs and also by streams. At first, it is a fine 
 white milky precipitate, but gradually crystals of calcite shape 
 themselves and grow out of it, with their vertical axes usually 
 at right angles to the surface of deposit. In a vertical stalactite, 
 consequently, the prisms radiate horizontally from the centre 
 outwards; on a horizontal surface of stalagmite they diverge 
 perpendicular to the floor. A mass of limestone, not originally 
 
CLASSIFICATION OF ROCKS 161 
 
 crystalline, may thus acquire a thoroughly crystalline internal 
 structure by the action of infiltrating water in dissolving the 
 carbonate of lime and redepositing it in a crystalline condition. 
 
 Limestones vary greatly in texture and purity. Some are 
 snow-white and distinctly crystalline; others are gray, blue, 
 yellow, or brown, dull and compact, and full of various im- 
 purities. They may usually be detected by the ease with which 
 they can be scratched, and their copious effervescence when a 
 drop of weak acid is put on the scratched surface. Pure lime- 
 stone dissolves entirely in hydrochloric acid, so that the amount 
 of residue is an indication of the proportion of insoluble im- 
 purity. Among the varieties of limestone the following may 
 be named: Oolite, a limestone composed of minute spherical 
 grains like the roe of a fish, each grain being composed of 
 concentrically deposited layers or shells of calcite (Fig. 66) ; 
 Pisolite, a similar rock, where the grains are as large as peas 
 (Fig. 67) ; Travertine or calcareous tufa, a white porous 
 crumbling rock which, by infiltration of carbonate of lime, 
 may acquire a compact texture, and become suitable for use as 
 building stone; Hydraulic limestone, containing 10 to 30 
 per cent of fine sand or clay, and having the property, after 
 being burnt, of hardening under water into a firm compact 
 mortar. 
 
 DOLOMITE, MAGNESIAS LIMESTOKE this substance has been 
 already referred to as a mineral (p. 140) ; but it also occurs 
 in large masses as a white or yellowish crystalline or compact 
 rock. The white varieties look like marble. The yellow and 
 brown kinds contain various impurities, and are coloured by 
 iron-oxide. Dolomite differs from limestone in its greater 
 hardness and feebler solubility in acid, in its frequently cellular 
 or cavernous texture, in its tendency to assume spherical, grape- 
 shaped, or other irregular concretionary forms (Fig. 75), and 
 in its proneness to crumble down into loose crystals. It occurs 
 in beds, not uncommonly associated with gypsum and rock-salt, 
 and in such conditions it may have been deposited first as 
 limestone which, by the chemical action of the magnesian salts 
 in the saline water, had its carbonate of lime partially replaced 
 by carbonate of magnesia. It is also found in irregular bands 
 traversing limestone which, probably by the influence of per- 
 
1G2 GEOLOGY 
 
 colating water containing carbonate of magnesia in solution, 
 has been changed into dolomite. 
 
 GYPSUM is not only a mineral (p. 140) but also a rock, white, 
 gray, brown, or reddish in colour, granular to compact, some- 
 times fibrous or coarsely crystalline in texture. It consists of 
 sulphate of lime, is easily scratched with the nail, and is not 
 affected by acids, being thus readily distinguishable from lime- 
 stone. It is found in beds or veins, especially associated with 
 layers of red clay and rock-salt, and in these cases has evidently 
 resulted from the evaporation of water containing it in solution, 
 such as that of the sea. The lime-sulphate being less soluble 
 
 Fig. 75. Concretionary forms assumed by Dolomite, Magnesian Lime- 
 stone, Durham. 
 
 than the other constituents is precipitated first. Hence in a 
 thick series of alternations of beds of gypsum (or anhydrite) 
 and rock-salt, each layer of sulphate of lime indicates a new 
 supply of water into the natural reservoirs where the evapora- 
 tion took place. The overlying bed of salt, usually much thicker 
 than the gypsum, points to the condensation of the water into 
 a strong brine, from which the salt was ultimately precipitated. 
 And the next sheet of sulphate of lime tells how, by the break- 
 ing down of the barrier, renewed supplies of salt water were 
 poured into the basin. 
 
CLASSIFICATION OF ROCKS 103 
 
 BOCK-SALT occurs in beds or layers, from less than an inch 
 to hundreds or even thousands of feet in thickness. One mass 
 of salt in Galicia is more than 4600 feet thick, and a still 
 thicker mass occurs near Berlin. When quite pure, rock-salt 
 is clear and colourless, but it is usually more or less mixed with 
 impurities, particularly with red clay, and in association with 
 beds of gypsum, as above remarked. It has been formed in 
 inland salt lakes or basins by the evaporation and concentration 
 of the saline water. It is being deposited at the present time 
 in the Dead Sea., the Great Salt Lake, and the salt lakes so 
 frequent in the desert regions of continents, where the drainage 
 does not flow outwards to the sea. 
 
 IRONSTONE. Various minerals are included under this name 
 as large rock-masses. One of the most important of them is 
 Hcematite (p. 133), which occurs in large beds and veins, as 
 well as filling up caverns in limestone. Limonite or bog-iron 
 ore is formed in lakes and marshy places (p. 134), and occurs 
 in beds among other sedimentary accumulations. Magnetite 
 (p. 134) is found in beds and huge wedge-shaped masses among 
 various crystalline rocks, as in Scandinavia, where it sometimes 
 forms an entire mountain. Carbonate of iron (Siderite, Sphae- 
 rosiderite, Clay-ironstone) occurs in concretions and beds among 
 argillaceous deposits (Figs. 61, 65, and p. 140). In the Coal- 
 measures, for example, it is largely developed, much of the 
 iron of Britain being obtained from this source. As many iron- 
 stones are largely due to the influence of plants and animals, 
 the rock is alluded to again on p. 166. 
 
 SILICEOUS SINTER a white powdery to compact and flinty 
 deposit from the hot water of springs in volcanic districts, con- 
 sisting of 84 to 91 per cent of silica, with small proportions of 
 alumina, peroxide of iron, lime, magnesia, and alkali, and from 
 5 to 8 per cent of water. It accumulates in basin-shaped 
 cavities round the mouths of hot springs and geysers, and 
 sometimes forms extensive terraces and mounds, as at the geyser 
 regions of Iceland, Wyoming, and New Zealand. 
 
 VEIN-QUARTZ a massive form of quartz, which occurs in 
 thin veins and in broad dyke-like reefs, traversing especially 
 the older rocks. 
 
164 GEOLOGY 
 
 (3) Rocks formed of the Remains of Plants or Animals. 
 
 In Chapter VIII an account was given of t the manner in 
 which extensive accumulations are now being formed of the 
 remains of plants and animals. Similar deposits have con- 
 stantly been accumulated from an early period in the history 
 of the earth. Eegarding them with reference to their mode 
 of origin, we observe that in some cases they have been piled 
 up by the unremitting growth and decay of organisms upon 
 the same site. In a thick coral-reef, for example, the living 
 corals now building on the surface are the descendants of those 
 whose skeletons form the coral-rock far below them. In 
 other cases, the remains of the organisms are broken up and 
 carried along by moving water, which deposits them elsewhere 
 as a sediment. Strictly speaking, these last deposits are frag- 
 mental, and might be classed with those described at p. 154; 
 they pass into ordinary sand, sandstone, clay, or shale. But- 
 it will be more convenient to class together all the rocks which 
 consist mainly of organic remains, whether they have been 
 directly built up by the organisms, or have only been formed 
 out of their detrital remains. 
 
 LIMESTONE. As carbonate of lime is so largely secreted by 
 animals in their hard parts which are more or less durable, 
 it is naturally the most common substance among rocks of 
 organic origin. The limestones that form so large a proportion 
 of the stratified rocks of the earth's crust have been, for the 
 most part, formed out of the remains of marine animals. The 
 following are some of the more important or interesting varieties 
 of this rock : Shell-marl., a soft white earthy crumbling de- 
 posit which is formed chiefly of fresh-water shells; by sub- 
 sequent infiltration it may be hardened into a compact stone, 
 when it is known as fresh-water limestone; Calcareous sand 
 a mass of broken-up shells, calcareous algae, and other calcareous 
 organisms which are often cemented by percolating water into 
 solid stone ; Coral rock a limestone formed by the continuous 
 growth o'f corals and cemented into a solid compact and even 
 crystalline rock by the washing of calcareous mud into its 
 interstices and the permeation of sea-water and rain-water 
 through it, whereby crystalline calcite is deposited within it; 
 Chalk a soft and white rock, which soils the fingers, formed 
 
CLASSIFICATION OF ROCKS 165 
 
 of a fine calcareous powder of remains of foraminifera, shells, 
 etc. (see Ooze, Ch.VIII) ; Crinoidal limestone composed chief- 
 ly of the calcareous joints of the marine creatures known as 
 crinoids, with foraminifera, shells, corals, and other organisms. 
 A limestone composed in great part of organic remains may 
 show little trace of its origin on a fresh fracture of the stone; 
 but a weathered surface will often reveal its true nature, the 
 fossils being better able to withstand the action of the atmos- 
 phere than the surrounding matrix which is accordingly re- 
 moved, leaving them standing out in relief (Fig. 76). 
 
 PEAT a yellow, brown, or black fibrous mass of compressed 
 and somewhat altered vegetation. It occurs in boggy places 
 in temperate latitudes where it largely consists of bog-mosses 
 
 Fig. 76. Weathered surface of crinoidal limestone. 
 
 and also of other marshy plants. Its upper parts are loose 
 and full of the roots of living plants, while the bottom portions 
 may be compact and black like clay, and with little trace of 
 vegetable structure. 
 
 LIGNITE or Brown Coal is a more compressed and chemically 
 changed condition of vegetation. It varies in colour from 
 yellow to deep brown or black, and may be regarded as an inter- 
 mediate stage between peat and coal. -It occurs in beds inter- 
 calated between layers of shale, clay, and sandstone. ' 
 
 COAL a compact, brittle, black, or dark brown stone, formed 
 of mineralised vegetation, and found in beds or seams usually 
 resting on clay, and covered with sandstone, shale, etc. (see 
 Figs. 79 and 140). There are many varieties of coal, differing 
 
166 GEOLOGY 
 
 from each other in the relative proportions of their constituents. 
 Coking-coal, such as is ordinarily used in England, contains 
 from 75 to 80 per cent of carbon, 5 or 6 per cent of hydrogen, 
 and 10 or 12 per cent of oxygen, with some sulphur and other 
 impurities. Anthracite, the most thoroughly mineralised con- 
 dition of vegetation, is a hard, brittle, lustrous substance, from 
 which the hydrogen and oxygen have been in great measure 
 driven away, leaving 90 per cent or more of carbon. 
 
 IRONSTONE. Reference has been made before to ironstone 
 iprecipitated from chemical solution. This precipitation is often 
 caused through the medium of decomposing organic matter. 
 Organic acids, produced by the decay of plants in marshy places 
 and shallow lakes, attack the salts of iron contained in the 
 rocks 1 or detritus of the bottom, and remove the iron in solution. 
 On exposure, the iron oxidises and is thrown down as a yellow 
 or brown precipitate of limonite or bog-iron ore (p. 134), 
 which is found in layers and concretions. Clay-ironstone, com- 
 posed of a mixture of carbonate of iron, with clay and carbona- 
 ceous matter, occurs abundantly both as nodules and in layers, 
 with remains of plants, shells, fishes, etc., in the Coal-measures 
 (Figs. 61, 65, and bed 13 in Fig. 80), and has, no doubt, been 
 also formed through the agency of organic acids which, passing 
 into carbonic acid, have given rise to the solution and subse- 
 quent deposit of the iron as carbonate mingled with mud and 
 with entombed plants and animals. 
 
 FLINT. Some siliceous deposits, due to organic agency, have 
 been already referred to in Chap. VIII. Besides these, mention 
 may be made of Flint, which occurs as dark lumps and irregular 
 nodular sheets in chalk and other limestones, frequently enclos- 
 ing urchins, shells, and other organisms, which are sometimes 
 converted into flint. Its mode or origin is not yet thoroughly 
 understood, but there is reason to regard it as due to the 
 abstraction of silica from sea-water, either directly, by such 
 animals as sponges, or indirectly, by the decomposition of ani- 
 mal remains. Chert is a more impure siliceous aggregate found 
 under similar conditions, especially among the older limestones. 
 
 GUANO a brown, light, powdery deposit, formed of the 
 droppings of sea-birds in rainless tracts of the west coasts of 
 South America and Africa. Containing much phosphate of 
 
CLASSIFICATION OF ROCKS 1G7 
 
 lime as well as ammoniacal salts, it has great commercial value 
 as an important manure. 
 
 BONE-BEDS deposits composed of fragmentary or entire 
 bones of fish, reptiles, or higher animals, as in the well-known 
 bone-bed of the Khaetic series (Ch. XXII). The floors of some 
 caverns are covered with stalagmite, so full of pieces of the 
 bones of cave-bears, hyaenas, and other extinct and living species, 
 as to be called Bone-breccia. Layers of stone, full of the copro- 
 lites (fossil excrement) or of the rolled bones of various verte- 
 brate animals have, in recent years, been largely worked as 
 sources of phosphate of lime for the manufacture of artificial 
 manures. 
 
 II. ERUPTIVE EOCKS. 
 
 Under this division are grouped all the massive rocks which 
 have been erupted from underneath into the crust or to the 
 surface of the earth. They are composed chiefly of silicates 
 of alumina, magnesia, lime, potash, and soda, with different 
 proportions of free silica, magnetic or other oxide of iron, and 
 phosphate of lime. The principal silicate is generally some 
 felspar, the number of eruptive rocks without felspar being 
 comparatively small. The felspar is, in different rocks, con- 
 joined with mica, hornblende, augite, magnetite, or other 
 minerals. 
 
 No perfectly satisfactory classification of the eruptive rocks 
 has yet been devised; they have been grouped according to 
 their presumed mode of origin, some being classed as plutonic 
 or Jiypogene., from their supposed origin, deep within the earth's 
 crust, others as volcanic, from having been ejected by volcanoes. 
 They have likewise been arranged according to their chemical 
 composition, and also with reference to their internal structure. 
 In the following enumeration of some of the more abundant 
 and important varieties, it may be enough to adopt an arrange- 
 ment in three sections, according to the nature of the pre- 
 dominant silicate: viz. (1) Orthoclase rocks; (2) Plagioclase 
 rocks; and (3) Olivine and Serpentine rocks. It has already 
 been pointed out that the original condition of many lavas and 
 other eruptive rocks has been that of molten glass, their present 
 stony structure being due to the more or less complete devitrifi- 
 
108 GEOLOGY 
 
 cation and disappearance of the glass by the development of 
 crystals and crystallites out of it during the process of cooling 
 and consolidation (p. 149). Though there is no evidence that 
 all crystalline eruptive rocks have once been in the state of 
 molten glass, it may be useful to begin with the vitreous va- 
 rieties, which we know to represent the earliest forms of many 
 that are now quite crystalline. 
 
 (1) Orthoclase Rocks. 
 
 In this section the prevalent silicate is Orthoclase, either in 
 its common dull, white, or pink form, or in the glassy condition 
 (sanidine). In many of the rocks, free quartz occurs either in 
 irregular crystalline blebs or in definite crystals, which fre- 
 quently take the form of double pyramids. Among other 
 minerals, horn-blende, white and black mica, and apatite are 
 of common occurrence. The rocks of this division are the 
 most acid of the eruptive series that is, they contain the 
 largest proportion of silica or silicic acid, sometimes more than 
 75 per cent. Some of them (granite) are only found as masses 
 that have consolidated deep beneath the surface; others 
 (trachyte, rhyolite, obsidian) are abundant as superficial vol- 
 canic products. 
 
 OBSIDIAN a black, brown, or greenish (sometimes yellow, 
 blue, or red) glass, breaking with a shell-like or conclwidal 
 fracture and into sharp splinters, which are translucent at the 
 edges. .Examined in a thin section under the microscope, the 
 rock is found to , owe its usual blackness to the presence of 
 minute opaque crystallites (Fig. 69) which are crowded through 
 it, not infrequently drawn out into streaky lines and curving 
 round any larger crystal that may be embedded in the mass 
 (Fig. 71 B). These arrangements, called flow-structure (p. 
 152, have evidently been caused by the movement of the rock 
 while still in a fused state, the crystallites and other objects 
 being borne onward by the currents of molten glass. In some 
 obsidians, little spherulites of a dull gray enamel-like substance 
 have made their appearance as stages in the devitrification of 
 the rock (Fig. 71) ; but the mass has consolidated before the 
 stony condition could be completed. In other instances, the 
 whole rock has passed into a stony enamel-like mass with 
 
CLASSIFICATION OF ROCKS 169 
 
 perlitic structure (pearlstone, p. 150). Where a still molten 
 obsidian has been frothed up by the expansion of steam or 
 gas through it, so as to become a spongy cellular substance* 
 which will float on water, it is called pumice. Obsidian occurs 
 in many volcanic regions, sometimes as streams of lava which 
 have been poured forth at the surface, sometimes in dykes and 
 veins, and often in fragments ejected with the other detritus 
 that now forms tuff. 
 
 TRACHYTE a compact porphyritic rock, consisting mainly 
 of orthoclase (sanidine), with some plagioclase and usually with 
 some hornblende, or with augite, mica, magnetite, or other 
 minerals ; having a peculiar matrix which, under the microscope, 
 is found to consist mainly of minute felspar-crystallites. Large 
 crystals of orthoclase (sanidine) are frequent, and also scales 
 of dark mica. This rock is found abundantly among some of 
 the younger volcanic regions of the world, where it occurs in 
 lava streams and also in intrusive sheets and dykes. QUARTZ- 
 TRACHYTE (Liparite, Rhyolite) is a rock composed of a com- 
 pact, often rough and somewhat porous base, through which 
 are scattered crystals of felspar and blebs of quartz, often also 
 with hornblende and mica. 
 
 FELSITE an exceedingly close-grained rock, composed of an 
 intimate mixture of quartz and orthoclase. The felspar often 
 occurs as large disseminated crystals, giving the porphyritic 
 structure. Where the quartz appears as distinct blebs or crystals 
 (sometimes double pyramids) the rock becomes QUARTZ- 
 PORPHYRY. The felsites and quartz-porphyries play an impor- 
 tant part among the eruptive rocks of older geological, time, 
 occurring both in the form of lavas erupted to the surface and 
 of intrusive masses that have consolidated below ground. 
 Many of them can be proved to have been originally in the 
 condition of molten glass which has been devitrified. Eocks 
 which show the characteristic closeness of grain characteristic 
 of the felsites are said to be felsitic or to have a felsitic ground 
 mass. 
 
 SYENITE a thoroughly crystalline rock, consisting essen- 
 tially of orthoclase and hornblende, and distinguished from 
 granite chiefly by the absence or small amount of quartz. It 
 occurs in bosses and veins which have been erupted into older 
 rocks. 
 
170 GEOLOGY 
 
 GRANITE a thoroughly crystalline (holo-crystalline) com- 
 pound of felspar, quartz, and mica, the individual minerals 
 being large enough to be distinctly recognised by the naked eye. 
 Sometimes large crystals of felspar are porphyritically scattered 
 through the rock. Granite occurs in large eruptive masses 
 which have been intruded into many different kinds of rocks, 
 also in smaller bosses and veins. Round the outside of a mass 
 of granite there frequently diverge from it dykes and veins 
 which, where they are of great width, may show the usual 
 granitic structure; but which, when of small dimensions, are 
 apt to appear as felsite or quartz-porphyry. There can be no 
 doubt that such fine-grained veins are actually portions of the 
 same mass of rock as the granite, so that granite and felsite 
 
 Fig. 77. Group of crystals of felspar, quartz, and mica, from a cavity 
 in the Mourne Mountain granite. 
 
 or quartz-porphyry are only different conditions of the same 
 substance, the differences being probably due to variations in 
 the circumstances under which the cooling and consolidation 
 took place. In the crystalline-granular structure so distinctive 
 of granite (granitic or granitoid,, p. 149) the constituent min- 
 erals have not had room to assume perfect crystallised shapes, 
 but occasionally they have been able to shoot out in perfect 
 crystals where cavities occur. Fig. 77, for example, shows a 
 group of the ordinary crystals of this rock which have crystal- 
 lised in a cavity of the granite of the Mourne Mountains, 
 Ireland. It is in such cavities also that the rarer minerals of 
 this rock, such as topaz and beryl, may be looked for. 
 
CLASSIFICATION OF ROCKS 171 
 
 (2) Plagiodase Rocks. 
 
 In this section the felspar is some variety of plagioclase, and 
 the other most frequent silicate is either augite or hornblende. 
 Though free quartz occurs in some of the rocks, they contain 
 generally so much less silica than the orthoclase rocks that 
 instead of being acid they are commonly basic compounds. A 
 range of texture can be observed in them similar to that char- 
 acteristic of the orthoclase series, from a true glass up to a 
 thoroughly crystalline granitoid rock. Some of them, more 
 especially the coarsely crystalline varieties, are probably of 
 deep-seated origin; others (and these include the great ma- 
 jority) are truly volcanic ejections which have risen in volcanic 
 pipes and fissures, and have been poured forth at the surface 
 as actual lava-streams. 
 
 BASALT-ROCKS a group of rocks consisting of plagioclase, 
 augite, olivine, and magnetite or titaniferous iron, to which 
 apatite and other minerals may be added. These rocks range 
 in texture from a black glass up to a coarsely crystalline mass 
 wherein the component minerals are distinctly visible to the 
 naked eye. Different names are employed to distinguish these 
 varieties. Basalt-glass (Tachylyte, Hyalomelan) is a general 
 epithet to denote the vitreous varieties. These are particularly 
 to be observed along the edges of dykes and other intrusive 
 masses, where they represent the outer surface of the basalt that 
 was suddenly chilled and consolidated by coming in contact with 
 the cold walls of the vent or fissure into which it was injected, 
 and where they no doubt show what was the original state of 
 the whole basalt before devitrification converted the rock into 
 its present crystalline structure (see Chap. IX). Basalt 
 a black, compact, heavy, homogeneous rock, breaking with a 
 conchoidal fracture, showing sometimes large porphyritic 
 crystals of plagioclase, olivine, or augite, but too fine-grained 
 for the component minerals of the base to be determined except 
 with the microscope. The coarser varieties, where the minerals 
 can be recognised with the naked eye, are known as Dolerite. 
 The basalt-rocks are pre-eminently volcanic lavas, occurring 
 both as intrusive masses that consolidated underground, and as 
 sheets that were poured out in successive streams at the surface. 
 The black, compact kinds (true basalt) are particularly prone 
 
172 
 
 GEOLOGY 
 
CLASSIFICATION OF ROCKS 173 
 
 to assume columnar forms (Fig. 78), whence columnar rocks 
 are sometimes spoken of as basaltic. In some varieties of basalt 
 the mineral leucite takes the part of the plagioclase; and in 
 others this is done by another mineral, nepheline. 
 
 DIABASE a name given to some ancient basalt-rocks in 
 which, owing to alteration of their augite or olivine, a greenish 
 chloritic discoloration has often taken place. The lavas of 
 early geological time are to a large extent diabase. 
 
 ANDESITE is closely allied to basalt; but contains no olivine. 
 It sometimes includes free quartz, and hornblende may be. sub- 
 stituted in it for augite. Hornblende-andesite and Augite-ande- 
 site are lavas which have been extensively erupted in later 
 geological time. 
 
 DIORITE a crystalline aggregate of plagioclase and horn- 
 blende, usually with magnetite and apatite, sometimes with 
 augite and mica. The hornblende is black or dark green and 
 often more or less decomposed, giving rise to a greenish chloritic 
 discoloration of the felspar. From its prevalent green colour, 
 the rock was formerly known as "greenstone." It occurs in 
 intrusive masses, and seems generally if not always to have 
 consolidated below ground instead of being poured out at the 
 surface. 
 
 GABBRO, DIALLAGE-ROCK a thoroughly crystalline granitoid 
 aggregate of plagioclase and the variety of augite known as 
 diallage, which appears in distinct brown or greenish crystals, 
 with a peculiar metalloidal or pearly lustre; it is found in 
 bosses associated with granite, gneiss, etc., and also sometimes 
 with volcanic rocks in centres of eruption. 
 
 (3) Olivine and Serpentine Rocks. 
 
 In this group may be included a comparatively small number 
 of rocks which consist principally of olivine, and which by 
 gradual alteration pass into serpentine (Fig. 58). OLIVINE- 
 ROCKS (Peridotites) are liable to remarkably rapid changes 
 of texture and composition. In some places they are mainly 
 made up of olivine, augite, or hornblende, magnetite, and brown 
 mica, but some of these minerals may disappear and some 
 felspar may take their place. They are intrusive masses which 
 appear to have been generally injected into the crust in con- 
 
174 GEOLOGY 
 
 nection with volcanic eruptions, rather than to have been poured 
 out at the surface in true lava-streams. 
 
 SERPENTINE a compact, dull, or faintly glimmering rock, 
 with a general dark dirty green colour, variously mottled, 
 greasy to the touch, easily scratched, and giving a white powder 
 which does not effervesce with acids. It is a massive form of 
 the mineral serpentine described on p. 139, frequently con- 
 taining disseminated crystals of the minerals bronzite, enstatite, 
 and chromic iron, and veins of a delicately fibrous silky variety 
 of serpentine known as chrysotile. Many serpentines were 
 originally olivine-rocks which, by hydration and alteration of 
 their magnesian silicates, have assumed their present characters. 
 Serpentine occurs in bosses, dykes, and veins, which were evi- 
 dently of eruptive origin and were at first probably olivine- 
 rocks; it is also found in thick beds associated with limestones 
 and crystalline schists, where it may be a metamorphosed sedi- 
 mentary rock. 
 
 III. THE SCHISTS AND THEIR ACCOMPANIMENTS. 
 
 This section includes a remarkable series of rocks of which 
 the leading feature is the possession of a schistose or foliated 
 character (Fig. 72 )\ They are, in their more typical varieties, 
 distinctly crystalline. Some of them shade off into ordinary 
 fragmental rocks, such as shale and sandstone; others agree 
 in chemical and mineral composition with some of the eruptive 
 rocks already enumerated, into which they may often be traced 
 by imperceptible gradations. 
 
 In the schists, therefore, we see an assemblage of rocks which, 
 though possessing distinct characters of their own, may yet be 
 observed to shade off into fragmental rocks on the one side, 
 and into eruptive rocks on the other. In Chapter XIII some 
 further account of them will be given, with special reference 
 to their probable origin, and to the grounds on which they have 
 been regarded as metamorphic or altered rocks. For the pres- 
 ent, in taking notice of their composition and structure, it will 
 be enough to state that in many cases they can be shown to 
 be more or less altered and crystalline transformations of what 
 were originally sedimentary rocks; and that in other instances 
 they represent original crystalline eruptive masses, which have 
 
CLASSIFICATION OF ROCKS 175 
 
 been subjected to such enormous pressure and shearing, that a 
 foliated structure and recrystallisation of minerals have been 
 superinduced in them. The essential feature which unites 
 masses of such different origin is the possession of that common 
 schistose structure which they have derived from having all 
 been alike subjected to the same kind of intense terrestrial 
 movements. 
 
 CLAY-SLATE a hard fissile clay-rock, through which minute 
 scales of mica and crystals or crystallites of other minerals have 
 been developed; generally bluish-gray to purple or green, and 
 splitting into thin parallel leaves. As this rock often contains 
 remains of marine animals and plants, and is interstratified 
 with bands of sandstone, grit, conglomerate, and limestone, it 
 was undoubtedly at first in the condition of soft mud on the 
 sea-bottom. Sometimes the organic remains in it are so curi- 
 ously elongated or distorted in one general direction as to show 
 that the rock has been drawn out by intense pressure and 
 shearing (Figs. 98, 103,. 104). The planes along which clay- 
 slate splits are generally independent of the original surfaces 
 of deposit, sometimes cross these at a right angle, and have 
 been superinduced by mechanical movements (Cleavage), as 
 explained in Chapter XIII. Different varieties of clay-slate 
 have received special names. Roofing slate is the fine compact 
 durable kind, employed for roofing purposes and also for the 
 manufacture of cisterns, chimneypieces, writing-slates; Alum' 
 slate dark, carbonaceous, and pyritous, the iron-disulphide 
 oxidising into sulphuric acid, and giving rise to an efflorescence 
 of alum; Wliet-slate Jionestone exceedingly hard, fine-grained, 
 and suitable for making hones; sometimes owing its hardness 
 to. the presence of microscopic crystals of garnet; Chiastolite- 
 slate containing disseminated crystals of chiastolite, and 
 found especially around eruptive bosses of granite. By increase 
 of its mica-flakes a clay-slate passes into a Phyllite, -which has a 
 more silvery sheen, and represents a farther stage of meta- 
 morphism. Phyllite, by increase of the mica, becomes Mica- 
 slate, so that a transition may be traced from sedimentary fossil- 
 iferous rocks through clay-slate and phyllite into thoroughly 
 crystalline schist. Clay-slate occurs extensively among the 
 older geological formations in all parts of the world. 
 
176 GEOLOGY 
 
 AMPHIBOLITES rocks composed mainly of hornblende, but 
 with quartz, orthoclase, and other minerals in minor propor- 
 tions; sometimes they are massive and granular (Hornblende- 
 rock), and in this condition doubtless represent eruptive rocks. 
 Gradations can be followed from such rocks (originally diorite, 
 diabase, etc.) into perfect schist (Hornblende-schist), so that 
 the development of the schistose structure can be traced from 
 rocks that were at first as structureless as any amorphous erup- 
 tive mass can be. Amphibolites occur among the crystalline 
 schists in most parts of the world as occasional bands or bosses, 
 which probably mark zones of basic igneous rock, either in- 
 truded into the accompanying masses, or contemporaneously 
 erupted with them. 
 
 CHLORIDE-SCHIST a scaly, schistose aggregate of greenish 
 chlorite with quartz, and often with felspar, mica, and octahedra 
 of magnetite (Fig. 54) ; it occurs in beds associated with gneiss 
 and other schists. Some chloritic schists may represent old 
 lavas or other erupted rocks which have been crushed down 
 and become schistose; others, especially where they contain 
 pebbles of quartz, etc., and are banded with quartzites and 
 schistose conglomerates, not improbably mark where fine vol- 
 canic ashes fell over a sea-bottom, and were then mingled and 
 interstratified with the ordinary sediment that happened to be 
 accumulating at the time. 
 
 MICA-SCHIST (MICA-SLATE) a schistose aggregate of quartz 
 and mica, the two minerals being arranged in irregular but 
 nearly parallel wavy folia. The rock splits along the laminaB 
 of mica, so that its flat surfaces have a bright silvery sheen, 
 and the quartz is not well seen except on the cross fracture, 
 where only the thin edges of the mica-plates present themselves. 
 Mica-schist is often remarkably crumpled or puckered a 
 structure bearing witness to the intense compression it has 
 undergone (Fig. 114). It abounds in most regions where 
 schists are extensively developed (Chapter XVI). Some mica- 
 schists contain fossil shells and corals (Bergen), and must thus 
 represent what were originally sedimentary deposits ; others may 
 be highly deformed eruptive rocks. 
 
 GNEISS a schistose aggregate of orthoclase, quartz, and 
 
CLASSIFICATION OF ROCKS 177 
 
 mica, varying in texture from a fine-grained rock up to. a 
 coarse crystalline mass which, in hand specimens, may not be 
 distinguishable from granite. There is no difference indeed 
 as regards composition between gneiss and granite; gneiss may 
 be called a foliated granite. There is good reason to believe 
 that some, if not all, true gneisses have been made out of granite 
 or allied rocks by the process of shearing above referred to. 
 Gneiss occurs abundantly among the oldest known rocks of the 
 earth's crust, and may be found in most large regions of crystal- 
 line schists (Chapter XVI). 
 
 A few rocks which are found associated with the schists, or 
 with evidence of metamorphism, may be noticed here marble, 
 quartzite, and schistose conglomerate. 
 
 MARBLE a crystalline granular aggregate of calcite, white 
 when pure, and having the texture of loaf-sugar, but passing, 
 into various colours according to the nature of the impurities. 
 It occurs in beds among the schists, and is no doubt a lime- 
 stone, formed either by chemical precipitation or by organic 
 agency, which has been metamorphosed by heat and pressure 
 into its present thoroughly crystalline character. Some of the 
 fossiliferous limestones through which the Christiania granite 
 rises have been changed into crystalline marble, but their 
 original corals and shells have not been wholly effaced (see 
 Chapter XIV). 
 
 QUARTZITE a hard, compact, granular rock, composed of 
 adherent quartz-grains, and breaking with a characteristic 
 lustrous fracture. It occurs in beds and thick masses, not in- 
 frequently associated with slates, mica-schists, and limestones; 
 it sometimes contains organic remains; and is evidently -an 
 indurated siliceous sand. 
 
 SCHISTOSE GRIT AND CONGLOMERATE. Interstratified with 
 clay-slates and mica-schists there are sometimes found beds of 
 grit and conglomerate, the grains and pebbles of which consist 
 of quartz or other durable material, imbedded in slate or schist. 
 The original fragmental character of such rocks admits of no 
 doubt; they were obviously at one time sheets of fine and coarse 
 gravel mixed with sandy mud; and their presence among 
 schistose rocks furnishes additional corroborative evidence of 
 
178 GEOLOGY 
 
 the original sedimentary character of some of these rocks. 
 The clay or mud which formed the matrix has been metamor- 
 phosed into a more or less thoroughly crystalline micaceous 
 substance, while in many cases the pebbles have been flattened 
 and pulled out of shape. Hence these rocks afford important 
 evidence as to the nature of the processes whereby the schists 
 have been produced. 
 
SEDIMENTARY ROCKS 179 
 
 PAET III. 
 
 THE STRUCTURE OF THE CRUST OF THE EARTH. 
 CHAPTER XII. 
 
 SEDIMENTARY ROCKS. 
 
 HAVING in the two foregoing chapters considered the 
 more important elementary substances of which the 
 earth's crust is composed and their combinations in 
 minerals and rocks, we have to inquire how these minerals and 
 rocks have been put together so as to build up the crust. A very 
 little examination will suffice to show us that the upper or outer 
 parts of the solid globe consist chiefly of sedimentary rocks. All 
 over the plains and low grounds of the earth's surface, which 
 cover so large a proportion of the whole area of the land, some 
 kind of sediment underlies the soil clay, sand, gravel, lime- 
 stone. It is for the most part only in hilly or mountainous re- 
 gions that anything has been pushed up from below so as to in- 
 dicate the nature of the materials underneath. But everywhere 
 we encounter proofs that the sedimentary rocks do not remain as 
 they were deposited. In the first place, most of them were laid 
 down on the sea-floor and they have been upraised into land. In 
 the next place, not only have they been upheaved, they have not 
 infrequently been bent, broken, and crushed until sometimes 
 their original condition can no longer be determined. Moreover 
 they have been invaded by masses of lava and other eruptive 
 rocks, which have been thrust in among them and have often 
 burst through them to form volcanoes at the surface. We must 
 now endeavour to form as clear a conception as possible of what, 
 after all these changes, the present structure of the crust actual- 
 ly is. In this chapter, therefore, we may examine some of the 
 leading characters of sedimentary rocks in the architecture of 
 the crust, more particularly those which have been determined 
 by the conditions under which the rocks were formed. In the 
 next chapter we shall consider some of the more important char- 
 acters which have been superinduced upon the rocks since their 
 formation. 
 
180 
 
 GEOLOGY 
 
 Fig. 79. Section of strat- 
 ified rocks. 
 
 STRATIFICATION. It has been shown (p. 154) that one of the 
 most distinctive features in sedimentary rocks is that they are 
 
 stratified that is, are arranged in 
 layers one above another. As those 
 at the bottom must have been depos- 
 ited before those at the top, a suc- 
 cession of layers of stratified rocks 
 forms a record of deposition, in 
 which the early stages are chronicled 
 by the lower, and the later stages by 
 the upper layers. An illustration of 
 this kind of record has already been 
 given in the introductory chapter. 
 As a further example, the accom- 
 panying section (Fig. 79) may be 
 taken. At the bottom lies a bed (a) 
 of dark shale or clay with fragments of crinoids, corals, shells, 
 and other marine organisms. Such a bed unmistakably points to 
 a former muddy sea-floor, on which the creatures lived whose re- 
 mains have been preserved in the hardened mud or shale. The 
 next bed (6) is one of limestone full of similar organic remains; 
 it shows that the supply of mud, which had previously made the 
 water turbid and had been slowly gathering in successive layers on 
 the bottom, now ceased. The water became clear and much better 
 fitted for the life of the crinoids, corals, and shells. These creat- 
 ures accordingly flourished abundantly, living and dying on the 
 spot generation after generation, until their accumulated remains 
 had built up a solid sheet of limestone several feet thick. But once 
 more muddy currents spread over the place, and from the cloud 
 of suspended mud there slowly settled down the layer of blue 
 clay (c) which overlies the limestone. As hardly any remains 
 of organisms are to be seen in it, we may infer that the inroad 
 of mud killed them off. Next, owing to some new shifting of 
 the currents, a quantity of sand was brought in and spread out 
 over the mud, forming the sandstone beds (d). The sea in 
 which these various strata were deposited was probably shallow; 
 or its floor may have been gradually rising. At all events, the 
 last layers of sand could have been only slightly below the sur- 
 face of the water, for they are immediately covered by a hardened 
 silt or fire-clay (e) which, from the abundant roots and rootlets 
 
SEDIMENTARY ROCKS 
 
 181 
 
 \ I I 
 
 ' ' 1 1 
 
 that run through it in all directions, was clearly once a soil 
 whereon plants grew. It was probably part 
 of a mud-flat, on which vegetation spread sea- 
 ward from the land where the water shallowed, 
 as happens at the present day among the trop- 
 ical mangrove swamps. The various plants 
 that grew on this soil have formed the coal- 
 seam (/), no doubt representing the growth of 
 a long period of time. But the existence of 
 the coal-jungle came to an end probably by a 
 sinking of the ground beneath the water. 
 Mud, once more carried hither from the neigh- 
 bouring land, settled down upon the submerged 
 vegetation and formed the clay (g). But 
 that land plants still abounded in the imme- 
 diate neighbourhood, is shown by their nu- 
 merous remains in this clay. We notice too 
 that the salts of iron dissolved in the water 
 were eliminated by the decaying plants and 
 animals and were precipitated in the form of 
 carbonate, so as to form concretions round 
 occasional dead shells, fishes, fern-fronds, and 
 seed-cones. What were the immediately suc- 
 ceeding events in this ancient history we can- pig. 80< Section 
 not tell; the layer next in order is a coarse ti^oThe'ds 6 *" 
 conglomerate (h), originally gravel, which 15. shale. 14. Seam 
 must have been swept along by a swift cur- 
 rent that tore away the upper part of the 
 clay-beds (g) and any strata which may once 
 have overlain them. 
 
 The whole stratified part of the earth's 
 crust is composed of materials which in this 
 way may be made to tell their story. In forc- 
 ing them to yield up their records of the 
 ancient changes of which they are memorials, 
 scope is afforded for the most accurate and 
 laborious investigation and for the closest reas- 
 oning from the facts collected. At the same 
 time, it is obvious that the pursuit is one 
 which constantly exercises the imagination, and that, in- 
 
 of sandstone. 13. 
 Shale with sep- 
 tarian nodules. 
 12. Sandstone. 
 11. Mudstone. 10. 
 Lime stone. 9. 
 Clay. 8. Sand- 
 stones. 7. Sandy 
 clays. 6. Lime- 
 stone with part- 
 ing of shale. 5. 
 Shale. 4. Lime- 
 stone. 3. Shale 
 with cement- 
 stone passing 
 down into sand- 
 stone (2), which 
 graduates into 
 fi n e conglomer- 
 ate (1). 
 
182 
 
 GEOLOGY 
 
 deed, it cannot be adequately followed unless, by the proper use 
 of the imagination, the former conditions of the earth's surface 
 are vividly realised. 
 
 The thinnest layers of a stratified rock form lamina, such as 
 the thin paper-like leaves into which shale can be split. A num- 
 ber of laminaB may be united in a stratum or bed which may 
 vary from less than an inch to several feet or yards in thickness. 
 It is only the finer kinds of sedimentary rock that, as a rule, are 
 laminated. In other cases a stratum or bed is the thinnest sub- 
 division ; it can usually be separated easily from those above and 
 below it, and it may generally be regarded as marking one con- 
 tinued phase of deposit, while the break between it and the next 
 bed above or below probably denotes an interruption of the de- 
 posit. The study of the relations of strata to each other is called 
 Stratigraphy. 
 
 Fig. 81. False-bedded sandstone. 
 
 Layers of deposit usually lie parallel with each other, their 
 flat surfaces marking the general floor of the water at the time 
 of their formation (Figs. 79, 80). But sometimes a series of 
 layers may be found inclined at various angles to what was ob- 
 viously the original general plane of deposition. In Fig. 81, 
 for example, a series of strata is presented, which are dis- 
 tinguished by a diagonal lamination. This is known as False 
 bedding or Current-bedding. As before explained in Chapter III 
 it has been caused by the pushing of layers of sediment over 
 the advancing front of a stratum, and may be compared to the 
 
SEDIMENTARY ROCKS 183 
 
 oblique bedding often to be seen in an earthwork, such as a 
 railway embankment, the upper surface of which may be in a 
 general sense parallel with the flat bottom of the valley, while 
 the successive layers of which the mound is made, are inclined at 
 angles of 30 or more. False bedding is interesting as afford- 
 ing some indication of the nature and direction of the currents 
 by which sediment has been transported. 
 
 PROOFS or FORMER SHORES. Along the margin of the sea, 
 of lakes, and of rivers, several interesting kinds of markings 
 may be seen impressed on surfaces of sand or mud from which 
 the water has retired. Every one who has walked on a tidal 
 
 Fig. 82. Ripple-marked surface of sandstone. 
 
 sea-beach is familiar with the Ripple-marks left by the retreating 
 tide upon the bare sands. They are produced by the oscillation 
 of the water driven into movement by wind playing over its 
 surface. They are usually effaced by the next advancing tide; 
 hence, out of the same sand new sets of ripple-marks are made 
 by each tide. But we can understand that now and then, under 
 peculiarly favourable conditions, the markings may not be de- 
 stroyed. If, for instance, they were made in a kind of muddy 
 sand, which, in the interval between two tides and under a strong 
 sun, could become hard and coherent on the surface, and if the 
 next tide advanced so quietly as not to disturb them, but to lay 
 
184 GEOLOGY 
 
 down upon them a fresh layer of sand or mud, they might be 
 covered up and preserved. They would then remain as a me- 
 morial of the shallow rippling water and bare sandy shore where 
 they had been |ormed. 
 
 Now evidence of this kind regarding the conditions of deposi- 
 tion occur abundantly among sedimentary rocks (Fig. 82). Eip- 
 ple-marked surfaces may be traced one over another for many 
 hundred feet in a thick series of sandstones. They bring clearly 
 to the mind that the strata on which they lie were accumulated 
 in shallow water, or along beaches that were often laid dry. 
 
 LAND-SURFACES. Other traces of exposure to the air may be 
 noticed, where ripple-mark is abundant, in what are termed Sun- 
 cracks, Foot-prints, and Rain-prints. Those who have observed 
 
 Fig. 83. Cast of a sun-cracked surface preserved in the next succeeding 
 layer of sediment. 
 
 what takes place in muddy places during dry weather will re- 
 member that, as the mud dries and contracts it splits up into a 
 network of cracks; and that, on its hardened surface, it retains 
 impressions of the feet of birds or of insects that may have 
 walked over it while still soft. The geological history recorded 
 at such places cannot be mistaken; first, the rainy period, with 
 the rush of muddy water down the slopes and the formation of 
 pools in which the mud is allowed to settle; then the season of 
 warm weather when the pools gradually dry up and birds seek 
 their edges to drink. If by any means a layer of sediment could 
 be laid down upon one of these desiccated basins so gently as not 
 to efface its peculiar markings, the cracked surface of mud, with 
 
SEDIMENTARY ROCKS 185 
 
 its footprints, would contain a perfectly intelligible record of the 
 changes which it had witnessed (Fig. 83). 
 
 Now surfaces of this kind abound among the sedimentary 
 rocks of the earth's crust. They are found upon strata which, 
 from the presence of marine organic remains in them, were cer- 
 tainly deposited under the sea. But these strata cannot have 
 accumulated in deep water; they must have been formed along 
 flat shores, where the sheets of sand and mud were liable from 
 time to time to be laid bare to the sun and wind, where animals 
 of various kinds left their footmarks or trails on the still soft 
 sediment, where the evaporation and desiccation were so rapid 
 
 Fig. 84. Rain-prints on fine mud. 
 
 as to cause the exposed mud to harden on the surface and to 
 crack up into irregular polygonal cakes, and where the next suc- 
 ceeding layers of sediment were deposit el so gently as to cover 
 up and preserve the sun-cracked surfaces. 
 
 One further piece of evidence to indicate land-surfaces, or, at 
 least, shore-surfaces, in a series of aqueous sedimentary strata, is 
 that furnished by Eain-prints. A brief shower of rain leaves 
 upon a smooth surface of fine sand or mud a series of small pits, 
 each of which is the imprint of a descending raindrop (Fig. 84). 
 Where this takes place along the edge of a muddy pool which is 
 rapidly being dried up, the prints of the drops may remain quite 
 distinct on the hardened surface of mud. And here, again, we 
 can suppose that if another layer of mud were gently deposited 
 
186 
 
 GEOLOGY 
 
 above this surface the rain-prints would be sealed up and pre- 
 served. We might even be able to tell from what quarter the 
 wind blew that brought the rain-cloud. If, for example, the 
 rain-prints were ridged up on one side in one general direction 
 this would show that the shower fell aslant and with some force, 
 and that the side on which the mud round the imprints was 
 forced up was that towards which the rain was driven. Such 
 indications of ancient weather may here and there be detected 
 among stratified rocks. 
 
 CONCRETIONS. Another original characteristic of many sedi- 
 mentary rocks is a concretionary structure, particularly observ- 
 
 Fig. 85. Vertical trees (Sigillaria) in sandstone, Swansea (Logan). 
 
 able in clays, limestones, and ironstones. In many cases, the 
 concretions have gathered round some fragment of a plant or an 
 animal. Clay-ironstone and impure limestone have been aggre- 
 gated into spherical or elliptical forms (septaria), which are of 
 frequent occurrence in clay or shale (Figs. 61, 65). Flint has 
 also gathered round some organic nucleus, which it has often 
 entirely replaced. But many concretions may be found where 
 no organic fragment as a starting-point can be detected. Some 
 of the most curious are the so-called Fairy-stones (Fig. 64), 
 
SEDIMENTARY ROCKS 187 
 
 found in alluvial clays, with so many imitative shapes, which 
 have been popularly supposed to be works of human or even pre- 
 ternatural construction. They have probably been produced by 
 the irregular cementing of clay, owing to the spread of car- 
 bonate of lime through it, carried down by permeating water. 
 Some of the most extraordinary concretionary masses are to be 
 seen in certain magnesian limestones, which appear to be built 
 up of petrified lumps of coral, bunches of grapes, cannon-balls, 
 and other objects (Fig. 75). In reality, all these diversified 
 figures are due to the irregularly varied way in which a concre- 
 tionary structure has been developed in the limestone. 
 
 ASSOCIATION AND ALTERNATION OF STRATA. Certain kinds 
 of sedimentary rocks are apt to occur together to the exclusion of 
 others. This association depends on the circumstances of depo- 
 sition. Ironstone concretions, for example, are much more fre- 
 quent among clays or shales than in any other strata, because it 
 was during the deposit of fine mud with abundant decomposing 
 organic matter that the most favourable conditions were sup- 
 plied for the precipitation of carbonate of iron. Clays and lime- 
 stones frequently alternate, as also do sandstones and conglomer- 
 ates, because the circumstances of deposition were somewhat 
 alike, (see Fig. 80). But we need not expect to encounter a bed 
 of coarse conglomerate in a group of fine clays, for the current 
 that was strong enough to sweep along the stones of the con- 
 glomerate was too powerful to allow the fine silt to lie undis- 
 turbed. For a similar reason, we should be surprised to meet 
 with a layer of well-stratified shale in a mass of conglomerate. 
 The agitated water in which these coarse materials were heaped 
 up would have swept away any fine sediment and prevented it 
 from being deposited. In all cases, the manner in which the 
 different kinds of sediment are associated with each other leads 
 us back directly to the original conditions of deposit, and is only 
 intelligible in proportion as these conditions are clearly realised. 
 
 EELATIVE AREAS OF STRATIFIED ROCKS. Moreover, some 
 kinds of sedimentary material must obviously spread over wider 
 areas than others. The coarse gravel and shingle of the present 
 beach do not extend far seawards ; they are confined to the mar- 
 gin of the land. Sand covers the sea-floor over a wider area; 
 and beyond the limits of the sand, in the deeper and stiller water, 
 mud is allowed to accumulate. Eoughly speaking, therefore, the 
 
188 GEOLOGY 
 
 area of the distribution of sediment is in inverse proportion to 
 the coarseness of the materials. The same law has regulated the 
 accumulation of detritus from early geological time. Coarse 
 conglomerates, which represent ancient shingles and gravels 
 thicken and thin out rapidly, and do not usually cover a large 
 area, though they may sometimes be traced for long distances 
 in the direction probably of the original coast or line of heaping 
 up of the shingle. They pass laterally and vertically into grit 
 and sandstones which have a much wider distribution, and then 
 again shade off into clays and shales that range also over large 
 areas. 
 
 CHRONOLOGICAL VALUE OP STRATA. No clue has yet been 
 found to determine the length of time required for the accumu- 
 lation of a stratum or group of strata ; but some indications are 
 afforded of relative lapse of time. Here and there, for instance, 
 where trunks of trees are met with standing in their positions 
 of growth, but imbedded in solid sandstone (Fig. 85). These 
 stems, sometimes 20 feet or more in height, prove that a 
 mass of sand of that depth must have been accumulated around 
 them before they had time to decay. We know little about the 
 durability of the submerged trees; but they probably could not 
 have lasted long unless covered up by sediment ; so that the mass 
 of strata in which they are enclosed may be supposed to have 
 been accumulated within a few years. The nature of the mate- 
 rial composing sedimentary rocks may likewise furnish indica- 
 tions of relative rate of deposition. Thus finely laminated clays 
 were evidently deposited with extreme slowness. Beds of lime- 
 stone, composed of the crowded remains of successive genera- 
 tions of marine creatures, must also have required prolonged 
 periods of time for their growth. On the other hand, thick beds 
 of sandstone preserving great uniformity of characters may not 
 improbably have been laid down with comparative rapidity. 
 
 A reliable inference can be drawn from the mere thicknesses 
 of strata as to the lapse of time which they represent. A mass 
 of sandstone 20 feet thick may have accumulated round a sub- 
 merged tree in a few years. On the other hand, a corresponding 
 depth of fine laminated clay may have required tenfold more time 
 for its deposition. But the same thickness of rock composed of 
 alternations of shale and limestone might represent a still longer 
 period. For it is obvious that the change from one kind of 
 
SEDIMENTARY ROCKS 189 
 
 sediment to another must often have been brought about by an 
 extremely gradual modification of the geography of the region 
 from which the supply of sediment was derived. Hence the 
 interval between two beds or groups of beds, differing much from 
 each other in mineral composition, may have been considerably 
 longer than the time required for the actual deposition of the 
 strata of either or both beds or groups of beds. 
 
 On any probable estimate, the deposition of sedimentary rocks 
 to a depth of many thousand feet and over areas many thousands 
 of square miles in extent, must have demanded enormous periods 
 of time. Side by side with the growth of mechanical sediments, 
 there must have been a corresponding wasting of land. Every 
 bed of conglomerate, sand, or mud represents at least an equiva- 
 
 Fig. 86. Hills formed out of horizontal sedimentary rocks. 
 
 lent amount of rock worn away from the land and transported 
 as sediment to the floor of the sea. During such prolonged 
 ages as these changes required, there was ample time for the 
 outburst of many successive volcanoes, for the passage of many 
 earthquake-shocks, and for the subsidence or upheaval of many 
 parts of the earth's crust. 
 
 PROOFS OF SUBSIDENCE. A mass of sedimentary material of 
 great thickness which, from the remains of sun-cracks and other 
 evidence, was obviously deposited in shallow water near land can 
 only have been accumulated on an area that was gradually sink- 
 ing. Suppose, for instance, that a hill formed out of such strata 
 rises a thousand feet above the valley at its foot (Fig. 86), and 
 that proofs of deposition in shallow water can be detected from 
 the lowest beds all the way up to the highest. The lowest beds 
 having once been close to the surface, as shown by the sun-cracks 
 
190 GEOLOGY 
 
 and other evidence, could only be covered with hundreds of feet 
 of similar strata by a gradual sinking of the ground, during 
 which fresh sediment was poured in, so that, although the origi- 
 nal bottom sank a thousand feet, the water may never have be- 
 come sensibly deeper, the rate of deposit of sediment having, on 
 the whole, kept pace with that of the subsidence. 
 
 OVERLAP. During such tranquil movements, as the area of 
 land lessens and that of the sea increases, the later sedimentary 
 accumulations must needs extend beyond the limits of the older 
 ones. Suppose, for instance, that such a sloping land-surface as 
 that represented in the section (x, Fig 87) were slowly to subside 
 
 Fig. 87. Section of overlap. 
 
 beneath the sea, the first-formed strata (a) will be covered and 
 overlapped by the next series ( b ) , and these in turn, as the sea- 
 floor sinks, will be similarly concealed by the following group 
 (c) . This structure, termed Overlap, may usually be regarded as 
 evidence of a gentle subsidence of the area of deposit. 
 . CONFORMABILITY, TlNCONFORMABiLiTY. When stratified de- 
 posits are laid down regularly and continuously upon each other, 
 with no interruption of their generally level position, they are 
 said to be conformable. In the section Fig. 80, for instance, the 
 series of sediments there represented has evidently been deposited 
 under the same general conditions. The nature of the sediment 
 has of course varied from time to time; limestones, shales, and 
 sandstones have alternated with each other ; but there has been no 
 marked interruption or disturbance in their sequence. Suppose, 
 however, that owing to subterranean movements, a series of rocks 
 (a in Fig. 88) is shifted from its original position and after be- 
 ing uplifted, is exposed to the wearing action of the sea, rivers, 
 air, rain, frosts, and the other agents concerned in the degrada- 
 tion of the surface of the land. If a new series of deposits (&) 
 is laid down upon the denuded edges of these rocks, the bedding 
 of the whole will not be continuous. The younger strata will rest 
 successively upon different parts of the older group, or, in other 
 words, will be unconformable. Such a relation or unconform- 
 
SEDIMENTARY ROCKS 
 
 191 
 
 ability (unconformity) implies a terrestrial disturbance, and 
 usually also the lapse of a long interval of time between the re- 
 spective periods of the older and younger rocks during which 
 denudation of the older strata took place. It serves to mark 
 one of the breaks or gaps in geological history. Unconformabili- 
 ties differ much from each other in regard to the length of in- 
 terval which they denote. In some cases, the blank may be of 
 comparatively slight moment ; in others, it is so vast as to include 
 the greater part of the time represented by the stratified rocks of 
 the earth's crust. 
 
 By means of unconformabilities the different ages of mountain- 
 chains are determined. If, for example, a mountain showed the 
 structure represented in Fig. 88, its upheaval must obviously have 
 taken place between the deposition of the two series of rocks. 
 Suppose the series a to represent Lower Silurian, and & Carboni- 
 ferous rocks, the date of the mountain would be between the 
 
 Fig. 88. Unconformability. 
 
 Lower Silurian and Carboniferous periods. If, in another moun- 
 tain, series b were unconformably overlain by a younger series, 
 say of Jurassic age, this mountain would thereby be shown to 
 have undergone a subsequent uplift in the long interval between 
 the Carboniferous and the Jurassic periods. 
 
 SUMMARY. In this Lesson some of the more characteristic 
 original features of sedimentary rocks have been considered. Of 
 these features, one of the most distinctive is the arrangement into 
 layers of beds, each of which is the record of a portion of geologi- 
 cal history, the oldest being below and the youngest above. The 
 smallest subdivision of these records is a lamina or thin leaf, 
 such as those into which shales may be split. A stratum or bed, 
 which may contain many laminae or none, is a thicker layer 
 separable with more or less ease from those below and above it. 
 Though strata lie on the whole parallel with each other, they 
 often show oblique current-bedding, especially in sandstones. 
 
192 GEOLOGY 
 
 Traces of shore-lines and of surfaces laid bare by the retirement 
 of the water in which they were deposited, are found in sun- 
 cracks, rain-pittings, and footprints. Not infrequently, instead 
 of being evenly spread out in layers, the sedimentary material 
 has been aggregated into variously-shaped concretions. Certain 
 kinds of sedimentary rocks are apt to occur together, such as clays 
 and limestones, clay-ironstones and shales, coals and fire-clays; 
 because the conditions under which they were respectively de- 
 posited were on the whole similar. As a rule, the finer the 
 detritus, the wider the area over which it is spread ; hence clays 
 generally cover wider tracts than conglomerates. No inference 
 can safely be drawn from the relative thickness of strata as to the 
 length of time which they respectively represent ; they must vary 
 widely in this respect, and it is quite conceivable that, in many 
 cases, the interval of time between the deposition of two succes- 
 sive beds of very different character and composition may have 
 been actually longer than the period required for the deposition 
 of two beds. A thick series of sedimentary deposits usually in- 
 dicates that the sea-bottom on which it was laid down was slowly 
 sinking. In subsiding, the later deposits spread beyond the 
 limits of the earlier ones, and thus present what is called an 
 overlap. Where they have been laid down continuously one upon 
 another they are said to be conformable; where one group has 
 been deposited on the disturbed and worn edges of an older series 
 the two are unconf ormable to each other. 
 
SEDIMENTARY ROCKS AFTER FORMATION 193 
 
 CHAPTER XIII. 
 
 SEDIMENTARY ROCKS AFTER FORMATION. 
 
 AFTER their deposition sedimentary materials have under- 
 gone various changes before assuming the aspect which 
 they now wear. 
 
 CONSOLIDATION. The most obvious of these changes is that, 
 instead of consisting of loose materials, gravel, sand, mud and so 
 on, they are now hard stone. This consolidation has sometimes 
 been the result of mere pressure. As bed was piled over bed, 
 those at the bottom would gradually be more and more com- 
 pressed by the increasing weight of those that were laid down 
 upon them, the water would be squeezed out, and any tendency 
 which the particles might have to cohere would promote the con- 
 solidation of the mass. Mud, for example, might in this way be 
 converted into clay, and clay in turn might be pressed into mud- 
 stone or shale. But besides cohesion from the pressure of over- 
 lying masses, sedimentary matter has often been bound together 
 by some kind of cement, either originally deposited with it or 
 subsequently introduced by permeating water. Among natural 
 cements, the most common are silica, carbonate of lime and per- 
 oxide of iron. In a red sandstone, for example, the quartz-grains 
 may be observed to be coated over with earthy iron peroxide, 
 which serves to unite them together into a more or less coherent 
 stone. The effect of weathering is not infrequently to remove 
 the binding cement and thereby to allow the stone to return to 
 its original condition of loose sediment. 
 
 JOINTS. Next to their consolidation into stone, the most com- 
 mon change which has affected sedimentary rocks is the produc- 
 tion in them of a series of divisional planes or fractures termed 
 Joints. Except in loose incoherent materials, this structure is 
 hardly ever absent. In any ordinary quarry of sandstone, lime- 
 stone, or other sedimentary rock, or along a natural cliff of the 
 
194 GEOLOGY 
 
 same materials, a little attentive observation will show that the 
 bare wall of rock forming the back of the quarry or the face of 
 the cliff has been determined by one or more natural fissures 
 in the stone, and that there are other fissures running parallel 
 with it through every outstanding buttress of rock. Moreover, 
 we may observe that these vertical or highly inclined lines of 
 fissure are cut across by others, more" or less nearly at a right 
 angle, and that the sides of the buttresses have been defined 
 by these transverse lines, just as the main face of rock has been 
 formed by the first set. Such lines of division are Joints. In 
 close-grained stone they may be imperceptible until it is quarried 
 or broken, when they reveal themselves as sharply defined, nearly 
 vertical fractures, along which the stone splits. There are usually 
 
 Fig. 89. Joints in a stratified rock. 
 
 at least two series of joints crossing each other at right angles 
 or obliquely, whereby a rock is divided into quadrangular blocks. 
 In the accompanying diagram (Fig. 89) a group of stratified 
 rocks is seen to be traversed by two sets of joints, one of which 
 (dip- joints, "cutters" of the quarrymen) defines the faces 
 that are in shadow, the other (strike- joints, "backs" of the 
 quarrymen) those that are in light. By help of these divisional 
 planes, it is possible to obtain large blocks of stone for building 
 purposes. The art of the quarryman, indeed, largely consists of 
 taking advantage of these natural lines of fracture, so as to 
 obtain his materials with the least expenditure of time and 
 labour, and in large masses. In nature also the existence of 
 joints is a fact of the highest importance. Eeference has already 
 
SEDIMENTARY ROCKS AFTER FORMATION 195 
 
 been made to the way in which they afford a passage for the 
 descent of water from the surface. It is in great measure along 
 joints that the underground circulation of water is conducted. 
 At the surface, too, where rocks yield to the decomposing influ- 
 ence of the weather, it is by their joints that they are chiefly 
 split up. Along these convenient planes of division, rain-water 
 trickles and freezes; the walls of the joints are separated, and 
 the space between them is slowly widened, until in the end it 
 opens into yawning rents, and portions of a cliff are overbalanced 
 and fall, while detached pinnacles are here and there isolated. 
 The picturesqueness of the scenery of stratified rock is, in great 
 measure, dependent upon the influence of joints in promoting 
 their dislocation and disintegration by air, rain, and frost. 
 
 In many cases, joints may be due to contraction. A mass of 
 sand or mud, as it loses water and as its particles are more firmly 
 united to each other, gradually occupies less room than at first. 
 In consequence of the contraction strains are set up in the stone, 
 and relief from these is eventually found in a system of cracks 
 or fissures. In other instances, joints have been produced by 
 the compression or torsion to which large masses of rock have 
 been exposed during movements of the earth's crust. 
 
 ORIGINAL HORIZONTALITY. As laid down upon the margin 
 or floor of the sea., on the bottoms of lakes, and on the beds or 
 alluvial plains of rivers, sedimentary accumulations are in general 
 nearly flat. They slope gently, indeed, seawards from a shelving 
 shore, and they gather at steeper angles on slopes of debris at 
 the foot of cliffs, or down the sides of mountains. But, taken 
 as a whole, and over wide areas, their original position is not far 
 removed from the horizontal. If we turn, however, to the sedi- 
 mentary rocks that form so large a part of the earth's crust, 
 and so much of the dry land, we find that although originally 
 deposited for the most part over the sea-bottom, they are now 
 inclined at all angles, and even sometimes stand on end. Such 
 situations, in which their deposition could never have taken 
 place, show that they have been disturbed. Not only have they 
 been upraised into land, but they have been tilted unequally, 
 some parts rising or sinking much more than others. 
 
 DIP. The inclination of bedded rocks from the horizon is 
 called their Dip. The amount of dip is reckoned from the plane 
 of the horizon. A face of rock standing up vertically above that 
 
196 
 
 GEOLOGY 
 
 plane is said to be at 90, while midway between that position 
 and horizontally it lies at an inclination of 45. The angle of 
 dip is accurately measured with an instrument called a Clino- 
 meter, of which there are various forms. One of the simplest 
 kinds is a brass half-circle graduated into 90 on each side of 
 the vertical, on which a pendulum is hung as in Fig. 91. The 
 
 / <: A 
 
 / . My, 
 
 Fig. 90. Dip and Strike. The arrow shows the direction of dip; the 
 line s s marks the strike. 
 
 instrument is held between the eye and the angle to be measured, 
 and the upper edge is made to coincide with the line of the 
 inclined rock. The pendulum, remaining vertical, points to the 
 angle of inclination from the horizon. A little practice, how- 
 ever, enables an observer to estimate the amount of dip by the 
 eye with sufficient accuracy for most purposes. The direction of 
 dip is the point of the compass toward which a stratum is 
 inclined (shown by the arrow in Fig. 90), and is best ascer- 
 
 Fig. 91. Clinometer. 
 
 tained with a magnetic compass. But here again a little experi- 
 ence in judging of the quarters of the sky without an instrument 
 will usually enable us to tell the direction of dip with as much 
 precision as may be required. 
 
 STRIKE. A mathematical line running at a right angle to 
 the direction of dip is called the Strike ( s s in Figs. 90, 92). 
 Where a series of strata dips due north or due south the strike 
 
SEDIMENTARY ROCKS AFTER FORMATION 197 
 
 is east and west; but the direction of strike changes with that 
 of the dip. Suppose, for example, that certain strata dip due 
 east, then veer round by south-east to south, and so on by west 
 and north, back to east again. The strike following this change 
 would describe a circle. In fact, the beds would be included in 
 a basin-shaped or dome-shaped arrangement and the strike would 
 be the lip of the basin or rim of the truncated dome. Though 
 the dip may slightly vary from place to place, still, if it remains 
 in the same general direction along the line of certain strata, 
 their strike is on the whole uniform. 
 
 OUTCROP. The actual edge presented by a stratum at the 
 surface of the ground is called its Outcrop. On a perfectly level 
 surface, strike and outcrop must coincide; but as ground is 
 
 P 
 
 Fig. 92. Dip, Strike, and Outcrop. 
 
 seldom quite level they usually diverge from each other, and do 
 so the more in proportion to the lowness of angle of dip and 
 the inequalities of the ground. This may be illustrated by a 
 diagram such as that given in Fig. 92, which represents a portion 
 of the edge of a table-land, deeply trenched by two valleys that 
 discharge their waters into the plain below (P). The arrows 
 point out that the strata dip due N. at 5. On the level plain, 
 the outcrop and the strike (s s) of the beds are coincident and 
 run due E. and W. But as the surface rises toward the high 
 ground and the deep valleys, the outcrop (o o) is observed to 
 depart mo^ and more from the strike till in some places the 
 two lines are at right angles; yet, as the dip remains the same, 
 
198 
 
 GEOLOGY 
 
 the strike is likewise unchanged, the sinuosities of the outcrop 
 being entirely due to the irregularities pf the surface of the 
 ground. 
 
 CUEVATURE. It requires no long observation to perceive that 
 in being tilted from their original more or less level positions, 
 stratified rocks have been thrown into curves. Suppose, for 
 instance, that in walking along a mile of coast-line, where all 
 the successive strata of a thick series are exposed to view, we 
 should observe such a section as is drawn in Fig. 93. Beginning 
 
 Fig. 93. Inclined strata shown to be parts of curves. 
 
 at A, we find the beds tilted up at angles of 70 which gradually 
 lessen, till at B they have sunk to 15. As there is no break in 
 the series, it is evident that the lines of bedding must be pro- 
 longed downward, and must once have been continued upward 
 in some such way as is expressed by the dotted lines. The 
 visible portion which is here shaded must thus form part of 
 a great curvature of the rocks. 
 
 But the actual curvature may often be seen on coast-cliffs, 
 ravines, or hillsides. In Fig. 94, for example, a simple arch is 
 shown from the Berwickshire coast, wherein hard beds of gray- 
 
SEDIMENTARY ROCKS AFTER FORMATION 
 
 199 
 
 wacke and shale have been folded. Again, in Fig. 95, the reverse 
 structure is exhibited, beds of grit and slate being there curved 
 into a trough. Where rocks dip away from a central line of 
 axis the structure is known as an Anticline ; where, on the other 
 hand, they dip towards an axis it is called a Syncline. In Figs. 
 94 and 95 these two structures are presented on so small a scale 
 as to be visible in a single section. More usually, however, it 
 is only by observing the upturned edges of strata that anticlines 
 and synclines can be detected. The dark part of Fig. 96 repre- 
 
 Fig. 94. Curved strata (anticlinal fold), near St. Abb's Head. 
 
 sents all that can be actually seen ; but the angles and direction 
 of dip leave no doubt that if we could restore the amount of 
 rock which has here been worn away from the surface of the 
 land, the present truncated ends of the strata would be pro- 
 longed upward in some such way as is indicated by the dotted 
 lines. By observations of this truncation of strata some of the 
 most interesting and important evidence is obtained of the 
 enormous extent to which the land has been reduced by the 
 removal of solid material from its surface. 
 
 PLICATION, SHEARING. From such simple curvatures as 
 those depicted in the foregoing diagrams, we may advance to 
 
200 GEOLOGY 
 
 more complex foldings, wherein the solid strata have been 
 doubled up and crumpled together, as if they had been mere 
 layers of carpet. So far is this plication sometimes carried, 
 that the lowest rocks are brought up and thrown over the highest, 
 the more yielding materials being squeezed into the most intricate 
 frillings and puckerings. It is in mountainous regions, where 
 the crust of the earth has been subjected to the most intense 
 corrugation, that these structures are best seen. We can form 
 some idea of the gigantic energy of the earth-movements that 
 produced them, when we see a whole mountain-range made up 
 of solid limestones or sandstones which have been bent, twisted, 
 
 Fig. 95. Curved strata (synclinal fold), near Banff. 
 
 crumpled, and inverted, as we might crush up sheets of paper 
 (Fig. 97). 
 
 So enormous has been the compression produced by impor- 
 tant movements of the earth's crust, that the solid rocks have 
 actually been squeezed out of shape or have undergone a process 
 of shearing. The amount of distortion may sometimes be meas- 
 ured by the extent to which shells or other organic remains are 
 pulled out in the direction of movement. In Fig. 98 the proper 
 shape of a trilobite (Angelina Sedgwickii) is given, and along- 
 side of it is a view of the same organism which has been elongated 
 
SEDIMENTARY ROCKS AFTER FORMATION 
 
 201 
 
 by the distortion of the mass of rock in which it lies. Further 
 results of shearing will be immediately referred to in connection 
 with the cleavage and metamorphism of rocks. 
 
 CLEAVAGE. One of the most important structures developed 
 by the great compression to which the rocks of the earth's crust 
 
 \ 
 
 I 
 
 Fig. 96. Anticlines (a a) and Synclines (66). 
 
 have been exposed is known as Cleavage. The minute particles 
 of rocks, being usually of irregular shapes, have been compelled 
 to arrange themselves with their long axes perpendicular to the 
 direction of pressure during the interstitial movements conse- 
 quent upon intense subterranean compression. Hence, a fissile 
 tendency has been imparted to a rock, which will now split into 
 leaves along the planes of rearrangement of the particles. This 
 superinduced tendency to split into parallel leaves, irrespective 
 
 Fig. 97. Section of folded and crumpled strata forming the Grosse Wind- 
 galle (10,482 feet), Canton Uri, Switzerland, showing crumpled and 
 inverted strata (after Heim). 
 
 of what may have been the original structure of the rock, con- 
 stitutes cleavage. It is well developed in ordinary roofing-slates. 
 Though the leaves or plates into which a slate splits resemble 
 those in a shale, they have no necessary relation to the layers 
 of deposition but may cross them at any angle. In Fig. 99, for 
 instance, the original bedding is quite distinct and shows that 
 
202 
 
 GEOLOGY 
 
 the strata have been folded by a force acting from the right and 
 left of the section; the parallel highly inclined lines traversing 
 the folds of the bedding represent the planes of cleavage. Where 
 
 Fig. 98. Distortion of fossils by the shearing of rocks; (a) a Trilobite 
 (Angelina Sedgwickii) distorted by shearing, the direction of move- 
 ment indicated by the arrows; (6) the same fossil in its natural form. 
 
 the material is of exceedingly fine grain, such as fine consolidated 
 mud the original bedding may be entirely effaced by the cleavage, 
 and the rock will, only split along the cleavage-planes. Indeed, 
 
 Fig. 99. Curved and cleaved rocks. Coast of Wigtonshire. The fine 
 parallel oblique lines indicate the cleavage, which is finer in the 
 dark shales and coarser in the thicker sandy beds. 
 
 the finer the grain of a rock, the more perfect may be its cleavage, 
 so that where alternations of coarser and finer sediment have 
 been subjected to the same amount of compression cleavage may 
 be perfect in the one and rudely developed in the other, as 
 is indicated in Fig. 99. 
 
SEDIMENTARY ROCKS AFTER FORMATION 
 
 203 
 
 Cleavage may be regarded as one of the first stages in the 
 mechanical deformation of a rock, and the production of schistose 
 metamorphism (Chap. XI) . Besides being compressed and having 
 its component particles rearranged in definite planes, the rock 
 may likewise reveal under the microscope that new minerals, 
 such for example as crystallites or minute flakes of some mica, 
 
 Fig. 100. Examples of normal Faults. 
 
 have been developed out of the general matrix, as may be seen 
 in common roofing-slate. By increasing stages of crystallisation 
 we trace gradations into phyllites and mica-schists. 
 
 DISLOCATION. Another important structure produced in rocks 
 after their formation is Dislocation. Not only have they been 
 folded by the great movements to which the crust of the earth 
 has been subjected, but the strain upon them has often been 
 so great that they have snapped across. Such ruptures of con- 
 
 Fig. 101. Sections to show the relations of Plications (a, 6) to reversed 
 
 Faults (c). 
 
 tinuity present an infinite variety in the position of the rocks 
 on the two sides. Sometimes a mere fissure has been caused, 
 the rocks being simply cracked across, but remaining otherwise 
 unchanged in their relative situations. But, in the great majority 
 of instances, one or both of the walls of a fissure have moved, 
 producing what is termed a FAULT. Where the displacement 
 has been small, a fault may appear as if the strata had been 
 sharply sliced through, shifted, and firmly pressed together again 
 
204 GEOLOGY 
 
 (a in Fig. 100). Usually, however, they have not only been cut, 
 but bent or crushed on one or both siies (6) ; while not infre- 
 quently the line of fracture is represented by a bank of broken 
 and crushed material (Fault-rock, c). The fracture is seldom 
 quite vertical ; almost always it is inclined at angles varying up 
 to 70 or more from the vertical. In by far the largest number 
 of faults, the inclination of the plane of the fissure, or what is 
 called the Hade of the fault, is away from the side which has 
 risen or toward that which has sunk. In the examples given 
 in Fig. 100, a, ~b, this relation is expressed; but in nature it 
 often happens. that the beds on two sides of a fault are entirely 
 different (Fig. 100, c), and consequently that the side of up- 
 throw or downthrow cannot be determined by the identification 
 of the two severed positions of the same bed. But if the hade 
 of the fault can be seen, we may usually be confident that the 
 strata on the upper or hanging side belong to a higher part of 
 the series than those on the lower side. Faults that follow this 
 rule (normal faults) are by far the most frequent. They occur 
 universally, and are probably for the most part caused by sub- 
 sidence in the earth's crust. In adjusting themselves to the 
 new position into which a downward movement brings them, 
 rocks must often be subject to such strains that their limit of 
 elasticity is reached, and they break across, one portion settling 
 down farther than the part next to it. In a normal fault, the 
 same bed can never be cut twice by a vertical line.. 
 
 In mountainous districts, however, and generally where the 
 rocks of the earth's crust have been disrupted and pushed over 
 each other, what are termed reversed faults occur. In these, the 
 hade slopes in the direction of upthrow, and a vertical line may 
 cut the same beds twice on opposite sides of the fracture (Fig. 
 101). Such faults may be observed more particularly where 
 strata have been much folded. A fold may be seen to have 
 snapped asunder, the whole being pushed over, and the upper 
 side being driven forward over the lower. 
 
 The amount of vertical displacement between the two frac- 
 tured ends of a bed is called the Throw of a fault. In Fig. 102, 
 for example, where bed a has been shifted from & to d, a vertical 
 line dropped from the end of the bed at & to the level of the 
 corresponding part of the bed at e will give the amount of the 
 subsidence of d, which is the throw. Faults may be seen with 
 
SEDIMENTARY ROCKS AFTER FORMATION 205 
 
 a throw of less than an inch mere local cracks and trifling 
 subsidences in a mass of rock ; in others the throw may be many 
 thousand feet. Large faults often bring rocks of entirely differ- 
 ent characters together, as, for instance, shales against lime- 
 stones or sandstones, or sedimentary against eruptive rocks. 
 Consequently they are not infrequently marked at the surface 
 by the difference between the form of ground characteristic of 
 the two kinds of rock. One side, perhaps, rises into a hilly or 
 undulating region, while the other side may be a plain. Com- 
 paratively seldom does a fault make itself visible as a line of 
 ravine or valley. On the contrary, most faults cut across valleys 
 or only coincide with them here and there. They run in straight 
 or wavy lines which, where the amount of displacement is great, 
 may be traced for many miles. The Scottish Highlands, for 
 example, are bounded along their southern margin by a great 
 
 Fig. 102. Throw of a Fault. 
 
 fault which places a thick series of sandstones and conglomerates 
 on end against the flanks of the mountains. This fault may 
 be traced across the island from sea to sea a distance of fully 
 120 miles, and by bringing two distinct kinds of rocks next each 
 other along a nearly straight line it has given rise to the boun- 
 dary between Highland and Lowland scenery which, in some 
 places, is so singularly abrupt. 
 
 In regions of the most intense terrestrial disturbance, tracts 
 of rock many square miles in area and hundreds or thousands of 
 feet in thickness, have been torn away and pushed upward and 
 forward until they have come to rest on rocks originally much 
 higher in geological position. Such displaced cakes or slices of 
 the earth's crust sometimes rest upon an almost horizontal or 
 gently inclined platform of undisturbed materials. Vertical or 
 contorted strata are thus placed above others which may be flat 
 or but little inclined. The plane of separation between the moved 
 and unmoved masses is really a dislocation, but to distinguish 
 
206 
 
 GEOLOGY 
 
 it from faults, which are generally placed at steep angles, it is 
 called a Thrust-plane. Structures of this kind on a colossal scale 
 are traceable for about 100 miles in the north-west of Scotland. 
 
 METAMORPHISM. The last structure, which will be mentioned 
 in this chapter as having been superinduced upon rocks, is con- 
 nected with the movements to which plication, cleavage, and 
 reversed faults are due. So enormous has been the energy with 
 which these movements have been carried on, that not only have 
 the rocks been crumpled, ruptured, and pushed over each other, 
 but they have undergone such intense shearing that as was 
 pointed out in Chap.XI, their original structure has been partially 
 or wholly effaced. They have been so crushed that their com 
 
 Fig. 103. Ordinary un- 
 altered red sandstone, 
 Keeshorn, Ross-shire 
 (magnified). 
 
 Fig. 104. Sheared red 
 sandstone forming now 
 a micaceous schist, 
 Keeshorn, Ross-shire 
 (magnified). 
 
 ponent particles have been reduced, as it were, to powder, and 
 have assumed new crystalline arrangements along the shearing- 
 planes or surfaces of movement. A sandstone, for example, 
 which in- its ordinary state shows, when magnified, such a struc- 
 ture as is represented in Fig. 103, when it has come within 
 the influence of this crushing process has its grains of quartz, 
 felspar, and other materials flattened and squeezed against each 
 other in one general direction as in cleavage', while out of the 
 crushed debris a good deal of new mica has been developed. 
 This change may be intensified until the component grains are 
 hardly recognisable, and the proportion of new mica has so in- 
 creased that the rock has become a mica-schist. Other new 
 minerals such as garnet, may likewise make their appearance, 
 until the rock assumes an entirely crystalline structure. Such 
 an alteration of the internal structure of a rock is known as 
 
SEDIMENTARY ROCKS AFTER FORMATION 207 
 
 Metamorphism. Where the change arises from mechanical 
 movements combined with chemical rearrangement, it usually 
 affects a wide district, and is then spoken of as regional meta- 
 "jiorpliism. 
 
 There are wide regions of the earth's surface where schists of 
 various kinds form the prevailing rock. Whether they have all 
 been produced by the shearing and alteration of previously- 
 formed rocks has not yet been determined. But that a large 
 number of schists are truly altered or metamorphosed rocks 
 admits of no doubt. Sandstones, shales, limestones, quartzites, 
 diorites, syenites, granites, in short, any old form of rock that 
 has come within the crushing and shearing movements here 
 referred to has been converted into schist. The gradation be- 
 tween the unaltered and the metamorphic condition can often 
 be clearly traced. Granite, by crushing, passes into gneiss, 
 diorite into hornblende-schist, sandstone into quartz-schist or 
 mica-schist, and so on. Even where it is no longer possible to 
 tell what the original nature of the metamorphosed material may 
 have been, there is usually abundant evidence that the rock has 
 undergone great compression. 
 
 SUMMARY. In this Lesson attention has been directed to 
 new structures produced in sedimentary rocks after their forma- 
 tion. Beginning with the simplest and most universal of these, 
 we find that sediments have been consolidated into stone, partly 
 by pressure, and partly by some kind of cement, such as silica 
 or carbonate of lime. In the process of consolidation and con- 
 traction, they have been traversed by systems of joints, or have 
 had these subsequently produced by the torsion accompanying 
 movements of the crust. Though at first nearly flat they have 
 by these movements been thrown into various inclined positions, 
 and more especially into undulating folds or more complicated 
 plication and puckering. So great has been the compression 
 under which they have been moved, that a cleavage has been 
 developed in them. They have also been everywhere more or 
 less fractured, the dislocations being due either to their gradual 
 subsidence or to excessive plication. Their most complete altera- 
 tion is seen in metamorphism, where, under the influence of 
 intense shearing, their original structure has been more or less 
 completely effaced, and a new crystalline rearrangement has 
 been developed in them, converting them into schists. 
 
208 GEOLOGY 
 
 CHAPTER XIV. 
 
 ERUPTIVE ROCKS AND MINERAL VEINS. 
 
 NOT only have sedimentary formations since their dep- 
 osition been hardened, plicated, fractured, and some- 
 times even turned into crystalline schists, but into the 
 rents opened in them new masses of mineral matter have been 
 introduced which, in many regions, have entirely changed the 
 structure of the crust below and the appearance of the surface 
 above. Broadly speaking, there are two ways in which these 
 new masses have been wedged into their places. First of all, 
 eruptive material in a molten, or at least in a viscous or plastic 
 condition, has been thrust upward into the cool and consolidated 
 crust of the earth; and in the next place, various ores and 
 minerals have been deposited from solution in cracks and fissures, 
 which they have entirely filled up. To each of these two kinds 
 of later rocks attention will be given in this chapter. 
 
 ERUPTIVE ROCKS. 
 
 The rise of eruptive matter thrust upwards from lower depths 
 within the planet is one of the causes by which the structure 
 of the crust has been most seriously affected. In Chapter IX 
 reference was made to some of the features connected with the 
 protrusion of molten rocks in the production of volcanoes, and 
 more particularly to those subterranean changes which, when 
 all the outer and ordinary tokens of a volcano have been swept 
 away, remain as evidence of former volcanic action, even in dis- 
 tricts where every symptom of volcanic activity has long vanished. 
 We have now to inquire, generally, in what forms eruptive matter 
 has been built into the earth's crust, and what changes it has 
 produced there, apart from those superficial manifestations which 
 are the visible signs of volcanic action. 
 
 When a mass of lava is forced upwards from the heated in- 
 
ERUPTIVE ROCKS AND MINERAL VEINS 209 
 
 terior of the earth towards the surface the form which it finally 
 takes and in which it cools and solidifies must depend upon the 
 shape of the rent or cavity into which it has been thrust. We 
 may compare such a mass to a quantity of melted iron escaping 
 from a blast-furnace. The shape taken by the iron will, of 
 course, be fixed by that of the mould into which it is allowed 
 to run. The crust of the earth, as was pointed out in the 
 previous chapter, has undergone extensive movements, whereby 
 its rocks have been crumpled and broken. It consequently pre- 
 sents in different parts very various degrees of resistance to any 
 force acting upon it from below. The eruptive materials have 
 sometimes risen in the fissures, sometimes have forced their way 
 between the beds and joints of the strata. According to the 
 form of the mould in which they have solidified, we may classify 
 the eruptive rocks of the crust into (1) bosses; (2) sheets; 
 (3) veins and dykes; and (4) necks. 
 
 BOSSES. These are circular, elliptical, or irregularly shaped 
 masses of rock which, while still in a liquid or viscous state, 
 have been ejected into irregular rents of the earth's crust and 
 have solidified there. They consist of various crystalline rocks, 
 more especially granite, syenite, quartz-porphyry, diorite, diabase, 
 and basalt-rocks, and vary in width from a few yards to several 
 miles. Being generally harder than the surrounding rocks, they 
 commonly stand up as prominent knobs, hills, or ridges. Their 
 presence at the surface, however, is due, not to their original 
 protrusion there, as in a volcanic cone, but to the removal of 
 the overlying part of the original crust under which they cooled 
 and consolidated. Every boss is thus a witness of the extensive 
 wearing away of the surface of the land (Fig. 105). 
 
 In some large bosses there may have been a complex system 
 of fissures in which the eruptive material rose. Forced upwards 
 into these, the molten rock would no doubt envelope separated 
 masses of the crust, and might bear them along with it in its 
 ascent. We may even conceive it to have melted down such 
 enveloped masses. Pushing the rocks aside and thrusting itself 
 into every available crack in them the eruptive mass would work 
 its way across the crust. Where it succeeded in opening a pas- 
 sage to the surface, ordinary volcanic phenomena would take 
 place, such as disruption of the ground, ejection of stones and 
 ashes, and outflow of lava. But, no doubt, in a vast number 
 
210 GEOLOGY 
 
 of cases, no such communication was ever effected. The eruptive 
 material paused in its upward passage and consolidated below 
 ground. 
 
 No rock affords more interesting bosses than granite. Two 
 features are especially well displayed by it the marginal veins 
 or dykes and the surrounding ring of metamorphism produced 
 in the rocks through which granite has risen. Granite has in- 
 vaded many different kinds of rocks, and has effected various 
 kinds of change in them. Bound its margin, large numbers 
 of veins or dykes of granite or quartz-porphyry often strike out 
 from it into the surrounding rocks. There can be no doubt 
 that these are portions of the granite material, squeezed into 
 cracks that opened in the crust around it during its ascent. 
 More important is the change that can be observed to have taken 
 place in the rocks immediately surrounding the boss. The 
 granite at the time of its protrusion was probably in a molten 
 or pasty condition and impregnated with hot water or steam 
 and vapours. For a distance varying from a few feet up to 
 two or three miles, according chiefly to the size of the granite 
 mass, the rocks next to it have undergone alteration, the nature 
 and amount of which appear to have been in great measure de- 
 pendent on the chemical and mineralogical composition of the 
 rocks themselves. This metamorphism consists partly in mere 
 induration, but still more in the development of new minerals, 
 or a new crystalline structure, even out of non-crystalline sedi- 
 mentary materials. The very same rock, which is elsewhere 
 a dark limestone full of shells, corals, or other organic remains, 
 becomes a white crystalline marble next the granite, with no 
 trace of any organisms, and so unlike its usual condition that 
 no one would readily believe it to be the same rock. Again, a 
 dark shaly sandstone or graywacke traced towards the granite 
 begins to show an increasing amount of mica. New minerals 
 likewise make their appearance, particularly garnets, until the 
 rock entirely loses its sedimentary structure and becomes a 
 garnetiferous mica-schist. Shales and slates, as they approach 
 the granite, likewise present a remarkable development of fine 
 mica-plates, and pass into argillaceous schists or phyllites, with 
 crystals of chiastolite or other minerals developed in them. The 
 alteration of rocks round eruptive masses is called Contact- 
 metamorphism. 
 
ERUPTIVE ROCKS AND MINERAL VEINS 211 
 
 What the cause may be of this remarkable alteration has not 
 yet been satisfactorily made out. In some bosses, the mere heat 
 of the eruptive material was probably sufficient to produce change. 
 There must often have been also a copious discharge of hot 
 vapours and water which would powerfully affect the adjacent 
 rocks. Silica and other substances might then be introduced, 
 leading to induration and new chemical rearrangements of the 
 constituents. The protrusion of enormous masses of granite 
 may also have given rise to mechanical movements in the earth's 
 crust, like those which have produced the shearing and schistose 
 structure, seen in regional metamorphism. 
 
 a * 
 
 Fig. 105. Outline and section of a Boss (a) traversing stratified 
 rocks (6). 
 
 SHEETS. Sometimes the easiest passage for the erupted ma- 
 terial from below has lain between the bedding of strata. The 
 molten rock, after ascending some fissure or pipe has found 
 its farther progress barred, and has escaped by forcing up the 
 overlying beds and thrusting itself in below them. On cooling 
 and consolidating, it appears as a sheet or bed intercalated 
 between older rocks. This structure is represented in Fig. 107. 
 Any one examining such a section on the ground, might naturally 
 regard the sheet s as a bed of lava erupted at the surface after 
 the formation of the strata a and before that of &. But various 
 features characteristic of intrusive or subsequently injected 
 sheets enable us to distinguish them from those which have been 
 poured out during the deposition of the strata among which 
 they lie. For example, intrusive sheets break across the strata 
 (as at d in Fig. 107) and send veins into them. They are 
 
212 
 
 GEOLOGY 
 
 commonly most close-grained along their edges, where they have 
 been most rapidly chilled by contact with the cool rocks ; while, 
 on the contrary, true lava-streams erupted at the surface are 
 generally most slaggy and scoriform on their upper and under 
 surfaces. Lastly, they have generally hardened and otherwise 
 altered the rocks above and below them, sometimes baking or 
 even fusing them. Where these characters are present, we may 
 confidently infer that, though a sheet of crystalline rock, so far 
 as visible at the surface, may seem to be regularly interstratified 
 between sedimentary beds, as if it had been contemporaneously 
 poured forth among them, it has nevertheless been thrust in 
 between them and may be of much younger date. 
 
 On the other hand, a truly contemporaneous sheet or group 
 of sheets marking the actual outpouring of lava-streams at the 
 
 Fig. 106. Ground-plan of Granite-boss with ring of Contact-Metamor- 
 phism ; (a) sandstones, shales, etc., dipping at high angles in the 
 direction of the arrows; (6) zone or ring within which these rocks 
 are metamorphosed; (c) granite sending out veins into &. 
 
 surface, during the deposition of the strata among which they 
 now lie, may be recognised by equally distinctive characters. 
 Thus they do not break across nor send veins into the overlying 
 or underlying beds, while their upper and under surfaces are 
 usually their most open cellular portions though they are often 
 more or less vesicular or amygdaloidal throughout. In Fig. 108 
 the beds marked 1, 2, 3, and 4 are sheets of different lavas 
 interstratified contemporaneously in the series of sandstones, 
 shales, limestones, and other strata among which they lie. Frag- 
 ments of them are not infrequently to be detected in the overlying 
 strata, which are thus shown to be of later origin, and bands 
 of tuff are commonly associated with them, just as showers 
 of ashes accompany the lava-streams of living volcanoes. As 
 
ERUPTIVE ROCKS AND MINERAL VEINS 213 
 
 an illustration of the way in which the evidence of ancient vol- 
 canic action may be gathered, the section given in Fig. 109 
 may be taken supplementary to the data given already in Chapter 
 IX. At the bottom of the section we stand on the slaggy upper 
 surface of a lava-stream (1) which was poured out under water, 
 for directly above it comes a seam of dark shale (2) representing 
 fine mud that was deposited from suspension in water. That 
 volcanic explosions still continued after the outflow of the lava, 
 is indicated by the abundant bits of slaggy lava and volcanic 
 detritus scattered through the shale, and that the scene of these 
 operations was the sea-floor is conclusively proved by the numer- 
 ous shells, crinoids, and other marine remains that lie in some 
 bands of the shale. The bottom must then have been muddy 
 
 a 
 
 Fig. 107. Intrusive Sheet. 
 
 and not so well suited as it afterwards became for the support 
 of life. Above the shale we find two feet of limestone (3) 
 which is entirely made up of fragments of marine organisms. 
 These creatures, when the water had cleared, continued to flourish 
 abundantly until their congregated remains formed a bed of 
 solid limestone. But from some change in the geography of 
 the region, currents bearing dark mud once more invaded this 
 part of the sea and threw down the material that now forms 
 the band of shale (4). The absence of organic remains in this 
 band probably shows the inroad of mud to have destroyed the 
 life that had previously been prolific. When this condition of 
 things had been brought about, renewed volcanic explosions took 
 place in the neighbourhood. First came showers of dust, ashes, 
 and stones, which fell over the sea, and are now represented by 
 the band of tuff (5). Then followed the outpouring of a stream 
 of lava (6), with its characteristic cellular structure. But this 
 did not quite exhaust the vigour of the volcano, for the band of 
 tuff (7) points to successive showers of dust and stones. When 
 
214 
 
 GEOLOGY 
 
 the explosion ceased, the deposition of dark mud, which had 
 been interrupted by the volcanic episode, was resumed, and the 
 band of shale (8) was laid down. From the fragments of ferns 
 and other plants in this shale it is clear that land was not far 
 off. The sea had evidently been gradually shallowing by the 
 
 Fig. 108. Interstratified or contemporaneous Sheets. 
 
 infilling of sediment and volcanic materials, and at last, on the 
 muddy flat, represented by the layer of fire-clay (9), marshy 
 vegetation sprang up into a thick jungle like the mangrove- 
 swamps of tropical shores at the present day. But after growing 
 long enough to form the bed of matted vegetable matter now 
 represented by the coal-seam (10), the 
 verdant jungle was invaded by the sea, 
 and sank under the muddy water that 
 threw down upon its submerged surface 
 the gray shale (11). In this shale we 
 detect interesting traces of the renewal 
 of volcanic activity, more especially in 
 occasional large blocks of lava, which 
 have evidently been ejected by some 
 volcanic explosion, as in the example 
 already cited in Chap. IX (Fig. 38). A 
 more vigorous volcanic outburst poured 
 out the stream of columnar lava (12) 
 which buried the whole and forms the 
 top of the section. 
 
 Tustrate" evMence t0 of VEINS AND DYKES. These have al- 
 
 contemporaneous vol- ready been referred to in Chapter IX as 
 came action. . . -, -, 
 
 part of the evidence for volcanic action. 
 
 We have here to consider how they occur in connection with the 
 protrusion of eruptive material within the crust of the earth. 
 Where the material so erupted has solidified in a vertical or 
 
ERUPTIVE ROCKS AND MINERAL VEINS 215 
 
 iiearly vertical fissure so as to form a wall-like mass, it is called 
 a dyke (Fig. 43 and d in Fig. 107). Otherwise the portions 
 of erupted rock that have consolidated in irregular rents are 
 known as veins. 
 
 Veins are of common occurrence round bosses of granite, where 
 they can be traced into the parent mass from which they have 
 proceeded (Fig. 106). They may likewise be observed in con- 
 nection with intrusive sheets and bosses of basalt and diorite, 
 from which they ramify outwards into the surrounding rocks. 
 Their occurrence there is one of the proofs of the intrusive 
 character and subsequent date of such sheets. 
 
 Dykes vary from less than a foot to 70 feet or upward in 
 breadth, and run in nearly straight courses sometimes for many 
 miles. They consist most usually of diabase, andesite, basalt, 
 or allied rock. Sometimes they have risen along lines of fault; 
 
 Fig. 110. Map of Dykes near Muirkirk, Ayrshire. 1. Silurian rocks. 
 2. Lower Old Red Sandstone. 3. Carboniferous rocks. /, /, /. Faults. 
 d, d. Dykes. 
 
 but in hundreds of instances in Great Britain, they do not appear 
 to be connected with any faults, but actually cross some of 
 the largest faults in the country without being deflected. The 
 remarkable way in which dykes have risen through a complicated 
 series of rocks and faults and have preserved their courses is 
 exemplified in Fig. 110. 
 
 Like intrusive sheets, but in a less degree, dykes harden or 
 otherwise alter the rocks on either side of them; they likewise 
 present a similar closeness of grain along their margins where 
 the molten rock was most rapidly chilled by coming in contact 
 
216 
 
 GEOLOGY 
 
 with the cold walls of the fissure. Sometimes, indeed, their 
 sides are coated with a thin crust of black glass, as if they 
 had been painted with tar. This glass represents the effect of 
 rapid cooling (see Basalt-glass, Chap. XI). No doubt the whole 
 rock of the dyke, at the time when it rose from below and filled 
 up the space between the two walls of its opened fissure, was 
 a molten glass. The portions that were at once chilled by contact 
 with the walls adhered as a layer of glass. But inside this 
 layer, the molten rock had more time to cool. In cooling, its 
 various minerals crystallised and the present crystalline struc- 
 ture was developed. But even yet, though most of the rock is 
 formed of crystalline minerals, portions of the original glass 
 may not infrequently be detected between them when thin sec- 
 tions are placed under the microscope. 
 
 Fig. 111. Section of a volcanic neck. The dotted lines suggest the 
 original form of the volcano. 
 
 NECKS. These are the filled-up pipes or funnels of former 
 volcanic vents. Their connection with volcanic action has been 
 already alluded to in Chap. IX. They are circular or elliptical in 
 ground-plan, and vary in diameter from a few yards up. to a 
 mile or more (see Figs. 40, 41, 42). They consist of some form 
 of lava (quartz-porphyry, basalt, diorite, etc.) or of the frag- 
 mentary materials that, after being ejected from the volcanic 
 chimney, fell back into it and consolidated there. They occur 
 more particularly in districts where beds of lava and tuff are 
 interstratified with other rocks. The necks represent the vents 
 from which these volcanic materials were ejected. In Fig. Ill, 
 for example, the bed of lava and tuff (b b) interstratified be- 
 tween the strata a a and c c have been folded into an anticline. 
 In the centre of the arch rises the neck (n) which has probably 
 been the chimney that supplied these volcanic sheets, and which 
 
ERUPTIVE ROCKS AND MINERAL VEINS 
 
 217 
 
 has been filled up with coarse tuff, and traversed with dykes 
 and veins of basalt (*). The dotted lines, suggestive of the 
 outline of the original volcano, may serve to indicate the con- 
 nection between the neck and its volcanic sheets, and also the 
 effects of denudation. 
 
 Necks are frequently traversed by dykes (* in Fig. Ill), as 
 we know also to be the case with the craters of modern volcanoes. 
 The rocks surrounding a neck are sometimes bent down round 
 it, as if they had been dragged down by the subsidence of the 
 material filling up the vent; they are also frequently much 
 hardened and baked. When we reflect upon the great heat of 
 molten lava and of the escaping gases and vapours, we may well 
 expect the walls of a volcanic vent to bear witness to the effects 
 of this heat. Sandstones, for instance, have been indurated into 
 quartzite, and shales have been baked into a hardened clay or 
 porcelain-like substance. 
 
 a a 3 4) 
 
 Fig. 112. Section of a Mineral vein. 
 MINERAL VEINS. 
 
 Into the fissures opened in the earth's crust there have been 
 introduced various simple minerals and ores which, solidifying 
 there, have taken the form of Mineral Veins. These materials 
 are to be distinguished from the eruptive veins and dykes above 
 described. A true mineral vein consists of one or more minerals 
 filling up a fissure which may be vertical, but is usually more or 
 less inclined, and may vary in width from less than an inch 
 up to 150 feet or more. The commonest minerals (or veinstones) 
 found in these veins are quartz, calcite, barytes, and fluor-spar. 
 The metalliferous portions (or ores) are sometimes native metals 
 
218 GEOLOGY 
 
 (gold and copper, for example), but are more usually metallic 
 oxides, silicates, carbonates, sulphides, chlorides, or other com- 
 binations. These materials are commonly arranged in parallel 
 layers, and it may often be noticed that they have been deposited 
 in duplicate on each side of a vein. In Fig. 112, for instance, 
 we see that each wall (w w) is coated with a band of quartz 
 (1, 1), followed successively by one of blende (sulphide of zinc, 
 2, 2), galena (sulphide of lead, 3, 3), barytes (4, 4) and quartz 
 (5, 5). The central portion of the vein (6) is sometimes empty 
 or may be filled up with some veinstone or ore. Eemarkable 
 variations in breadth characterise most mineral veins. Some- 
 times the two walls come together and thereafter retire from 
 each other far enough to allow a thick mass of mineral matter 
 to have been deposited between them. Great differences may 
 also be observed in the breadth of the several bands composing 
 a vein. One of these bands may swell out so as to occupy the 
 whole breadth of the vein, and then rapidly dwindle down. The 
 ores are more especially liable to such variations. A solid mass 
 of ore may be found many feet in breadth and of great value; 
 but when followed along the course of the vein, may die away 
 into mere strings or threads through the veinstones. 
 
 The duplication of the layers in mineral veins shows that the 
 deposition proceeded from the walls inwards to the centre. In 
 the diagram (Fig. 112) it is evident that the walls were first 
 coated with quartz. The next substance introduced into the 
 vein was sulphide of zinc, a layer of which was deposited on 
 the quartz. Then came sulphide of lead, and lastly, quartz again. 
 The way in which the quartz-crystals project from the two sides 
 shows that the space between them was free, and, as above 
 stated, it has sometimes remained unfilled up. 
 
 There appears to be now no reason to doubt that the sub- 
 stances deposited in mineral veins were mainly introduced dis- 
 solved in water. Not improbably heated waters rose in the 
 fissures, and as they cooled in their ascent, they coated the walls 
 with the minerals which they held in solution. These minerals 
 may have been abstracted from the surrounding rocks by the 
 permeating water ; or they may have been carried up from some 
 deeper source within the crust. During the process of infilling, 
 or after it was completed, a fissure has sometimes reopened, and 
 a new deposition of veinstones or ores has taken place. Now 
 
ERUPTIVE ROCKS AND MINERAL VEINS 219 
 
 and then, too> land-shells and pebbles are found far down in 
 mineral veins, showing that during the time when the layers of 
 mineral matter were being deposited, the fissures sometimes com- 
 municated with the surface. 
 
 SUMMARY. In this chapter it has been shown that, in many 
 cases, the rents in the earth's crust have been filled up with 
 mineral matter introduced into them, either (i) in the molten 
 state, or (ii) in solution in water, (i) The forms assumed by 
 the masses of eruptive rock injected into the crust of the earth 
 have depended upon the shape of the fissures into which the 
 melted matter has been poured, as the form of a cast-iron bar 
 is regulated by that of the mould into which the melted metal is 
 allowed to run. Taking this principle of arrangement, we find 
 that eruptive rocks may be grouped into (1) Bosses, or irregu- 
 larly-shaped masses, which have risen through fissures or orifices, 
 and now, owing to the removal of the rock under which they 
 solidified, form hills or ridges. The eruptive material sends 
 out veins into the surrounding rocks which are sometimes con- 
 siderably altered, forming a metamorphic ring round the eruptive 
 rock. (2) Sheets or masses which have been thrust between 
 the bedding-planes of strata. These resemble truly interstratified 
 beds, but the difference between the two kinds of structure can 
 be readily appreciated. Interstratified beds mark the occurrence 
 of volcanic phenomena at the surface during the time of the 
 formation of the strata among which they occur. Intrusive 
 sheets, on the other hand, are always subsequent in date to the 
 rocks between which they lie. (3) Veins and dykes, consisting 
 of eruptive rock which has been thrust between the walls of 
 irregular rents or straight fissures. (4) Necks, or the filled-up 
 pipes of former volcanic vents, (ii) Mineral veins are masses 
 of mineral matter which has been deposited, probably from 
 aqueous solution, between the walls of fissures in the earth's 
 crust, and consists of bands of veinstones (quartz, calcite, barytes, 
 etc.) and ores (native metals, or oxides, sulphides, etc., of 
 metals) . 
 
220 GEOLOGY 
 
 CHAPTER XV. 
 
 FOSSIL REMAINS. 
 
 IN AN" earlier part of this volume (Chapter VIII) attention 
 was called to the various circumstances under which the 
 remains of plants and animals may be entombed and pre- 
 served in sedimentary accumulations. When these remains have 
 thus been buried they are known as Fossils. 
 
 NATURE AND USE OF FOSSILS. The word " fossil," meaning 
 literally " dug up," was originally applied to all kinds of mineral 
 substances taken out of the earth ; but it is now exclusively used 
 for the remains or traces of plants and animals imbedded by 
 natural causes in any kind of rock, whether loose and incoherent, 
 like blown sand, or solid, like the most compact limestone. It 
 includes not only the actual remains of the organisms. The 
 empty mould of a shell which has decayed out of the stone that 
 once enveloped it, or the cast of the shell which has been entirely 
 replaced by inorganic sand, mud, calcite, silica, etc., are fossils. 
 The very impressions left by organisms, such as the burrow or 
 trail of a worm in hardened mud, and the footprints of birds 
 and quadrupeds upon what is now sandstone, are undoubted 
 fossils. In short, under this general term is included whatever 
 bears traces of the form, structure, or presence of organisms 
 preserved in the sedimentary accumulations of the surface, or 
 in the rocks underneath. 
 
 In geological history fossils are of fundamental importance. 
 They enable us to investigate conditions of geography, of climate, 
 and of life in ancient times, when these conditions were very- 
 different from those which now prevail on the earth's surface. 
 They likewise furnish the ground on which the several epochs 
 of geological history can be determined, and on which the stages 
 of that history in one country can be compared with those in 
 another. So valuable and varied is the evidence supplied by 
 
FOSSIL REMAINS 221 
 
 fossils to the geologist, that he regards them as among the most 
 precious documents accessible to him for unravelling the past 
 history of the earth. Some knowledge of the structure and 
 classification of plants and animals is essential for an intelligent 
 appreciation of the use of fossils in geological inquiry. An 
 effort is made in this work to give the learner such aid as is fairly 
 within its scope to arrive at an intelligent understanding ; but it 
 must be understood that for adequate information on this sub- 
 ject recourse should be had to text-books of Botany and Zoology. 
 
 CONDITIONS FOR THE PRESERVATION OF ORGANIC EEMAINS. 
 It is obvious that all kinds of plants and animals have not the 
 same chances of being preserved as fossils. In the first place, 
 only those, as a rule, are likely to become fossils whose remains 
 can be kept from decay and dissolution by being entombed in 
 some kind of deposit. Hence land-animals and plants have, 
 on the whole, less chance of preservation than those living in 
 the sea, because deposits capable of receiving and securing their 
 remains are exceptional on land, but are generally distributed 
 over the floor of the sea. We should expect, therefore, that 
 among the records of past time, traces of marine should largely 
 preponderate over traces of terrestrial life. Now this is every- 
 where the case. We know relatively little of the assemblages 
 of plants and animals which in successive epochs have lived upon 
 the dry land, but we have a comparatively large amount of infor- 
 mation regarding those which have tenanted the sea. For this 
 reason, marine fossils are more valuable than terrestrial, in 
 comparing the records of the successive epochs of geological 
 history in different parts of the globe. 
 
 In the second place, from their own chemical composition and 
 structure, plants and animals present extraordinary differences 
 in their aptitude for preservation as fossils. Where they possess 
 no hard parts, and are liable to speedy decay, we can hardly 
 expect that they should leave behind them any enduring relic 
 of their existence. Hence a large proportion, both of the vege- 
 table and animal kingdoms, may at once be excluded as in- 
 herently unlikely to occur in the fossil condition. Of course, 
 under exceptional circumstances, traces of almost any organism 
 may be preserved, and therefore we should probably not be 
 justified in saying that by no chance might some recognisable 
 
222 GEOLOGY 
 
 vestige of it be found fossil. Nothing seems more perishable 
 than the tiny gnats and other forms of insect life that fill the 
 air on a summer evening. Yet many of these short-lived flies 
 have been sealed up within the resin of trees (amber), and their 
 structure has been admirably preserved. Such exceptional in- 
 stances, however, only bring out more distinctly how large a 
 proportion of the living tribes of the land must utterly perish, 
 and leave no recognisable record of their ever having existed. 
 
 But, where there are hard parts in an organism, and especially 
 where, from their chemical composition, they can for some time 
 resist decay, they may, under favourable conditions, be buried 
 in sedimentary deposits, and may remain for indefinite ages 
 locked up there. It is obvious, therefore, that animals possessing 
 hard parts are much the most likely to leave permanent relics 
 of their presence, and ought to occur most frequently as fossils. 
 It is these animals whose remains are preserved in peat-mosses, 
 river-gravels, lake-marls, and on the sea-floor at the present time. 
 Yet, if we were to judge of the extent of the whole existing 
 animal kingdom solely from the fragmentary remains so pre- 
 served, what an utterly inadequate conception of it we should 
 form ! So, too> if we estimate the variety of the living creatures 
 of past time merely from the evidence of the fossils that have 
 chanced to be preserved among the rocks, we shall probably 
 arrive at quite as erroneous a conclusion. There can be no 
 doubt that from the earliest time only an insignificant fraction 
 of the varied life of each period has been preserved in the fossil 
 state, as is unquestionably the case at the present day. 
 
 DURABLE PARTS OF PLANTS. The essential parts of the solid 
 framework of plants consist of the substances known as cellulose 
 and vasculose, which, when kept in dry air, or when waterlogged 
 and buried in stiff mud, may remain undecomposed for long 
 periods. The timber beams in the roofs and floors of old build- 
 ings are evidence that, under favourable conditions, wood may 
 last for many centuries. Some plants eliminate carbonate of 
 lime from solution in water, and form with it a solid substance 
 which requires no further treatment to enable it to endure for 
 an indefinite period, when screened from the action of water. 
 Still more durable are the remains of those plants which abstract 
 silica and build it up into their framework, such as the diatoms 
 of which the frustules become remarkably permanent fossils, in 
 
FOSSIL REMAINS 223 
 
 the form of diatom-earth or tripoli-powder, which is made up 
 of them. 
 
 DURABLE PARTS OF ANIMALS. The hard parts of animals 
 may be preserved with little or no chemical change, and remain 
 as durable relics. The hard horny integuments of insects, arach- 
 nids, Crustacea, and some other animals, are composed essentially 
 of the substance called chitin, which can long resist decomposi- 
 tion, and which may therefore be looked for in the sedimentary 
 deposits of the present time, as well as of former periods. The 
 chitin of some fossil scorpions, admirably preserved among the 
 Carboniferous rocks of Scotland, can hardly be distinguished 
 from that of the living scorpion. Many of the lower forms of 
 animal life secrete silica, and their hard parts are consequently 
 easily preserved, as in the case of radiolaria and sponges. In the 
 great majority of instances, however, the hard parts of inverte- 
 brates consist mainly of carbonate of lime, and are readily pre- 
 served among sedimentary deposits. The skeletons of corals, the 
 plates of echinoderms, and the shells of molluscs, are examples 
 of the abundance of calcareous organisms, and the frequency 
 of their remains in the fossil state shows how well fitted they 
 are for preservation. Among vertebrates the hard parts consist 
 chiefly of phosphate of lime. In some forms (ganoid fishes and 
 crocodiles, for example) this substance is partly disposed outside 
 the body (exo-skeleton) in the form of scales, scutes, or bony 
 plates. But more usually it is confined to the internal skeleton 
 (endo-skeleton). It is mainly by their bones and teeth that 
 the higher vertebrates can be recognised in the fossil state. 
 Sometimes the excrement has been preserved (Coprolites), and 
 may furnish information regarding the food of the animals, 
 portions of undigested scales, teeth, and bones being traceable 
 in it (Fig. 65). 
 
 FOSSILISATION, The process by which the remains of a plant 
 or animal are preserved in the fossil state is termed Fossilisation. 
 It varies greatly in details, but all these may be reduced to three 
 leading types. 
 
 1. Entire or partial preservation of the original substance. 
 In rare instances, the entire animal or plant has been preserved, 
 of which the most remarkable examples are those where carcases 
 of the extinct mammoth have been sealed up in the frozen mud 
 and peat of Siberia, and have thus been preserved in ice. Insects 
 
224 
 
 GEOLOGY 
 
 have been involved in the resin of trees, and may now be seen, 
 embalmed like mummies, in amber. More usually, however, a 
 variable proportion of the organic matter has passed away, and 
 its more durable parts have been left, as in the carbonisation 
 of plants (peat, lignite, coal) and the disappearance of the 
 organic matter from shells and bones, which then become dry 
 and brittle and adhere to the tongue. 
 
 2. Entire removal of the original substance and internal struc- 
 ture,, only the external form being preserved. When a dead 
 animal or plant has been entombed, the mineral matter in which 
 it lies hardens round it and takes a mould of its form. This 
 may be accomplished with great perfection if the material is 
 sufficiently fine-grained and solidifies before the object within 
 has time to decay. Carbonate of lime and silica are especially 
 well adapted for taking the moulds of organisms, but fine mud, 
 
 Fig. 113. Common Cockle (Cardium edule) ; (a) side view of both 
 valves; (&) mould of the external form of one valve taken in plaster 
 of Paris; (c) side view of cast in plaster of Paris of interior of the 
 united valves. 
 
 marl, and sand, are also effective. The organism may then 
 entirely decay, and its substance may be gradually removed by 
 percolating water, leaving a hollow empty space or mould of its 
 form. Such moulds are of frequent occurrence among fossil- 
 iferous rocks, and are specially characteristic of molluscs, the 
 shells of which are so abundant, and occur imbedded in so many 
 different kinds of material. Sometimes it is the external form 
 of the shell that has been taken, the shell itself having entirely 
 disappeared; in other cases a cast of the interior of the shell 
 has been preserved. How different these two representations 
 of the same shell may be is shown in Fig. 113, wherein a repre- 
 sents a side view of the common cockle, while c is a cast of the 
 
FOSSIL REMAINS 225 
 
 interior of the shell in plaster of Paris. The contrast between 
 a mould of the outside and inside of the same shell is shown by 
 the difference between & and c, which are both impressions taken 
 in plaster. 
 
 After the decay and removal of the substance of the enclosed 
 organisms, the moulds may be filled up with mineral matter, 
 which is sometimes different from the surrounding rock. The 
 empty cavities have formed convenient receptacles for any deposit 
 which permeating water might introduce. Hence we find casts 
 of organisms in sand, clay, ironstone, silica, limestone, pyrites, 
 and other mineral substances. Of course, in such cases, though 
 the external form of the original organism is preserved, there 
 is no trace of internal structure. No single particle of the cast 
 may ever have formed part of the plant or animal. 
 
 3. Partial or entire petrifaction of organic structure by molec- 
 ular replacement. Plants and animals which have undergone 
 this change have had their substance gradually removed and 
 replaced, particle by particle, with the mineral matter. This 
 transformation has been effected by percolating water contain- 
 ing mineral solutions, and has proceeded so tranquilly, that 
 sometimes not a delicate tissue in the internal structure of a 
 plant has been displaced, and yet so rapidly, that the plant had 
 not time to rot before the conversion was completed. Accordingly, 
 in true petrifactions, that is, plants or animals of which the 
 structure has been more or less perfectly preserved in stone, the 
 petrifying material is always such as may have been deposited 
 from water. The most common substance employed by nature 
 in the process of petrification is carbonate of lime, which, as 
 we have seen, is almost always present in the water of springs 
 and rivers. Organic structures replaced by this substance are 
 said to be calcified. Frequently the carbonate of lime has as- 
 sumed, more or less completely, a crystalline structure after its 
 deposition, and in so doing has generally injured or destroyed 
 the organic structure which it originally replaced. Where the 
 calcareous matter of an organism has been removed by percolat- 
 ing water, as often happens in sands, gravels, or other porous 
 deposits, the fossil is said to be decalcified. Another abundant 
 petrifying medium in nature is silica, which, in its soluble form, 
 is generally diffused in terrestrial waters, where humous acids 
 or organic matter are present in solution. The replacement of or- 
 
226 GEOLOGY 
 
 game structures by silica, called silicification, furnishes the most 
 perfect form of petrifaction. The interchange of mineral mat- 
 ter has been so complete that even the finest microscopic 
 structures have been faithfully preserved. Silicified wood is an 
 excellent example of this perfect replacement. Sulphides, which 
 are often produced by the reducing action of decaying organic 
 matter upon sulphates, occur also as petrifying media, the most 
 common being the iron sulphide, usually in the less stable form 
 a marcasite, but sometimes as pyrite. Carbonate of iron like- 
 wise frequently replaces organic structures; the clay-ironstones 
 of the Carboniferous system abound with the remains of plants, 
 shells, fishes, and other organisms which have been converted 
 into siderite (Fig. 62). 
 
 The chief value of fossils in geology is to be found in the 
 light which they cast upon former conditions of geography and 
 climate, and in the clue which they furnish to the relative ages 
 of different geological formations. 
 
 1. HOW FOSSILS INDICATE FORMER CHANGES IN GEOGRAPHY. 
 
 Terrestrial plants and animals obviously point to the existence 
 of land. If their remains are found in strata wherein most of 
 the fossils are marine, they usually show that the deposits were 
 laid down upon the sea-floor not far from land. But where 
 they occur in the positions in which they lived and died, they 
 prove that their site was formerly a land-surface. The stumps 
 of trees remaining in their positions of growth, with their roots 
 branching out freely from them in the clay or loam underneath, 
 undoubtedly mark the position of an ancient woodland. If, with 
 these remains, there are associated in the same strata wing-cases 
 of beetles, bones of birds and of land-animals, additional cor- 
 roborative evidence is thereby obtained as to the existence of 
 the ancient land. More usually, however, it is by deposits left 
 on lake-bottoms that the land of former periods of geological 
 history is known. As already pointed out (Chapter IV), the 
 fine mud and marl of lakes receive and preserve abundant relics 
 of the vegetation and animal life of the surrounding regions. 
 As illustrations of lacustrine formations, from which most of 
 our knowledge of the contemporary terrestrial life is obtained, 
 reference may be made to the Molasse of Switzerland, the lime- 
 stones and marls of the Limagne d'Auvergne, and the vast depth 
 of strata from which so rich an assemblage of plant and animal 
 
FOSSIL REMAINS 227 
 
 remains has been obtained in the Western Territories of the 
 United States (see Chapter XXV). Alternations of buried 
 forests or peat-mosses, with lake deposits, show how lakes have 
 successively increased and diminished in volume. The frequent 
 occurrence of a bed of lacustrine marl at the bottom of a peat- 
 bog proves how commonly shallow lakes have been filled up and 
 displace by the growth of marshy vegetation. 
 
 Remains of marine plants and animals almost invariably 
 demonstrate that the locality in which they are found was once 
 covered by the sea. Exceptions to this rule are so few as hardly 
 to be worthy of special notice, as, for instance, when molluscs, 
 crustaceans, and other forms of marine life are carried up by 
 sea-birds to considerable elevations, where, after their soft parts 
 have been eaten, their hard shells and crusts may be preserved 
 in truly terrestrial deposits, or when sea-shells, tossed up by 
 breakers above the tide-line, are swept inland by wind. 
 
 Rolled fragments of shells, mingled in well-rounded gravel and 
 sand, point to some former shore where these materials were 
 ground down by beach-waves. Fine muddy sediment containing 
 unbroken shells, echinoderms, crustaceans, and other relics of the 
 sea indicate deeper water beyond the scour of waves, tides, and 
 currents. Beds of limestone, full of corals and crinoids, mark the 
 site of a clear sea, in which these organisms were allowed to 
 flourish undisturbed for many generations. It may often be 
 observed that the fossils, which are abundant and large in a 
 limestone, become few in number and small in size in an over- 
 lying bed of shale or clay ; or that they wholly disappear in the 
 argillaceous rock. The meaning of this can hardly be mistaken. 
 The clear water in which the marine creatures were able to build 
 up the limestone was at last invaded by some current carrying 
 mud. Consequently, while the more delicate forms perished, 
 others continued to live on in diminished numbers and dwarfed 
 development, until at last the muddy sediment settled down so 
 thickly that the animals, whose hard parts might have been pre- 
 served, were driven away from that area of the sea-bottom. 
 
 2. How FOSSILS INDICATE FORMER CONDITIONS OF CLIMATE. 
 The remains of plants or animals characteristic of tropical coun- 
 tries may be taken to bear witness to a tropical climate at the 
 time which they represent. If for example, a deposit were found 
 containing leaves of palms and bones of tigers, lions and ele- 
 
228 GEOLOGY 
 
 phants, we should infer that it was formed in some tropical coun- 
 try, such as the warmer parts of Africa or Asia. On the other 
 hand, were a stratum to yield leaves of a small birch and willow, 
 with bones of reindeer, musk-ox, and lemming, we would regard 
 it as evidence of a cold climate. Such inferences, however, must 
 be based either upon the occurrence of the very same species as 
 are now living, and the characteristic climate of which is known, 
 or upon assemblages of plants or animals which may be compared 
 with corresponding assemblages now living. We may be toler- 
 ably confident that the existing reindeer has always been restrict- 
 ed to a cold climate, and that the living elephants have as char- 
 acteristically been confined to warm climates. But it would be 
 rash to assume that all deer prefer cold and all elephants choose 
 heat. The bones of an extinct variety of elephant and one of 
 rhinoceros, have long been known as occurring even up within 
 the Arctic regions, and when these remains were first found the 
 conclusion was naturally drawn that they proved the former ex- 
 istence of a warm climate in the far north. But the subse- 
 quent discovery of entire carcases of the animals covered with 
 a thick mat of woolly hair, showed that they were adapted for 
 life in a cold climate, and their occurrence in association with the 
 remains of animals which still live in the Arctic regions, proved 
 beyond doubt that the original inference regarding them was 
 erroneous. In drawing conclusions as to climate from fossil 
 evidence, it is always desirable to base them upon the concurrent 
 testimony of as large a variety of organisms as possible, and to 
 remember that they become less and less reliable in proportion 
 as the organisms on which they are founded depart from the 
 species now living. 
 
 3. How FOSSILS INDICATE GEOLOGICAL CHRONOLOGY. As 
 the result of careful observations all over the world, it has been 
 ascertained that in the youngest strata the organic remains are 
 nearly or quite the same as species now living, but that, as we 
 proceed into older strata, the number of existing species dimin- 
 ishes, and the number of extinct species increases, until at last 
 no living species is to be found. Moreover, the extinct species 
 found in younger strata disappear as we trace them back into 
 older rocks, and itieir places are taken by other extinct species. 
 Every great series of fossiliferous rocks is thus characterised by 
 its own peculiar assemblage of species. Not only do the species 
 
FOSSIL REMAINS 229 
 
 change ; the genera, too, disappear one by one as we follow them 
 into older rocks, until among the earliest strata only a few of the 
 living genera are represented. Whole families and orders of ani- 
 mals which once flourished have utterly vanished from the living 
 world, and we only know of their existence from the remains of 
 them preserved among the rocks. 
 
 A certain definite order of succession has been observed among 
 the organic remains imbedded in the stratified rocks of the earth's 
 crust, and this order has been found to be broadly alike all over 
 the world. The fossils of the oldest fossiliferous rocks of 
 Europe, for instance, are like those of the oldest fossiliferous 
 rocks of Asia, Africa, America, and Australasia, and those of each 
 succeeding series of rocks follow the same general sequence. It 
 is obvious, therefore, that fossils supply us with an invaluable 
 means of fixing the relative position of rocks in the series of 
 geological formations. Whether or not the same type of fossils 
 was always contemporaneous over the whole planet cannot be de- 
 termined ; but it generally occupied the same place in the proces- 
 sion of life. Hence stratified formations, which may be quite 
 unlike each other in regard to the nature of their component ma- 
 terials, if they contain similar organic remains, may be compared 
 with each other, and classed under the same name. 
 
 Fossils Characteristic of particular subdivisions of the series of 
 geological formations are known as type-fossils, of which the fol- 
 lowing are examples : 
 
 Lepidodendra and Sigillarise, characteristic of Old Red Sandstone and 
 Carboniferous rocks. 
 
 Cycads, characteristic of Mesozoic rocks. 
 
 Graptolites, characteristic of Silurian rocks. 
 
 Trilobites Cambrian to Carboniferous rocks (Figs. 
 
 118, 125, etc.). 
 
 Cystideans, characteristic of Silurian rocks (Fig. 123). 
 
 Blastoids Carboniferous rocks (Fig. 150). 
 
 Hippurites Cretaceous rocks. 
 
 Orthoceratites " Palaeozoic rocks (Figs. 130, 137.) 
 
 Ammonites Mesozoic rocks (Figs. 167, 176, 190). 
 
 Cephalaspid fishes " Silurian, Old Red Sandstone. 
 
 Ichthyosaurus and Plesiosaurus Mesozoic rocks (Fig. 180). 
 
 Iguanodon Cretaceous rocks (Fig. 192). 
 
 Toothed birds Cretaceous rocks. 
 
 Nummulites, Palseotherium, Anoplotherium, Deinocerata, character- 
 istic of older Tertiary rocks. 
 
 Mastodon, Elephas, Equus, Cervus, Hyaena, Apes, characteristic of 
 younger Tertiary and Recent rocks. 
 
230 . GEOLOGY 
 
 By attentive study and comparison, the fossiliferous rocks in 
 different countries have been subdivided into sections, each 
 characterised by its own f acies or type of organic remains. Con- 
 sequently, beginning with the oldest and proceeding upward to 
 the youngest, we advance through natural chronicles of the suc- 
 cessive tribes of plants and animals which have lived on the 
 earth's surface. These chronicles, consisting of sandstones, 
 shales, limestones, and the other kinds of stratified deposits, 
 form what is called the Geological Record. In order to establish 
 their true sequence in time, their Order of Superposition must 
 first be determined; that is, it is requisite to know which lie at 
 the bottom, and must have been formed first, and in what order 
 the others succeed them. When this fundamental question has 
 once been settled, then the fossils characteristic of each group of 
 strata serve as a guide for recognising that group wherever it may 
 be found. 
 
 While fossils enable us to divide the Geological Record into 
 chapters, they also show how strikingly imperfect this record is 
 as a history of the plants and animals that have lived on the 
 surface of the earth, and of the revolutions which that surface 
 has undergone. We may be sure that the progress of life, from 
 its earliest appearance in lowly forms of plant or animal, has been 
 continuous up to the present condition of things. But in the 
 Geological Record there occur numerous gaps. The fossils of one 
 group of rocks are succeeded by a more or less completely differ- 
 ent series in the next group. At one time it was supposed that 
 such breaks in the continuity of the record marked terrestrial 
 convulsions which caused the destruction of the plants and ani- 
 mals of the time, and were followed by the creation of new tribes 
 of living things. But evidence has every year been augmenting 
 to indicate that no such general destruction and fresh creation 
 ever took place. The gaps in the record mark no real interrup- 
 tion of the life of the globe. They are rather to be looked upon 
 as chapters that have been torn out of the annals, or which never 
 were written. We have already learnt in Chapter VIII how 
 many chances there must be against the preservation of any- 
 thing like a complete record of the life of the globe at any par- 
 ticular time. It is also clear that even where the chronicle may 
 have been comparatively full, it is exposed to many dangers after- 
 wards. The rocks containing it may be hidden beneath the sea, 
 
FOSSIL REMAINS 
 
 231 
 
 or raised up into land and entirely worn away, or entombed be- 
 neath volcanic ejections, or so crushed and crumpled as to become 
 no longer legible. 
 
 Taking fossils as a guide, geologists have partitioned the fos- 
 siliferous rocks into what are called stratigraphical subdivisions 
 as follows : A bed, or limited number of beds, in which one or 
 more distinctive species of fossils occur, is called a zone 
 or horizon, and may be named after its most typical fossil. Thus 
 in the Lias, the zone in which the ammonite known as Am- 
 monites Jamesoni occurs, is spoken of as the " zone of Am- 
 monites Jamesoni" or "Jamesoni-zone." Two or more zones, 
 united by the occurrence in them of a number of the same char- 
 acteristic species or genera, form what are known as Beds or an 
 Assise. Two or more of such beds or assises may be termed a 
 Group or Stage. Where the number of assises in a stage is large 
 they may be subdivided into Sub-stages or Sub-groups. The 
 stage or group will then consist of several sub-stages, and each 
 sub-stage or sub-group of several assises. A number of groups 
 or stages is combined into a Series, Section, or Formation, and 
 a number of series, sections, or formations constitute a System. 
 A number of systems are connected together to form each of the 
 great divisions of the Geological Eecord. This classification will- 
 be best understood if placed in tabular form, as in the subjoined 
 subdivisions, which occur in the Cretaceous System. 1 
 
 Stratigraphical com- 
 ponents. 
 
 A stratum, layer, seam, 
 or bed, or a number 
 of such minor subdi- 
 visions, characterised 
 by some distinctive 
 fossil. 
 
 Two or more zones 
 
 Two or more sets of 
 connected beds or 
 assises 
 
 Descriptive Names. 
 
 = Zone or horizon, 
 
 Beds or an assise. .. . 
 
 I Group or stage, which 
 I may be subdivided 
 I into sub-groups or 
 sub-stages 
 
 Series, section, or for- 
 mation 
 
 Two or more groups or 
 stages 
 
 Several related forma- 
 tions 
 1 For an account of the Cretaceous System, see Chapter XXIV. 
 
 System 
 
 Examples from the Cre- 
 taceous System. 
 
 Zone of Pecten asper. 
 
 Warminster beds. 
 
 Cenomanian stage, com- 
 prising the Rotho- 
 magian and Caren- 
 tonian sub-stages. 
 
 Neocomian formation. 
 Cretaceous System. 
 
232 GEOLOGY 
 
 The names by which the larger subdivisions of the Geological 
 Record are known have been adopted at various times and on no 
 regular system. Some of them are purely lithological ; that is, 
 they refer to the mere mineral nature of the strata, apart alto- 
 gether from their fossils, such as Coal-measures, Chalk, Green- 
 sand, Oolite. These names belong to the early years of the prog- 
 ress of geology, before the nature and value of organic remains 
 had been definitely realised. Other epithets have been suggested 
 by localities where the strata occur, as London Clay, Oxford Clay, 
 Mountain Limestone. The more recent names for the larger 
 divisions have, in general, been chosen from districts where the 
 strata are typically developed, or where they were first critically 
 studied, e.g. Silurian, Devonian, Permian, Jurassic. In some 
 cases, the larger subdivisions have received names from some dis- 
 tinguishing feature in their fossil contents, as Eocene, Miocene, 
 Pliocene. 1 But it is mainly to the minor sections that the char- 
 acters of the fossil contents have supplied names. 
 
 The designation of any particular group of strata has gradually 
 come to acquire a chronological meaning. Thus we speak of the 
 Oolites or Oolitic formations of England, and include under these 
 terms a thick series of limestones, clays, sandstones, and other 
 strata, replete with organic remains, and containing the records 
 of a long interval of geological time. But we also speak of the 
 Oolitic period a phrase which, in the strict grammatical use 
 of the word, is of course incorrect, but which conveniently desig- 
 nates the period of geological time during which the great series 
 of Oolites was deposited, and when the abundant life of which 
 they contain the remains flourished on the surface of the earth. 
 This chronological meaning has indeed come to be the more 
 usual sense in which the names of the major subdivisions of the 
 Geological Record are generally employed. Such adjectives as 
 Devonian and Jurassic do not so much suggest to the mind of 
 the geologist Devonshire and the Jura Mountains, from which 
 they were taken, nor even the rocks to which they are applied, 
 as the great sections of the earth's history of which these rocks 
 contain the memorials. He compares the Jurassic or Devonian 
 rocks of one country with those of another, studies the organic 
 remains contained in them, and then obtains materials for form- 
 
 1 For the meanings of these names see Chapter XXV. 
 
FOSSIL REMAINS 233 
 
 ing some conception of what were the conditions of geography 
 and climate, and what was the general character of the vegetable 
 and animal life of the globe, during the periods which he classes 
 as Jurassic and Devonian. 
 
 SUMMARY. Fossils are the remains or traces of plants and 
 animals which have been imbedded in the rocks of the earth's 
 crust. From the exceptional nature of the circumstances in 
 which these remains have been entombed and preserved, only a 
 comparatively small proportion of the various tribes of plants 
 and animals living at any time upon the earth is likely to be 
 fossilised. Those organisms which contain hard parts are best 
 fitted for becoming fossils. The original substance of the organ- 
 ism may, in rare cases, be preserved; more usually the organic 
 matter is partially or wholly removed. Sometimes a mere cast 
 of the plant or animal in amorphous mineral matter retains the 
 outward form without any trace of the internal structure. In 
 other instances, true petrifaction has taken place, the organic 
 structure being reproduced in calcite, silica, or other mineral by 
 molecular replacement. 
 
 Fossils are of the utmost value in geology, inasmuch as they 
 indicate (1) former changes in geography, such as the existence 
 of ancient land-surfaces, lakes, and rivers, the former extension 
 of the sea over what is now dry land, and changes in the currents 
 of the ocean; (2) former conditions of climate, such as an Arctic 
 state of things as far south as Central France, where bones of 
 reindeer and other Arctic animals have been found; (3) the 
 chronological sequence of geological formations, and, conse- 
 quently, the succession of events in geological history, each great 
 group of strata being characterised by its distinctive fossils. This 
 is the most important use of fossils. Having ascertained the 
 order of superposition of fossiliferous rocks, that is, the order in 
 which they were successively deposited, and having found what 
 are the characteristic fossils of each subdivision, we obtain a 
 guide by which to identify the various rock-groups from district 
 to district, and from country to country. By means of the evi- 
 dence of fossils the stratified rocks of the Geological Eecord have 
 been divided into sections and subsections, to which names are 
 applied that have now come to designate not merely the rocks 
 and their fossils, but the period of geological time during which 
 these rocks were accumulated and these fossils actually lived. 
 
234 GEOLOGY 
 
 PART IV. 
 
 THE GEOLOGICAL BECOED OF THE HISTOEY OP 
 THE EAETH. 
 
 CHAPTEE XVI. 
 
 EARLIEST CONDITIONS OF THE GLOBE. 
 
 THE foregoing chapters have dealt chiefly with the materials 
 of which the crust of the earth consists, with the processes 
 whereby these materials are produced or modified, and 
 with the methods pursued by geologists in making their study of 
 these materials and processes subservient to the elucidation of 
 the History of the Earth. The soils, rocks, and minerals be- 
 neath our feet, like the inscriptions and sculptures of a long-lost 
 race of people, are in themselves full of interest, apart from 
 the story which they chronicle ; but it is when they are made to 
 reveal the history of land and sea, and of life upon the earth, 
 that they are put to their noblest use. The investigation of the 
 various processes whereby geological changes are carried on at 
 the present day is undoubtedly full of fascination for the student 
 of nature ; yet he is conscious that it gains enormously in interest 
 when he reflects that in watching the geological operations of the 
 present day he is brought face to face with the same instruments 
 whereby the very framework of the continents has been piled 
 up and sculptured into the present outlines of mountain, valley, 
 and plain. 
 
 The highest aim of the geologist is to trace the history of the 
 earth. All his researches, remote though they may seem from 
 this aim, are linked together in the one great task of unravelling 
 the successive mutations through which each area of the earth's 
 surface has passed, and of discovering what successive races of 
 plants and animals have appeared upon the globe. The investiga- 
 
EARLIEST CONDITIONS OF THE GLOBE. 235 
 
 tion of facts and processes, to which the previous pages have been 
 devoted, must accordingly be regarded as in one sense introduc- 
 tory to the highest branch of geological inquiry. We have now to 
 apply the methods and principles already discussed to the eluci- 
 dation of the history of our planet and its inhabitants. Within 
 the limits of this volume only a mere outline of what has been 
 ascertained regarding this history can be given. I shall arrange 
 in chronological order the main phases through which the globe 
 seems to have passed, and present such a general summary of the 
 more important facts regarding each of them as may, P hope, 
 convey an adequate outline of what is at present known regard- 
 ing the successive periods of geological history. 
 
 As the primitive stages of mankind upon the earth and the 
 early progress of every race fade into the obscurities of mythology 
 and archaeology, so the story of the primeval condition of our 
 globe is lost in the dim light of remote ages, regarding which 
 almost all that is known or can be surmised is furnished by the 
 calculations and speculations of the astronomer. If the earth's 
 history could only be traced out from evidence supplied by the 
 planet itself, it could be followed no further back than the 
 oldest portions of the earth now accessible to us. Yet there can 
 be no doubt that the planet must have had a long history before 
 the appearance of any of the solid portions now to be seen. That 
 such was the case is made almost certain by the traces of a 
 gradual evolution or development which astronomers have been 
 led to recognise among the heavenly bodies. Our earth being 
 only one of a number of planets revolving round the sun, the 
 earliest stages of its separate existence must be studied in refer- 
 ence to the whole planetary system of which it forms a part. 
 Thus, in compiling the earliest chapter of the history of the 
 earth, the geologist turns for evidence to the researches of the 
 astronomer among stars and nebulas. 
 
 In recent years, more precise methods of inquiry, and, in par- 
 ticular, the application of the spectroscope to the study of the 
 stars, have gone far to confirm the speculation known as the 
 Nebular Hypothesis. According to this view, the orderly related 
 series of heavenly bodies, which we call the Solar System, existed 
 at one time, enormously remote from the present, as a Nebula 
 that is, a cloudy mass of matter, like one of those nebulous, 
 faintly luminous clouds which can be seen in the heavens. This 
 
236 GEOLOGY 
 
 nebula probably extended at least as far as the outermost plane- 
 tary member of the system is now removed from the sun. It 
 may have consisted entirely of incandescent gases or vapours, 
 or of clouds of stones in rapid movement, like the stones that 
 from time to time fall through our atmosphere as meteorites, and 
 reach the surface of the earth. The collision of these stones 
 moving with planetary velocity would dissipate them into vapour, 
 as is perhaps the case in the faint luminous tails of comets. At 
 all events, the materials of the nebula began to condense, and in 
 so doing threw off, or left behind, successive rings (like those 
 around the planet Saturn), which, in obedience to the rotation 
 of the parent nebula, began to rotate in one general plane around 
 the gradually shrinking nucleus. As the process of condensation 
 proceeded, these rings broke up, and their fragments rushed to- 
 gether with such force as not improbably to generate heat enough 
 to dissipate them .again into vapour. They eventually condensed 
 into planets, sometimes with a further formation of rings, or 
 with a disruption of these secondary rings, and the consequent 
 formation of moons or satellites round the planets. The outer 
 planets would thus be the oldest, and, on the whole, the coolest 
 and least dense. Towards the centre of the nebula the heaviest 
 elements might be expected to condense, and there the high tem- 
 perature would longest continue. The sun is the remaining in- 
 tensely hot nucleus of the original nebula, from which heat is 
 still radiated to the furthest part of the system. 
 
 When a planetary ring broke up, and by the heat thereby 
 generated was probably reduced to the state of vapour, its ma- 
 terials, as they cooled, would tend to arrange themselves in ac- 
 cordance with their respective densities, the heaviest in the centre, 
 and the lightest outside. In process of time, as cooling and con- 
 traction advanced, the outer layers might grow quite cold, while 
 the inner nucleus of the planet might still be intensely hot. 
 Such, in brief, is the well-known Nebular Hypothesis. 
 
 Now the present condition of our earth is very much what, ac- 
 cording to this hypothesis or theory, it might be expected to be. 
 On the outside comes the lightest layer or shell in the form of an 
 Atmosphere, consisting of gases and vapours. Below this gaseous 
 envelope which entirely surrounds the globe lies an inner en- 
 velope of water, the ocean, which covers about two-thirds of the 
 earth's surface, and is likewise composed of gases. Underneath 
 
EARLIEST CONDITIONS OF THE GLOBE 237 
 
 this watery covering, and rising above it in dry land, rests the 
 solid part of the globe, which, so far as accessible to us, is com- 
 posed of rocks twice or thrice the weight of pure water. But 
 observations with the pendulum at various heights above the sea 
 show that the attraction of the earth as a whole indicates that 
 the globe probably has a density about five and a half times that 
 of water. Hence we may infer that its inner nucleus not im- 
 probably consists of heavy materials, and may be metallic. There 
 is thus evidence of an arrangement of the planet's materials in 
 successive spherical shells, the lightest or least dense being on the 
 outside, and the heaviest or most dense in the centre. 
 
 Again, the outside of the earth is now quite cool; but abun- 
 dant proof exists that at no great distance below the surface the 
 temperature is high. Volcanoes, hot springs, and artificial bor- 
 ings all over the world testify to the abundant store of heat within 
 the earth. Probably at a depth of not more than 20 miles from 
 the surface the temperature is as high as the melting-point of 
 any ordinary rock at the surface. By far the largest part of the 
 planet, therefore, is hotter than molten iron. We need have no 
 hesitation in admitting it to be highly probable that the earth 
 was formerly in the state of incandescent vapour, and that it has 
 ever since that time been cooling and contracting. Its present 
 shape affords strong presumption in favour of the opinion that 
 the globe was once in a plastic condition. The flattening at the 
 poles and bulging at the equator, or what is called the oblately 
 spheroidal figure of the planet, is just the shape which a plastic 
 mass would have assumed in obedience to the influence of the 
 movement of rotation, imparted to it when detached from the 
 parent nebula. 
 
 At present a complete rotation is performed by the earth in 
 twenty-four hours. But calculations have been made with the 
 result of showing that originally the rate of rotation was much 
 greater. Fifty-seven millions of years ago it was about four 
 times faster, the length of the day- being only six and three-quar- 
 ter hours. The moon at that time was only about 35,000 miles 
 distant from the earth, instead of 239,000 miles as at present. 
 Since these early times the rate of rotation has gradually been 
 diminishing, and the figure of the earth has been slowly tending 
 to become more spherical, by sinking in the equatorial and rising 
 in the polar regions. 
 
238 GEOLOGY 
 
 Of the first hard crust that formed upon the surface of the earth 
 no trace has yet been found. Indeed, there is reason to suppose 
 that this original crust would break up and sink into the molten 
 mass beneath, and that not until after many such formations and 
 submergences did a crust establish itself of sufficient strength to 
 form a permanent solid surface. Even though solid, the surface 
 may still have been at a glowing red-heat, like so much molten 
 iron. Over this burning nucleus lay the original atmosphere, 
 consisting -not merely of the gases in the present atmosphere, 
 but of the hot vapours which subsequently condensed into the 
 ocean, or were absorbed into the crust. It was a hot, vaporous 
 envelope, .under the pressure of which the first layers of water 
 that condensed from it may have had the temperature of molten 
 lead. As the steam passed into water, it would carry down with 
 it the gaseous chlorides of sodium, magnesium, and other vapours 
 in the original atmosphere, so that the first ocean was probably 
 not only hot, but intensely saline. 
 
 Eegarding these early ages in the earth's history we can only 
 surmise, for no direct record of them has been preserved. They 
 are sometimes spoken of as pre-geological ; but geology really 
 embraces the whole history of the planet, no matter from what 
 sources the evidence may be obtained. Deposits from this orig- 
 inal hot saline ocean have been supposed to be recognisable in the 
 very oldest crystalline schists ; but for this supposition there does 
 not appear to be any good ground. The early history of our 
 planet, like that of man himself, is lost in the dimness of an- 
 tiquity, and we can only speculate about it on more or less 
 plausible suppositions. 
 
 When we come to the solid framework of the earth we stand 
 on firmer footing in the investigation of geological history. The 
 terrestrial crust, or that portion of the globe which is accessible 
 to human observation, has been found to consist of successive 
 layers of rock, which, though far from constant in their occur- 
 rence, and though often broken and crumpled by subsequent 
 disturbance, have been recognised over a large part of the globe. 
 They contain the earth's own chronicle of its history, which has 
 already been referred to as the Geological Eecord, and the sub- 
 division of which into larger and minor sections, according 
 mainly to the evidence of fossils, was explained in the preceding 
 chapter. 
 
EARLIEST CONDITIONS OF THE GLOBE 239 
 
 Had the successive layers of rock that constitute the Geological 
 Kecord remained in their original positions, only the uppermost, 
 and therefore most recent, of them would have been visible, and 
 nothing more could have been learnt regarding the underlying 
 layers, except in so far as it might have been possible to explore 
 them by boring into them. But the deepest mines do not reach 
 greater depths than between 3,000 and 4,000 feet from the sur- 
 face. Owing, however, to the way in which the crust of the 
 earth has been plicated and fractured, portions of the bottom 
 layers have been pushed up to the surface, and those that lay 
 above them have been thrown into vertical or inclined positions, 
 so that we can walk over their upturned edges and examine them, 
 bed by bed. Instead of being restricted to merely the uppermost 
 few hundred feet of the crust, we are enabled to examine many 
 thousand feet of its rocks. The total mean thickness of the ac- 
 cessible fossiliferous rocks of Europe has been estimated at 75,000 
 feet, or upwards of 14 miles. This vast depth of rock has been 
 laid bare to observation by successive disturbances of the crust. 
 
 The main divisions of the Geological Eecord and, we may also 
 say, of geological time, are five: (1) Archaean, embracing the 
 periods of the earliest rocks, wherein no traces of organic life 
 occur; (2) Palaeozoic (ancient life) or Primary, including the 
 long succession of ages during which the earliest types of life 
 existed; (3) Mesozoic (middle life) or Secondary, comprising a 
 series of periods when more advanced types of life nourished ; (4) 
 Cainozoic (recent life) or Tertiary, embracing the ages when the 
 existing types of life appeared, but excluding man; and (5) 
 Quaternary or Post-tertiary and Eecent, including the time since 
 man appeared upon the earth. It must not be supposed that 
 each of these five divisions was of the same duration. The 
 Palaeozoic ages were probably vastly more prolonged than those 
 of any later division; while the Quaternary periods must com- 
 prise a very much briefer time than any of the other four groups. 
 
 Each of these main sections is further subdivided into systems 
 or periods, and each system into formations as already explained. 
 Arranged in their order of sequence, the various divisions of the 
 Geological Eecord may be placed as in the accompanying Table. 
 
240 GEOLOGY 
 
 THE GEOLOGICAL RECORD, 
 
 or, Order of Succession of the Stratified Formations of the Earth's Crust 
 
 Recent and Prehistoric. 
 Pleistocene or Glacial. 
 
 a 
 
 Pliocene. 
 Miocene. 
 Oligocene. 
 
 Eocene. 
 
 Cretaceous. 
 Danian. 
 Senonian. 
 Turonian. 
 Cenomanian. 
 Gault (Albian). 
 Neocomian. 
 
 Jurassic. 
 Purbeckian. 
 Portlandian. 
 Kimmeridgian. 
 Corallian. 
 Oxfordian. 
 Bathonian. 
 Bajocian. 
 Liassic. 
 
 Triassic. 
 Rhsetic. 
 
 Keuper or Upper Trias. 
 Muschelkalk. 
 Bunter or Lower Trias. 
 
EARLIEST CONDITIONS OF THE GLOBE 
 
 241 
 
 Permian. 
 
 Upper Red Sandstones, clays, and gypsum. 
 Magnesian Limestone (Zechstein). 
 Marl-Slate (Kupferschiefer). 
 Lower Red Sandstones, breccias, etc. (Rothliegende), 
 
 Carboniferous. 
 Coal-Measures. , 
 Millstone-Grit. 
 Carboniferous Limestone series. 
 
 Devonian and Old Red Sandstone. 
 
 Devonian 
 Type 
 
 Old Red 
 
 Sandstone 
 Type 
 
 Upper Cypridina and Goniatite beds. 
 Middle Stringocephalus (Eifel) Limestone. 
 Lower Spirifer Sandstone, etc. 
 
 Upper Yellow and Red Sandstones, with Holop- 
 
 tychius, Pterichthys major, etc. 
 Lower Sandstones, flagstones, and conglomerates, 
 
 with Cephalaspis, Coccosteus, Asterolepis, etc. 
 
 Silurian. 
 
 Ludlow group. 
 
 Wenlock group. 
 
 Upper Llandovery group. 
 f Lower Llandovery group. 
 I Caradoc and Bala group. 
 1 Llandeilo group. 
 [Arenig group. 
 
 .a & 
 
 fc 
 O rt 
 
 Upper Tremadoc Slates. 
 
 8 | 
 
 O M 
 
 a $ 
 
 oi T3 
 
 Lingula Flags. 
 
 11 
 
 2 1 
 
 Lower Menevian group. 
 
 Oj P-4 
 
 P-l H 
 
 11 
 
 Harlech group. 
 
 O 
 
 5 
 
 
 Longmyndian Uriconian. 
 
 [Dalradian.] 
 
 Torridonian. 
 
 Archaean Lewisian, Hebridean. 
 
 THE PRE-CAMBRIAN PERIODS. 
 
 Owing to the revolutions which the crust of the earth has un 
 dergone, there have been pushed up to the surface, from under 
 neath the oldest fossiliferous strata, certain very ancient crystal- 
 
242 GEOLOGY 
 
 line rocks which form what is termed the Archaean system. As 
 already mentioned, these rocks have by some geologists been sup- 
 posed to be a part of the primeval crust of the planet, which 
 solidified from fusion. By others they have been thought to 
 have been formed in the boiling ocean, which first condensed 
 upon the still hot surface of the globe. In truth, we are still 
 profoundly ignorant as to the conditions under which they arose. 
 We have hardly any means of ascertaining in what order they 
 were formed. We know no method of determining whether those 
 of one region belong to the same period as those of another. N or 
 can we always be sure that what have been called Archaean rocks 
 may not belong to -a, much later part of the Geological Eecord, 
 their peculiar crystalline structure having been superinduced 
 upon them by some of those subterranean movements described 
 in Chapter XIII. 
 
 Of Archaean rocks the most abundant is gneiss, passing on the 
 one hand into granite, and on the other into micaceous and 
 argillaceous schists, with interstratified bands of various horn- 
 blendic, pyroxenic, and garnetiferous rocks, limestone, dolomite, 
 serpentine, quartzite, graphite, hematite, magnetite, etc. These 
 various materials are more or less distinctly bedded. But the 
 beds are for the most part inconstant, swelling out into 
 thick zones, and then rapidly diminishing and dying out. This 
 bedding somewhat resembles that of sedimentary rocks, and the 
 manner in which the limestone and graphite occur, recalls the 
 way in which limestone and coal are found in the fossiliferous 
 formations. The inference has accordingly been drawn that the 
 Archaean crystalline bands were really deposited as chemical pre- 
 cipitates or mechanical sediments on the floor of the primeval 
 ocean, and have since been more or less crystallised and disturbed. 
 But from what has been brought forward in Chapter XIII, re- 
 garding the totally new structures which have been developed in 
 rocks by subterranean movement, it is evident that a bedded ar- 
 rangement and a crystalline texture, like those of the Archaean 
 gneisses and schists, have sometimes been induced in rocks by ex- 
 cessive crumpling, fracture, and shearing. How far, therefore, 
 the apparent bedding of Archaean rocks is their original condi- 
 tion, or is the result of subsequent disturbance, is a question that 
 cannot yet be answered. 
 
 The alternations of gneiss and other crystalline masses form 
 
EARLIEST CONDITIONS OF THE GLOBE 243 
 
 bands which are usually placed on end or at high angles, and 
 are often intensely crumpled and puckered, having evidently un- 
 dergone enormous crushing (Fig. 114). Attempts have been 
 made to subdivide them into groups or series, according to their 
 apparent order of succession and lithological characters. But 
 such subdivisions, even where practicable, are probably only of 
 local value. As a rule, those members of the system which, if 
 the succession of beds may be trusted, are the lowest and oldest, 
 present coarser crystalline characters than those which seem to 
 be higher and later. They often consist of massive granitic 
 gneiss, with abundant veins and bands of the coarsely crystalline 
 variety of granite, known as pegmatite. The apparently higher 
 rocks are less coarsely crystalline gneiss, and often mica-schists 
 and other schistose masses. 
 
 Fig. 114. Fragment of crumpled Schist. 
 
 No unquestionable relic of organic existence has been met with 
 among Archaean rocks. Some of the Archaean limestones of Can- 
 ada have yielded a peculiar mixture of serpentine and calcite, 
 with a structure which is regarded by some able naturalists as 
 that of a reef -building f oraminifer. It occurs in masses, and is 
 supposed by these writers to have grown in large, thick sheets or 
 reefs over the sea-bottom. By most observers, however, this 
 supposed organism (to which the name of Eozoon has been 
 given) is now regarded as merely a mineral segregation, and vari- 
 ous undoubted mineral structures are pointed to in illustration 
 and confirmation of this view. 
 
 Archaean rocks cover a large area in Europe. Among the 
 Hebrides and along the north-west coast of the Scottish High- 
 lands, where they are most largely developed, they consist of a 
 
244 GEOLOGY 
 
 very ancient group of rocks, of which the most conspicuous are 
 various forms of gneiss (Lewisian, Hebridean, Fundamental), 
 overlain by a much younger series of dull red sandstones, con- 
 glomerates, and dark gray shales (Torridon), which lie on the 
 gneiss unconformably, and are covered unconformably by the 
 oldest Cambrian strata, containing annelid tracks and the trilo- 
 bite Olenellus. The gneiss gives rise to a singular type of scen- 
 ery. Over much of that region it forms hummocky bosses of 
 naked rock, with tarns and peat-bogs lying in the hollows, seldom 
 rising into mountains, but forming the platform which supports 
 a singular group of red (Torridon) sandstone mountains. Here 
 and there, it mounts up into solitary hills or groups of hills. The 
 highest point it reaches on the mainland is among the mountains 
 on the east side of Loch Maree, in Eoss-shire, where it attains an 
 elevation of 3,000 feet. Some of its masses in that region were 
 mountains at the time of the deposition of the overlying Torridon 
 sandstone, which when removed by denudation reveals a very 
 ancient system of hills and valleys. In the Island of Harris 
 the gneiss sweeps upwards into rugged mountainous ground, of 
 which the highest summits rise more than 2,600 feet out of the 
 Atlantic, and are visible far and wide as a notable landmark. 
 The different varieties of gneiss are associated with dykes of 
 various basic igneous rocks, and bands of pegmatite. Eocks of 
 similar character appear likewise in the west of Ireland ; while in 
 Anglesey, and possibly in the south-west of England, other scat- 
 tered bosses of them rise to the surface. 
 
 Later than the Archaean gneiss, but older than the lowest Cam- 
 brian strata, are certain groups of sedimentary (partly also 
 igneous) rocks which at present can only be grouped under the 
 common term Pre- Cambrian. The Torridon sandstone reaches 
 a thickness of 8,000 or 10,000 feet, and is almost entirely con- 
 fined to the west of the counties of Sutherland and Eoss. It 
 there forms a remarkable group of pyramidal mountains, to 
 which their nearly horizontal stratification gives a characteristic 
 architectural aspect. Traces of organic remains (annelid tracks, 
 etc.) have been found in these strata. 
 
 The term " Dalradian " has recently been applied to a thick 
 series of metamorphosed sedimentary and igneous rocks forming 
 the Central and Southern Highlands of Scotland. They must 
 be of great thickness, but their true geological position is not yet 
 
EARLIEST CONDITIONS OF THE GLOBE 245 
 
 ascertained. They may possibly contain altered representatives 
 of the old gneiss, Torridon sandstone and Cambrian quartzites 
 and limestones of the north-west. 
 
 On the borders of Wales and Shropshire a thick series of 
 sedimentary rocks (Longmyndian) forms the Longmynd country. 
 It appears to be Pre-Cambrian, and may be partly the equivalent 
 of the Torridon sandstone of the north-west. It is underlain by 
 a group of felsitic lavas and tuffs named Uriconian. 
 
 On the continent of Europe, Archaean rocks have their greatest 
 extension in Scandinavia, where they evidently belong to the 
 same ancient land as that of which the Hebrides and Scottish 
 Highlands are fragments. They range through Finland far into 
 Russia, appearing in the centre of the chain of the Ural Moun- 
 tains. They form likewise the nucleus of the Carpathians and 
 the Alps, and appear in detached areas in Bavaria, Bohemia, 
 France, and the Pyrenees. They are estimated to occupy an area 
 of more than 2,000,000 of square miles in the more northerly 
 part of North America, stretching from the Arctic regions south- 
 wards to the great lakes. Both in the Old and New World, the 
 Archaean rocks are chiefly exposed in the northern tracts of the 
 continents. The areas which they there overspread were proba- 
 bly land at an early geological period, and it was the waste of 
 this land that mainly supplied the original materials out of which 
 the enormous masses of stratified rocks were formed. Various 
 thick accumulations of sedimentary and igneous rocks have been 
 ascertained to lie in North America, as in Europe, between the 
 Archaean gneisses and the base of the Cambrian system. 
 
 In the southern hemisphere also ancient gneisses and other 
 schists rise from under the oldest fossiliferous formations. In 
 Australia and in New Zealand they cover large tracts of country, 
 and appear in the heart of the mountain ranges. It thus appears 
 that all over the world the oldest known rocks are gneisses 
 and similar or allied crystalline masses, having a remarkable 
 uniformity of character. 
 
246 GEOLOGY 
 
 CHAPTEE XVII. 
 
 PALAEOZOIC PERIODS CAMBRIAN. 
 
 THE portion of geological history which treats of those ages 
 in which the earliest known types of plants and animals 
 lived is termed Palaeozoic. Of the first appearance of 
 organic life upon our planet we know nothing. Whether plants 
 or animals came first, and in what forms they came, are ques- 
 tions to which as yet no satisfactory answer can be given. The 
 oldest discovered fossils are assuredly not vestiges of the first 
 living things that peopled the globe. There is every reason, in- 
 deed, to hope that as researches in all parts of the world are 
 pushed into older and yet older rocks, still more ancient organ- 
 isms may be discovered. But it is in the highest degree im- 
 probable that any trace of the earliest beginnings of life will ever 
 be found. The first plants and the first animals were probably 
 of a lowly kind, with no hard parts capable of preservation in 
 the fossil state. Moreover, the sedimentary rocks which may 
 have chronicled the first advent of organised existence are hardly 
 likely to have escaped the varied revolutions to which all parts of 
 the crust of the earth have been exposed. They have more prob- 
 ably been buried out of sight, or have been so crushed, broken, 
 and metamorphosed, that their original condition, together with 
 any fossils they may have enclosed, is no longer to be recognised. 
 The first chapters have been, as it were, torn out from the 
 chronicle of the earth's history. 
 
 The Palaeozoic rocks, which contain the earliest record of 
 plant and animal life, consist mainly of the hardened mud, sand, 
 and gravel of the sea-bottom. Here and there they include beds, 
 or thick groups of beds, of limestone composed of marine shells, 
 crinoids, corals, and other denizens of salt water. They are thus 
 essentially the chronicles of the sea. But they also contain 
 occasional vestiges of shores, and even of the jungles and swamps 
 
PALAEOZOIC PERIODS CAMBRIAN 247 
 
 of the land, with a few rare glimpses into the terrestrial life of the 
 time. Everywhere they abound in evidence of shallow water; 
 for though chiefly marine, they appear to have been accumulated 
 not far from land. We may believe that in the earliest periods, 
 as at the present day, the sediment washed away from the land 
 has been deposited on the sea-floor, for the most part at no great 
 distance from the coast. 
 
 The land from the waste of which the Palaeozoic rocks were 
 formed lay in Europe and North America chiefly towards the 
 north. It no doubt consisted of Archa3an rocks, such as still 
 rise out from under the oldest Paleozoic formations. As already 
 mentioned, the north-west Highlands of Scotland, part of the 
 table-land of Scandinavia, and most of North America to the 
 north of the great lakes, are probably portions of that earliest 
 land, which, after being deeply buried under later geological ac- 
 cumulations, have once more been laid bare to the winds and 
 waves. We can form some conception of the bulk of the primeval 
 northern land by noting the thickness of sedimentary rocks that 
 were formed out of its detritus during the Palaeozoic periods. 
 The older half of the Palaeozoic rocks in the British Islands, for 
 example, is at least 16,000 feet or 3 miles thick, and covers an 
 area of not less than 60,000 square miles. This material, derived 
 from the waste of the Archaean rocks, would make a table-land 
 larger than Spain, with an average height of 5,000 feet, or a 
 mountain chain 1,800 miles long, with an average elevation of 
 16,000 feet. Of the general form and height of the northern 
 land that supplied this vast mass of sedimentary matter nothing 
 is known. Perhaps it was lofty; but it may have been slowly 
 uplifted, so that its rise compensated for the ceaseless degrada- 
 tion of its surface. 
 
 Abundant evidence of volcanic action has been preserved 
 among the Palaeozoic rocks in the form of piles of lavas and tuffs. 
 We find also many indications of upward and downward move- 
 ments of the crust of the earth. The mere fact of the super- 
 position of many thousands of feet of shallow-water strata, one 
 above another, is a proof of gradual subsidence. For it is evi- 
 dent that the accumulation of such a thickness of sediment, and 
 the continuance of a shallow sea over the area of deposition, could 
 only take place during a progressive subsidence. 
 
 The life of the Palaeozoic periods, so far as known from the 
 
248 GEOLOGY 
 
 fossils which have been obtained from the rocks, appears to have 
 been far more uniform over the whole globe than at any subse- 
 quent epoch in geological history. For instance, the same species 
 of fossils are found in corresponding rocks in Britain, Eussia, 
 United States, China, and Australia, The climate of the globe 
 at that ancient date was doubtless more uniform than it after- 
 wards became, and was probably also generally warmer. Palae- 
 ozoic fossils, obtained from high northern latitudes, are precisely 
 similar to those that abound in England, whence it may be in- 
 ferred that not only was there a greater uniformity of climate, 
 but that the great cold which now characterises the Arctic regions 
 did not then exist. 
 
 In the earlier Palaeozoic periods, the animal life of the globe 
 appears to have been entirely invertebrate, the highest known 
 types being chambered shells, of which our living nautilus is a 
 representative. In the middle periods vertebrate life appeared. 
 The earliest known vertebrate forms are fishes akin to some 
 modern sharks and to the sturgeon, the polypterus of the 
 Nile, and the gar-pike of American lakes. The most highly 
 organised forms of existence upon the earth's surface in the 
 later Palaeozoic periods were amphibians a class of animals 
 represented at the present day by frogs, toads, newts, and 
 salamanders. It is evident, however, that the number and 
 kinds of animal remains preserved in Palaeozoic rocks afford 
 only an imperfect record of the animal life of these early ages. 
 Whole tribes of creatures no doubt existed of which no trace 
 whatever has yet been recovered. An accidental discovery may 
 at any moment reveal the former presence of some of these van- 
 ished forms. For example, the examination of a fossil tree- 
 trunk imbedded among the coal-strata of Nova Scotia, led to the 
 finding of the first and as yet almost the only traces of Palaeozoic 
 land-shells, though thousands of species of marine shells, belong- 
 ing to the same period, had long been known. Every year is en- 
 larging our knowledge in these respects, but from the very nature 
 of the circumstances in which the records of the rocks were 
 formed, we cannot expect this knowledge ever to be more than 
 fragmentary. 
 
 The Palaeozoic rocks are divided into five systems which in 
 the order of their age have been named; (1) Cambrian; (2) 
 Silurian; (3) Devonian; (4) Carboniferous; (5) Permian. 
 
PALAEOZOIC PERIODS CAMBRIAN 249 
 
 CAMBRIAN. 
 
 The strata containing the earliest organic remains were for- 
 merly known as Graywacke, from the rock which is specially 
 abundant among them. They were also termed Transition, from 
 the supposition that they were deposited during a transitional 
 period, between the time when no organic life was possible on the 
 earth's surface and the time when plant and animal life abounded. 
 But the late Sir Koderick Murchison, who first explored them, 
 showed that they contain a series of formations, each character- 
 ised by its own assemblage of organic remains. He called them 
 the Silurian system, after the name of the old British tribe the 
 Silures, who lived on the borders of England and Wales, where 
 these rocks are especially well developed. This name has now 
 been adopted all over the world as the designation of those 
 stratified formations which contain the same or similar organic 
 remains to those found in the typical region described by 
 Murchison. 
 
 While the succession of the rocks and fossils was established by 
 that geologist in South Wales, and in the border counties of 
 Wales and England, Professor Sedgwick was at work among sim- 
 ilar rocks in North Wales. These were at first believed to be 
 all older than those called Silurian, and were accordingly named 
 "CAMBRIAN, after the old name for Wales, Cambria. In the end, 
 however, it was found that throughout a large part of them the 
 same fossils occurred, as in the Silurian series, and they were 
 accordingly claimed as Silurian. Much controversy has since 
 been carried on regarding the limits and names to be assigned to 
 these rocks, and geologists are not yet agreed upon the nomen- 
 clature that should be followed. Murchison and his followers 
 claimed the Cambrian as the lowest portion of the Silurian sys- 
 tem, while Sedgwick and his disciples maintained that the lower 
 half of the Silurian system should be included in his Cambrian 
 series. There can be no doubt that the first succession of organic 
 remains established among these ancient members of the great 
 Paleozoic series of formations was that worked out by Murchison 
 and named by him Silurian. But it has been found convenient to 
 retain the name Cambrian for the oldest group of fossiliferous 
 formations. It may be well to repeat that these words, like all 
 those adopted by geologists to distinguish the successive rock- 
 
250 GEOLOGY 
 
 groups of the earth's crust, have acquired a chronological mean- 
 ing. We speak, not only of Cambrian and Silurian strata and 
 Cambrian and Silurian fossils, but of Cambrian and Silurian 
 time. The terms are used to denote those particular periods in 
 the history of the earth when Cambrian and Silurian strata were 
 respectively deposited, and when Cambrian and Silurian fossils 
 were the living denizens of sea and land. 
 
 The rocks of which the Cambrian system is composed, like 
 those of the whole of the Lower Palaeozoic formations, present 
 considerable uniformity over the whole globe. They consist of 
 gray and reddish grits, sandstones, graywackes, quartzites, and 
 conglomerates, with thick groups of shale, slate, or phyllite. 
 These sedimentary accumulations attain a great thickness in some 
 countries. In Wales they have been estimated by some observers 
 to be at least 20,000 feet in depth. Their ripple-marks, pebble- 
 beds, and frequent alternations of coarse and fine sediment, point 
 to their having probably been laid down in comparatively shallow 
 water, during a period of prolonged subsidence of the sea-bottom. 
 Tfyey include tuffs and basic lavas which indicate contemporane- 
 ous submarine eruptions. 
 
 With regard to the occurrence of fossils among the older 
 Pakeozoic formations, and indeed among stratified rocks in gen- 
 eral, it is worthy of notice that they are far from being equally 
 distributed ; that, 'on the contrary, they occur by preference in 
 certain kinds of material rather than in others. Grits and 
 sandstones, for instance, are comparatively unfossiliferous, while 
 fine shales, slates, and limestones are often crowded with fossils. 
 It is not that life was probably on the whole more abundant at 
 the time of the deposition of some kinds of strata, but that the 
 local conditions for its growth and for the subsequent entomb- 
 ment and preservation of its remains were then more favourable. 
 At the present time, for example, dredging operations show the 
 most remarkable variations between different and even adjacent 
 parts of the sea-bottom as regards the abundance of marine life. 
 Some tracts are almost lifeless, while others are crowded with a 
 varied and prolific fauna. We can easily understand that if, from 
 the nature of the bottom, plants and smaller animals cannot 
 flourish on a particular tract, the larger kinds that feed on them 
 will also desert it. Even if organisms live and die in some num- 
 bers over a part of the sea-bed, the conditions may not be suita- 
 
PALAEOZOIC PERIODS CAMBRIAN 251 
 
 ble there for the preservation of their remains. The rate of 
 deposit of sediment, for instance, may be so slow that the remains 
 may decay before there is time for them to be covered up ; or the 
 sediment may be unfitted for effectually preserving them, even 
 when they are buried in it. We must not lose sight of these facts 
 in our explorations of the Geological Eecord. A relation has 
 always existed between the abundance or absence of fossils in a 
 sedimentary rock and the circumstances under which the rock 
 was originally formed. 
 
 The oldest fossiliferous strata (Cambrian or Primordial) con- 
 tain a remarkable assemblage of animal remains, which, being the 
 earliest traces of the animal life of the 
 globe, might have been anticipated to belong 
 to the very lowest tribes of the animal king- 
 dom. But they are by no means of such 
 humble organisation. On the contrary, they 
 include no representatives of many of the 
 groups of simpler invertebrates, which we 
 may be sure were nevertheless living at the 
 same time. Not only so, but some of the 
 fossils belong to comparatively high grades 
 injthe scale of invertebrate life, such as 
 chambered molluscs. From this incom- 
 pleteness, and from the wide differences in 
 the organic grade of the forms actually pre- 
 served in the rocks, we may reasonably infer 
 that only a most 'meagre representation of 
 the life of that time has come down to us in Fig. 115. Fucoid-like 
 the fossil 'state. Some of the fossils, more- 'Tfn^Tinniwum) 
 over, have been so indistinctly preserved from Cambrian 
 that considerable difficulty is experienced in 
 deciding to what sections of the animal or vegetable kingdoms 
 they should be assigned. 
 
 Among the markings which have given rise to much discus- 
 sion allusion may be made to plant-like impressions some of 
 which, like Eophyton (Fig. 115), have been claimed as sea-weeds. 
 Others, however, may only be irregular wrinklings of the sur- 
 
 1 The fractional numbers inserted within parentheses in the titles of 
 the figures of fossils indicate how much the figures have been reduced 
 or magnified. Thus J reduced one-third ; f magnified four times. 
 
252 GEOLOGY 
 
 faces of deposit, and of no organic origin at all (see p. 259). 
 Another puzzling impression is that called Oldhamia (Fig. 116), 
 which has been variously referred to the Hydrozoa, the Sertularia, 
 the Polyzoa, and the calcareous Alga3. 
 
 Fig. 116. Oldhamia radiata (natural size), Ireland. 
 
 Some of the most characteristic older Palaeozoic organisms 
 belong to the Hydrozoa, and are embraced under the general title 
 of Graptolites a name given to them from their fancied re- 
 semblance to quill-pens. They were conrp&sed of a horny or 
 chitinous substance, and hence they commonly present themselves 
 
 Fig. 117. Hydrozoon from the Cambrian rocks (Dictyograptus (Dicty- 
 onema) sociale), natural size. 
 
 merely as black streaks upon the stone. Each graptolite was a 
 colony comprising many individuals which occupied each its own 
 cell. The cells are in some kinds placed in a row on one side of 
 a supporting rod or axis ; in other kinds there is a row of cells on 
 
PALAEOZOIC PERIODS CAMBRIAN 
 
 253 
 
 both sides (see Fig. 121). Some varieties are straight, others 
 curved or spiral. Some are simple branches, others are com- 
 posed of two or more branches, while in certain types a large 
 number of separate branches is united in one common centre. 
 One of the most ancient hydrozoa is Dictyograptus (Dictyonema, 
 Fig. 117) a characteristic fossil of the Cambrian rocks of Scan- 
 dinavia. The graptolites are more especially characteristic of 
 the Silurian system. 
 
 The Echinodermata had their representatives in Cambrian 
 time, though the remains of these are few and infrequent. The 
 
 Fig. 118. Cambrian Trilobites. (a) Paradoxides bohemicus (natural 
 size) ; (6) Agnostus princeps ( f ) ; (c) Olenus micrurus (natural 
 size) ; (d) Ellipsocephalus Eofti (natural size). 
 
 great tribe of the Crinoids or Sea-lilies (p. 261) had already 
 established itself on the floor of the Cambrian sea, where also 
 there were representatives of Cystideans and star-fishes. 
 
 Numerous kinds of Sea-worms (Annelids) crawled over the 
 sandy and muddy bottom and shores of the Cambrian ocean. 
 These creatures have left no trace of their bodies, which, like 
 those of their representatives in the present ocean, were soft and 
 unfitted for preservation. But the burrows they made in wet sand 
 
254 GEOLOGY 
 
 or mud, and the trails they left upon the soft surfaces over which 
 they moved, have been abundantly preserved (see Fig. 124). 
 These markings afford unquestionable proof of the presence of 
 creatures which have otherwise utterly disappeared. 
 
 Among the most abundant and characteristic fossils of the 
 older stratified rocks of the earth's crust are those to which the 
 general name of Trilobites has been given. These long-extinct 
 animals were crustaceans, having a more or less distinctly 
 three-lobed body, at one end of which was the head or cephalic 
 shield, usually with a pair of fixed compound eyes ; at the other 
 end the caudal shield or tail ; while between the two shields was 
 the ringed or jointed body, the rings of which were movable, so 
 that the animal could bring the two shields together or coil itself 
 up. It will be seen from the different genera represented in Figs. 
 118 and 125 how varied were the forms which they assumed. In 
 the shapes and relative sizes of the shield and segmented body, 
 in the number of the body-rings, in the development of spines, 
 and in other features, the most wonderful variety is traceable 
 among the trilobites even of the oldest f ossiliferous strata. Some 
 of the earliest genera were also the largest; Paradoxides some- 
 times reaching a length of nearly two feet. Yet contemporane- 
 ous with this large creature were some diminutive forms. A 
 few genera (among them Agnostm) were blind; but most pos- 
 sessed eyes furnished with facets, which in some forms are four- 
 teen in number, while in others they are said to amount to 15,000. 
 The peculiar crescent-shaped eye on each side of the head is well 
 shown in some of the forms represented in Figs. 118 and 125. 
 The trilobites appear to have particularly swarmed on sandy and 
 muddy bottoms, for their remains are abundant in many sand- 
 stones and shales. 
 
 Another form of crustacean life represented in the early Palae- 
 ozoic ocean was that of the Phyllopods animals furnished with 
 bivalve shell-like carapaces, which protected the head and upper 
 part of the body, while the jointed tail projected beyond it. 
 Most of them were of small size (see Fig. 126). The character- 
 istic Cambrian genus is Hymenocaris. 
 
 Of all the divisions of the animal kingdom none is so inv 
 portant to the geologist as that of the Mollusca. When one 
 walks along the shores of the sea at the present time, by far the 
 most abundant remains of the marine organisms to be there ob- 
 
PALAEOZOIC PERIODS CAMBRIAN 255 
 
 served are shells. They occur in all stages of freshness and de- 
 cay, and we may trace even their comminuted fragments forming 
 much of the white sand of the beach. So in the geological forma- 
 tions, which represent the shores and shallow sea-bottoms of 
 former periods, it is mainly remains of the marine shells that 
 have been preserved. From their abundance and wide diffusion, 
 they supply us with a basis for the comparison of the strata of 
 different ages and countries, such as no other kind of organic re- 
 mains can afford. 
 
 It is interesting and important to find that among the fossils 
 of the oldest fossiliferous rocks the remains of molluscan shells 
 occur, and that they are of kinds which can be satisfactorily re- 
 ferred to their place in the great series of the Mollusca. The most 
 abundant of them are representatives of the Brachiopods or 
 Lamp-shells. Among these are species of the genera Lingula (Lin~ 
 gulella, Fig. 119) and Discina which have a peculiar interest, in- 
 asmuch as they are the oldest known molluscs, 
 and are still represented by living species in 
 the ocean. They have persisted with but little 
 change during the whole of geological time, 
 from the early Palaeozoic periods downwards, 
 for the living shells do not appear to indicate Fig. 119^Cambri- 
 any marked divergence from the earliest forms. ^(Lingul^la^^a- 
 They possess horny shells which are not hinged visii, natural 
 together by teeth. A more highly organised 
 order of brachiopods possesses two hard calcareous shells 
 articulated by teeth on the hinge-line. ' These forms, 
 apparently later in their advent, soon vastly outnumbered 
 the horny lingulids and discinids. So abundant are they both in 
 individuals and in genera and species among the older Palaeozoic 
 rocks, that the period to which these rocks belong is sometimes 
 spoken of as the " Age of Brachiopods." 
 
 The ordinary bivalve shells or Lamellibranchs had their re- 
 presentatives even in Cambrian times. From that early period 
 they have gradually increased in numbers, till they have attained 
 their maximum at the present time. Among the known Cam- 
 brian genera are Ctenodonta allied to the living " ark-shells/' and 
 Modiolopsis, probably representing some of the modern mussels. 
 
 The Gasteropods or common univalve shells, now so abundant 
 in the ocean, made their advent not later than Cambrian time, 
 
256 GEOLOGY 
 
 for the remains of the genus Bellerophon (Fig. 129) are found 
 in the group of strata known as the Lingula-flags in Wales. 
 
 The highest division of the molluscs, the Cephalopods, to 
 which the living nautilus and cuttle-fish belong, is but poorly re- 
 presented at the present time. But during the Palaeozoic and 
 Secondary periods it flourished exuberantly, both as regards 
 number of individuals and variety of forms. It is divisible into 
 two great families. In one of these the shell is usually internal 
 and is never chambered; in the other the shellis chambered and 
 external, the chambers being connected by a tube or siphuncle. 
 The former family includes all the living cuttle-fishes, squids, and 
 the paper-nautilus; th-s latter comprises only one living repre- 
 sentative the pearly nautilus. It is to the family of chambered 
 cephalopods that the Palaeozoic forms are all referable. In some 
 the shell was straight, in others it was variously curved. Only 
 scanty traces of cephalopodan life have yet been found among 
 the Cambrian rocks. But occasional examples of the important 
 genus Orthoceras (see Fig. 130) show that this great division of 
 the molluscs had even in the earliest Palaeozoic ages appeared 
 upon the earth. 
 
 As the term Cambrian denotes, the rocks to which this name 
 is applied are well developed in Wales. There, and in the bor- 
 der English counties, they attain a depth of perhaps more than 
 20,000 feet. They are found also in the east of Ireland, while 
 in the north-west of Scotland they appear to be represented by 
 massive red sandstones and conglomerates. The peculiar Pri- 
 mordial fauna has been widely recognised in both hemispheres. 
 It occurs at intervals from France to Russia, and from Sweden 
 to Bohemia and Sardinia. It is well represented in the United 
 States and Canada, and it is met with even as far east as China. 
 
 The following Table gives the commonly accepted subdivisions 
 of the Cambrian rocks in Britain. 
 
 Tremadoc group dark gray slates. 
 Lingula Flags bluish and black 
 flags, and sandstones. 
 
 W OJ 
 
 W> ~ 
 
PALAEOZOIC PERIODS CAMBRIAN 
 
 257 
 
 * 
 
 Menevian group sandstones, shales, slates, 
 and grits. 
 
 Harlech group purple, red, and gray 
 flags, sandstones, slates, and conglomerates. 
 (Horizon of Olenellus.) 
 
 Recently the existence of Olenellus a genus of trilobite characteris- 
 tic of the lowest platform of the Cambrian system has been found in the 
 midland counties of England and in the north-west Highlands of Scot- 
 land. In the latter region the occurrence of this fossil fixes the strati- 
 graphical position of the well-known quartzites as Lower Cambrian. 
 
258 GEOLOGY 
 
 CHAPTER XVIII. 
 
 THE SILURIAN PERIOD. 
 
 THE origin and use of the term SILURIAN" have already been 
 fully explained. The rocks embraced under this term 
 form a mass of strata which in some countries (Wales and 
 Scotland) must be many thousand feet thick. Like the Cam- 
 brian system below, into which they graduate downward, they 
 consist mainly of graywackes, sandstones, shales, or slates; but 
 they are marked by the occasional occurrence of bands of lime- 
 stone a rock which from this part of the geological record ap- 
 pears in increasing quantity onwards to recent times. Some 
 highly characteristic bands of dark carbonaceous shale are in 
 some countries persistent for long distances, and contain abun- 
 dant graptolites. Not infrequently these dark shales are full of 
 pyritous impregnations, which, when the rock weathers, give rise 
 to the efflorescence of alum or the formation of chalybeate 
 springs ; such bands are sometimes called alumschists. In Wales, 
 the Lake District of the north of England, and to a less degree in 
 the south of Scotland, there are remains of submarine volcanic 
 eruptions of Silurian time in the form of intercalated sheets of 
 tuff and beds of different lavas. 
 
 In certain regions (Russia, New York) Silurian rocks have 
 undergone little change since the time of their deposition ; but, 
 as a rule, they have been more or less indurated, plicated, and 
 dislocated (Wales, Lake District, etc.), while in some countries 
 (Norway, Scotland) they have been so crushed and metamor- 
 phosed as to have assumed the character of schistose rocks 
 (phyllites, mica-schists, etc.). 
 
 Murchison subdivided his Silurian system into two great sec- 
 tions, Lower and Upper. This classification still holds, though 
 the limits and nomenclature of the several component groups 
 have not been exactly maintained. The arrangement of the 
 
THE SILURIAN PERIOD 259 
 
 various subdivisions, as followed in Britain, is shown in the table 
 in Chapter XVIII. 
 
 Taking the fossils of the Silurian system as a whole, we find 
 that they prolong and amplify the peculiar type of life found 
 to characterise the Cambrian system. They include both flora 
 and fauna. The flora, however, is exceedingly meagre. It con- 
 sists almost entirely of sea-weeds, which occur usually in the 
 form of fucoid-like impressions. But, as already remarked in 
 reference to the so-called plants of the Cambrian rocks, many 
 of the supposed vegetable remains are almost certainly not such. 
 Some of the supposed remains may be tracks left by worms, 
 crustaceans, or other marine creeping or crawling creatures, 
 upon soft mud or sand; others 
 may be casts of hollows made 
 by trickling water or yielding 
 sediment; while others seem to 
 be the result of some peculiar 
 crumpling or puckering of the 
 strata. But undoubted remains 
 of sea-weeds do occur. Some 
 of these are delicate branching 
 forms, like some still living, as 
 shown in the organism figured 
 in Fig. 120 from the Upper Fig. 120. An Upper Silu- 
 Silurian series. Among the ria ?. ? 7 e .a-weed (Chondrites 
 TT ci-i vensvnilis) , natural size. 
 
 Upper Silurian strata, also, traces 
 
 of land-vegetation have been detected in the form of spores and 
 stems of cryptogamous plants. Lycopods or club-mosses and 
 ferns appear to have been the chief types in the earliest ter- 
 restrial floras; at least, it is remains referable to them that 
 chiefly occur in the older Palaeozoic rocks. They reached a great 
 development in the Carboniferous period, in the account of 
 which a fuller description of them will be given. We can dimly 
 picture the Silurian land with its waving thickets of fern, above 
 which lycopod trees raised their fluted and scarred stems, threw 
 out their scaly moss-like branches, and shed their spiky cones. 
 
 The fauna of the Silurian period has been more abundantly 
 preserved than that of the Cambrian, and appears to have been 
 more varied and advanced. Among its simpler forms were 
 Foraminifera and sponges. A foraminifer (of which there were 
 
2GO 
 
 GEOLOGY 
 
 no doubt representatives in Cambrian times, and there are still 
 many living types in the present ocean, see Fig. 33) is generally 
 a 'minute animal, composed of a jelly-dike substance which, 
 possessing no definite organs, has in some kinds the power of 
 secreting a hard calcareous or horny shell, through openings or 
 pores (foramina) in which filaments from the jelly-like mass 
 are protruded. By other kinds, grains of sand are cemented 
 together to form a protecting shell. It is these calcareous and 
 sandy coverings which occur in the fossil state and prove the 
 presence of foraminifera in the older oceans of the globe. 
 Sponges also are known to have existed in the Cambrian and 
 Silurian seas, and their remains have been met with in all parts 
 of the Geological Record down to the present day. It is, of 
 
 Fig. 121. Graptolites from Silurian rocks. A, Rastrites Linncei. B, 
 Monograptus pnodon. C, Diplograptus pristis, D, Puyllograptus ty- 
 pus. E$ Didymograptus Murchisonii (all natural size). 
 
 course, only where these animals secrete hard durable parts that 
 they can be detected as fossils. A sponge is a mass of soft, 
 transparent, jelly-like substance, perforated by tubes or canals, 
 and supported on an internal network of minute calcareous or 
 siliceous spicules, or of interlacing horny fibres. Most fossil 
 sponges are calcareous or siliceous, and their hard parts, being 
 durable, have been preserved sometimes in prodigious numbers 
 and in wonderful perfection. The common sponge of domestic 
 use is an example of the horny type. 
 
 The Hydrozoa were abundantly represented in the Silurian 
 seas by graptolites, of which there were many different kinds. 
 Some of the more characteristic of these are shown in Fig. 121. 
 
THE SILURIAN PERIOD 261 
 
 They abound in certain bands of shale, both in the Lower and 
 Upper Silurian series, the double forms (such as C, Fig. 121) 
 being more characteristic of the Lower division, while the single 
 forms run throughout the system. 
 
 Corals abound in some parts of the Silurian seas. Their 
 remains chiefly occur in the limestones, doubtless because these 
 rocks were formed in comparatively clear water, in which the 
 corals could flourish. But they differed in structure from the 
 familiar reef-building corals of the present day. The great 
 majority of them belonged to the family of the Eugose corals, 
 now only sparingly represented in the waters of the present 
 ocean. As their name denotes, they were particularly marked 
 by their thick rugged walls. Many of them were single inde- 
 pendent individuals; some lived together in colonies; while 
 
 Fig. 122. Silurian Corals, (a) Rugose Coral (Omphyma turbinatum, 
 y 2 ). (&) Alcyonarian Coral (Heliolites inter stinctus, natural size). 
 
 others were sometimes solitary, sometimes gregarious. A typical 
 example of these rugose forms is Omphyma, shown in Fig. 122 
 (a). Other genera were Cyathaxonia, Cyathophyllum, and 
 Zaphrentis. There were likewise less numerous and more deli- 
 cate compound forms belonging to what are known as the 
 Tabulate corals (Favosites, Halysites) , while another type 
 (Heliolites, Fig. 122, b) represented in ancient times the Alcy- 
 onarian corals (Heliopora) of the present time. 
 
 Crinoids or stone-lilies played an important part in the earlier 
 seas of the globe. In some regions they lived in such abun- 
 dance on the sea-floor that their aggregated remains formed 
 solid beds of limestone hundreds of feet thick, and covering 
 thousands of square miles. As their name denotes, crinoids are 
 
262 GEOLOGY 
 
 lily-shaped animals, having a calcareous, jointed flexible stalk 
 fixed to the bottom and supporting at its upper end the body, 
 which is composed of calcareous plates furnished with branched 
 calcareous arms (see Figs. 149, 165, 173). It is these hard 
 calcareous parts which have been so abundantly preserved in 
 the fossil state. Eemains of crinoids are found in various parts 
 of the Silurian system (Dendrocrinus, Glyptocrinus) chiefly in 
 the limestones, but not in such abundance and variety as in 
 later portions of the Palaeozoic formations (compare pp. 274, 
 286, and Figs. 149, 165, and 173). Allied to the crinoids 
 were the Cystideans, a curious order of echinoderms, with 
 rounded or oval bodies enclosed in calcareous plates, possessing 
 only rudimentary arms, and a comparatively small and short 
 jointed stalk. They 'first appeared in the Cambrian period 
 
 Fig. 123. Silurian Echinoderms. (a) Cystidean (Pseudocrinites quad- 
 rifasdatus, natural size). (&) Star-fish (Palceasterina stellata, 3|). 
 
 (Protocystites) , but attained their chief development during 
 Silurian time, thereafter diminishing in numbers. They are 
 thus characteristically Silurian types of life. One of them 
 is represented in Fig. 123 (a). Star-fishes and brittle-stars 
 likewise occur as fossils among the Silurian rocks. These 
 marine creatures, still represented in our present seas, possess 
 hard calcareous plates and spines, which, being imbedded in 
 a tough leathery integument, have not infrequently been pre- 
 served in their natural position as fossils. Some of the genera 
 of star-fishes found in the Silurian system are Palceaster, 
 
THE SILURIAN PERIOD 263 
 
 Palceasterina (Fig. 123, &), Pdlceochoma. Brittle-stars were 
 represented by Protaster. 
 
 In the Silurian system are found many tracks and burrows 
 like those of the Cambrian rocks, indicative of the presence 
 of different kinds of sea-worms. Throughout great thicknesses 
 of strata, indeed, these markings are sometimes the only or 
 chief fossils to be found. Names have been given to the differ- 
 ent kinds of burrows (Arenicolites, Scolitlius, Lumbricaria, Fig. 
 124), and of trails (Palceo chorda, Palceophycus) . There were 
 likewise representatives of the 
 familiar Serpula, which is 
 found so abundantly on the 
 present sea-bottom, encrusting 
 shells and stones with a calca- 
 reous protecting tube, inside of 
 which the annelide lives. This 
 tube has been preserved in the 
 
 fossil state in rocks of all ages. Fiff> 124. Filled-up Burrows or 
 
 TTiP TrilnVu'f^c w"hir>li "ha^ al Trails left by a sea- worm on the 
 ine irilObltes, WHICH nad. al- bed of the Silurian sea (Lumbri- 
 
 ready appeared in Cambrian caria antiqua, %) . 
 time, attained their maximum development during the Silurian 
 period. A few of the primordial or Cambrian types continued 
 to live into this period, but many new genera appeared. In 
 the Lower Silurian series some of the more abundant genera 
 are Asaplius, Ampyx, Ogygia, and Trinucleus; in the Upper 
 Silurian division characteristic genera are Calymene, Phacops, 
 Encrinurus, Illcenus, and Homalonotus (Fig. 125). Trilobites 
 continued to nourish, but in gradually diminishing variety, 
 during the Devonian and Carboniferous periods, after which 
 they seem to have died out. They are thus a distinctively 
 Palaeozoic type of life, each great division of the Paleozoic 
 rocks being characterised by its own varieties of the type. 
 
 Phyllopod crustaceans likewise attained to greater variety 
 during the Silurian period; some of the more frequent genera 
 are Ceratiocaris (Fig. 126), Discinocaris, and Caryocaris. The 
 Phyllopods attained their maximum development during Palaeo- 
 zoic time, but they have continued in existence ever since, 
 and are at present represented by a number of genera, some 
 of which live in the sea, others in fresh water. 
 
264 
 
 GEOLOGY 
 
 The Mollusca are far more abundant and varied in the 
 Silurian than in the Cambrian rocks. Among the more char- 
 acteristic Silurian genera of Brachiopods are Airy pa, Leptcena, 
 Orthis, Pentamerus,, Rhynclionella, and Strophomena (Fig. 127). 
 Among the lamellibranchs we find the Cambrian genera 
 
 Fig. 125. Lower and Upper Silurian Trilobites. (a) Asaphus ty- 
 rannus (J) ; (6) Ogygia Buchii (J) ; (c) Illcenus barriensis (J) ; 
 (d) Trinucleus concentricus (natural size) ; (e) Homalonotus del- 
 phinocephalus (S). 
 
 Ctenodonta and Modiolopsis, with new forms such as Orthonota 
 (Fig. 128), CleidopJiorus and Amlonychia. 
 
 The Gasteropods played an important part in the fauna of 
 the Silurian sea, for upwards of 1300 species of them have 
 
THE SILURIAN PERIOD 
 
 265 
 
 been found in Silurian rocks. Among the more frequent genera 
 are Belleroplion (Fig. 129), Opliileta, Holopea, Murchisonia, 
 Platyschisma. 
 
 Numerous representatives of the 
 chambered cephalopods have been 
 found in the Silurian rocks, especially 
 in the upper division. Among the 
 more frequent genera are Orthoceras 
 (straight, Fig. 130 a), Cyrtoceras 
 (curved), Ascoceras (globular or pear- 
 shaped), Lituites (coiled, Fig. 130 &), 
 and also Nautilus, a genus which has 
 
 g. iztj 
 
 Phyllopod Crustacean 
 
 ( Ceratiocaris papi- 
 
 lio). 
 
 persisted through the greater part of geological time to the pres- 
 ent day, and now remains the only representative of the cham- 
 bered cephalopods formerly so abundant. 
 
 Fig. 127. Silurian Brachiopods. (a) Atrypa reticularis (natural size), 
 Caradoc beds to Lower Devonian; (6) Orthis actonice (natural size) ; 
 (c) Rhynchonella lorealis (natural size) ; (d) Pentamerus galeatus 
 (natural size). 
 
 Remains of Fishes detected in the Upper Silurian rocks are 
 the earliest traces of vertebrate life yet known. They consist 
 partly of plates which are regarded as portions of the bony 
 covering of certain placoderms or bone-plated forms (Pteraspis, 
 
266 
 
 GEOLOGY 
 
 Cephalaspis, AucJienaspis) ; partly of curved spines and shag- 
 reen-like fragments. The creatures of which these are relics 
 appeared as forerunners of the remarkable assemblage of fishes 
 in the next geological period (see p. 270). All the animal 
 remains hitherto enumerated are relics of the inhabitants of 
 the sea. Of the land-animals of the time nothing was known 
 until the year 1884, when, by a curious coincidence, the dis- 
 covery was made of the remains of scorpions in the Silurian 
 rocks of Sweden, Scotland, and the United States, and of an 
 insect allied to the living cockroach (Palceoblattina) in those 
 of France. If scorpions and insects existed during this ancient 
 period we may be sura that other forms of terrestrial life were 
 also present. A new interest is thus given to the prosecution 
 of the search for fossils among the older formations. 
 
 Fig. 128. Silurian L a m e 1 1 i- 
 branch (Orthonota semisulcata 
 natural size). 
 
 Fig. 129. Silurian 
 Gasteropod (Bellero 
 phon dilatatus, &). 
 
 Putting together evidence furnished by rocks and fossils of 
 the Silurian system, we get a glimpse of the aspect of the 
 globe during the early geological period which they represent. 
 The rocks bring before us the sand, mud, and gravel of the 
 bottom of the sea, and tell of some old land from which these 
 materials were worn away. The detritus carried out from the 
 shores of that land was laid down upon the sea-bottom just 
 as similar materials are being disposed of at the present day. 
 The area occupied by Silurian rocks marks out the tracts then 
 covered by the sea. Following these upon a map we perceive 
 that vast regions of the existing continents were then parts 
 of the ocean-floor. In Europe, for example, Silurian rocks un- 
 derlie the greater part of the British Islands, whence they 
 stretch northwards across a large part of Scandinavia and 
 the basin of the Baltic. They rise to the surface in many places 
 
THE SILURIAN PERIOD 
 
 2G7 
 
 on the continent from Spain to the Ural Mountains. They 
 are found forming parts of some of the great mountain-chains 
 of the globe, as, for instance, in the Cordilleras of South 
 America, in the Alps, and in the Himalayas. Even at the 
 antipodes they are met with as thick masses in Australia and 
 New Zealand. It is evident that the geography of the globe 
 in Silurian times was utterly unlike what it is now. A large 
 
 Fig. 130. Silurian Cephalopods. (a) Orthoceras emeritum (S) ; (6) 
 Trochoceras (Lituites) cornu-arietis (i). 
 
 part of the present land was then covered with shallow seas, 
 in which the Silurian sedimentary rocks were laid down. There 
 would seem to have been extensive masses of land in the boreal 
 part of the northern hemisphere connecting the European, 
 Asiatic, and American continents. Along the coast-line of the 
 northern land and across the shallow seas lying to the south 
 of it, the same species of marine organisms migrated freely 
 between the Old and the New Worlds. 
 
 The following Table shows the subdivisions which have been 
 made in the Silurian system of Britain. 
 
 Ludlow group (mudstone and Aymestry limestone) 
 Kirkby Moor and Bannisdale flags and slates. 
 
 Wenlock group (shales and limestones) Denbighshire 
 and Coniston grits and flags. 
 
 Upper Llandovery group May Hill sandstones. 
 
 s| 
 
 II 
 
 Lower Llandovery group grits and sandstones. 
 
 Bala and Caradoc group sandstones, slates, and grits, 
 with Bala (Coniston) limestone. 
 
 Llandeilo group dark argillaceous and sometimes cal- 
 careous flagstones and shales. 
 
 Arenig group dark slates, flags, and sandstones. 
 
GEOLOGY 
 
 CHAPTER XIX. 
 
 DEVONIAN AND OLD RED SANDSTONE. 
 
 THE DEVONIAN system, which comes next in order, was 
 named by Sedgwick and Murchison after the county of 
 Devon where they studied its. details. In Europe, and 
 likewise in the eastern part of North America, it occurs in two 
 distinct types, which bring before us the records of two very 
 different conditions in geography of these regions during the 
 time when the. rocks composing the system were being deposited. 
 The ordinary type, which occurs all over the world, represents 
 the tracts that were covered by the sea, and has preserved the 
 remains of many forms of the marine life of the period. It 
 is that to which the name Devonian is more particularly ap- 
 plicable. The less frequent type, is characterised by thick 
 accumulations of sandstones, flagstones, and conglomerates that 
 were laid down in lakes and Inland seas, and contain a distinct 
 assemblage of land and fresh-water fossils. This lacustrine 
 type is known by the name of OLD BED SANDSTONE. 
 
 In their general character the Devonian rocks resemble those 
 of the Silurian system underneath. In Central Europe, where 
 they attain a thickness of many thousand feet, their lower 
 division consists mainly of sandstones, grits, graywackes, slates, 
 and phyllites. The central zone contains thick masses of lime- 
 stone, often full of corals and shells, while the upper portions 
 comprise thin-bedded sandstones, shales, and limestones. These 
 various strata represent the sediments intermittently laid down 
 upon .the bottom of the sea which then covered the greater 
 part of Europe. Here and there, they include bands of diabase 
 and tuff, which show that submarine volcanic eruptions took 
 place during their deposition. 
 
 In the north-west of Europe, however, the floor of the Silurian 
 sea was irregularly ridged up into land, and large lakes were 
 
DEVONIAN AND OLD RED SANDSTONE 
 
 2G9 
 
 formed, into which rivers from the ancient northern continent 
 poured enormous quantities of gravel, sand, and silt. The sites 
 of these lakes can be traced in Scotland, the north of England, 
 and Ireland. Similar evidence of land and lake-waters is 
 found in New Brunswick and Nova Scotia. That some of the 
 larger lakes were marked by lines of active volcanoes is well 
 shown in Central Scotland, where the piles of lava and ashes 
 left by the eruptions are more than 6000 feet thick. 
 
 The occurrence of both marine and lacustrine deposits is of 
 the highest interest, for, on the one hand, we learn what kinds 
 of animals lived in the sea in succession to those that peopled 
 the Silurian waters, and, on the other hand, we meet with 
 the first abundant remains of the vegetation that covered the 
 
 Fig. 131. Plants of the Devonian period, (a) Psilophyton (^4) ; (I) 
 Palceopteris ( % ) . 
 
 land, and of the fishes that inhabited the fresh waters. The 
 terrestrial flora of the Devonian period has been only sparingly 
 preserved in the marine strata; but occasional drifted speci- 
 mens occur to show that land was not very distant from the 
 tracts on which these strata were laid down. In the lacustrine 
 series or Old Eed Sandstone of Britain more abundant remains 
 have been met with, but the chief sources of information re- 
 garding this flora are to be sought in New Brunswick and 
 Gaspe, where upwards of 100 species of plants have been dis- 
 covered. Both in Europe and in North America, the Devonian 
 vegetation was characterised by the predominance of ferns, 
 lycopods (Lepidodendron, etc.), and calamites. It was essen- 
 tially acrogenous that is, it consisted mainly of flowerless 
 
270 GEOLOGY 
 
 plants like our modern ferns, club-mosses, and horse-tail reeds. 
 Traces of coniferous plants, however, show that on the upland 
 of the time pine-trees grew, the stems of which were now and 
 then swept down by floods into the lakes or the sea. 
 
 While the general aspect of the flora was uniformly green 
 and somewhat monotonous, the fauna had now become increas- 
 ingly varied. We know that these early woodlands were not 
 without insect life, for neuropterous and orthopterous wings 
 have been preserved in the strata. Some of these remains 
 indicate the existence of ancient forms of ephemera or May-fly, 
 one of which was so large as to have a spread of wing measur- 
 ing 5 inches across. -There were likewise millipedes, which 
 fed on the decayed wood of the forests. Traces of land-snails 
 too have been detected among the fossil vegetation. It is evi- 
 dent, however, that the plant and animal life of the land has 
 only been sparingly preserved; and though our knowledge of 
 it has in recent years been largely increased, we shall probably 
 never discover more than a mere fragmentary representation of 
 what the original terrestrial flora and fauna really were. 
 
 The lake-basins of the Old Red 
 Sandstone have yielded large 
 numbers of remains of the 
 fishes of the time. These are 
 members of the remarkable or- 
 der of Ganoids the earliest 
 known type of fishes which, 
 though so abundant in early 
 geological time, is represented 
 Fig. 132. Overlapping scales ^ ^ nrpqpnt rlav hv rvnlv a 
 of an Old Red Sandstone ai aa J D J ' n v a 
 
 fish (Holoptychius Ander- few widely scattered species, such 
 
 as the sturgeon, the polypterus 
 
 of the Nile, and the bony pike or garpike of the American lakes. 
 These modern forms are denizens of fresh water, and there 
 is reason to believe that their early ancestors were also in- 
 habitants of lakes and rivers, though many of them may also 
 have been able to pass out to the sea. The ganoids are so 
 named from the enamelled scales and plates of bone in which 
 they are encased. In some of the fossil forms, this defensive 
 armour consisted of accurately fitting and overlapping scales 
 (-Figs. 132, 133) ; in others, the head with more or less of the 
 
DEVONIAN AND OLD RED SANDSTONE 
 
 271 
 
 body was protected by large and thick plates of bone (Fig. 134). 
 Examples of both these kinds of armature are to be observed 
 among the fishes of the Old Eed Sandstone. Some of the 
 
 Fig. 133. Scale-covered Old Red Sandstone fishes, (a) Osteolepis; 
 (&) Acanthodes (both reduced). 
 
 most characteristic scale-covered genera are Osteolepis, Diplop- 
 terus, Glyptolcemus, Holoptychius, Acanthodes (Figs. 132, 133). 
 
 Fig. 134. Plate-covered Old Red Sandstone fishes, (a) Cephalaspis; 
 (6) Pterichthys (both reduced). 
 
 The acanthodians (Fig. 133, &), distinguished by the thorn- 
 like spines supporting their fins, reached their greatest develop- 
 ment during the Devonian period. Of the plate-covered ganoids 
 or placoderms some of the most characteristic were the curious 
 Cephalaspis (Fig. 134, a), with its head-buckler shaped like a 
 saddler's awl, the Pteraspis, which, with Cephalaspis., had al- 
 ready appeared in the Silurian period, the Coccosteus and 
 Pterichthys (Fig. 134, I). Some of the contemporaries of 
 
272 GEOLOGY 
 
 these creatures attained a great size. Thus the Asterolepis had 
 its head and shoulders encased in a buckler, which in some 
 examples is 20 inches long by 16 broad. Still larger were some 
 of its American allies, one of which, the Dinichthys, had a 
 head-buckler 3 feet long armed with formidable teeth. 
 
 One of the fishes of the Old Eed Sandstone, named Dipterus, 
 has recently been found to have a singular modern representa- 
 tive in the barramunda or mud-fish (Ceratodus) of the Queens- 
 land rivers in Australia. Dipterus resembled the ganoids in its 
 external enamel and strong bony helmet, but its jaws present 
 the characteristic teeth, and its scales have the rounded or 
 "cycloid" form of 'Ceratodus. That some of these fishes 
 swarmed in the waters of the Old Eed Sandstone is shown by 
 the prodigious numbers of their remains occasionally preserved 
 in the sandstones and flagstones. Their bodies lie piled on 
 each other in such numbers, and often so well preserved, as to 
 show that probably the animals were 
 suddenly killed, and were covered up 
 with sediment before their remains had 
 time to decay and to be dispersed by the 
 currents of water. Perhaps earth- 
 quake shocks, or the copious discharge 
 of mephitic gases, or other sudden bane- 
 ful influence, may have been the cause 
 of the extensive destruction of life in 
 these ancient waters. 
 
 That some of the fishes found their 
 
 Fig. 135. Devonian wa ? to the sea > as our modern salmon 
 Eurypterid Crustacean does, is indicated by the occasional oc- 
 (Pterygotus, reduced). currence of their rema i ns amon g those 
 
 of the truly marine fauna of the Devonian rocks. But the rarity 
 of their presence there, compared with their prodigious abun- 
 dance in some parts of the Old Eed Sandstone, probably serves 
 to show that they were essentially inhabitants of the lakes and 
 rivers of the land. 
 
 Among the animals that appear to have been migratory 
 between the sea and the terrestrial waters, were the curious 
 forms known as Eurypterids, which, though generally classed 
 with the crustaceans, had many affinities with the arachnids 
 or scorpions. One of the most remarkable of these creatures 
 
DEVONIAN AND OLD RED SANDSTONE 
 
 273 
 
 was faePterygotus, of which the general form is shown in Fig. 
 135. Most of the species are small, though one of them found 
 in Scotland must have attained a length of 5 or 6 feet. 
 
 But it is the marine or Devonian fauna which is most widely 
 spread over the globe, and from its extensive distribution is 
 
 136. Devonian Trilobites. (a) Bronteus flalellifer (J) ; (6) 
 Dalmanites rugosa (S) ; (c) Homalonotus armatus (4) ; (d) Harpes 
 macrocephalus (|). 
 
 of most importance to the geologist. Taken as a whple, it 
 presents a general resemblance to that of the Silurian period 
 which it succeeded. Some of the Silurian species survived in 
 
 Fig. 137. Devonian Corals, (a) Cyathophyllum ceratites (J) ; (6) 
 Calceola sandalina (). 
 
 it, and new species of the old genera made their appearance. 
 But important differences are to be observed between the faunas 
 of the two systems, showing the long lapse of time, and the 
 changes which it brought about in the life of the globe. 
 
 It is specially interesting to mark how some of the char- 
 
274 
 
 GEOLOGY 
 
 acteristic Silurian types dwindle and finally die out in the 
 Devonian system. One of the best examples of this survival 
 and disappearance is supplied by the Graptolites. It will bo 
 remembered how prodigiously abundant these creatures were 
 in the Silurian seas. They are met with also in scattered 
 specimens in the lower and middle divisions of the Devonian 
 system, but their rarity there affords a striking contrast to 
 their profusion among the Silurian strata, and they seem to 
 have entirely died out before the end of the Devonian period, 
 for no traces of them occur in the later parts of the system, 
 
 Fig. 138. Devonian Brachiopods. (a) Uncites gryphus () ; (6) 
 String ocephalus Burtini () ; (c) Spirifera disjuncta (Verneuillii) (i). 
 
 and they have never been met with in any later geological 
 formation. 
 
 Again, Trilobites, which form such a predominant and strik- 
 ing feature of the Silurian fauna, occur in greatly diminished 
 number and variety among the Devonian rocks. Most of the 
 Silurian genera are absent, some of the most frequent Devonian 
 types being PTiacops, Cryphceus, Homalonotus, Dalmanites, and 
 Bronteus (Fig. 136). We shall find that this peculiarly Pateo- 
 
DEVONIAN AND OLD RED SANDSTONE 275 
 
 zoic type of Crustacea finally died out in the next or Carbon- 
 iferous period. But while the trilobites were waning, the euryp- 
 terids, already referred to, appeared and attained a great 
 development. 
 
 In the clearer parts of the sea vast numbers of rugose corals 
 flourished, and, with other calcareous organisms, built up solid 
 masses of limestone. Some of the characteristic genera were 
 CyatJiopliyllum (Fig. 137), Acervularia, Cystiphyttum, and the 
 curious Calceola which, after being successively placed among 
 the lamellibranchs and the brachiopods, is now regarded as a 
 rugose coral with an opercular lid. With these were likewise 
 associated vast numbers of crinoids, of which the genera 
 Cyathocrinus and Cupressocrinus were especially characteristic. 
 
 The Brachiopods reached their maximum of development 
 in the Devonian seas, upwards of 60 genera and 1100 species 
 
 Fig. 139. A, Devonian Lamellibranch (Cucullcea Hardingii, ). B, 
 Devonian Cephalopod (Clymenia Sedgwickii, ). 
 
 having been described from Devonian rocks. Comparing them 
 with those of the Silurian system, we notice that some of the 
 most characteristic Silurian types, such as forms of Orthis and 
 Strophomena, became fewer in number, while forms of Pro- 
 ductus and Chonetes increased. The most abundant families 
 were those of the Spirifers (Uncites, Cyrtia, Athyris, Atrypa) 
 and Ehynchonellids (Fig. 138). Two distinctively Devonian 
 brachiopods were StringocepJialus and Rensseleria, allied to the 
 still living Terebratula. The former is especially characteristic 
 of one of the Middle limestones. 
 
 The other mollusca appear to have been well represented in 
 the Devonian seas. Of the Lamellibranchs, Pterinea is par- 
 ticularly abundant in the lower part of the system, Cucullcea 
 
276 
 
 GEOLOGY 
 
 Fig. 139, A) in the upper part. The Devonian cephalopods in- 
 cluded many species of the genera Orthoceras, Cyrtoceras, 
 Clymenia, Goniatites, and Bactntes (Fig. 139, B). 
 
 The Devonian system in Europe is subdivided as in the sub- 
 joined Table : 
 
 Upper 
 
 Lower 
 
 Middle < 
 
 Pilton and Pickwell-Down group of England Upper 
 Old Red Sandstone of Scotland. 
 
 Famennian and Frasnian sandstones, shales, and lime- 
 stones of the North of France and Belgium Psani- 
 mites de Condroz. 
 
 Cypridina-shales, Spirifer sandstone, Rhynchonella 
 culoides beds of Germany. 
 
 Ilfracombe and Plymouth limestones, grits, and con- 
 glomerates of Devonshire. [No middle Old Red Sand- 
 stone.] 
 
 Limestone of Givet, and Calceola shales of North of 
 France. 
 
 Stringocephalus-limestone of the Eifel Calceola-group 
 of Germany. 
 
 Linton slates and sandstones of Devon and Cornwall 
 Lower Old Red Sandstone of Scotland and Wales. 
 
 Coblenzian, Taunusian, and Gedinnian rocks of the Ar- 
 dennes and Taunus. 
 
THE CARBONIFEROUS PERIOD 277 
 
 CHAPTER XX. 
 
 THE CARBONIFEROUS PERIOD. 
 
 THE next great division of the Geological Eecord has re- 
 ceived the name of CARBONIFEROUS, from the beds of 
 coal (Latin Carlo) which form one of its most conspicu- 
 ous features. The rocks of which it consists reach sometimes a 
 thickness of fully 20,000 feet, and contain the chronicle of a 
 remarkable series of geographical changes which succeeded the 
 Devonian period. They include limestones made up in great 
 part of corals, crinoids, polyzoa, brachiopods, and other calcare- 
 ous organisms which swarmed in the clearer parts of the sea ; 
 sandstones often full of coaly streaks- and remains of terrestrial 
 plants; dark shales not infrequently charged with vegetation, 
 and containing nodules and seams of clay-ironstone; and seams 
 of coal varying from less than an inch to several feet or yards 
 in thickness, and generally resting on beds of fire-clay. 
 
 These various strata are disposed in such a way as to afford 
 clear evidence of the physical geography of large areas of the 
 earth's surface during .the Carboniferous period. The lime- 
 stones attain a thickness of sometimes several thousand feet, 
 with hardly any intermixture of sedimentary material. They 
 consist partly of aggregated masses of corals and coralloid 
 animals, which grew on the sea-floor somewhat after the man- 
 ner of modern coral-reefs ; partly of aggregated stems and joints 
 of crinoids, which must have flourished in prodigious numbers 
 on the bottom, mixed with fragments of other organisms, the 
 whole being aggregated into sheets of solid stone. The Car- 
 boniferous or Mountain Limestone, which forms the lower part 
 of the Carboniferous system, stretches from the west of Ireland 
 eastwards for a distance of 750 miles, across England, Wales, 
 Belgium, and Rhineland into Westphalia. In the basin of 
 the Meuse it is not less than 2500 feet thick, and in Lancashire, 
 where it attains its maximum development, it exceeds 6000 
 
278 GEOLOGY 
 
 feet. Such an enormous accumulation of organic remains 
 shows that, during the time of its deposition, a wide and clear 
 sea extended over the centre of Europe. But as the limestone 
 is traced northwards, it is found to diminish in thickness. 
 Beds of sandstone, shale, and coal begin to make their appear- 
 ance in it, and rapidly increase in importance, as they are 
 followed away from the chief limestone area; while the lime- 
 stone itself is at last reduced in Scotland to a few beds, each 
 only a yard or two in thickness. From this change in the 
 character of the rocks, the inference may be drawn that, while 
 the sea extended from the west of Ireland eastwards into 
 Westphalia, land lyin to the north supplied sand, mud, and 
 drifted plants, which, being scattered over the sea-floor, pre- 
 vented the thick limestone from extending northwards. These 
 detrital materials now form the masses of sandstone and shale 
 that take the place of the limestone in the north of England 
 and in Scotland. The northward extension of a few limestone 
 beds full of marine organisms serves to mark a time when, for 
 a longer or shorter interval, the water cleared, sand and mud 
 ceased to be carried so far southward, and the corals, crinoids, 
 and other limestone-building creatures were able to spread them- 
 selves farther over the sea-floor. But the thinness, of such 
 intercalated limestones also indicates that the intervals favour- 
 able for their formation were comparatively short, the sandy 
 and muddy silt being once again borne southward from the 
 land, killing off or driving away the limestone-builders and 
 spreading new sheets of sand and mud over the site. 
 
 There can be no doubt that, while these changes were in 
 progress, the whole wide area of deposition in Western and 
 Central Europe was undergoing a gradual depression. The 
 sea-bottom was sinking, but so slowly that the growth of lime- 
 stone and the deposit of sediment probably on the whole kept 
 pace with it. The actual depth of the water may not have varied 
 greatly even during a subsidence of several thousand feet. That 
 this was the case may be inferred from the structure of the 
 limestone itself. We have seen that this rock sometimes ex- 
 ceeds 6000 feet in thickness. Had there been no subsidence of 
 the sea-floor during the accumulation of so thick a mass of 
 organic debris, it is evident that the first beds of limestone 
 must have been begun at a depth of at least 6000 feet below the 
 
THE CARBONIFEROUS PERIOD 279 
 
 surface of Ithe sea, and that, by the gradual increase of cal- 
 careous matter, the sea was eventually filled up to that amount, 
 f it was not filled up entirely. But we can hardly suppose 
 that the same kinds of organisms could live at a depth of 6000 
 feet and also at or near the surface. We should expect to find 
 the organic contents of the lower parts of the limestone entirely 
 different from those in the upper parts. But though there are 
 differences sufficient to admit of the limestone being separated 
 into stages, each marked by its own distinctive assemblage of 
 fossils, the general character or facies of the organisms remains 
 so uniform and persistent throughout, as to make it quite certain 
 that the conditions under which the creatures lived on the 
 bottom and built up the limestone continued with but little 
 change during the whole time when the 6000 feet of rock were 
 being deposited. As this could not have been the case had 
 there been a gulf of 6000 feet to fill up, we are led to conclude 
 that the bottom slowly subsided until its original level, on which 
 the limestone began to form, had sunk at least 6000 feet. 
 
 This conclusion is borne out by many other considerations. 
 Thus the sedimentary strata that replace the limestone on its 
 northern margin are also several thousand feet thick. But from 
 bottom to top they abound with evidence of shallow-water con- 
 ditions of deposition. Their repeated alternations of sandstone, 
 grit (even conglomerate), and shale; the presence in them of 
 constant current-bedding; the frequent occurrence of ripple- 
 marked and sun-cracked surfaces; the preservation of abundant 
 remains of terrestrial vegetation some of it evidently in its 
 position of growth prove that the mass of sediment was not 
 laid down in a deep hollow of the sea-bottom, but in shallow 
 waters not far from the margin of the land. 
 
 But probably the most interesting evidence of long-continued 
 subsidence during the Carboniferous period is furnished by the 
 history of the coal-seams. Coal is composed of compressed and 
 mineralised vegetation. In Britain each layer of coal is usually 
 underlain by a bed of fire-clay, or at least of shale, through 
 which roots and rootlets, descending from the under surface 
 of the coal-seam, branch freely. There can be little doubt 
 that each bed of fire-clay is an old soil, while the coal lying 
 upon it represents the matted growth of vegetation which that 
 
280 
 
 GEOLOGY 
 
 soil supported. Hence the association of a fire-clay and a coal- 
 seam furnishes distinct evidence of a terrestrial surface. 1 
 
 In many regions the Carboniferous system comprises a series 
 of sandstones, shales, and other strata, many thousands of feet 
 in thickness, throughout which, on suc- 
 cessive platforms, there lie -hundreds of 
 seams of coal. If each of these seams 
 marks a former surface of terrestrial 
 vegetation, how is this succession of 
 buried land-surfaces to be accounted 
 for? There is obviously but one so- 
 lution of the problem. The area over 
 which the coal-seams extend must have 
 been slowly sinking. During this 
 subsidence, sand, mud, and silt were 
 transported from the neighbouring 
 land, and in such quantity as to fill up 
 the shallow waters. On the muddy flats 
 thus formed, the vegetation of the flat 
 marshy swamps spread seaward. There 
 may not improbably have been pauses 
 in the downward movement, during 
 which the maritime jungles and forests 
 continued to flourish and to form a 
 thick matted mass of vegetable matter. 
 When the subsidence recommenced, 
 this mass of living and dead vegetation 
 was carried down beneath the water 
 and buried under fresh deposits of sand 
 and mud. As the weight of sediment 
 increased, the vegetable matter would 
 be gradually compressed and would 
 slowly pass into coal. But eventually 
 another interval of rest or of slower 
 subsidence would allow the shallow sea once more to be silted 
 up. Again the marsh-loving plants from the neighbouring 
 swampy shores would creep outward and cover the tract with 
 
 ^ In some Continental coal-fields there is evidence that coal has like- 
 wise been formed out of matted vegetation which has been swept down by 
 floods and buried under sand, gravel, and other sediment. 
 
 Fig. 140. Section of 
 part of the Cape Bre- 
 ton coal-field, showing 
 a succession of buried 
 trees and land-sur- 
 faces, (a) sandstones; 
 (6) shales; (c) coal- 
 seams; (d) under- 
 clays or soils. 
 
THE CARBONIFEROUS PERIOD 281 
 
 a new mantle of vegetation, which, on the renewal of the down- 
 ward movement, would be submerged and buried. 
 
 In the successive strata of a coal-field, therefore, we are pre- 
 sented with the records of a prolonged period of subsidence, 
 probably marked by longer or shorter intervals of rest. 
 These more stationary periods are indicated by the coal- 
 seams, and perhaps their relative duration may be inferred 
 from the thickness of the coal. A thick coal-bed not 
 improbably marks a time of rest, when the vegetation was 
 allowed to flourish unchecked, or when at least the sinking 
 was so imperceptible that the successive generations of plants 2 
 springing up on the remains of their predecessors, contrived 
 to keep themselves above the level of the water. 
 
 In the present world there is no vegetable growth now in 
 progress quite like that of the coal-seams of the Carboniferous 
 period. Perhaps the nearest analogy is supplied by the man- 
 grove-swamps of certain tropical coasts. In these tracts, the 
 mangrove trees grow seaward, dropping their roots and radicles 
 into the shallow waters, and gradually forming a belt of swampy 
 jungle several miles broad. That the coal-jungles extended 
 into the sea is shown by the occurrence of marine shells and 
 other organisms in the coal itself. But there were probably 
 also wide swamps wherein the water was fresh. A single coal- 
 seam may sometimes be traced over an area of more than 1000 
 square miles, showing how widespread and uniform were the 
 conditions in which it was formed. 
 
 During the subterranean movements that marked the Car- 
 boniferous period, the Devonian physical geography was en- 
 tirely remodelled. The lake-basins of the Old Eed Sandstone 
 were effaced, and the sea of the Carboniferous limestone spread 
 over their site. Much of the Devonian marine area was up- 
 ridged into land, and the rocks eventually underwent that 
 intense compression and plication which have given them their 
 cleaved, crumpled, and metamorphic aspect, and in connec- 
 tion with which they were invaded by granite and intersected 
 with mineral veins. It is deserving of remark that volcanic 
 action, which played so notable a part in Devonian time, was 
 continued, but with diminished vigour, in the Carboniferous 
 period. During the earlier half of the period, volcanic out- 
 bursts were frequent in different parts of Britain, particularly 
 
282 
 
 GEOLOGY 
 
 in Derbyshire, the Isle of Man, central and southern Scotland, 
 and the south-west of Ireland. The lava and ashes ejected in 
 some of these areas during the time of the Carboniferous 
 Limestone form conspicuous groups of hills. 
 
 Of the plant and animal life of the Carboniferous period 
 much is now known from the abundant remains which have 
 been preserved of the terrestrial surfaces and sea-floors of the 
 time. Beginning with the flora, we have first to notice its gen- 
 eral resemblance to that of the Devonian period. Many of 
 the genera of the older time survived in the Carboniferous 
 jungles; but other forms appear in vast profusion, which have 
 not been met with in any Devonian or Old Eed Sandstone strata. 
 The Carboniferous flora, like that which preceded it, must have 
 
 Fig. 141. Carboniferous Ferns. 
 (6) Sphenopteris artemisice folia 
 lonchitica (4). 
 
 a) 
 
 Neuropteris macrophylla (i) ; 
 (c) Alcthopteris (Pecopteris) 
 
 been singularly monotonous, consisting as it did almost entirely 
 of flowerless plants. Not only so, but the very same species and 
 genera appear to have then ranged over the whole world, for 
 their remains are found in Carboniferous strata from the 
 Equator to the Arctic Circle. Ferns, lycopods, and equisetaceae, 
 constituted the main mass of the vegetation. The ferns recall 
 not a few of their modern allies, some of the more abundant 
 kinds being Sphenopieris, Neuropteris, and Pecopteris (Fig. 
 141). Among the lycopods the most common genus is Lepido- 
 dendron, so named from the scale-like leaf-scars that wind round 
 its stem (Fig. 1,42). Its smaller branches, closely covered with 
 small pointed leaves, and bearing at their ends little cones or 
 
THE CARBONIFEROUS PERIOD 
 
 283 
 
 spikes (Lepidostrobus) , remind one of the club-mosses of our 
 moors and mountains; but instead of being low-growing or 
 creeping plants, like their modern representatives, they shot 
 up into trees, sometimes 50 feet or more in height. Equisetaceaj 
 abounded in the Carboniferous swamps, the most frequent genus 
 being Calamites, the jointed and finely-ribbed stems of which 
 are frequent fossils in the sandstones and shales (Fig. 143, a). 
 This plant probably grew in dense thickets in the sandy and 
 muddy lagoons, and bore as its foliage slim branches, with 
 whorls of pointed leaves set round the joints (AsteropJiyllites., 
 Fig. 143, &). The Sigillarioids were among the most abundant, 
 and at the same time, most puzzling members 
 of the Carboniferous flora. They do not 
 appear to have any close modern allies, and 
 their place in the botanical scale has been a 
 subject of much controversy. The stem of 
 these trees, sometimes reaching a height of 50 
 feet or more, was fluted, each of the parallel 
 ribs being marked by a row of leaf-scars, 
 hence the name Sigillaria., from the seal-like 
 impressions of the scars (Fig. 144). These 
 surface-markings disappeared as the tree 
 grew, and in the lower part of the trunk they 
 passed down into the pitted and tubercular 
 surface characteristic of the roots (Stig- 
 maria), still so abundant in their position of 
 growth in fire-clay, and also as drifted 
 broken specimens in sandstones and shales. 
 Another plant that took a prominent part in 
 the Carboniferous flora was that named Cor- 
 daites (Fig. 145). Its true botanical place 
 is still matter of dispute; some writers placing it with 
 the lycopods, others with the cycads, or even among the conifers. 
 It bore parallel-veined leaves somewhat like those of a yucca, 
 which, when they fell off, left prominent scars on the stem, 
 and it also carried spikes or buds (Carpolithes, Fig. 145). All 
 the plants now enumerated probably grew on the lower grounds 
 and swamps. But on the higher and drier tracts of the in- 
 terior there grew araucarian pines (Dadoxylon, Araucarioxylon), 
 the trunks of which, swept down by floods, were imbedded in 
 
 ( Lepidode n d r o n 
 Sternbergii, J). 
 
284 
 
 GEOLOGY 
 
 some of the sands of the time and now appear petrified in the 
 sandstones. 
 While the terrestrial vegetation of the Carboniferous period 1 
 
 Fig. 143. Carboniferous Equisetaceous Plants, (a) Calamites Lindleyl 
 ( C. Mougeoti, Lindl, |) ; (6) Asterophyllites densifolius (J). 
 
 has been so abundantly entombed, the fauna of the land has 
 been but scantily preserved. That air-breathers existed, how- 
 ever, has been made known by the finding of specimens of scor- 
 
 Fig. 144. Sigillaria with Stigmaria roots (much reduced). 
 
 pions, myriapods, true insects, and amphibians. Within the 
 last few years vast numbers of the remains of scorpions have 
 been discovered in the Carboniferous rocks of Scotland. These 
 
THE CARBONIFEROUS PERIOD 285 
 
 ancient forms (Eoscorpius) presented a remarkably close re- 
 semblance to the living scorpion,, and so well have they been 
 preserved among the shales that even the minutest parts of 
 their structure can be recognised. They possessed stings like 
 their modern descendants, whence we may infer the presence 
 of other forms of life which they killed. The Carboniferous 
 woodlands had plant-eating millipedes, and their silence was 
 broken by the hum of insect-life ; for ancestral forms of dragon- 
 flies (Libellulce) , May-flies (Ephemeridce), stone-flies ( Perlidce), 
 white-ants (Termidce), cockroaches (Blattida), spectre-insects 
 (Phasmidce) , crickets (Gryttidce), locusts (Acrydiidce) , and 
 curious transitional forms between modern types that are quite 
 distinct have been detected, chiefly among the shales and coals 
 3f the Coal-measures. Some of these insects attained a great 
 size; a single wing of one of them (Megaptilus) must have 
 
 Fig. 145. Cordaites alloidius (|), with Carpolithes attached. 
 
 measured between 7 and 8 inches in length. While detached 
 wings and more or less complete bodies have been found as 
 rare and precious discoveries in many coal-fields in Europe 
 and America, it is at Commentry in France that remains of 
 insects have been met with in largest numbers no fewer than 
 1300 specimens having there been disinterred, most of them 
 admirably preserved. In the interior of decaying trees early 
 forms of land-snails lived, having a striking resemblance to 
 some kinds that are still to be found in our present woodlands 
 (Pupa) . 
 
 The lagoons in which the coal-growths flourished were ten- 
 anted by numerous forms of animal life. Among these were 
 various mussel-like molluscs (Antliracomya,, Fig. 154, Anthra- 
 
286 GEOLOGY 
 
 cosia), which were possibly restricted to fresh water. But wher- 
 ever the sea-water penetrated, it carried some of its character- 
 istic life with it, particularly Lingula, Discina, Aviculopecten, 
 Goniatites, and other marine shells. The fishes of the lagoons 
 were chiefly ganoids (Hegalichthys, Rhizodus, Fig. 158, Cheiro- 
 dus, Strepsodus, etc.). But some of the rays and sharks from 
 the sea made their way into these waters, for their spines are 
 occasionally found among the coal-seams and shales (Oyracan- 
 thus, Pleuracanthus, Fig. 158). That the larger fishes lived 
 upon the smaller ones is shown by a curious and interesting 
 piece of evidence. Many of the shales are full of small oblong 
 bodies which contain in their interior the broken and undi- 
 gested scales and bones of small fishes. From their contents, 
 their peculiar external form and markings, and their phos- 
 phatic composition, these bodies (coprolites) are recognised 
 as the excrement of some of the larger fishes, and the teeth 
 and scales within them serve to show what were the smaller 
 forms on which these fishes fed (see Fig. 65). 
 
 During the Carboniferous period, and indeed throughout the 
 later parts of the Paleozoic ages, the most highly organised 
 creatures living on the globe, so far as we at present know, 
 belonged to the Amphibia the great class which includes 
 our modern frogs, toads, and salamanders. 
 They belonged, however, to an order that has 
 long been entirely extinct the Labyrintho- 
 donts, so named from the labyrinthine folds 
 ^erous~Foramini~ f "the internal substance of their teeth. They 
 fer (FusuUna cy- we re somewhat like the existing salamander 
 
 hndrica, f) . .,, , ,. , . ., 
 
 in form, with weak limbs and a long tail. 
 Their skulls were encased in strong plates of bone, and 
 they likewise carried protective bony scutes on the under sides 
 of their bodies. Those found in Carboniferous rocks are mostly 
 small in size, but some of them, measuring perhaps 7 or 8 feet 
 in length, must have been the monsters of the lagoons in 
 which they lived. Some of the leading genera are Archego- 
 saurus, Anthracosaurus, Loxomma, Dendrerpeton, Baphetes. 
 
 The marine life of the Carboniferous period has been ex- 
 tensively preserved in the Carboniferous Limestone, which, 
 as already stated, consists of little else than aggregated remains 
 of organisms. In walking over the surface of the beds of this 
 
THE CARBONIFEROUS PERIOD 287 
 
 limestone, one treads upon the floor of the sea in Carboniferous 
 times, with its corals, crinoids, and shells crowded and crushed 
 upon each other. Beginning with the most lowly of these 
 organisms, we may observe abundant remains of foraminifera, 
 which in some portions of the limestone constitute the greater 
 part of the rock. One of their most characteristic forms, named 
 Fusulina (Fig. 146), enters largely into the structure of the 
 limestone across the Old World from Russia to China and 
 Japan, and likewise in North America. Another, called Sacam- 
 mina, abounds as aggregates of little globular bodies in some 
 parts of the limestone of Britain. Corals have been preserved 
 
 Fig. 147. Carboniferous Rugose Corals, (a) Zaphrentis Enniskilleni 
 (i) ; (&) Lithostrotion junceum (natural size). 
 
 in prodigious numbers; indeed, some parts of the limestone are 
 almost entirely made up of them. Most of them are rugose 
 kinds, characteristic genera being Zaphrentis, Lithostrotion, 
 (Fig. 147), ClisiopJiyllum, Lonsdaleia. With these there occur 
 also tabulate forms,, including Chcetetes, Alveolites, Favosites, 
 etc. Of the sea-urchins, the plates and spines of the genus 
 Archceocidaris (Fig. 148) are specially frequent. But the most 
 common echinoderms are members of the great order of crinoids, 
 which must have grown in thick groves over many square miles 
 of the sea-bottom. So prodigiously numerous were they that 
 their remains have been aggregated into beds of limestone 
 
288 
 
 GEOLOGY 
 
 hundreds of feet in thickness, hence known as crinoidal or 
 encrinite limestone (Fig. 76). The general plant-like form 
 of these animals is shown in Fig. 149. But usually the cal- 
 careous joints and plates fell asunder. Frequent genera are 
 named Platycrinus, Poteriocrinus, Cyathocrinus. The Carbon- 
 
 Fig. 148. Carbonifer- 
 ous Sea-U r c h i n 
 (A rchceocid aris 
 Vrei, natural size), 
 (a) Single plate; 
 (6) Portion of 
 spine. 
 
 Fig. 149. Carboniferous 
 Crinoid ( Woodocrinus 
 expansus, J). 
 
 iferous seas were tenanted by a peculiar extinct order of echino- 
 derms known as Blastoids or Pentremites (Fig. 150), distin- 
 guished from true crinoids by the want of free arms, and by 
 the arrangement of the plates forming the cup. These crea- 
 
 Fig. 150. Carboniferous Blastoid 
 (Cup of Pentremite, magnified), 
 (a) View from above; (6) Side 
 
 Fig. 151. Carbonifer- 
 ous Trilobite (Phil- 
 lip s i a der~biensis, 
 natural size). 
 
 tures are characteristically Carboniferous, though they are 
 found also in the higher part of the Silurian system and in 
 Devonian rocks. 
 
THE CARBONIFEROUS PERIOD 289 
 
 The Crustacea of the Carboniferous period presented a strong 
 contrast to those of earlier geological time. In particular, 
 the great family of the Trilobites, so characteristic of the older 
 Palaeozoic systems, now died out altogether. Instead of its 
 numerous types in the Silurian and Devonian rocks, it is repre- 
 sented in the Carboniferous system by only four genera, all 
 the species of which are small (Phillipsia, Fig. 151, Griffi- 
 thides, Brachymetopus) , and none of which rises into the next 
 succeeding system. The most abundant crustaceans were 
 ostracods an order still abundantly represented at the pres- 
 ent day. They are minute forms enclosed within a bivalve 
 shell or carapace which entirely invests the body. Many of 
 these live in fresh water; the Cypris, for example, being abun- 
 dant in ponds and ditches. Others 
 are marine, while some are brackish- 
 water forms. In the Carboniferous 
 lagoons, as at the present time, they 
 lived in enormous numbers ; their little 
 seed-like valves are crowded together 
 in some parts of the shale which repre- 
 sents the mud of these lagoons; some- 
 times they even form beds of lime- 
 stone. Doubtless, they served as food 
 to the smaller fishes whose remains are 
 usually to be found where the ostracod Fi |; ol ^n~ 
 valves are plentiful. One of the prin- Mcrrisii, natural size), 
 cipal genera is Leperditia. There were 
 
 likewise long-tailed shrimp-like crustaceans (Anihrapalcemon, 
 Palceocrangon) , and king-crabs (Prestwichid) ; while in the ear- 
 lier part of the period Eurypterids still survived in the waters. 
 
 Some of the most delicately beautiful fossils of the Carbon- 
 iferous limestone belong to the Polyzoa. These animals, of 
 which familiar living examples are the common sea-mats of 
 our shares, are characterised by their compound calcareous or 
 horny framework studded with minute cells, each of which is 
 occupied by a separate individual, though the whole forms 
 one united colony. One of the most abundant Carboniferous 
 genera is Fenestella (Fig. 152). So numerous are the polyzoa 
 in some bands of limestone as to constitute the main part of 
 the stone. Their delicate lace-like fronds are best seen where 
 
290 
 
 GEOLOGY 
 
 the rock has been exposed for a time to the action of the 
 weather; they then stand out in relief and often retain per- 
 fectly their rows of cells. 
 
 Fig. 153. Carboniferous Brachiopods. (a) Productus semireticulatus 
 (5) ; (&) Streptorhynchus crenistria (J) ; (c) Spirifera striata (J). 
 
 The Brachiopods, so preponderant among the molluscs of 
 the earlier division of Palaeozoic time, now decidedly wane 
 
 Fig. 154. Carboniferous Lamellibranchs. (a) Edmondia sulcata (i) ; 
 (&) Anthracomya Adamsii () ; (c) Aviculopecten fallax (). 
 
 before the great advance of the more highly organised lamelli- 
 branchs and gasteropods. Some of the most characteristic 
 genera (Fig. 153) are Productus, Spirifera, Streptorhynchus, 
 
THE CARBONIFEROU PERIOD 
 
 291 
 
 Ehynclionella, Athyris, Chonetes, Terebratula, Lingula, Dis- 
 cina. Some of the species appear to range over the whole world, 
 for they have been met with across Europe, in China, Aus- 
 tralia, and North America. Among these cosmopolitan forms 
 are Productus semireticulatus, Productus longispinus, Strepto- 
 
 Fig. 155. Carboniferous Gasteropoda, (a) Euomphalns pentangulatus 
 (I) ; (&) Bellerophon tenuifascia (). 
 
 rliynchus crenistria, Spirifera glabm, Terebratula Jiastata (Fig. 
 153). 
 
 Some of the more common Lamellib ranch molluscs (Fig. 
 154) belong to the genera Aviculopecten, Leda, Nucula, Ed- 
 mondia, Modiola, Anthracomya. Among the Gasterpods Euom- 
 phalus, Pleurotomaria, Loxonema, and Bellerophon (Fig. 155) 
 are not infrequent. A pteropod (Conularia, Fig. 156) may be 
 gathered in great numbers in some parts of the Carboniferous 
 
 Fig. 156. Carbon- 
 iferous Pteropod 
 (Conularia quad- 
 risulcata, ). 
 
 Fig. 157. Carboniferous CepK- 
 alopods. (a) Orthoceras gold- 
 fussianum (i) ; (&) Ooni- 
 atites sphcericus (natural 
 size). 
 
 Limestone. The Cephalopods were represented by numerous 
 species of Orthoceras, Nautilus, and Goniatites (Fig. 157). 
 
 Eemains of fishes are not infrequent in the Carboniferous 
 Limestone. But they present a striking contrast to those of 
 the black shales and ironstones of the Coal-measures. They 
 
292 
 
 GEOLOGY 
 
 consist for the most part of teeth or of spines belonging to 
 large predatory sharks. These teeth were placed as a kind of 
 pavement and roof in the mouth, and were used as effective 
 instruments for crushing the hard parts of the animals, on 
 which these larger creatures preyed. If, as is probable, the 
 sharks fed upon the ganoid fishes of the time, they must have 
 required a powerful apparatus of teeth for crushing the hard, 
 bony armour in which these fishes were encased. Of the com- 
 moner genera of sharks, which have been named from the forms 
 of their teeth the only hard parts of their structure that 
 have survived the following may be mentioned: Cochliodus, 
 
 Fig. 158. Carboniferous Fishes, (a) Tooth of Rhisodus Hiblerti (J) ; 
 (&) Tooth of Orodus ramosus (J) ; (c) Ichthyodorulite or Fin-spine 
 of Pleuracanthus Icevissimus (J). 
 
 Orodus, Psammodus, Petalodus. The small ganoids that so 
 abound in the black shales, ironstones, and coal-seams which 
 represent the deposits of the sheltered lagoons of the coal- 
 jungles, are hardly to be found in the thick limestone, whence 
 we may infer that they were inhabitants of the quiet shore- 
 waters, and did not venture out into the open sea, where the 
 sharks found their congenial element. But the occasional occur- 
 rence of the teeth and spines of sharks in the Coal-measure 
 shales and coal-seams shows that these monsters now and then 
 made their way into the inland waters, where they would find 
 abundant food. 
 
THE CARBONIFEROUS PERIOD 
 
 293 
 
 The Carboniferous system iri Europe presents at least two 
 well-marked subdivisions. In the lower section the strata are 
 in large measure marine, for they include the Carboniferous 
 Limestone; in the upper part they consist mainly of sand- 
 stones, shales, fire-clays, and coal-seams, constituting what are 
 called the Coal-measures, or coal-bearing division of the system. 
 The subjoined Table shows the order of succession of the rocks 
 in Britain: 
 
 Coal-Measures. At the top, red and gray sandstones, 
 clays, and thin limestone, resting upon a great 
 thickness of white, gray, and yellow sandstones, . 
 clays, shales, and fire-clays, with numerous work- 
 Lagoon type. -{ able coal-seams, and with a lower subdivision of 
 coal-bearing beds, among which there occur marine 
 fossils (Orthoceras, Posidonomya, etc.) Thickness 
 in South Wales, 12,000 feet; South Lancashire, 8,- 
 000 feet; Central Scotland, 3,000 feet. 
 
 Millstone Grit. Grits, flagstones, sandstones, and 
 shales, with thin seams of coal and occasional bands 
 containing marine fossils. Thickness 400-1000 feet, 
 increasing in Lancashire to 5500 feet. 
 
 Carboniferous Limestone. Consisting typically of 
 massive marine limestones and shales, but passing 
 laterally into sandstones and shales, with thin coal- 
 seams, which indicate alternations of marine and 
 brackish water conditions. Thickness in South 
 Wales, 500 feet, increasing northwards to more 
 than 4000 feet in Derbyshire, and to upwards of 
 6000 feet in Lancashire, but diminishing north- 
 wards into Scotland. 
 
 The base of the Carboniferous Limestone series 
 passes down conformably into the Upper Old Red 
 Sandstone. 
 
 The Carboniferous system occupies a number of detached 
 areas on the European continent. Its largest tract extends from 
 the north of France, through Belgium, into Westphalia. The 
 most important coal-fields of Europe belonging to this system 
 are those of Belgium, Westphalia, the north of France, Saar- 
 briicken, St. Etienne in Central France, Bohemia, and the 
 Donetz in Southern Eussia, In North America, the Coal-meas- 
 ures of the eastern United States reach a thickness of 4000 
 feet in Pennsylvania, and contain many valuable seams of coal. 
 They increase in thickness northwards, reaching a maximum 
 
 Marine type, 
 but passing 
 northwar d s 
 into that of 
 the lagoons. 
 
294 GEOLOGY 
 
 of 8000 feet in Nova Scotia. They are underlain by bands of 
 conglomeratic strata, answering to the English Millstone grit, 
 below which comes a group of beds with marine fossils (sub- 
 carboniferous), probably representing the Carboniferous Lime- 
 stone of Europe. In Australia and New Zealand also thick masses 
 of sedimentary strata contain recognisable Carboniferous organic 
 remains. In New South Wales they include a valuable suc- 
 cession of coal-seams. 
 
THE PERMIAN ROCKS 295 
 
 CHAPTEE XXI. 
 
 THE PERMIAN" ROCKS. 
 
 THE prolonged subsidence during which the Coal-measures 
 were accumulated was at last brought to an end by a 
 series of great terrestrial disturbances, whereby the 
 lagoons and coal-growing swamps were in great measure effaced 
 from the geography of Europe. So abrupt in some regions is 
 the discordance between the Coal-measures and the next series 
 of strata, that geologists have naturally been led to regard this 
 break as one of great chronological importance, serving as 
 the boundary between two distinct systems. Nevertheless, so 
 far as the evidence of fossils goes, there is no such interruption 
 of the Geological Record as might be supposed from this strati- 
 graphical unconformability, many of the Carboniferous types 
 of life having survived the terrestrial disturbances. Again, 
 though the discordance among the strata is, in many parts of 
 Europe, particularly in England, most striking, yet it is by no 
 means universal. On the contrary, some localities (Autun in 
 France, and the Bohemian coal-field, for example) escaped the 
 upheaval and prolonged denudation which elsewhere have pro- 
 duced so marked a hiatus in the chronicle. And in these places 
 a gradual passage can be traced from the strata and fossils of 
 the Coal-measures into those of the next succeeding division 
 of the series, no sharp line being there discoverable, nor any 
 evidence to warrant the separation of the overlying strata as 
 an independent system distinct from the Carboniferous. 
 Hence, by many geologists, the rocks now to be described are 
 regarded as the upper part of the Carboniferous system. 
 
 To these overlying rocks the name of PERMIAN was given, 
 from the Russian province of Perm, where they are well de- 
 veloped. They consist of red sandstones, marls, conglomerates, 
 and breccias, with limestones and dolomites. In Germany they 
 
296 GEOLOGY 
 
 are often called Dyas, because they are there easily grouped in 
 two great divisions. The coarsest strata breccias and con- 
 glomerates are composed of rounded and angular fragments 
 of granite, diorite, gneiss, graywacke, sandstone, and other 
 crystalline and older Palaeozoic rocks, which must have been 
 upheaved and exposed to denudation before Permian time. The 
 sandstones are usually bright brick-red in colour, owing to the 
 presence of earthy peroxide of iron which serves to cement 
 the particles of sand together. The shales or marls are coloured 
 by the same pigment. So characteristic indeed is the red 
 colour of the rocks that they form part of a great series of 
 strata, originally known as the New Red Sandstone. Generally, 
 greenish or whitish spots and streaks occur in the red beds, 
 marking where the iron-oxide has been reduced and removed 
 by decaying organic matter. Eed strata are, as a rule, singularly 
 barren of organic remains, probably because the water from 
 which the iron-peroxide was precipitated must have been un- 
 fitted for the support of life. The red Permian rocks are there- 
 fore generally unfossiliferous. Among them, however, occur 
 dark shales or "marl-slate," which have yielded numerous re- 
 mains of fishes. The limestones too are f ossiliferous, but they 
 are associated with unfossiliferous dolomite, gypsum, anhydrite, 
 and rock-salt. In some places seams of coal also occur. 
 
 These various rocks tell distinctly the story of their origin. 
 They could not have been deposited in the open sea, but rather in 
 basins more or less shut off from it, wherein the water was 
 charged with iron and was liable to concentration, with the con- 
 sequent precipitation of its solutions. The beds of anhydrite, 
 gypsum, and rock-salt are memorials of these processes. The 
 dolomite may at first have been laid down as limestone which 
 afterwards was converted into dolomite by the action of the 
 magnesian salts in the concentrated water. In such intensely 
 saline and bitter solutions, animal life would not be likely to 
 flourish, and hence, no doubt, the poverty of fossils in the Per- 
 mian series of rocks. But it is observable that where evidence 
 occurs of the cessation of ferruginous, saliferous, and gypseoup 
 deposition, fossils not infrequently appear. The brown Marl- 
 Slate, for example, and the thick beds of limestone are some- 
 times abundantly f ossiliferous, and indeed are almost the only 
 bands of rock in the whole series where organic remains occur. 
 
THE PERMIAN ROCKS 
 
 297 
 
 They were probably deposited during intervals when the bar- 
 riers of the inland seas or salt-lakes were broken down, or, at 
 least, when from some cause the waters came to be connected 
 with the open sea, and when a portion of the ordinary marine 
 fauna swarmed into them. Volcanic action showed itself during 
 Permian time in many parts of Western, Central, and Southern 
 Europe. There was a group of small volcanoes in the south of 
 Scotland. Great eruptions took place in Germany, notably in 
 the area of the present Yosges mountains, and the region of 
 volcanic activity extended across the region where now the Alps 
 
 Fig. 159. Permian Plants, (a) Callipteris conferta (4); (&) Wai- 
 chia piniformis (i). 
 
 stand, as far south as Cannes on the shores of the Mediterranean. 
 Hence, from the very circumstances in which their remains 
 have been entombed and preserved, the flora and fauna of Per- 
 mian times are comparatively little known. The total number 
 of species and genera obtained from Permian rocks, hardly more 
 than 300 in all, forms a singular contrast to the ample assem- 
 blages which have been recovered from the older systems. But 
 that the land of these times was still richly clothed with vegeta- 
 tion and the sea abundantly stocked with animal life, there can 
 be no doubt. The flora appears to have closely resembled that 
 
298 
 
 GEOLOGY 
 
 of the Carboniferous period, a considerable proportion of the 
 species of plants being survivals from the Carboniferous jungles 
 and forests. The Lepidodendra, Sigillariae, and Calamites, 
 tvhich had been such conspicuous members of all the Palaeozoic 
 floras, now appear in diminishing number and variety, and final- 
 ly die out. With their cessation, new features arise in the 
 vegetation. Among these may be mentioned the abundance of 
 tree-ferns, which, though they sparingly existed even as far back 
 as Devonian times, now attained a conspicuous development 
 (Psaronius, Caulopteris) . The genus of ferns called Callip- 
 teris likewise played a, prominent part in the Permian wood- 
 
 Fig. 160. Permian Brachiopods. (a) Producing horridus (reduced); 
 (6) Strophalosia Ooldfussi; (c) Camarophoria humbletonensis (). 
 
 lands (Fig. 159, a). But perhaps the most remarkable feature 
 in the flora was the abundance of its conifers, and the appearance 
 of the earliest forms of cycads. The yew-like conifer Walchia 
 (Fig. 159, &) if we may judge from the abundance of its re- 
 mains, flourished in great profusion on the drier grounds, 
 mingled with others that bore cones (Utttnannia) . The cycads, 
 which now made their advent, continued during Mesozoic time 
 to give the leading character to the vegetation of the globe. 
 
 The scanty relics of the Permian fauna, as above stated, have 
 been almost wholly preserved in those strata which were de- 
 posited during temporary irruptions of the open sea into the 
 
THE PERMIAN ROCKS 299 
 
 inland salt-basins of the time. Some of the Carboniferous 
 genera of brachiopods still survived Productus, Spirifera, and 
 Strophalosia being conspicuous (Fig. 160). Among the lamel- 
 libranchs Axinus, Bakevellm, and Schizodus are frequent forms 
 
 Fig. 161. Permian Lamellibranchs. (a) Bakevellia, tumida (natural 
 .size) ; (&) Schizodus Schlotheimi (natural size). 
 
 (Fig. 161). Among the higher molluscs,, which have been but 
 sparingly preserved in the rocks, the old types of OrtJioceras, 
 Cyrtoceras, and Nautilus are still to be noticed. In Europe, the 
 fishes of the time have been chiefly sealed up in the marl-slate 
 or copper-shale (Kupferschiefer) ; two of the most frequent 
 genera being Palceoniscus and Platysomus (Fig. 162). 
 
 Fig. 162. Permian Ganoid Fish (Platysomus striatus, i). 
 
 Labyrinthodonts continued to abound in the waters. Some 
 of the Carboniferous genera still survived, but with these were 
 associated many new forms, most of which have been discovered 
 in the strata overlying the true Coal-measures of Bohemia (Fig. 
 163). But a great onward step in the advance of animal organ- 
 isation was made in Permian time by the appearance of the 
 
300 GEOLOGY 
 
 earliest known lizard Proterosaurus, which, like the living 
 crocodile, had its teeth implanted in distinct sockets. 
 
 In Britain the Permian strata rest unconformably on the 
 Carboniferous system, which must have been greatly disturbed 
 
 Fig. 163. Permian Labyrinthodont (Branchiosaurus salamandroides, 
 
 natural size). 
 
 and enormously denuded before they were deposited. They 
 consist of the following subdivisions: 
 
 Upper red sandstones, clays and gypsum (50 to 100 feet thick in the 
 east of England, but swelling out west of the Pennine Chain to 
 600 feet). 
 
 Magnesian limestone a mass of dolomite ranging up to 600 feet in 
 thickness, and the chief repository of the Permian fossils; remark- 
 able for the curious concretionary forms assumed by many of its 
 beds on the coast of Durham (Fig. 75). [Zechstein of Germany.] 
 
 Marl-slate a hard brown shale with occasional limestone bands. [Kup- 
 f erschiefer] . 
 
 Lower red and variegated sandstones with conglomerates and breccias. 
 This division attains a thickness of 3000 feet in Cumberland, but is 
 hardly represented in the east of England. [Rothliegende of Ger- 
 many.] 
 
 In Germany, where the Dyas or twofold development of the 
 Permian rocks is so well displayed, the lower subdivision, called 
 Eothliegende, consists of great masses of conglomerate with 
 sandstones, shales, thin limestones, and important intercalations 
 of contemporaneous volcanic rocks, both lavas and tuffs. The 
 upper section is composed chiefly of limestone called Zechstein, 
 and answering to the Magnesian limestone of England. With 
 it are associated beds of anhydrite, gypsum, rock-salt, and 
 bituminous limestone, and underneath it lies the celebrated 
 Kupferschiefer or copper-shale a black bituminous shale, 
 about two feet thick, which has long been extensively worked 
 on the flanks of the Harz Mountains for the ores of copper with 
 
THE PERMIAN ROCKS 301 
 
 which it is impregnated, and which is the great repository for 
 the fossil fishes of the Permian period. This remarkable band 
 of rock was probably deposited in one of the inland basins, which 
 at first may have maintained a free communication with the 
 open sea. But eventually mineral springs, not improbably con- 
 nected with the volcanic action of the time, brought up such an 
 abundant supply of dissolved metallic salts as to kill the fish 
 and render the water unsuitable for their existence. The 
 metallic salts were reduced and precipitated as sulphides round 
 the organisms, and impregnated the surrounding mud. In the 
 overlying succession of strata, we see how the area was once 
 more overspread by the clearer and opener water, which brought 
 in the fauna of the Zechstein, and then how the basin gradually 
 came to be shut off once more, until from its concentrated 
 waters the various beds of anhydrite, gypsum, and rock-salt were 
 thrown down. 
 
 In the heart of France, at Autun, the Coal-measures pass up- 
 ward into Permian strata, as already stated. That area appears 
 to have escaped the disturbance which in Western Europe placed 
 the Permian unconformably upon the Carboniferous rocks. It 
 presents a mass of sandstones, shales, coal-seams, and some bands 
 of magnesian limestone, the whole having a thickness of more 
 than 3,000 feet referred to the Permian system. The plants in 
 the lower part of this group of strata are unmistakably Car- 
 boniferous, but Permian forms appear in increasing numbers as 
 the beds are followed upwards until the highest stage presents a 
 predominant Permian flora. Besides the characteristic Permian 
 fishes these strata have yielded remains of several salamander- 
 like animals (Protriton or Branchiosaurus, Melanerpeton), and 
 of some labyrinthodonts (Actinodon, Euchirosaurus, etc.). 
 
302 GEOLOGY 
 
 CHAPTEK XXII. 
 
 MESOZOIC PERIODS TRIASSIO. 
 
 THE great series of red strata referred to in the foregoing 
 chapter as overlying the Carboniferous system in England 
 was called " New Eed Sandstone," to distinguish it from 
 the " Old Eed Sandstone " which underlies that system. But 
 the progress of geology on the European continent eventually 
 proved that, notwithstanding their general similarity of litho- 
 logical character, two series of rocks had been comprised under 
 the general title of New Eed Sandstone. The older of these, 
 separated from the rest under the name of Permian, was placed 
 at the top of the great succession of Paleozoic formations. The 
 younger division (still sometimes spoken of as New Eed Sand- 
 stone) was called Trias, and was regarded as the first system 
 in the great Mesozoic or Secondary succession. 
 
 Essentially the Permian strata form merely the upper part of 
 the Carboniferous system. Their types of life are fundamentally 
 Palaeozoic, but, as we have seen, both the flora and fauna are 
 marked by a decrease in the number and variety of old forms, 
 and by the advent of the precursors of a new order of things. 
 Conifers and cycads now began to replace the early types of 
 lepidodendron and sigillaria; amphibians became more abun- 
 dant, and saurians now took their place at the head of the 
 animal world. 
 
 But when we ascend into the Trias, though in Europe the 
 physical conditions of deposition remained much the same as 
 in Permian time, we meet with a decided contrast in the 
 organic remains. A new and more advanced phase of develop- 
 ment presents itself in that richer and more varied assemblage 
 of plant and animal life which characterised Mesozoic or Sec- 
 ondary time. 
 
 The word TRIAS has reference to the marked threefold division 
 of the rocks of this system in Germany. In that country, and 
 
MESOZOIC PERIODS TRIASSIC 303 
 
 generally in Western Europe, the rocks consist of bright red 
 sandstones and marls or clays, with beds of gypsum, anhydrite, 
 rock-salt, dolomite, and limestone. These rocks, so closely re- 
 sembling the Permian series below, had evidently a similar 
 origin. They were in large part deposited in inland seas or 
 salt-lakes, wherein, by evaporation and concentration of the 
 water, the dissolved salts were precipitated upon the bottom, and 
 where, consequently, the conditions must have been extremely 
 unfavourable for the presence of living things. The sites of 
 these inland basins can still be partially traced. They extended 
 at least as far west as the north of Ireland. One or more of 
 them lay across the site of the plains of Central England. 
 Others were dotted over the lowlands of middle Europe. The 
 largest of them occupied an extensive area now traversed by 
 the Ehine. It stretched, on the one hand, from Basel to the 
 plains of Hanover, and, on the other, from the highlands of 
 Saxony and Bohemia across the site of the Vosges Mountains 
 westward to the flank of the Ardennes. The continent must 
 then have been somewhat like the steppes of Southern Eussia 
 . a region of sandy wastes and salt-lakes, with a warm and dry 
 climate. Probably higher land rose to the north, as in earlier 
 geological times, for traces of its vegetation have been found in 
 Sweden. But southwards lay the more open sea, spreading 
 over part at least of the site of the modern Alps, and thence 
 probably across much of Asia to the Indian and Pacific Oceans. 
 So long as only the deposits of the salt-basins had been ex- 
 plored, it was but natural that comparatively little should be 
 known of the flora and fauna of the Triassic period. The 
 climate around these lakes was perhaps not a very salubrious 
 one, and hence there may have been only a scanty terrestrial 
 fauna in their immediate vicinity, while the waters of the lakes 
 themselves were unsuited for the support of life. It is not 
 surprising, therefore, that the strata deposited in these tracts are 
 on the whole unf ossilif erous ; that, indeed, fossils only abound 
 where there are indications that, owing to some temporary de- 
 pression or breaking down of the barriers, the open sea spread 
 into these basins, and carried with it the organisms whose re- 
 mains gathered into beds of limestone. But over the tracts 
 that lay under the open sea, a more abundant marine fauna 
 lived and died. It is in the records of that sea-bottom, rather 
 
304 
 
 GEOLOGY 
 
 than in those of the salt-basins, that we must seek for the evi- 
 dence of the general character of the life over the globe, and 
 for the fossil data with which to compare together the Triassic 
 rocks of distant regions. 
 
 There are traces of contemporaneous volcanic action among 
 the Triassic strata. A little group of volcanoes appears to have 
 existed during Triassic time in South Devonshire; but in the 
 region of the Eastern Alps, especially around Predazzo *in the 
 Tyrol, evidence of far more extensive eruptions exists. 
 
 The flora of the Triassic period has been preserved chiefly in 
 the dark shales and thin coal-seams formed in some of the inland 
 
 Fig. 164. Triassic Plants, (a) Horse-tail Reed (Equisetum columnare, 
 I) ; (6) Conifer (Voltzia heterophylla, ) ; (c) Cycad (Pterophyl- 
 lum Blasii, |). 
 
 basins. So far as known to us it consisted chiefly of ferns, 
 equisetums or horse-tails, conifers, and cycads. Among the 
 ferns a few Carboniferous genera still survived, but some of the 
 most characteristic forms were tree-ferns. The oldest known 
 true horse-tails are met with in the Trias (Fig. 164, a). The 
 most abundant conifer is the cypress-like Vollzia (Fig. 164, I). 
 Cycads, already a feature in the vegetation of the Permian sys- 
 tem, now increase in number and variety. During the Mesozoic 
 ages they continued to be the most characteristic members of the 
 terrestrial flora, insomuch that this division of geological time 
 is sometimes spoken of as the "Age of Cycads/' Some of the 
 
MESOZOIC PERIODS TRIASSIO 305 
 
 more common cycads in the Triassic rocks are Pterophyllum, 
 Zamites, and Podozamites (Fig. 164, c). 
 
 The red gypseous and saliferous strata, for the reason already 
 given, are on the whole unfossiliferous. Here and there, foot- 
 prints of amphibians, preserved on the sandstones, give us a 
 glimpse of the higher forms of life that moved about on the 
 margin of the salt-lakes. The beds of limestone, which repre- 
 sent intervals when, for a time, the sea over- 
 spread the lakes, contain sometimes abundant 
 fossils. But they are numerous in individuals 
 rather than in species or genera, as if the con- 
 ditions for life in those waters were still some- 
 what unfavourable. On the other hand, the 
 limestones laid down in the opener sea are 
 crowded with a varied fauna. One of the most 
 typical fossils of the Trias is the crinoid En- 
 crinus liliiformis (Fig. 165), one of the most 
 familiar fossils of the limestones (Muschelkalk) 
 which in Germany form the central division of 
 the system. Among the lamellibranchs, Myo- 
 phoria, Avicula, Pecten, Cardium, Pullastra, 
 Daonella, and Monotis are characteristic genera (Fig. 166), 
 some species such as Avicula contorta, Pecten valoniensis, 
 and Cardium rhceticum being eminently useful in tracing 
 the upper parts of the Trias (Kha3tic) all over Europe 
 from Italy to Scandinavia. One of the most distinctive features 
 of the Triassic fauna is its development of cephalopod life. In 
 the limestones of the middle subdivision in Germany, a few 
 species, of cephalopods occur, the two prevalent forms being 
 species of Nautilus and the ammonite Ceratites (Fig. 167). 
 But when we turn to the Trias of the Eastern Alps, which repre- 
 sents the deposits of the more open sea, we meet with a remark- 
 able abundance and variety of cephalopods, and with a striking 
 admixture of ancient and more modern types. For example, 
 the venerable genus Orthoceras, which occurs even down in the 
 Cambrian rocks, is found also here as a survival from Pabeozoic 
 time. But new types now appeared. In particular, the tribe 
 of Ammonites, so pre-eminently typical of the molluscan life 
 of the Mesozoic seas, is represented by numerous genera and 
 species (Arcestes, Tracliyceras, Pinacoceras, Pliylloceras, be- 
 
306 
 
 GEOLOGY 
 
 sides Ceratites above referred to). Among the fishes of the 
 Trias, the genera Acrodus, Ceratodus, Gyrolepis, Hy~bodus, and 
 Pholidophorus may be mentioned. Labyrinthodonts still 
 haunted the lagoons and sandy shores (Mastodonsaurus, Trema- 
 tosaurus) ; but they no longer remained the most important 
 
 Fig. 166. Triassic Lamellibranchs. (c) Avicula contorta (natural 
 size) ; (6) Pecten valoniensis (i) ; (c) Cardium rhceticum (natural 
 size) ; (d) Myophoria vulgaris (i). 
 
 members of the animal world. Various early types of lizards 
 now took their places in the ranks of creation (Hyperodapedon, 
 Telerpeton, Fig. 168). A strange order of Triassic reptiles was 
 
 Fig. 167. Triassic Cephalopoda, (a) Nautilus bidorsatus Ck) ', (&) 
 Ceratites nodosus (reduced). 
 
 characterised by the jaws having the form of a beak, somewhat 
 like that of a turtle ; Dicynodon, one of these forms, carried two 
 huge tusks in the upper jaw. A remarkable and long-extinct 
 
MESOZOIC PERIODS TRIASSIC 
 
 307 
 
 order of reptiles, that of the Deinosaurs, made its first appear- 
 ance in Triassic time. These creatures were marked by peculi- 
 arities of structure that linked them both with true reptiles 
 and with birds, while in size they sometimes resembled ele- 
 
 Fig. 168. Trias- 
 sic Lizard (Te- 
 lerpeton elginen- 
 se, 1). 
 
 Fig. 169. Triassic Croco- 
 dile (Scutes of Stagonole- 
 pis elginensis, J). 
 
 phants and rhinoceroses. They seem to have walked mainly 
 on their hind feet, the three-toed or five-toed bird-like imprints 
 of which are numerous on some beds of sandstone. They are 
 characteristically Mesozoic types of life. An- 
 other not less typically Mesozoic form, that 
 of the Plesiosaurs, likewise began in Triassic 
 time ; but it will be more particularly alluded 
 to in the following chapter. The earliest 
 known crocodiles have been found in Triassic 
 rocks; some of the scutes or scales of one 
 of these animals are shown in Fig. 169. But 
 the most important advance in the fauna of 
 the globe during the Triassic period was the 
 first appearance of mammalian life. De- 
 tached teeth and lower jaws have been met 
 with in the uppermost parts of the Triassic 
 system, which have been identified as possess- 
 ing structures like those of some of the ( T ) 
 marsupial animals of Australia (Microlestcs, Fig. 170). It is 
 interesting to know that the earliest representatives of the great 
 class of the mammalia belonged to one of its lowest divisions. 
 They were small creatures probably resembling the OrnitJio- 
 rliynclius and Echidna of Australia. 
 
 Fig. 170. Trias- 
 sic Marsupial 
 (M i c r o I e s - 
 tes Moorei). 
 (o) Lower mo- 
 lar tooth, outer 
 side (| ) ; (6) 
 Ditto (nat. 
 size) ; (c) Dit- 
 to, front side 
 
308 
 
 GEOLOGY 
 
 The Triassic strata of the inland basins (England, Germany, 
 France, etc.) have been subdivided into the following groups: 
 
 Red, green, and gray marls, black shales, sandstones, 
 bone-beds, and in Germany sometimes thin seams 
 of coal. Characteristic fossils are Gardium rhceti- 
 cum, Avicula contorta, Pecten valoniensis, Pullastra 
 arenicola, Acrodus, Ceratodus, Hybodus, Saurians, 
 Microlestes. 
 
 Red, gray, and green marls, with beds of rock-salt and 
 gypsum. 
 
 Red sandstones and marls (England) ; gray sand- 
 stones and dark marls and clays, with thin seams 
 of earthy coal (Germany). 
 
 Limestones and dolomites, with bands of anhydrite, 
 gypsum, and rock-salt. The limestones are the 
 great repository of the fossils. This subdivision is 
 absent or only feebly represented in England. 
 
 Mottled red and green sandstones, marls, and some- 
 times pebble-beds. 
 
 Rhsetic. 
 
 Keuper or up- 
 per Trias. 
 
 Muschelkalk or 
 Middle Trias. 
 
 Bunter or Low- 
 er Trias. 
 
 The salt-beds of Cheshire have long been worked for com- 
 mercial purposes. The lower bed is sometimes more than 100 
 feet thick; but the salt deposits of Germany are much more 
 important. Thus at Sperenberg, 20 miles south of Berlin, a 
 boring was put down through about 290 feet of gypsum, and 
 then through upwards of 5,000 feet of rock-salt, without reach- 
 ing the bottom of the deposit. 
 
 The alternation of bands of rock-salt with thin layers of 
 anhydrite or of gypsum no doubt marks successive periods of 
 desiccation and inflow; in other words, each seam of sulphate 
 of lime (which is the least soluble salt, and is therefore thrown 
 down first) seems to indicate a renewed supply of salt water from 
 outside, probably from the open sea, while the overlying rock- 
 salt shows continued evaporation, during which the water be- 
 came a concentrated solution and deposited a thicker layer of 
 sodium chloride. Sometimes the concentration continued until 
 still more soluble salts, such as chlorides of potassium and mag- 
 nesium, were also eliminated. These phenomena are well dis- 
 played at the great salt-mines of Stassfurt, on the north flank 
 of the Harz Mountains. The lowest rock there found is a mass 
 of pure, solid, crystalline rock-salt of still unknown thickness, 
 but which has been pierced for about 1,000 feet. This rock 
 is separated into layers, averaging about 3% inches in thick- 
 
MESOZOIC PERIODS TRIASSIC 309 
 
 ness, by partings of anhydrite % inch thick or less. If each 
 of these "year rings," as the German miners call them, repre- 
 sented the deposit formed during the dry season of a single 
 year, then the mass of 1,000 feet would have taken more than 
 3,000 years for its formation. But there do not appear to be 
 any good grounds for believing that each band marks one year's 
 accumulation. Above the rock-salt lie valuable deposits of the 
 more soluble salts, particularly chlorides of potassium and mag- 
 nesium, with sulphates of lime and magnesia. The compound 
 known as Carnallite (a double chloride of potassium and mag- 
 nesium) is now the chief source of the potash salts of commerce. 
 
 In the Ehaetic beds of England, one of the most interesting 
 bands is the so-called "bone-bed" a thin layer of dark sand- 
 stone, charged with bones, teeth, and scales of fishes and sauri- 
 ans. A thin seam of limestone in the same group contains 
 wings and wing-cases of insects. 
 
 The Trias of the Eastern Alps reaches a thickness of many 
 thousand feet, and forms great ranges of mountains. The lower 
 division runs throughout the Alps with considerable uniformity 
 of character, so that it forms a useful platform from which to 
 investigate the complicated geological structure of these moun- 
 tains. The Upper Trias consists of several thousand feet of 
 shales, marls, limestones, and dolomites, while the Ehaetic group 
 swells out into- a great succession of limestones and dolomites. 
 During the time when the Triassic sea stretched over the site 
 of the Alps there were evidently considerable oscillations of 
 level, and there likewise occurred extensive volcanic eruptions, 
 whereby large masses of lavas and tuffs were ejected. These 
 rocks now form conspicuous hills in the Tyrol. 
 
 Triassic rocks have been traced in Beloochistan, the salt range 
 of the Punjab, Northern Kashmir, and Western Thibet. They 
 cover a large area of North America, and have been recognised 
 in Australia and New Zealand. Eocks which have been assigned 
 to the same geological period occur in South Africa, and have 
 there yielded a remarkable series of reptilian remains. 
 
310 GEOLOGY 
 
 CHAPTEE XXIII. 
 
 THE JURASSIC PERIOD. 
 
 THE system which follows the Trias has received its name, 
 JURASSIC, from the Jura Mountains, where it is well 
 developed. It contains the record of a great series of 
 geographical changes, which in Europe entirely effaced the 
 inland basins and sandy wastes of the previous period, and dur- 
 ing which sedimentary rocks were accumulated that now extend 
 in a broad belt across England, from the coasts of Dorset to 
 those of Yorkshire, cover an enormous area of France and Ger- 
 many, and sweep along both sides of the Alps and the Apennines. 
 These strata vary greatly in composition and thickness as they 
 are traced from country to country. In one district they pre- 
 sent a series of limestones which, as they are followed into an- 
 other area, pass into shales or sandstones. The widespread uni- 
 formity of lithological character, so marked among the Palaeozoic 
 systems, gives place in the Mesozoic series to greater variety. 
 Sandstones, shales, and limestones alternate more rapidly with 
 each other, and are more local in their extent. They indicate 
 greater vicissitudes in the process of deposition, more frequent 
 alternations of sea and land, and not improbably greater differ- 
 ences of climate than in Palaeozoic time. 
 
 The flora of the Jurassic period is marked by the same general 
 characters as that of the Trias ferns, equisetums, conifers, and 
 cycads, being its distinguishing elements. Cycads now abound 
 (Pterophyllum, Zamites, Cycadites, and many others, Fig. 171). 
 Among the conifers are the remote ancestors of our " Puzzle- 
 monkeys," introduced from Chili and now so common as orna- 
 mental garden shrubs (Araucaria imbricata), and of our pines 
 and firs. This vegetation flourished luxuriantly over the area 
 of Britain; on the site of Yorkshire it grew so densely as to 
 give rise to thick peaty accumulations, which now form beds 
 
THE JURASSIC PERIOD 
 
 311 
 
 of coal. It went far northward, for its remains have been abun- 
 dantly preserved in Spitzbergen, where numerous cycads have 
 been found among them. These plants unquestionably grew 
 and flourished within the Arctic Circle, so that, though the 
 climates of the globe were already beginning to emerge from 
 
 Fig. 171. Jurassic Cycad (Cycadeoidea, microphylla, J) 
 
 the greater uniformity of Palaeozoic time, the Arctic regions still 
 enjoyed a temperature like that of sub-tropical countries at the 
 present time. 
 
 The animal world during the Jurassic period, if we may judge 
 of it from its fossil remains, must have been much more varied 
 alike on land and in the sea than during the previous ages of the 
 earth's history. From the circum- 
 stances in which the strata were de- 
 posited, relics of the life of the land 
 are frequently met with, besides 
 abundant records of that of the sea. 
 A characteristic feature of the period 
 was the profusion of corals, which 
 at different times spread over much 
 of the site of modern Europe. They 
 were no longer the rugose forms 
 so distinctive of the Paleozoic seas, 
 but true reef-building astraeids, be- 
 longing to the genera Isastrcea, Thamnastrcea, Montlivaltia, The- 
 cosmilia, etc. (Fig. 172). Crinoids were still abundant, though 
 less so than in the Carboniferous limestone sea ; the old forms were 
 
 Fig. 172. Jurassic reef- 
 building Coral ( Isastrcea 
 explanata, J). From the 
 Corallian. 
 
312 
 
 GEOLOGY 
 
 now replaced by others, among which the most conspicuous was 
 the Pentacrinite (Fig. 173) a genus still living in our present 
 seas. Sea-urchins swarmed on some parts of the sea-floor; 
 
 Fig. 173. Jurassic Crinoid (Pentacrinus fasciculosus, ). 
 
 among their more frequent genera are Cidaris (Fig. 174), Dia- 
 dema, Hemicidaris, Acrosalenia, Glyptichus, Pygaster. Of the 
 
 contrasts between the Mesozoic and 
 Paleozoic faunas, one of the most 
 marked is to be found among the 
 brachiopods. Except the persistent in- 
 articulate types which have lived on 
 from Cambrian time to the present day 
 (Crania, Lingula, Discina), the num- 
 reef _ erous and varied forms which played so 
 important a part in the life of the 
 Palaeozoic seas died out almost entirely 
 at the close of the Palaeozoic period. The ancient Spirifers and 
 Leptaenids lingered on until the Jurassic period, and then dis- 
 
 Fig. 174. Jurassic . 
 
 urchin (Cidaris florigem 
 wt, ), Corallian. 
 
THE JURASSIC PERIOD 313 
 
 appeared. On the other hand, the genera Rhynchonella and 
 Terebratula, which occupied a subordinate place in earlier ages, 
 now became the chief representatives of the brachiopods. They 
 abounded throughout Mesozoic time, but they have gradually 
 diminished in number since then, and at the present day each 
 genus survives only in a small number of species. With the 
 decay of the brachiopods, the other divisions of the mollusca 
 proportionately advanced. The lamellibranchs attained a great 
 development in Mesozoic time, some characteristic genera being 
 Gervillia, Exogyra, Lima, Ostrea, Pecten, Pinna, Astarte, Hip- 
 
 Fig. 175. Jurassic Lamellibranchs. (a) Trigonia monilifera (), 
 Kimmeridge Clay; (&) Plicatula spinosa (), Middle Lias; (c) Gry- 
 phwa arcuata (incurva) (|), Lower Lias. 
 
 popodium, Trigonia (Fig. 175). Some of the oysters were par- 
 ticularly abundant, Gryphcea, for instance, being so plentiful in 
 some bands of limestone as to give the name of " Gryphite 
 limestone" to them. But undoubtedly the distinctive feature 
 of the molluscan fauna of Mesozoic time was the great develop- 
 ment of the cephalopods. The chambered division was repre- 
 sented by an extraordinary variety of Ammonites (Fig. 176), 
 and the cuttle-fishes by many species of Belemnite (Fig. 177). 
 The ammonites have been made use of to mark off the formations 
 into distinct zones; for, as a rule, the vertical range of each 
 
314 
 
 GEOLOGY 
 
 species is comparatively small. The band of strata characterised 
 by a particular species of ammonite is called the zone of that 
 species, e.g. Zone of Amm. planorbis, which is the lowest zone 
 of the Lower Lias. 
 
 Fig. 176. Jurassic Ammonites, (a) Ammonites striatus (i), Middle 
 Lias; (6) A. communis (), Upper Lias; (c) A. cordatus (|), 
 Lower Calcareous Grit; (d) A. Jason (J), Oxford Clay. 
 
 Another striking contrast is presented by the Jurassic crus- 
 tacea when compared with those of the Palaeozoic ages. The 
 ancient order of trilobites, so abundant in the seas of the older 
 
 Fig. 177. Jurassic Belemnite (B. hastatus, natural size), Middle Oolite. 
 
 time, had now entirely disappeared ; the eurypterids, which took 
 their place upon the scene as the trilobites were on the wane, 
 had likewise vanished. In their stead there now came abundant 
 
THE JURASSIC PERIOD 315 
 
 ten-footed Crustacea, including both long-tailed forms the 
 ancestors of our lobsters, prawns, shrimps, and cray-fish and 
 short-tailed forms that heralded the coming of our living crabs 
 (Fig. 178). Among the Jurassic strata there occasionally oc- 
 cur thin bands, which have received the name of " insect-beds " 
 
 Fig. 178. Jurassic Crustacean (Scapheus ancylochelis) . 
 
 from the numerous insect-remains which they contain. The 
 neuroptera are most frequent, but orthoptera and coleoptera also 
 occur. Among these remains are forms of dragon-fly, May-fly, 
 grasshopper, and cockroach. The wing-cases of beetles also 
 are not uncommon; and there has been found the wing of a 
 butterfly the oldest example of a lepidopterous insect yet 
 known. 
 
 Fig. 179. Jurassic Fish (Pholidophorus Bechei, J), Lower Lias. 
 
 Fishes abounded in the waters of the Jurassic time. Those of 
 which the remains have been preserved are chiefly small ganoids 
 (Plwlidopliorus, Dapedius, Lepidotus, Pycnodm, Fig. 179), 
 with no representatives of the huge bone-cased placoderms of 
 earlier time. There were likewise various tribes of sharks and 
 rays (Hybodus, Acrodus, Squaloraia). 
 
316 GEOLOGY 
 
 But taking the Jurassic fauna as a whole, undoubtedly its 
 most striking character was given by the extraordinary develop- 
 ment of its reptiles. So remarkably varied was the reptilian 
 life throughout the Mesozoic period that this part of the earth's 
 history has been called the " Age of Reptiles." There were forms 
 which haunted the sea, others that frequented the rivers, some 
 that lived on the land, some that flew through the air. Never 
 before or since has there been such a profusion of reptilian 
 types. Some of these are still represented at the present time. 
 The Jurassic Teleosaurus and Steneosaurus, for example, have 
 their counterparts now in the living crocodile and alligator. 
 The modern turtles, too, are descendants of those which lived in 
 Jurassic times. But it is the long-extinct types that fill us 
 with astonishment. One of the most abundant of them is that 
 of the enaliosaurs or sea-lizards, of which the two leading forms 
 were the Ichthyosaurus and Plesiosaurus. The former creature 
 
 Fig. 180. Jurassic Sea-lizard (Ichthyosaurus communis, ^), Lias. 
 
 (Fig. 180), occasionally more than 24 feet long, somewhat re- 
 sembled a whale in shape and bulk, its head being joined by no 
 distinct neck to the body, which tapered into a long tail. It 
 swam by means of two pairs of strong paddles, and probably 
 steered itself by a fin on the tail. Its eyes were large, and had 
 a ring of bony plates round the eyeball, which remain distinct 
 in the fossil state. Its jaws were arnied with numerous strong 
 pointed teeth, not set in distinct sockets. This reptile probably 
 lived chiefly in the sea, feeding there upon the abundant ganoid 
 fishes which its huge protected eyes enabled it to track even into 
 the deeper water. But it no doubt also sought the land, and 
 was able to waddle along the shore or to lie there basking in 
 the sunshine. The Plesiosaurus, in many respects like the 
 Ichthyosaurus, was distinguished by its proportionately shorter 
 tail, longer neck, smaller head, larger paddles, and the insertion 
 of the teeth in distinct sockets. It probably haunted the la- 
 goons, rivers, and shallow seas of the time. Its long swan-like 
 
THE JURASSIC PERIOD 317 
 
 neck enabled it to lie at the bottom and raise its head to the 
 surface to breathe, or/ when at the surface, to send down its 
 powerful jaws and catch its prey at the bottom. 
 
 Still more extraordinary were the Pterosaurs or flying rep- 
 tiles strange bat-like creatures with disproportionately large 
 heads, and large eyes like those of the Ichthyosaurus. The outer- 
 most finger of each forefoot was prolonged to a great length, 
 and supported a membrane with which the animals could fly. 
 The bones were hollow and filled with air like those of birds. 
 Various forms of these winged lizards are found in the Jurassic 
 rocks, the most typical being Pterodactylus and Scaphognatlius 
 (Fig. 181) ; others are Dimorphodon, Rhamphorhynchtis, 
 Eliamplwceplialus, and Dorygnathus. 
 
 Fig. 181. Jurassic Pterosaur, or flying reptile (Scaphognathus crassiroS' 
 tris), Middle Oolite. 
 
 The Deinosaurs, which have already been noticed as having 
 appeared in Triassic time, attained a far greater development in 
 the Jurassic period. These hugest land-reptiles now reached 
 their maximum in size and variety. One of their genera, 
 Megalosaurus, is believed to have been 25 feet long, and to have 
 walked on its massive hind legs along the margins of the shallow 
 waters in search of the smaller animals on which it preyed. 
 Another form, Ceteosaurus, which may have been as much as 50 
 feet from the snout to the tip of the tail, and stood some 10 
 feet high, fed on the vegetation that shaded the rivers and la- 
 goons where it lived. Still more gigantic were some deinosaurs, 
 
318 
 
 GEOLOGY 
 
 of which the remains have been found in the Jurassic rocks of 
 North America. Brontosaurus, about 50 'feet or more in length 
 with a short body, long neck and tail, and small head, had enor- 
 mous feet, each of which made an imprint measuring about a 
 square yard in area. Stegosaurus, another sluggish deinosaur, 
 was protected by numerous huge plates and spines of bone on its 
 
 Fig. 182. Jurassic Bird (Arcliccoptcryx macroura, about |), Solenhofen 
 Limestone Middle Jurassic. 
 
 back, some of the latter more than 3 feet long. The largest of 
 all these monsters, and, so far as yet known, the most colossal 
 animal that ever walked on the earth, was the Atlaniosaurus, 
 which is believed to have been not much less than 100 feet in 
 length, and 30 feet or more in height. 
 
 In another respect, the fauna of the Jurassic period stands 
 out from those that preceded it ; it contained the earliest known 
 birds. These interesting prototypes differed much from modern 
 birds, more particularly in the possession of certain_ peculiarities j 
 
THE JURASSIC PERIOD 
 
 319 
 
 of structure that linked them with reptiles. They had teeth 
 in their jaws, and some of them carried long lizard-like tails, 
 each vertebra of which bore a pair of quill-feathers. The best 
 known genus is Archceopteryx (Fig. 182), found in the litho- 
 graphic limestone of Solenhofen. 
 
 Marsupials, which, so far as yet known, made their appear- 
 ance in Triassic time, continued to be the only representatives 
 
 Fig. 183. Jurassic Marsupial (Phascolotherium Bucklandi). (a) 
 Teeth magnified; (6) Jaw, natural size. 
 
 of the mammalia during the Jurassic period, at least no other 
 types have yet been discovered among the fossils. Lower jaws 
 and detached teeth (Fig. 183) have been obtained from two dis- 
 tinct platforms in England the Stonesfield Slate and Purbeck 
 beds and have been referred to a number of genera which 
 find their nearest modern representatives in the Australian 
 bandicoots and in the American opossums (Phascolotherium^ 
 Stereognatlms, SpalacotJienum, Plagiaulax) . 
 
 The Jurassic system has been arranged in the following sub- 
 divisions : 
 
 as 
 
 si 
 
 I. 
 
 8. Purbeckian 
 
 7. Portlandian. 
 
 Am- 
 
 Am- 
 
 o L 
 
 Upper fresh-water beds (Purbeck). 
 
 Middle marine beds 
 
 Lower fresh-water beds 
 
 Limestones and calcareous freestones (Portland 
 Stone) ; Cerithium portlandicum, Ammonites 
 giganteus, Trigonia gibbosa. 
 
 Sandstones and marls (Portland Sand) ; 
 
 monites biplex, Exogyra bruntrutana. 
 
 6. Kimmeridgianf Dark shales and clays (Kimmeridge Clay) 
 \ monites decipiens, Exogyra virgula. 
 
 Coral rag (limestone with corals), clays, and 
 calcareous grits; Thamnastrcea, Isastraea, 
 Cidaris ftorigemma, Ammonites cordatus (Fig. 
 176). 
 
 Blue and brown clay (Oxford Clay) ; Ammon- 
 ites Jason (Fig. 176). 
 
 Calcareous sandstone (Kellaways Rock Cal- 
 lovian) ; Ammonites calloviensis. 
 
 Corallian 
 
 4. Oxfordian 
 
320 
 
 GEOLOGY 
 
 3. Bathonian . . 
 
 2. Bajocian 
 (Inferior Oolite) 
 
 1. Liassic 
 
 Shelly limestones, clays, and sands (Cornbrash, 
 Bradford Clay, and Forest Marble). Am- 
 monites discus. 
 
 Shelly limestones (Great or Bath Oolite), 
 Stonesfield Slate; Ammonites gracilis. 
 
 Fuller's Earth. 
 
 Marine calcareous freestones and grits (Chel- 
 tenham), containing zones of Ammonites Par- 
 kinsoni, A. Humphriesianus, A. Sowerbyi, A. 
 Hurchisonw ; represented in Yorkshire, by 
 800 feet or more of estuarine sandstones, 
 shales, and limestones, with beds of coal. 
 
 Sandy beds and clays (Upper Lias, Toarcian) ; 
 Ammonites communis, A. jurensis, A. serpcn- 
 tinus. 
 
 Limestones, sands, clays, and ironstones (Mid- 
 dle Lias, Marlstone) ; Ammonites margarita- 
 tus, A. spinatus, A. Jamesoni. 
 
 Thin blue and brown limestones, and dark 
 shales (Lower Lias, Sinemurian and Hettan- 
 gian) ; Ammonites planorbis, A. raricostatus, 
 A. Bucklandi. 
 
 1. The LIAS, so called originally by the Somerset quarrymen 
 from its marked arrangement into " layers," extends completely 
 across England from Lyme Kegis to Whitby. It can be divided 
 into three distinct sections : (a) A lower group of thin blue lime- 
 stones and dark shales with limestone nodules, the limestones 
 being largely used for making cement. This is one of the chief 
 platforms for the reptilian remains, entire skeletons of ichthyo- 
 saurus, plesiosaurus, etc., having been exhumed at Lyme Eegis; 
 (&) Marlstone or Middle Lias hard argillaceous or ferru- 
 ginous limestones which form a low ridge or escarpment rising 
 from the plain of the Lower Lias ; in Yorkshire contains a thick 
 series of beds of earthy carbonate of iron, which are extensively 
 mined as a source for the manufacture of iron; (c) Clays and 
 shales surmounted by sandy beds (Upper Lias Stands). The 
 organic remains of the Lias are abundant and well preserved. 
 They are chiefly marine; but that the rocks containing them 
 were deposited near land is indicated by the numerous leaves, 
 branches, and fruits imbedded in them, and by the various in- 
 sect-remains that have been obtained from them. In Germany, 
 where the Lias is well developed and presents a general re- 
 semblance to the English type, it is known as the Lower or 
 
THE JURASSIC PERIOD 321 
 
 Black Jura. It is still better shown in France, where its three 
 stages attain in Lorraine a united thickness of more than 600 
 feet. To the south, however, in Provence, it reaches the great 
 thickness of 2,300 feet. 
 
 2. The BAJOCIAN stage, so named from Bayeux in Normandy, 
 where it is well displayed, has long been known in England 
 under the name of Inferior Oolite. It presents two distinct 
 types in this country, being a thoroughly marine formation in 
 the southwestern counties and passing northward into a series 
 of strata which were accumulated in an estuary, and which con- 
 tain the chief repositories of the Jurassic flora. Among the 
 estuarine beds of Yorkshire a few thin coal-seams occur, which 
 have been worked to some extent. On the continent, this divi- 
 sion is characteristically marine; it reaches its greatest develop- 
 ment in Provence, where it is 950 feet thick. It runs through 
 the Jura Mountains, where it is made up of more than 300 feet 
 of strata, chiefly limestone. In Germany the strata from the 
 top of the Lias to the base of the Callovian group that is, the 
 two stages of Bajocian and Bathonian are classed together as 
 the Middle or Brown Jura, or Dogger. 
 
 3. The BATHONIAN stage is named from Bath, where its sub- 
 divisions are admirably exposed. At its base is a local argil- 
 laceous band known as Fuller's Earth, because long used for 
 fulling cloth. The chief member of the stage in the south-west 
 of England is the Great or Bath Oolite, a succession of lime- 
 stones, often oolitic, with clays and sands. The Stonesfield Slate 
 is the name locally given to some thin-bedded limestones and 
 sands forming the lower part of the Great Oolite, and of high 
 geological interest from having supplied among their fossils re- 
 mains of land-plants, numerous insects, bones of enaliosaurs and 
 deinosaurs, and of small marsupials. The Great Oolite abounds 
 in corals, and contains numerous genera of mollusca, fishes, and 
 reptiles. The Cornbrash (so named from its friable (brashy) 
 character, and from its forming good soil for corn) is one of the 
 most persistent bands in the English Jurassic system, retaining 
 its characters all the way from the south-western counties to near 
 the Humber. 
 
 4. The OXFORDIAN stage, sometimes called the Middle or Ox- 
 ford Oolite, consists of a lower zone of calcareous sandstone, 
 known as the Kellaways rock or Callovian, from the name of a 
 
322 GEOLOGY 
 
 place in Wiltshire, and of a thick upper stiff blue and brown 
 clay, called, from, the locality where it is well developed, the 
 Oxford Clay, and containing numerous ammonites, belemnites, 
 and oysters, but no corals. In Germany, the strata from the 
 base of the Callovian to the top of the Purbeckian group are 
 known as the Malm or White Jura. 
 
 5. The CORALLIAN stage, so named from the corals with which 
 it abounds, is one of the most distinctive in the Jurassic system. 
 It is traceable across the greater part of England, over the 
 continent of Europe from Normandy to the Mediterranean, 
 through the east of France, and along the whole length of the 
 Jura Mountains and the flank of the Swabian Alps. While it 
 was being formed, the greater part of Europe lay beneath a 
 shallow sea, the floor of which was clustered over with reefs of 
 coral. 
 
 6. The KJMMERIDGIAN group or stage is typically displayed 
 at Kimmeridge on the coast of Dorsetshire, whence its name. 
 It there consists of dark shales, some of which are so highly 
 bituminous as to burn readily, and which will probably be 
 eventually of commercial value as a source for the distillation 
 of mineral oil. This group of strata has yielded a larger num- 
 ber of reptilian genera and species than any other in the 
 Mesozoic systems of Britain plesiosaurs, ichthyosaurs, ptero- 
 saurs, deinosaurs, turtles, and crocodiles. It is well developed 
 in France and Germany. 
 
 7. The PORTLANDIAN stage, so called from the Isle of Port- 
 land where it is well seen, consists of a lower set of sandy beds 
 (Portland Sand), and a higher and thicker series of limestones 
 and calcareous freestones, some of the beds containing abundant 
 nodules and layers of flint. These rocks are prolonged into 
 France near Boulogne-sur-Mer. 
 
 8. The PURBECKIAN group or stage is best seen in the Isle of 
 Purbeck, hence its name. It lies on an upraised surface of 
 Portlandian beds, showing that after the deposition of these 
 beds there was some disturbance of the sea-bed, portions of 
 which were uplifted partly into land and partly into shallow 
 brackish and fresh waters. The Purbeck beds are subdivided 
 into three sub-stages : the lowest consisting of fresh-water lime- 
 stones, with layers of ancient soil (" dirt-beds "), in which the 
 stumps of cycadaceous trees still stand in the positions in which 
 
THE JURASSIC PERIOD 323 
 
 they grew (Fig. 171) ; the middle sub-stage contains oysters and 
 other marine shells which prove that owing to subsidence the 
 area sank under the sea; while in the higher subdivision fresh- 
 water fossils reappear. Among the more interesting organisms 
 yielded by the Purbeck beds are the remains of numerous insects 
 and of the marsupials already referred to, which chiefly occur as 
 lower jaws in a stratum about 5 inches thick. When the bodies of 
 dead animals float out to sea the first bones likely to drop out 
 of the decomposing carcases are the lower jaws; hence the 
 greater frequency of these bones in the fossil state. Strata be- 
 longing to the Purbeckian stage and including red and green 
 marls, with dolomite and gypsum, are found in north-western 
 Germany, showing in that region also the elevation of the floor 
 of the Jurassic sea into detached basins. 
 
 In India, a mass of strata 6,300 feet thick is found in Cutch, 
 and from its fossils is believed to represent the European Juras- 
 sic system from the Bajocian up to the top of the Portlandian 
 stage. In Australia and New Zealand, recognisable Jurassic 
 fossils have also been found, showing the extension of the 
 Jurassic system even to the Antipodes. In North America, 
 Jurassic rocks are not largely developed; but in Colorado they 
 have yielded an abundant series of organic remains, includ- 
 ing fishes, tortoises, pterosaurs, deinosaurs, crocodiles, and mar- 
 supials. 
 
324 GEOLOGY 
 
 CHAPTER XXIV. 
 
 THE CRETACEOUS PERIOD. 
 
 THE CEETACEOUS system received its name in Western 
 Europe, because in England and in Northern France its 
 most conspicuous member is a thick mass of white chalk 
 (Latin, Creta). It covers a far more extensive area of the 
 surface of this continent than any of the preceding systems. 
 Its western extremity reaches to the north of Ireland and the 
 Western Islands of Scotland. It covers a large part of the 
 east and south of England, stretching thence into France, where 
 it forms a broad band, encircling the Tertiary basin of Paris. It 
 sweeps across Belgium into Westphalia, underlies the vast plain 
 of Northern Germany and Denmark, whence it is prolonged into 
 Southern Russia, where it overspreads many thousands of square 
 miles. It flanks most of the principal mountain-chains of Eu- 
 rope the Pyrenees, Alps, Apennines, and Carpathians. It 
 spreads far and wide -over the basin of the Mediterranean Sea, 
 extending across vast tracts of Northern Africa, and from the 
 Adriatic athwart Greece and Turkey into Asia Minor, whence it 
 is prolonged through the Asiatic continent. 
 
 As most of the rocks of the system are of marine origin, we 
 at once perceive how entirely different the Cretaceous geography 
 must have been from that of the present day, and to what a great 
 extent the existing land of the Old World lay then below the sea. 
 But in tracing out the distribution of the rocks, geologists have 
 found that the Cretaceous sea did not sweep continuously across 
 Europe. On the contrary, as they have ascertained, the old 
 northern land still rose over the site of Northern Britain and 
 Scandinavia, while to the south of it a wide depression extended 
 across the area of Southern Britain, Northern France, Belgium, 
 and the North German plain, eastwards to Bohemia and Silesia. 
 This vast northern basin was the theatre of a remarkable suc- 
 cession of geological revolutions. While its eastern portions, 
 
THE CRETACEOUS PERIOD 325 
 
 during the earlier part of the Cretaceous period, were submerged 
 under the sea, its western tracts were the site of the delta of a 
 great river, probably descending from the land that still lay 
 massed towards the north. During the later ages of the period, 
 the whole of this area formed a broad and long gulf or inlet, 
 the southern margin of which seems to have been defined by the 
 ridge of old rocks that runs from the headlands of Brittany 
 through Central France, the Black Forest, and the high grounds 
 of Bohemia. South of that ridge lay the open ocean which 
 extended all over Southern Europe and the north of Africa, and 
 spread eastwards into Asia. 
 
 Bearing in mind this peculiar disposition of sea and land, 
 we can understand why the development of the Cretaceous sys- 
 tem, alike in regard to its deposits and its fossils, should be so 
 different in the area of the northern basin from that of the 
 southern regions. In the one case we meet with the local and 
 changing accumulations of a comparatively shallow and some- 
 what isolated portion of the sea-bed, wherein are mingled abun- 
 dant traces of the proximity of land. In the other we are 
 presented with evidence of a wide open sea, where the same 
 kinds of deposits and the same forms of marine life extend 
 with little change over vast distances. Obviously, it is not 
 the local type of the northern basin, but the more general and 
 widespread type of Southern Europe that should be taken for 
 the distinctive characteristics of the Cretaceous system. But 
 the northern basin was the first to be systematically explored, 
 and is still the best known, and hence its features have not un- 
 naturally usurped the place of importance which ought properly 
 to be assigned to the other wider area. 
 
 Eegarding the period as a whole, let us first consider the 
 general character of its distinguishing flora and fauna, and then 
 pass on to trace the history of the period as revealed by the 
 succession of strata. The plants of the Cretaceous system show 
 that the vegetable kingdom had now made a most important 
 advance in organisation. In the lower half of the system the 
 fossil plants yet found are on the whole like those of the Jurassic 
 rocks that is, they include some of the same genera of ferns, 
 cycads, and conifers which these rocks contained. But already 
 the ancestors of our common trees and flowering plants must 
 have made their appearance, for in the upper half of the system 
 
326 
 
 GEOLOGY 
 
 their remains occur in abundance. This earliest dicotyledonous 
 flora numbered among its members species of maple, alder, 
 aralia, poplar, myrica, oak, fig, walnut, beech, plane, sassafras, 
 laurel, cinnamon, ivy, dogwood, magnolia, gum-tree, ilex, buck- 
 thorn, cassia, credneria, and others. The modern aspect of this 
 assemblage of plants is in striking contrast to the more antique 
 look of all the older floras. There were likewise species of pine 
 (Pinus), Calif ornian pine (Sequoia), juniper, and other coni- 
 
 Fig. 184. Cretaceous Plants, (a) Quercus rinkiana () ; (6) Cinna- 
 momum sezannense () ; (c) Ficus atavina () ; (a) Sassafras re- 
 curvata () ; (e) Juglans arctica (i). 
 
 fers, various cycads, forms of screw-pine (Pandanus), palms 
 (Sabal), and numerous ferns (Gleichenia, Asplenium, etc.). 
 This flora spread over the land, surrounding the northern Creta- 
 ceous basin, and extended northwards even as far as North 
 Greenland, from which nearly 200 species of Cretaceous plants 
 have been obtained. The inference may be deduced that the 
 
THE CRETACEOUS PERIOD 
 
 327 
 
 climate of the globe must then have been much warmer than at 
 present. The luxuriant vegetation disinterred from the Creta- 
 ceous rocks of North Greenland includes more than forty kinds 
 of ferns, besides laurels, figs, magnolias, and other plants, which 
 
 Fig. 185. Cretaceous Foraminifera. (a) Textularia ~baudouiniana 
 (-T) 5 (6) QloUgerina, cretacea (V 1 ); (c) Rotalina voltziana (^). 
 
 show that, though the winters were no doubt dark, they must 
 have been extremely mild. There could have been no perpetual 
 frost and snow in these Arctic latitudes in Cretaceous times. 
 
 Foraminifera abound in some of the Cretaceous limestones, 
 indeed, in some places they form almost the only constituent of 
 these rocks. They are plentiful in the 
 white chalk of England, France, and Bel- 
 gium, one of the more frequent genera 
 being Gloligerina (Fig. 185) which still 
 lives in enormous numbers in the Atlan- 
 tic, and forms at the bottom of that 
 ocean a gray ooze not unlike chalk (Fig. 
 33). Sponges lived in great numbers in 
 the Cretaceous sea. Their minute sili- 
 ceous spicules are abundant in the Chalk, 
 and even entire sponges enveloped in flint 
 are not uncommon (Ventriculites, Fig. 
 186). Sea-urchins are among the most 
 familiar fossils of the Chalk, and must 
 have lived in great numbers on the Cre- 
 taceous sea-bottom. Some of their 
 genera are still living, and have been 
 recent years from great depths in the ocean 
 
 Fig. 186. Cretaceous 
 Sponge ( Ventricu- 
 lites decurrens, J). 
 
 dredged up in 
 Among the more 
 
 characteristic Cretaceous types are Anancliytes, Holaster, Mi- 
 craster, and Echinoconus (Fig. 187). The brachiopods were 
 still represented chiefly by the ancient genera Terebmtula and 
 
328 
 
 GEOLOGY 
 
 Rliynclionella. Lamellibranchs abounded, especially the genera 
 Ostrea, Exogyra, Inoceramus (Fig. 188), Lima, Pecten, and 
 the various forms of Hippuritids. These last (Hippurites, 
 Radiolites, Caprina, etc., Fig. 189) are specially characteristic, 
 being, so far as we know, confined to the Cretaceous system; 
 hence their occurrence serves to indicate the Cretaceous age of 
 the rock containing them. They have been imbedded in such 
 
 Fig. 187. Cretaceous Sea-urchins, (a) Echinoconus conicus, ( = 
 Galerites albo-galerus) , under surface and side view; (6) Ananchytes 
 ovatus (J), side view and under surface; (c) Micraster cor-arguinum 
 (5), upper and under surface. 
 
 numbers in the limestones of the south of Europe as to give the 
 name of hippurite-limestone to these rocks. They are compara- 
 tively infrequent in the strata of the northern Cretaceous basin. 
 Probably the most distinctive feature in the molluscan life of 
 the Cretaceous seas was the extraordinary variety in the devel- 
 
THE CRETACEOUS PERIOD 
 
 329 
 
 opment of the cephalopods. This is all the more remarkable 
 from the fact that before the next geological period the great 
 majority of these types appear to have become extinct. The 
 ammonites and belemnites, which played so important a part 
 
 Fig. 188. Cretaceous Lamellibranchs. (a) Trigonia aliformis (1) ; 
 (6) Inoceramus sulcatus (i) ; (c) Nucula bivirgata (natural size). 
 
 in the fauna of Mesozoic time, died out about the close of that 
 long succession of periods. At least in Europe, while their re- 
 mains continue to present themselves up to the top of the Greta- 
 
 Fig. 189. Cretaceous Lamellibranchs (Hippurites). (a) Radiolites 
 acuticostata () ; (6) Hippurites toucasiana (S) ; (c) Caprina Aguil- 
 loni (J) ; (d) Caprotina toucasianus (fe). 
 
 ceous system, they disappear entirely from the overlying strata. 
 It is curious to observe that while these important tribes were 
 
330 
 
 GEOLOGY 
 
 about to vanish, other cephalopods of new and varied types 
 nourished contemporaneously with them. Never before or since, 
 indeed, have the cephalopodan types been so manifold (Fig. 
 
 Fig. 190. Cretaceous Cephalopods. (a) B acuities anceps (J) ; (&) Pty- 
 choceras emericianus (\) ; (c) Toxoceras Utulerculatus (I) ; (d!) 
 Hamites rotundus (J) ; (e) Anculoceras renauxianus ( T V) ; (f) Sca- 
 phites cequalis () ; (g) Crioceras villiersianus (J) ; (h) Helicoceras 
 annulatus; (i) Ammonites inflatus (k) ', (k) Turrilites catenatus (J). 
 
 190). For instance Baculites is a straight chambered shell re- 
 minding us of the ancient OrtJioceras. In Toxoceras the shell 
 is bent into the form of a bow. In Hamites it is long, tapering, 
 and curved upon itself like a hook. In Ancyloceras it is coiled 
 
THE CRETACEOUS PERIOD 331 
 
 at the posterior end, the other being bent back upon itself ; while 
 in Scaphites the coils are adherent. In Ptychoceras the shell is 
 long, tapering, and bent once back on itself, the two portions be- 
 ing in contact. In Crioceras it is coiled, and the coils are not 
 adherent, as they are in the ammonites. In Helicoceras the 
 shell is coiled spirally, the coils remaining free, while in Tur- 
 rilites they are adherent. 
 
 The fishes of the Cretaceous period are chiefly known by 
 teeth belonging to various genera of sharks (Otodus, Lamna, 
 Oxyrhina) . But they also include the earliest known representa- 
 tives of the modern osseous or teleostean fishes, such as the 
 herring, salmon, and cod (Osmeroides, Enchodus, Beryx, Fig. 
 191, Syllcemus, etc.). 
 
 Fig. 191. Cretaceous Fish (Beryx lewesiensis, I). 
 
 Already reptilian life seems to have been on the decline, at 
 least there is much less variety and abundance of it in the 
 Cretaceous system than in that which immediately preceded it. 
 Turtles and tortoises continued to haunt the low shores of the 
 time. Ichthyosaurs, plesiosaurs, pterosaurs, and deinosaurs still 
 lived, but in diminishing numbers, and they are not known to 
 have survived the Cretaceous period. One of the most remarkable 
 of the deinosaurs, and interesting from being one of the last 
 of its race, is that known as Iguanodon (Fig. 192). Only scat- 
 tered teeth and bones of this animal were known, until a few 
 years ago the fortunate discovery of a number of entire skeletons 
 in Belgium enabled its structure to be almost completely made 
 known, and threw much fresh light on the osteology of the 
 deinosaurs. It was a herbivorous and probably amphibious 
 creature, able, no doubt, to walk along the shores, with an un- 
 
332 GEOLOGY 
 
 wieldy gait, on its long hind legs, and balancing itself by its 
 strong massive tail, which was doubtless a powerful instrument 
 of propulsion through the water. Its extraordinary fore legs, 
 with the strong spurs on the digits, must have been formidable 
 weapons of defence against its carnivorous contemporaries. An- 
 other gigantic reptile, the Mosasaurus, believed to have been 75 
 feet long, was furnished with fin-like paddles for swimming. 
 Several kinds of crocodiles have also been disinterred from 
 Cretaceous rocks in Europe. 
 
 Fig. 192. Cretaceous deinosaur (Iguanodon, about 
 
 Still more remarkable is the assemblage of remains of animal 
 life exhumed from corresponding rocks in the Western Terri- 
 tories of North America. Among these the Discosaurus was a 
 snake-like animal some 40 feet long, with a swan-like neck sup- 
 porting a slim head which it could raise 20 feet out of the water, 
 or dart to the bottom and catch its prey. The pythonomorphs 
 or sea-serpents were especially numerous. 
 
 The remains of true birds have been obtained from the Cre- 
 taceous rocks both of Europe and North America. They are re- 
 lated to the living ostrich, but some of them were furnished with 
 teeth set in a continuous groove (Hesperornis), others had large 
 teeth in distinct sockets (Ichthyornis). 
 
 The following are the principal subdivisions of the Cretaceous 
 system in Europe in descending order. The stages are based 
 upon more or less well-marked fossil evidence, but they are 
 
THE CRETACEOUS PERIOD 
 
 333 
 
 Danian 
 
 Senonian , 
 
 I 
 
 Turonian 
 
 Cenomanian 
 
 also for the most part to be distinguished by lithological char- 
 acters : 
 
 Pisolitic limestone of Paris basin; Chalk of 
 Hainault, Ciply, Maestricht, Faxoe in Denmark, 
 and the south of Sweden; absent in England 
 (Belemnitella, Baculites). 
 
 Chalk-with-flints of Norwich, Brighton, Flam- 
 borough Head, and Dover, north of France 
 (Belemnitella, Harsupites, Micraster) ; sand- 
 stones of Westphalia and Saxony. 
 
 Chalk-without-flints of Dover and north of France 
 (Holaster planus, Terebratulina gracilis, Inoce- 
 ramus labiatus) ; sandstones, limestones, and 
 marls of Saxony and Bohemia ; Hippurite lime- 
 stone of Southern France and Mediterranean 
 basin. 
 
 Gray Chalk of Folkestone (Holaster subglobosus, 
 Belemnitella plena], Chalk-Marl, red chalk of 
 Hunstanton, Glauconitic Marl and Upper Green- 
 sand (Ammonites rostratus, Pecten asper) ; 
 Chalk of Rouen ; earthy limestones and marls in 
 Hanover replaced southwards by plant-bearing 
 sandstones, clays, and thin coal-seams ; Hippurite 
 limestones of Southern Europe. 
 
 Gault (Ammonites cristatus, A. denarius, A. 
 auritus) . 
 
 In Southern England a fluviatile (partly marine) 
 succession of sands and clays (Wealden), sur- 
 mounted by sands, clays, and limestones (Lower 
 Greensand), in Northern England a series of 
 clays and limestones, with marine fossils (upper 
 part of Speeton clay) ; limestones and marls of 
 Neuchatel; compact crystalline limestones in 
 Provence (Ammonites Dehayesi, A. nisus in 
 upper division; abundant Ancyloceras with 
 Pecten cinctus in middle ; Ammonites noricus, A. 
 astieramus, Ostrea Couloni in lower). 
 
 It will be remembered that towards the close of the Jurassic 
 period the floor of the sea in the western part of the European 
 area was gently raised, some of the younger Jurassic marine 
 limestones being ridged up into islets or low land, with lakes or 
 estuaries in which the Purbeck beds were deposited. This ter- 
 restrial condition of the geography was maintained and ex- 
 tended in the same region during the early part of the Cretaceous 
 period. The geological history of Europe as revealed by the 
 
 Albian 
 
 Neocomian 
 
334 GEOLOGY 
 
 various subdivisions in the foregoing Table may be briefly given, 
 NEOCOMIAN (from Neocomum, the old Latin name of Neu- 
 chatel in Switzerland). This stage in the south of England 
 and thence eastwards across Hanover consists of a mass of sand 
 and clay sometimes 1,800 feet thick, representing the delta of a 
 river. Only a portion of this delta remains, but as it extends in 
 an east and west direction for at least 200, and from north to 
 south for perhaps 100 miles, its total area may have been 20,000 
 square miles, indicating a large river comparable with the 
 Quorra of the present day. This stream not improbably de- 
 scended from the north or north-west. It carried down the 
 drifted vegetation of the land, with occasional carcases of the 
 iguanodons and other terrestrial or amphibious creatures of the 
 time. From theii great development in the Weald of Sussex 
 these delta-deposits have been called Wealden. They attain 
 there a thickness of more than 1,800 feet and consist of the 
 following subdivisions in descending order. 
 
 Weald Clay 1000 feet. 
 
 Hastings Sand group, comprising 
 
 3. Tunbridge Wells Sand 140 to 380 " 
 
 2. Wadhurst Clay 120 to 180 " 
 
 1. Ashdown Sand 400 to 500 " 
 
 Beyond the area overspread by the sand and mud of the 
 delta, the ordinary marine sediments accumulated, with their 
 characteristic organic remains. We find these sediments in 
 Yorkshire (upper part of Speeton clay), which must then have 
 lain beyond the estuary of the river. They stretch thence east- 
 wards through North- Western Germany, and are found at the 
 base of the Cretaceous system through France into Switzerland. 
 The Lower Greensand which overlies the Wealden group in the 
 south of England contains marine fossils, and points to the 
 submergence of the delta. 
 
 ALBIAN (from the department of the Aube in France). In 
 England this stage nearly corresponds to the band of dark, stiff, 
 blue clay known as the Gault. Extending over the Wealden 
 sands and clays, the Gault (100 to 200 feet or more in thick- 
 ness), with its abundant marine fossils, shows how thoroughly 
 the Wealden delta was now submerged beneath the sea. 
 
 CENOMANIAN (from Coenomanum, the old Latin name of the 
 
THE CRETACEOUS PERIOD 335 
 
 town of Mans in the department of Sarthe, France) . This stage 
 comprises a group of impure chalky, glauconitic, and sandy 
 deposits lying at -the base of the Chalk in England and the north 
 of France. The subdivisions of this stage in England are shown 
 in the following table in descending order : 
 
 Gray chalk forming the base of the Chalk. 
 Chalk Marl (Red Chalk of Hunstanton). 
 Glauconitic Marl. 
 Upper Greensand. 
 
 Certain sandy portions of this group have been called the Upper 
 Greensand. The Glauconitic (or Chloritic) Marl is an impure, 
 dull white, or yellowish chalk, with grains of glauconite and 
 phosphatic nodules. The Chalk-Marl is an impure band of 
 chalk sometimes overlain by a zone of Gray Chalk which forms 
 the base of the true Chalk-without-flints. All these deposits 
 indicate the accumulations of a shallow sea, probably not far 
 from land. Traced eastwards into Germany, they undergo great 
 changes in their lithological characters, passing at last in Saxony 
 and Bohemia into sandstones and clays full of remains of terres- 
 trial vegetation, and even including some thin seams of coal. It 
 is in these beds that the oldest dicotyledonous plants in Europe 
 have been found. It is evident that land existed in the heart of 
 Germany during this stage of the Cretaceous period. In South- 
 ern France, on the other hand, the corresponding strata are 
 massive hippurite-limestones which sweep through the great 
 Mediterranean basin, and show how large an area of Southern 
 Europe then lay under the sea. 
 
 TURONIAN (from Touraine). This stage includes the lower 
 part of the Chalk, above the Gray Chalk. The thick mass of 
 white crumbly limestone known as the Chalk has been referred 
 to as the most conspicuous member of the Cretaceous system in 
 the west of Europe. It has long been grouped into two parts, 
 a lower band of " Chalk-without-flints," and an upper band of 
 " Chalk-with-flints." The former corresponds, on the whole, 
 with the Turonian stage. The Chalk is a remarkably pure lime- 
 stone, composed chiefly of crumbled foraminifera, urchins, mol- 
 luscs, and other marine organisms. It must have been laid down 
 in a sea singularly free from fine sediment; but there is no evi- 
 dence that this sea was one of great depth. On the contrary, 
 
330 GEOLOGY 
 
 though the Chalk itself resembles the Globigerina ooze of the 
 deeper parts of the Atlantic Ocean, the characters of its forami- 
 nifera and other organic remains indicate comparatively shal- 
 low-water conditions. The basin in which it was laid down 
 shallowed eastwards, where, from the evidence of sandstones, 
 coal-seams, and plants, there was land at the time; while, prob- 
 ably, towards the west there was connection with the open sea. 
 The total thickness of the Chalk, including the Cenomanian, 
 Turonian, and Senonian stages, exceeds 1,200 feet. 
 
 SENONIAN (from Sens, in the department of Yonne). This 
 stage corresponds generally with the original English subdivision 
 of Upper Chalk, or Chalk-with-flints. Its most conspicuous 
 feature is the presence of the layers of nodules or irregular lumps 
 of black flint which mark the stratification of the Chalk. The 
 origin of these concretions has been the subject of much discus- 
 sion among geologists, and it cannot be said to have been even 
 yet satisfactorily solved. Some marine plants (diatoms) and 
 animals (radiolarians, sponges, etc.) secrete silica from sea- 
 water, and build it up into their framework. But the flints are 
 not mere siliceous organisms, though organic remains may often 
 be observed enclosed within them. They are amorphous lumps 
 of dark silica, containing a little organic matter. By some 
 process, not yet well understood, these aggregations of silica have 
 gathered usually round organic nuclei, such as sponges, urchins, 
 shells, etc. The decomposition of organic matter on the sea-floor 
 may have been the principal cause in determining the abstrac- 
 tion and deposition of silica. Not infrequently an organism, 
 such as a brachiopod or echinus, originally composed of car- 
 bonate of lime, has been completely transformed into flint. 
 
 The Chalk is well exposed along the sea-cliffs of the east and 
 south of England. It forms the promontories of Flamborough 
 Head, Dover, Beachy Head, and the Needles in the Isle of 
 Wight. The white cliffs of Kent are repeated on the opposite 
 coast of France, where the Chalk with all its lithological and 
 palseontological characters reappears, and whence it extends 
 through Northern France into Belgium. 
 
 DANIAN (from Denmark). This stage has not been recog- 
 nised in England. Its component chalky strata occur in scat- 
 tered patches over Northern France, Belgium, and Denmark, to 
 the south of Sweden. 
 
THE CRETACEOUS PERIOD 337 
 
 The Cretaceous hippurite-limestones of Southern Europe and 
 the basin of the Mediterranean are prolonged through Asia 
 Minor into Persia, where they cover a vast area. They have 
 been found likewise on the flanks of the Himalaya Mountains, 
 so that the open Cretaceous sea must have stretched right across 
 the heart of the Old World. In the Indian Deccan a great ex- 
 tent of country, estimated at 200,000 square miles, lies buried 
 under horizontal or nearly horizontal sheets of lava, which have 
 a united thickness of from 4,000 to 5,000 feet, and were erupted 
 during the Cretaceous period. These eruptions, from the pres- 
 ence of interstratified layers containing remains of fresh-water 
 shells, land-plants, and insects, are believed to have taken place 
 on land and not under the sea. 
 
 Cretaceous rocks cover an enormous area in North America. 
 They attain no great thickness in the Eastern States, but they 
 thicken southwards, until in Texas they present massive lime- 
 stones indicative of deeper and clearer water than elsewhere in 
 that region. They attain gigantic proportions in Colorado, 
 Utah, and Wyoming, whence they are prolonged northwards into 
 the British territories, with a maximum thickness of 11,000 to 
 13,000 feet. They have yielded a remarkably abundant and 
 varied series of organic remains. In their upper parts (Lara- 
 mie group) they contain a large assemblage of land-plants, half 
 of which are allied to still living American trees, and in some 
 places these plants are aggregated into valuable seams of coal. 
 The numerous reptilian and bird remains found in these strata 
 have been already noticed. 
 
 Rocks assigned to the Cretaceous system cover a wide region 
 of Queensland, and also attain a considerable thickness in New 
 Zealand. 
 
888 GEOLOGY 
 
 CHAPTER XXV. 
 
 THE TERTIARY PERIODS. 
 
 THE Cretaceous system closes the long succession of Secon- 
 dary or Mesozoic formations. The rocks which come 
 next in order are classed as Tertiary or Cainozoic. When 
 these names were originally chosen, geologists in general be- 
 lieved not only that the divisions into which they grouped the 
 stratified rocks of the earth's crust corresponded on the whole 
 with well-defined periods of time, but that the abrupt transi- 
 tions, so often traceable between systems of rocks, served to mark 
 geological revolutions, in which old forms of life as well as old 
 geographical conditions disappeared and gave place to new. One 
 of the most notable of such breaks in the record was supposed 
 to separate the Cretaceous system from all the younger rocks. 
 This opinion arose from the study of the geology of Western 
 Europe, and more especially of South-Eastern England and 
 North-Western France. The top of the Chalk, partly worn 
 down by denudation, was found to be abruptly succeeded by the 
 pebble-beds, sands, and clays of the lower Tertiary groups. No 
 species of fossils found in the Chalk were known to occur also 
 in the younger strata. It was quite natural, therefore, that the 
 hiatus at the top of the Cretaceous system should have been re- 
 garded as marking the occurrence of some great geological catas- 
 trophe and new creation, and, consequently, as one of the great 
 divisional lines of the Geological Eecord. 
 
 More detailed investigation, however, has gradually over- 
 thrown this belief. In Northern France, Belgium, and Den- 
 mark various scattered deposits (Danian, p. 333) serve to bridge 
 over the gap that was supposed to separate Mesozoic and Caino- 
 zoic formations. In the Alps no satisfactory line could be found 
 to separate undoubtedly Cretaceous strata from others as obvi- 
 ously Tertiary. And in various parts of the world, especially in 
 Western North America, other testimony was gradually ob- 
 
THE TERTIARY PERIODS 339 
 
 tained to show that no general convulsion marked the end of 
 the Secondary and beginning of the Tertiary periods, but that 
 the changes on the earth's surface proceeded in the same orderly 
 connection and sequence as during previous and subsequent geo- 
 logical ages. The break in the continuity of the deposits in 
 Western Europe only means that in that part of the world the 
 record of the intervening ages has not been preserved. Either 
 strata containing the record were never deposited in the region 
 in question, or, having been deposited, they have subsequently 
 been removed. 
 
 Bearing in mind, then, that such geological terms are only 
 used for convenience of classification and description, and that 
 what is termed Mesozoic time glided insensibly into what is 
 called Cainozoic, we have now to enter upon the consideration of 
 that section of the earth's history comprised within the Tertiary 
 or Cainozoic periods. The importance of this part of the geo- 
 logical chronicle may be inferred from the following facts. 
 During Tertiary time the sea-bed was ridged up into land to 
 such an extent as to give the continents nearly their existing area 
 and contour. The crust of the earth was upturned into great 
 mountain ranges, and notably into that long band of lofty ground 
 stretching from the Pyrenees right through the heart of Europe 
 and Asia to Japan. Some portions of the Tertiary sea-bed now 
 form mountain peaks 16,000 feet or more above the sea. The 
 generally warm climate of the globe, indicated by the world-wide 
 diffusion of the same species of shells in Palaeozoic and less 
 conspicuously in Mesozoic time, now slowly passed into the 
 modern phase of graduated temperatures, from great heat at the 
 equator to extreme cold around the poles. At the beginning of 
 the Tertiary or Cainozoic periods the climate was mild even far 
 within the Arctic Circle, but at their close it became so cold 
 that snow and ice spread far southward over Europe and North 
 America. 
 
 The plants and animals of Tertiary time are strikingly modern 
 in their general aspect. The vegetation consists, for the most 
 part, of genera that are still familiar in the meadows, woodlands, 
 and forests of the present day. The assemblage of animals, too, 
 becomes increasingly like that of our own time as we follow the 
 upward succession of strata in which the remains are preserved. 
 In one strongly marked feature, however, does the Tertiary fauna 
 
340 GEOLOGY 
 
 stand contrasted alike with everything that preceded and fol- 
 lowed it. If the Secondary periods could appropriately be 
 grouped together under the name of the " Age of Keptiles," Ter- 
 tiary time may not less fitly be called the " Age of Mammals." 
 As the manifold reptilian types died out, the mammals, in ever- 
 increasing complexity of organization, took their place in the ani- 
 mal world. By the end of the Tertiary periods they had reached 
 a variety of type and a magnitude of size altogether astonishing, 
 and far surpassing what they now present. The great variety 
 of pachyderms is an especially marked feature among them. 
 
 The rocks embraced under the terms Cainozoic or Tertiary 
 have been classified according to a principle different from any 
 followed with regard to the older formations. When they began 
 to be sedulously studied, it was found that the percentage of 
 recent species of shells became more numerous as the strata were 
 followed from older to newer platforms. The French naturalist 
 Deshayes determined the proportions of these species in the dif- 
 ferent Tertiary groups of strata, and the English geologist Lyell 
 proposed a scheme of classification based on these ratios. His 
 names, with modifications as to their application, have been gen- 
 erally adopted. They are compounds of the Greek xawos, recent, 
 with affixes denoting the proportion of living species. 
 
 To the oldest Tertiary deposits, containing only about 3 per 
 cent of living species of shells, the name Eocene (dawn of the 
 recent) was given. The next series, containing a larger number 
 of living species, has received the name of Oligocene (few 
 recent). The third division in order is named Miocene, to 
 indicate that the living species, though in still larger proportions, 
 are yet a minority of the whole shells. The overlying series 
 forming the uppermost of the Tertiary divisions is termed Plio- 
 cene (more recent), because the majority are now living species. 
 The same system of nomenclature has been retained for the next 
 overlying group, which forms the lowest member of the Post- 
 tertiary or Quaternary series. This group is called Pleistocene 
 (most recent), and all the species of shells in it are still living at 
 the present time. It must not be supposed that the mere per- 
 centage of living or of extinct species of shells in a deposit always 
 affords satisfactory evidence of geological age. Obviously, there 
 may have been circumstances favourable or unfavourable to the 
 existence of some shells on the sea-bottom which that deposit 
 
THE TERTIARY PERIODS 341 
 
 represents, or to the subsequent preservation of their remains. 
 The system of classification by means of shell-percentages must 
 be used with some latitude, and with due regard to other evidence 
 of geological age. 
 
 EOCENE. 
 
 In Europe great geographical changes took place at the close 
 of the Cretaceous period. The wide depression in which the 
 Chalk had been deposited was gradually and irregularly elevated, 
 and over its site a series of somewhat local deposits of clay, sand, 
 marl, and limestone was laid down, partly in small basins of the 
 sea-floor, and partly in estuaries, rivers, or lakes. In Southern 
 Europe, however, the more open sea maintained its place, and 
 over its floor were accumulated widespread and thick sheets of 
 limestone which, from the crowded nummulites which they con- 
 tain, are known as Nummulitic Limestone. These characteristic 
 rocks extend all over the basin of the Mediterranean, stretching 
 far into Africa and sweeping eastwards through the Alps, Car- 
 pathians, and Caucasus, across Asia to China and Japan. In 
 North America the rocks classed as Eocene present two contrast- 
 ed types. Down the eastern and western borders of the Con- 
 tinent, from the coast of New Jersey into the Gulf of Mexico 
 on the one side, and along the coast ranges of California and 
 Oregon on the other, they are marine deposits, though occasion- 
 ally presenting layers of lignite with terrestrial plants. Over the 
 vast plateaux which support the Eocky Mountains, however, they 
 are of lacustrine origin, and show that in what is now the heart 
 of the Continent the bed of the Cretaceous sea was upraised into 
 a succession of vast lakes, round which grew a luxuriant vegeta- 
 tion. In these lakes a total mass of Eocene strata, estimated at 
 not less than 12,000 feet, was deposited, entombing and preserv- 
 ing an extraordinarily abundant and varied record of the plant 
 and animal life of the time. 
 
 The flora of Eocene time points to a somewhat tropical 
 climate. Among its plants are many which have living repre- 
 sentatives now in the hotter parts of India, Australia, Africa, and 
 America. Above the ferns (Lygodium, Asplenium, etc.) which 
 clustered below, rose clumps of palms, cactuses, and arc-ids; 
 numerous conifers and other evergreens gave the foliage an 
 umbrageous aspect, while many deciduous trees ancestors of 
 
342 
 
 GEOLOGY 
 
 some of the familiar forms of our woodlands raised their 
 branches to the sun. Among the conifers were many cypress- 
 like trees (Callitris, Cupressinites) , pines (Pinus, Sequoia), and 
 yews (Salisburia or Ginko). Species of 
 aloe (Agave), sarsaparilla (Smilax), and 
 amomum were mingled with fan-palms 
 (Sabal, Chamcerops) and screw-pines 
 (Pandanus, Nipa), together with early 
 forms of fig (Ficus), elm (Ulmus), 
 poplar (Populus), willow (Salix), hazel 
 (Corylus), hornbeam (Carpinus), chest- 
 Fig. 193.-E o rne Plant nut (Castanea), beech (Fagus), plane 
 (Petrophiloides Rich- (Platanus) , walnut (Juglans), liquid- 
 ,. ^onii), natural size. ^^ magnolia? p ro teaceJus plants (Fig. 
 
 193) resembling those of Australia and the Cape, water-bean 
 (N elumbium), water-lily (Victoria), maple (Acer), gum- 
 tree (Eucalyptus), cotoneaster, plum (Prunus), almond (Amyg- 
 dalus), laurel (Laurus), cinnamon tree (Cinnamomum) 9 and 
 many more. 
 
 Fig. 194. Eocene Molluscs, (a) Valuta luctatrix (g) ; (6) Oliva 
 Branderi (natural size) ; (c) Cerithium tricarinatum (). 
 
 The fauna likewise points to the extension of a warm cli- 
 mate over regions that are now entirely temperate. This is 
 particularly noticeable with regard to the mollusca. The species 
 are, with perhaps a few exceptions, all extinct, but many of the 
 genera are still living in the warmer seas of the globe. Some of 
 
THE TERTIARY PERIODS 343 
 
 the most characteristic forms are species of Nautilus, Oliva, Va- 
 luta, Conus, Mitra, Cyrena, Cytherea, Chama. The genus of 
 Foraminifera, called Nummulites from the resemblance of the 
 organism to a piece of money, is enormously abundant in the 
 limestones above referred to as nummulitic limestones. It must 
 have nourished in vast profusion over the floor of the sea, which 
 in older Tertiary time' spread across the heart of the Old World 
 from the Atlantic to the Pacific Oceans. Some of the most com- 
 mon fish-remains found in the Eocene strata belong to the 
 genera Lamna, Otodus, MylMates, Pristis. Eeptilian life, which 
 enjoyed such a preponderance during the Mesozoic ages, is con- 
 spicuously diminished in the Eocene deposits alike in number of 
 
 Fig. 195. Eocene Mammal (Palceotherium magnum, 
 
 individuals and variety of structure. 'The genera are chiefly tur- 
 tles, tortoises, crocodiles, and sea-snakes, presenting in their 
 general assemblage a decidedly modern aspect compared with the 
 reptilian fauna of the Secondary rocks. Remains of birds aje 
 comparatively rare as fossils. We have seen that the earliest 
 known type has been obtained in the Jurassic system, and that 
 others have been found in the Cretaceous rocks. Still more mod- 
 ern forms occur in Eocene strata ; they include one (Argillornis) 
 which may have been a forerunner of the living gannet ; another, 
 of large size (Dasornis), akin to the gigantic extinct ostrich-like 
 moa (Dinornis) of New Zealand; a third (Agnopterus) shows 
 an affinity with the flamingo ; while the buzzard, woodcock, quail, 
 pelican, ibis, and African hornbill are represented by ancestral 
 forms. That the early type which linked birds with reptiles was 
 still living is shown by the remains of one curious genus (Odon- 
 topteryx) which had serrated jaws in which the teeth were pro- 
 jections of the bony substance. 
 
GEOLOGY 
 
 ^ut, as stated above, it was chiefly in higher forms of life that 
 the fauna of early Tertiary time stood out in strong contrast 
 with that of previous ages of geological history. The mammalia 
 now took- the leading place in the animal world, which they have 
 retained ever since. Among the Eocene mammals reference may 
 here be made to the numerous tapir-like creatures which then 
 flourished (Coryphodon, PalceotJierium, Fig. 195, Anchiloplius, 
 etc.). Some of the forms were intermediate in character be- 
 tween tapirs and horses, and included the supposed ancestors 
 
 Fig. 196. Skull of Tinoceras ingens (about T V). 
 of the modern horse small pony-like animals, with three, four, 
 and even traces of five toes on each foot. Many of the mammals 
 of Eocene time presented more or less close resemblance to 
 wolves, foxes, wolverines, and other modern forms. There 
 were likewise true opossums. Numerous herds of hog-like ani- 
 mals (Hyopotamus) and of hornless deer and antelopes wandered 
 over the land, while in the woodlands lived early ancestors of 
 our present squirrels, hedgehogs, bats, and lemurs. Among 
 these various tribes which recall existing genera, others of 
 strange and long-extinct types roamed along the borders of the 
 great lakes in Western North America. The Tillodonts were 
 a remarkable order, in which the characters of the ungulates, 
 rodents, and carnivores were curiously combined. These ani- 
 
THE TERTIARY PERIODS 
 
 345 
 
 mals, perhaps rather less in size than the living tapir, had 
 skeletons resembling those of carnivores, but with large prom- 
 inent incisor teeth like those of rodents, and with molar teeth 
 possessing grinding crowns like those of ungulates. Still more 
 extraordinary were the forms to which the name of Deinocerata 
 has been given (Deinoceras, Tinoceras, Fig. 196). These were 
 somewhat like elephants in size, and like rhinoceroses in general 
 build, but the skull bore a pair of horn-like projections on the 
 snout, another pair on the forehead, and one on each cheek. 
 
 The Eocene rocks of England are confined to the south-eastern 
 part of the country, from the coast of Hampshire into Norfolk. 
 They vary in character from district to district, sands and 
 gravels being replaced by clays according to the conditions in 
 which the sediment was accumulated. They are prolonged into 
 the north of France and Belgium. Arranged in tabular form, 
 they may be grouped 1 as follows : 
 
 
 ENGLAND. 
 
 FRANCE AND BELGIUM. 
 
 I 
 
 ft 
 
 p 
 
 Barton Clay and Upper 
 Bagshot Sands. 
 
 Marine gypsum and marls of Paris; 
 sands and calcareous sandstones of 
 Belgium (Wemmelian). 
 Sands (marine), with estuarine and 
 fresh- water limestones, etc. (Sables 
 Moyens). 
 
 Middle. 
 
 Bracklesham Beds (leaf- 
 beds of Alum Bay and 
 Bournemouth), Middle 
 Bagshot beds. 
 
 Calcaire-grossier divided into (3) 
 upper limestones, with marine and 
 fresh- water fossils ; (2) middle 
 limestones, with marine shells and 
 terrestrial vegetation; (1) lower 
 glauconitic marine limestones and 
 sands. 
 Sandstones and sands (Bruxellian) of 
 Belgium. 
 
 1 
 
 Lower part of Bagshot 
 sands. 
 London Clay. 
 Oldhaven Beds. 
 Woolwich and Reading 
 Beds. 
 Thanet Beds. 
 
 Paniselian sands of Belgium. 
 Ypresian clays and overlying sands of 
 Belgium. Absent in Paris basin. 
 Landenian gravels and sands of Bel- 
 gium. 
 Sands of Bracheux (Paris basin), 
 Heersian beds of Belgium, marls 
 of Meudon; fresh-water limestones 
 of Rilly and Sezanne. Limestone 
 of Mons in Belgium. 
 
346 GEOLOGY 
 
 In striking contrast with these comparatively thin and locally 
 developed deposits are those of the Alps, Southern Europe, and 
 the basin of the Mediterranean. Masses of nummulitic lime- 
 stone and sandstone, several thousand feet thick, have been 
 upraised, folded, and fractured, and now form important parts 
 of the great mountain chains which run through Europe and 
 the north of Africa. Similar rocks have been uplifted along 
 the flanks of the great chain of heights that sweeps through 
 the heart of Asia, reaching in the Himalaya range a height of 
 16,500 feet above the sea-level. We thus learn not only that a 
 large part of the existing continents lay under the sea during 
 Eocene time, but that the principal mountain-chains of the 
 Old World have been upheaved to their present altitudes since 
 the beginning of the Tertiary periods. The great Eocene lake- 
 basins of North America so remarkable a feature in the 
 geography survived till a much later part of Tertiary time. 
 
 OLIGOCENE. 
 
 Under this name geologists have placed a group of strata 
 usually of comparatively insignificant thickness, chiefly of fresh- 
 water and estuarine, but partly also of marine origin, which, 
 in Western and Central Europe, show how the bays and shallow 
 seas of that region in the Eocene period were gradually obliter- 
 ated and replaced by land and by sheets of fresh water. They 
 attain in Switzerland a thickness of several thousand feet of 
 sandstones, conglomerates, and marls, almost entirely of lacus- 
 trine origin, and forming a group of massive mountains (Rigi, 
 Rossberg) . A large lake occupied -their site and continued to 
 be an important feature in the geography of Central Europe 
 during this and the following geological period. Other sheets 
 of fresh water were scattered over the west of Europe. One of 
 the largest of these lay in Central France, over the old district 
 known as the Limagne d'Auvergne. In Germany, lacustrine 
 and terrestrial deposits, including numerous seams of lignite 
 or brown coal, are separated by a group of strata full of marine 
 shells, foraminifera, etc., showing how the lakes and wood- 
 lands were submerged beneath the sea. In the Paris basin, 
 and in the Isle of Wight, the strata are chiefly of fresh-water 
 origin, but contain occasional marine intercalations. Evidently 
 
THE TERTIARY PERIODS 347 
 
 the Oligocene period, throughout the European area, was one 
 of considerable oscillation in the earth's crust. During this 
 time, too, the volcanic eruptions took place whereby the great 
 sheets of basalt forming the terraced hills of the north of Ire- 
 land and Western Islands of Scotland were thrown out. 
 
 An epoch of frequent change in the relative positions of sea 
 and land is one in which there may be exceptional facilities for 
 the preservation of a record of the plants and animals of the 
 time. Oligocene strata have accordingly a peculiar interest 
 from the abundant remains they contain of the contemporaneous 
 terrestrial plants and animals. The land flora of that period 
 is probably better known than that of any other section of the 
 Geological Record, chiefly from the extraordinary abundance 
 of its remains which have been preserved in the sediments of 
 
 Fig. 197. Oligocene Molluscs, (a) Ostrea ventilabrum (I); (6) Cor- 
 bula subpisum (f) ' (c) Paludina lenta (natural size). 
 
 the ancient Swiss lake. Judging of it from these remains, 
 we learn that it was in great measure made up of evergreens, 
 and in various ways resembled the existing vegetation of tropical 
 India and Aiistralia and that of sub-tropical America, Its 
 fan-palms, feather-palms, conifers, evergreen oaks, laurels, and 
 other evergreen trees, gave a peculiarly verdant umbrageous 
 character to the landscape in all seasons of the year, while 
 numerous proteaceous shrubs glowed with their bright blooms 
 on the lower grounds. 
 
 Of the terrestrial fauna numerous remains have been found 
 in the lacustrine deposits of the time. We know that the borders 
 of the lakes in Central France were frequented by many dif- 
 ferent kinds of birds paroquets, trogons, flamingoes, ibises, 
 pelicans, maraboots, cranes, secretary birds, eagles, grouse, and 
 other forms. This association of birds recalls that around the 
 lakes of Southern Africa at the present time. The mammals 
 
348 GEOLOGY 
 
 appeared in still more numerous and abundant types. Pachy- 
 derms abounded, including the Anoplotherium a slender, 
 long-tailed animal, about the size of an ass, with two toes on 
 each foot; certain transitional types of ungulates, with affinities 
 to the pigs, peccaries, and chevrotains (Anthracotherium, Cheer o- 
 potamus, Hyopotamus, etc.) ; various forms of the tapir family, 
 and of dogs, civets, martens, marmots, bats, moles, and shrews. 
 The carnivora still presented marsupial characters, and in not 
 a few of the animal types features of structure were combined 
 which are now only found in distinct genera. The Eocene 
 pala3otheres and the Oligocene anoplotheres appear to have died 
 out before the end of the Oligocene period. The fresh water 
 teemed with molluscs, belonging chiefly to genera that still 
 live in our rivers and lakes, such as Unio, Cyrena, Paludina, 
 Planorbis, Limncea, Helix, and others (Fig. 197). 
 
 In the Isle of Wight the highest Eocene strata were followed 
 by a group of fresh-water, estuarine, and marine deposits, for- 
 merly classed as Upper Eocene, but placed here in the Oligo- 
 cene division. They are arranged in the following manner in 
 descending order : 
 
 Hamstead series clays, marls, and shelly layers, with fresh-water and 
 estuarine shells and land plants. Atyout 260 feet. 
 
 Bembridge series marls and limestone, with fresh-water shells be- 
 low, and estuarine shells above. About 110 feet. 
 
 Osborne series clays, marls, sands, and limestones, with abundant 
 fresh-water shells. About 100 feet. 
 
 Headon series consisting of an upper and lower division, containing 
 fresh and brac&ish water fossils and a middle group in which marine 
 shells and corals occur. About 150 feet. 
 
 These Isle of Wight strata, having a total depth of more 
 than 600 feet, were for many years the only known examples in 
 Britain referable to this portion of the Geological Eecord, and 
 they are still the only beds in this country which in their 
 abundant molluscs allow a comparison to be made between them 
 and corresponding rocks on the Continent. But at Bovey 
 Tracey in Devonshire a small lake-basin has been discovered, 
 the deposits of which have yielded a large number of terrestrial 
 plants comparable with those found in the Oligocene strata 
 of Switzerland and Germany. Between the great sheets of 
 basalt, also, that form the plateaux of Antrim and the Inner 
 Hebrides, numerous remains of a similar vegetation have been 
 
THE TERTIARY PERIODS 
 
 349 
 
 discovered. There can be no doubt that these volcanic rocks 
 were poured out over the surface of the land, and that the 
 plants, whose remains have been disinterred from the inter- 
 calated layers of tuff and hardened clay, grew upon that land. 
 The basalts and other lavas, even after the great denudation 
 which they have undergone, are still in some places more than 
 3000 feet thick. They were poured out in wide-spreading sheets 
 that completely buried the previous topography and extended 
 as vast lava-plains, like those of younger date, which form so 
 impressive a feature in the scenery of Montana, Idaho, and 
 Oregon, in Western North America. 
 
 In the Paris basin, the Oligocene strata follow immediately 
 upon the Eocene group described on p. 345. They consist of 
 
 (1) a lower division of gypsum (65 feet) and marls, with 
 terrestrial shells, and remains of palaBotheres and anoplotheres ; 
 
 (2) a middle band of marl, limestone, and sand, with lacustrine 
 and estuarine shells; and (3) an upper division, in which the 
 most conspicuous members are the sands and hard siliceous 
 sandstone of Pontainebleau. 
 
 In Northern Germany the subjoined succession of strata in 
 descending order has been noted. 
 
 Upper 
 Middle 
 
 Lower 
 
 Marine marls, clays, and sands. 
 
 Brown coal of the Lower Rhine, with abundant terres- 
 trial vegetation and some marine bands. 
 
 Sands and Septaria-clay, with abundant marine fauna; 
 occasionally a brown-coal group occurs. 
 
 Marine beds of Egeln, with marine shells and corals. 
 
 Amber beds of Konigsberg, containing 4 or 5 feet of 
 glauconitic sand, with abundant pieces of amber, 
 which is the fossil resin of different species of conifer- 
 ous trees. A large number of species of insects has 
 been enclosed and preserved in the amber. 
 
 Lower brown coal sands, sandstones, clays, and con- 
 glomerates, with interstratified seams of brown coal 
 and an abundant terrestrial flora, in which coniferse 
 are prominent. 
 
850 GEOLOGY 
 
 CHAPTER XXVI. 
 
 TERTIARY PERIODS CONTINUED. 
 
 THE geological period at which we are now arrived, one 
 of the most important in the history of the configura- 
 tion of the existing continents, embraced that portion 
 of geological time during which the great mountain-chains of 
 the globe were uplifted into their present commanding positions. 
 There is good reason to believe that these lines of elevation 
 are of great geological antiquity, and that they have again and 
 again been pushed upward during great terrestrial disturb- 
 ances. But the intervals between these successive upthrusts 
 were probably often of immense duration, so that the mountains, 
 being exposed . to continuous and prolonged denudation, were 
 worn down, sometimes perhaps almost to the very roots. In 
 all probability the nucleus of the line of the Alps, for example, 
 dates back to a remote geological period. But only in Tertiary 
 time did it attain its present dimensions. We have seen that, 
 during the Eocene period, the sea of the nummulitic limestone 
 extended over at bast a considerable part of the Alpine region, 
 and that, as the limestone now forms crumpled and dislocated 
 mountainous masses, the great upheaval of the chain must 
 have taken place after Eocene time. Not improbably the proc- 
 ess was a prolonged one, advancing in successive uplifts with 
 intervals of rest. The final upheaval that gave the Alps their 
 colossal bulk did not take place until the Miocene period or 
 later, for the Miocene strata have been involved in the earth- 
 movements, and have been thrust up, bent, and broken. Nor 
 were the terrestrial convulsions confined to Central Europe, all 
 over the globe there seem to have been extensive disturbances. 
 The Eocene sea-bed with its thick accumulations of nummulite- 
 limestone was ridged up into land, and portions of it were 
 carried upward on the flanks of the mountains, in the 
 layas to a height of 16,500 feet above the sea. 
 
TERTIARY PERIODS CONTINUED 
 
 351 
 
 While these revolutions were taking place in its topography, 
 Europe continued to enjoy a climate which, to judge from the 
 remains of plants and animals preserved in the Miocene rocks, 
 must still have been of a somewhat tropical character. The 
 flora that clothed the slopes of the Alps was not unlike that of 
 the forests of India and Australia at the present time. Palms 
 of various kinds still flourished all over Central and Western 
 Europe, mingled with conifers, laurels, e^rgreen oaks, mag- 
 nolias, myrtles, mimosas, acacias, sumac! s, figs, oaks, and 
 various still living genera of proteaceous / shrubs (Fig. 198). 
 But there is evidence of the incoming o/. a more temperate 
 
 Fig. 198. Miocene Plants; (a) Magnolia Inglefieldi (J) ; (6) Rhua 
 Meriani (natural size) ; (c) Ficus decandolleana (i) ; (d) Quercus 
 ilicoides (). 
 
 climate, for, in the higher parts of the Miocene series of strata, 
 the vegetation was characterised by the abundance of its beeches, 
 poplars, hornbeams, elms, laurels, pondweeds, etc. 
 
 Eemains of the terrestrial fauna have been well preserved 
 in the deposits that gathered over the floors of the lakes. We 
 know, for instance, that in the woodlands surrounding the 
 large Miocene lake of Switzerland insect life was remarkably 
 abundant. From the proportions of the different kinds that 
 have been exhumed, it has been inferred that the total insect 
 population was then more varied in some respects than it is 
 
352 GEOLOGY 
 
 now in any part of Europe, wood-beetles being especially 
 numerous and large. In the thick underwood, frogs, toads, 
 lizards, and snakes found their food. Through the forests 
 there roamed antelopes, deer, and three-toed horses, while opos- 
 sums, apes, and monkeys (PliopitTiecus, Dryopitliecus, Oreo- 
 pithecus) gamboled among the branches. Wild cats, bears 
 (Hycenarctos) , and sabre-toothed lions (Machairodus) were 
 among the prominent carnivores of the time. But the most 
 striking denizens of these scenes were undoubtedly the huge 
 proboscidian creatures among which the Mastodon and- Deino- 
 therium took the lead. The mastodon (Fig. 199) was a large 
 and long-extinct form of elephant, which, besides tusks in 
 the upper jaw, had often also a pair in the lower jaw. The 
 
 Fig. 199. Mastodon augustidens 
 
 deinotherium (Fig. 200) possessed two large tusks in the lower 
 jaw which were curved downwards. This huge animal prob- 
 ably frequented the rivers of the time, using its powerful curved 
 tusks to dig up roots, and perhaps to moor itself to the banks. 
 Contemporaneous with these colossal pachyderms were species 
 of rhinoceros, hippopotamus, and tapir. The rivers were 
 haunted by crocodiles, turtles, beavers, and otters; while the 
 seas were tenanted by ancestors of our living morse, sea-calf, 
 dolphin, and lamantin. It is strange to reflect that such an 
 assemblage of animals should once have found a home all over 
 Europe. 
 
 The deposits referable to the Miocene period in Europe 
 indicate a great change in the geography of the region since 
 
TERTIARY PERIODS CONTINUED 353 
 
 Eocene and Oligocene times. While most of the Continent 
 remained land, with large lakes scattered over its surface, 
 certain tracts had subsided beneath shallow seas which pene- 
 trated here and there by long arms into the very heart of 
 the region. Britain continued to be a land surface, and as 
 such was continuously exposed to denudation. Instead of the 
 formation of new deposits, there was an uninterrupted waste 
 of those already existing. So vast indeed has been the destruc- 
 tion of the Tertiary strata of Britain that it has evidently 
 been in progress for an enormous period of time. Much of 
 it, no doubt, took place during the long interval required else- 
 where for the accumulation of the Miocene series of rocks. 
 
 Fig. 200. Skull of Deinotherium giganteum (reduced). 
 
 Not only were the soft sands and clays of the older Tertiary 
 groups of south-eastern England worn away from hundreds of 
 square miles which they originally covered, but even the hard 
 basalt-sheets of Antrim and the Inner Hebrides were so cut 
 down by the various agents of denudation that wide and deep 
 valleys were carved out of them, and hundreds of feet of solid 
 rock were gradually removed from their surface. 
 
 While Britain remained land, arms of the sea spread over 
 what is now Belgium, and the basins of the Loire, Indre, and 
 Cher, stretching across Southern France to the Mediterranean, 
 passing along the northern base of the Alps, running into the 
 valley of the Ehine as far north as Mainz, sweeping eastwards 
 round the eastern end of the Alps, and expanding into the 
 
354 GEOLOGY 
 
 broad gulf of Vienna among the submerged heights of Austria 
 and Hungary. 
 
 The strata that tell this story of submergence contain an 
 abundant assemblage of marine shells, many of which belong 
 to genera that now live in warmer seas than those which at 
 present bathe the coasts of Europe. Among them are Can- 
 cellaria, Cyprcea, Mitra, Murex, Strombus, Area, Cardita, 
 Cytherea, Pectunculus, Spondylus, together with genera, such 
 as Ostrea, Pecien, Cardium, Tapes, Tellina, which are familiar 
 in the Northern seas. 
 
 The district of France, formerly called Touraine, is largely 
 overspread with shelly sands and marls, rarely more than 50 
 feet thick, and locally known as " Faluns." These deposits 
 represent the floor of the shallow Miocene strait which extended 
 across France. They have yielded upwards of 300 species of 
 shells, the general character of which marks a warmer climate 
 than now exists in Southern Europe. The tableland of Spain, 
 with its northern mountainous border, rose along the southern 
 margin of this strait which connected the Atlantic and the 
 Mediterranean. Through this broad passage the large cetaceans 
 of the time passed freely from sea to sea, for their bones are 
 found in the upraised sea-bottom. The carcases of the mammals 
 that then lived among the Pyrenees mastodons, rhinoceroses, 
 lions, giraffes, deer, apes, and monkeys were likewise swept 
 down into the sea. The deposits of the shallow Miocene straits 
 and bays thus supply, us with evidence of the position of the 
 land and the character of its inhabitants. Eastwards the sea 
 appears to have deepened over the region now occupied by the 
 gulf of Genoa and the encircling mountain ranges, for the 
 Miocene deposits of that part of the basin of the. Mediter- 
 ranean, consisting almost wholly of blue marls, are said to 
 reach the great thickness of more than 10,000 feet. Beyond 
 that depression the sea once more shallowed across the site of 
 South-Eastern Europe. In the Vienna basin its deposits are 
 well developed and consist of two divisions : ( 1 ) a lower group 
 (Mediterranean or marine stage) of limestones, marls, clays, and 
 sands, containing an abundant assemblage of shells, some of which 
 belong to species still living in the present Mediterranean Sea, 
 or off the west coast of Africa, and also numerous remains of 
 land-plants which again recall the living floras of India and 
 
TERTIARY PERIODS CONTINUED 355 
 
 Australia; and (2) an upper group (Sarmatian or Cerithium 
 stage) of sands, gravels, and clays in which the shells and 
 terrestrial plants point to a much more temperate climate than 
 that indicated by the lower beds. 
 
 On the northern side of the Swiss Alps, the lake which was 
 formed by the uplifting of the Eocene sea-floor, and in which 
 so thick a succession of Oligocene strata was laid down, event- 
 ually disappeared among the terrestrial movements that sub- 
 merged so much of Europe beneath the Miocene sea. Marine 
 bands containing undoubted Miocene shells extend across 
 Switzerland : but among them there are such abundant remains 
 of terrestrial vegetation as to show that the land was not far 
 off. No doubt the Alps, not yet uplifted to their ultimate 
 height, rose along the southern borders of the strait that ran 
 across Central Europe, and bore on their slopes luxuriant forest- 
 growths. In Switzerland, however, we learn that before the 
 close of the Miocene period the sea was once more excluded 
 from the district, and another lake made its appearance. The 
 marls, limestones, and sandstones accumulated in this lake 
 (CEningen Beds) are among the most interesting geological 
 deposits in Europe, from the great number and perfect preser- 
 vation of the plants, insects, fishes, and mammals which have 
 been obtained from them. A large part of our knowledge 
 regarding the terrestrial vegetation and animal life of the 
 Miocene period has been derived from these strata. 
 
 Passing beyond the European area, we find that some of the 
 characteristic vegetation of Miocene time spread northwards far 
 within the Arctic Circle. In Spitzbergen and in North Green- 
 land, an abundant series of plant-remains has been discovered, 
 including a good many which occur also as fossils in the Miocene 
 deposits of Central Europe. More than half of them are trees, 
 among which are thirty species of conifers, also beeches, oaks, 
 planes, poplars, maples, walnuts, limes, and magnolias. This 
 flora has been traced as far as 81 45' north latitude, where the 
 last expedition sent out from England found a seam of coal 
 25 to 30 feet thick, covered with black shales full of plant- 
 remains. 
 
 Miocene deposits occupy a considerable area in North Amer- 
 ica. In the Eastern States, they are of marine origin and follow 
 generally the tract of the underlying Eocene beds. In the 
 
356 GEOLOGY 
 
 Western States and Territories they are lacustrine, and show 
 that the lakes which covered so wide an expanse in early Ter- 
 tiary time still existed, but in greatly diminished proportions. 
 They have preserved many interesting relics of the terrestrial 
 life of the period three-toed horses, tapiroid animals, hogs 
 as large as rhinoceroses, true rhinoceroses, huge elephant-like 
 creatures allied to deinoceras and tapir, stags, camels, beavers, 
 wolves, bears, and lions. In India, also, thick masses of sedi- 
 mentary rock occur containing remains of mastodon, deino- 
 therium, and other Miocene animals. 
 
 PLIOCENE. 
 
 The last division of the Tertiary series of formations lays 
 before us the history of the geological changes that brought 
 about the present general distribution of land 'and sea, and 
 completed the existing framework of the continents. Con- 
 trasted with the previous Tertiary groups, it is, on the whole, 
 insignificant in thickness and extent, and it probably records 
 the passing of a much less period of time, during which the 
 amount of terrestrial revolution was comparatively trifling. 
 Only in the basin of the Mediterranean are there any European 
 Pliocene strata worthy of note on account of their thickness. 
 The floor of that sea slowly subsided until sands, clays, and 
 accumulated shelj-beds had been piled up to a depth of several 
 thousand feet. An important volcanic episode then took place. 
 Etna, Vesuvius, and the other volcanoes of Central Italy began 
 their eruptions. Thick masses of Pliocene sediments were 
 ridged upon both sides of the Apennines, and in Sicily were 
 upheaved to a height of nearly 4000 feet above the present sea- 
 level. This elevation of the Pliocene sea-bed in the Mediter- 
 ranean area was not improbably connected with other 
 movements within the European region. The shallow firths and 
 bays which still indented the Continent were finally raised into 
 dry land, and the Alps may then have received their final uplift. 
 While the European Pliocene deposits have their maximum 
 thickness in the Mediterranean basin, they elsewhere represent 
 the sediments of shallow seas and of lakes and rivers. 
 
 The flora of the Pliocene period affords evidence of the con- 
 tinued advance of a more temperate climate. The tropical 
 
TERTIARY PERIODS CONTINUED 
 
 357 
 
 types of vegetation one by one retreated southwards in the 
 European region, leaving behind them a vegetation that par- 
 took of the characters of those of the present Canary Islands, 
 of North America, and of Eastern Asia and Japan, but which, 
 as time wore on, approached more and more to the present 
 
 Fig. 201. Pliocene Plants. (A) Populus canescens; (B) Salix alba; 
 ((7) Glyptostrobus europceus; (D) Alnus glutinosa; (E) Platanus 
 aceroides (all natural size except E, which is i). 
 
 European flora (Fig. 201). It included species of bamboo, 
 sarsaparilla (Smilax), ^lyptostrobus, taxodium, sequoia, mag- 
 nolia, tulip tree (Liriodendron) , maple (Acer), buckthorn 
 (Rliamnus), sumach (Rhus), plum (Prunus), laurel (Laurus), 
 cinnamon-tree (Cinnamomum), sassafras, fig (Ficus), elm 
 
358 
 
 GEOLOGY 
 
 (Ulmus), willow (Salix), poplar (Populus), alder (Alnus), 
 birch (Betula), liquidambar, oak (Quercus), evergreen oak 
 (Quercus ilex), plane (Platanus) , walnut (Juglans), hickory 
 (Carya), and other now familiar trees. 
 
 The fauna presented likewise evidence that the climate, dur- 
 ing at least the earlier part of the Pliocene period, still continued 
 warm enough to permit tribes of animals to roam over Europe, 
 the descendants of which are now confined to regions south of 
 the Mediterranean basin. Some of the huge mammalian types 
 that had survived from an earlier time now died out; such was 
 
 Fig. 202. Pliocene Marine Shells, (a) Rhynchonella psittacea (natural 
 size) ; (6) Panopcea norvegica (J) ; (c) Purpura lapillus (i) ; (d) 
 Trophon antiquum (|). 
 
 the case with the deinotherium and mastodon. Herds of pachy- 
 dermatous animals formed a distinguishing feature of the 
 fauna rhinoceroses, hippopotamuses, and elephants, with 
 troops of herbivorous quadrupeds gazelles, antelopes, deer, 
 giraffes, horses, oxen, and strange long-extinct types linking 
 together genera that are now quite distinct. There were, like- 
 wise, carnivores (wild-cats, bears, hyenas, etc.), and many 
 monkeys. The remains of monkeys have been found fossil in 
 Europe 1-1 farther north than their descendants now live. 
 
TERTIARY PERIODS CONTINUED 
 
 359 
 
 The shells of the Pliocene deposits afford important evidence 
 regarding the gradual change of climate. The great majority 
 of them belong to still living species (Fig. 202). They con- 
 sequently supply an excellent basis for comparison with the 
 existing distribution of the same species. When the deposits 
 containing them are examined with reference to the present 
 habitats of the species, it is found that the percentage of what 
 are now northern shells increases from the lower to the higher 
 parts of the series. In Pliocene time, each species no doubt 
 flourished only in that part of the sea where it found its con- 
 genial temperature and food. We infer that its requirements 
 are still the same at the present day, in other words, that the 
 temperature of the regions within which the species is now 
 confined affords, on the whole, an indication of the temperature 
 of the areas within which it lived in the Pliocene seas. On 
 this basis of comparison, the inference has been drawn that 
 the climate in the northern hemisphere, after becoming tem- 
 perate, passed on to a more rigorous stage. In the end thor- 
 oughly Arctic conditions spread over most of Europe and a 
 large part of North America, during the period that succeeded 
 the Pliocene. 
 
 In Britain Pliocene deposits are almost entirely confined to 
 the counties of Norfolk and Suffolk. They consist of various 
 shelly sands, gravels, and marls, which have long been known as 
 " Crag." Arranged in descending order, the following are 
 the recognised subdivisions : 
 
 ' Upper fresh-water, estuarine, and Lower fresh-water 
 sands and silts, with layers of peat, having a total 
 depth of 10 to 70 feet. Among the terrestrial plants 
 are cones of Scotch fir (Pinus sylvestris) and 
 spruce (Abies), leaves of water-lily (Nymphcea al- 
 60), yellow pond-lily (Nuphar luteum) , hornwort 
 (Ceratophyllum) , blackthorn (Prunus spinosa), 
 Forest - Bed bog-bean (Menyanthes trifoliata) , oak, and hazel, 
 Group -j with land and fresh-water shells, and many mam- 
 mals, including species of wolf, fox, machairodus, 
 hyaena, glutton, bear, seal, horse, rhinoceros, hip- 
 popotamus, pig, ox, musk-sheep, deer, beaver, trogon- 
 therium (a huge extinct kind of beaver), mole, 
 elephant (E. antiquus, E. meridionalis, E. primi- 
 genius) , etc. This group of strata is found at the 
 base of the sea-cliff of boulder-clay in Norfolk, 
 and extends under the present sea. 
 
360 
 
 GEOLOGY 
 
 Chillesford 
 Group 
 
 Norwich (flu- 
 vio-marine or 
 mammalife r - 
 ous) Crag 
 
 Red Crag 
 
 Lenham Beds 
 (Diestian) 
 
 St. Erth Beds 
 
 White (Suffolk 
 or Coralline) 
 Crag 
 
 Sands and clays occurring as a thin local deposit in 
 Suffolk, 6 to 16 feet thick, with marine shells, about 
 two-thirds of which still live in Arctic waters (My a 
 truncata, Cyprina islandica, Astarte borealis, Tel- 
 Una olliqua). 
 
 Shelly sand and gravel, 5 to 10 feet thick, containing 
 93 per cent of still living species of shells and bones 
 and teeth of mastodon, elephant (E. meridionalis, E. 
 antiquus) , hippopotamus, rhinoceros, etc. The pro- 
 portion of northern shells is 14.6 per cent, and the 
 following species are included Rhynchonella psit- 
 tacea, Scalaria grcenlandica, Panopwa norvegica, As- 
 tarte borealis. About twenty species of land or 
 fresh-water shells also occur. 
 
 A local and inconstant accumulation, 25 feet thick, of 
 red and dark brown ferruginous shelly sand, with 
 numerous species of shells of which 10.7 per cent 
 are northern forms. Some of the characteristic shells 
 of the deposit are Trophon antiquum, Valuta 
 Lamberti, Purpura lapillus, Pectunculus glycimeris, 
 Cardium edule. 
 
 Sands and ironstones filling hollows of the Chalk of the 
 North Downs, more than 600 feet above the sea, and 
 containing nearly 200 species of fossils, all of which, 
 save 22, have been found in the Coralline Crag. 
 
 A local deposit of clays and gravels found at St. Erth 
 in Cornwall, with abundant and well-preserved 
 shells, probably of older Pliocene age, about 40 per 
 cent being of extinct species. 
 
 Shelly sands and, clays containing 84 per cent of still 
 living shells, whereof 5 per cent are northern species. 
 One of the characteristics of the deposit is the large 
 number (140 species) of coral-like polyzoa (coral- 
 lines or bryozoa), whence one of the names given to 
 this subdivision. 
 
 On the Continent the youngest Tertiary deposits cover com- 
 paratively small areas and mark some of the last tracts occupied 
 by the sea. Thus, in the Vienna basin there is evidence that 
 the sea, shut off from the main ocean, and partly converted 
 into an inland sea, like the Caspian, was gradually filled up 
 
TERTIARY PERIODS CONTINUED 361 
 
 with sediment and raised into land. Along the northern borders 
 of the Mediterranean Sea, thick masses of marine Pliocene 
 strata show the prolonged depression of that region during 
 Pliocene time, and its subsequent elevation. In the south of 
 France these strata, lying unconfonnably on everything older 
 than themselves, reach a height of 1150 feet above the sea. 
 Along both sides of the Apennine chain, Pliocene blue marls, 
 clays, and sands, known as the sub-Apennine beds, have been 
 uplifted into a range of low hills. These deposits swell out 
 southwards, reaching their greatest thickness (2000 feet or 
 more) in Sicily, which was probably the region of maximum 
 subsidence during Pliocene time. Here and there, in the 
 Italian strata of this period, remains of terrestrial vegetation 
 and land-animals are abundantly preserved. One of the most 
 
 Fig. 203. Helladotherium Duvernoyi (-fa) a gigantic animal belong- 
 ing to the same family as the living giraffe, Pikermi, Attica. 
 
 noted localities for these fossils is the upper part of the valley 
 of the Arno. 
 
 Perhaps the most curious and interesting assemblage of the 
 land-fauna of Europe during Pliocene time has been found in 
 some hard red clays, alternating with gravels, at Pikermi in 
 Attica. Thirty-one genera of mammals have there been ob- 
 tained, of which twenty-two are extinct. The ruminants, spe- 
 cially well represented among these remains, include species 
 of giraffe, helladotherium (Fig. 203), antelopes, gazelles, and 
 other forms allied to, but distinct from, any living genera. 
 There are likewise the bones of gigantic wild boars, several 
 
362 GEOLOGY 
 
 species of rhinoceros, mastodon, deinotherium, porcupine, hyaena, 
 various extinct carnivores, and a monkey. 
 
 In India a somewhat similar fauna has been obtained from 
 a massive series of fresh-water sandstones, known as the Siwalik 
 group. A large proportion of the remains belong to existing 
 genera of animals, such as macaque, bear, elephant, horse, hippo- 
 potamus, giraffe, ox, porcupine, goat, sheep, and camel. Various 
 extinct types were contemporary with these animals, two of 
 the most extraordinary of them being the Sivatherium and 
 Bramatherium colossal, four-horned creatures allied to our 
 living antelopes and prong-bucks. 
 
POST-TERTIARY PERIODS 363 
 
 CHAPTEE XXVII. 
 
 POST-TERTIAEY PEKIODS. 
 
 WE HAVE now arrived at the last main division of the 
 Geological Record, that which is named POST-TER- 
 TIARY or QUATERNARY, and which includes all the 
 formations accumulated from the close of the Tertiary periods 
 down to the present day. But no 1 sharp line can be drawn at 
 the top of the Tertiary groups of strata. On the contrary, it is 
 often difficult, or indeed impossible, satisfactorily to decide 
 whether a particular deposit should be classed among the younger 
 Tertiary or among the Post-tertiary groups. In the latter, all 
 the molluscs are believed to belong to still living species, and 
 the mammals, although also mostly still of existing species, 
 include some which have become extinct. These extinct forms 
 are numerous in proportion to the antiquity of the deposits in 
 which they have been preserved. Accordingly, a classification 
 of the Quaternary strata has been adopted, in which the older 
 portions, containing a good many extinct mammals, have been 
 formed into what is termed the Pleistocene, Post-pliocene, or 
 Glacial group, while the younger deposits, containing few or 
 no extinct mammals, are termed Recent. 
 
 The gradual refrigeration of climate which is revealed to 
 us by the shells of the crag was prolonged and intensified 
 in Post-tertiary time. Ultimately the northern part of the 
 northern hemisphere was covered with snow and ' ice, which 
 extended into the heart of Europe and descended far southward 
 in North America. The previous denizens of land and sea were 
 in large measure driven out or even in many cases wholly extir- 
 pated by the cold, while northern forms advanced southward to 
 take their places. The reindeer, for instance, roamed in great 
 numbers across Southern France, and Arctic vegetation spread 
 all over Northern and Central Europe, even as far as the 
 Pyrenees. After the cold had reached its climax, the ice-fields 
 
364 GEOLOGY 
 
 began to retreat, and the northern flora and fauna to retire 
 before the advance of the plants and animals which had been 
 banished by the increasingly severe temperature. And at last 
 the present conditions of climate were reached. The story of 
 this Ice Age is told by the Pleistocene or Post-pliocene forma- 
 tions, while that of the changes which immediately led to the 
 establishment of the present order of things is made known 
 in the Recent deposits. 
 
 PLEISTOCENE, POST-PLIOCENE, OR GLACIAL. 
 
 The evidence from which geologists have unravelled the his- 
 tory of the Ice Age or cold episode which came after the Tertiary 
 periods in the northern hemisphere may here be briefly given. 
 All over Northern Europe and the northern part of North 
 America the solid rocks, where of hardness sufficient to retain 
 it, are found to present a characteristic smoothed, polished, and 
 striated surface. Even on crags and rocky bosses that have 
 remained for long periods exposed to the action of the weather, 
 this peculiar worn surface may be traced; but where they have 
 been protected by a covering of clay, these markings are often 
 as fresh as when they were first made. The groovings and fine 
 striae do not occur at random, but in every district run in one 
 or more determinate directions. The faces of rock that look 
 one way are rounded off, smoothed, and polished; those that 
 face to the opposite quarter are more or less rough and angular. 
 The quarter to which the worn faces are directed corresponds 
 with that to which the striae and grooves on the rock-surfaces 
 point. There can be no doubt that all this smoothing, polishing, 
 grooving, and striation has been done by land-ice ; that the trend 
 of the striae marks the direction in which the ice moved, those 
 faces of rock which looked towards the ice being ground away, 
 while those that looked away from it escaped. By following 
 out the directions of the rock-striae we can still trace the march 
 of the ice across the land (see Chapter VI). 
 
 As the ice travelled, it carried with it more or less detritus, 
 as a glacier does at the present day. Some of this material 
 may have lain on the surface, but probably most of it was 
 pushed along at the bottom of the ice. Accordingly, above 
 the ice-worn surfaces of rock, there lies a great deposit of 
 
POST-TERTIARY PERIODS 365 
 
 clay and boulders, evidently the debris that accumulated under 
 the ice-sheet and was left on the surface of the ground when 
 the ice retired. This deposit, called Boulder-Clay or Till, bears 
 distinct corroborative testimony to the movement of the ice. 
 It is always more or less local in origin, but contains a variable 
 proportion of stones which have travelled for a greater or less 
 distance, sometimes for several hundred miles. When these 
 stones are traced to their places of origin, which are often not 
 hard to seek, they are found to have come from the same quarter 
 as that indicated by the striation of the rocks. If, for example, 
 the ice-worn bosses of rock show the ice to have crept from 
 north to south, the boulders will be found to have a northern 
 origin. The height to which striated rock-surfaces and scattered 
 erratic blocks can be traced affords some measure of the depth 
 of the ice-sheet. 
 
 From this kind of evidence it has been ascertained that the 
 whole of Northern Europe, amounting in all to probably not 
 less than 770,000 square miles, was buried under one vast ex- 
 panse of snow and ice. The ice-sheet was thickest in the north 
 and west, whence it thinned away southward and eastward. 
 Upon Scandinavia it was not improbably between 6000 and 
 7000 feet thick. It has left its mark at heights of more than 
 3000 feet in the Scottish Highlands, and over North-Western 
 Scotland it was probably not less than 5000 feet thick. Where 
 it abutted upon the range of the Harz Mountains, it appears 
 to have been still not far short of 1500 feet in thickness. 
 
 This vast mantle of ice was in continual motion, creeping 
 outward and downward from the high grounds to the sea. The 
 direction taken by its principal currents can still be followed. 
 In Scandinavia, as shown by the rock-stria3 and the transport 
 of boulders, it swept westward into the Atlantic, eastward into 
 the Gulf of Bothnia, which it completely filled up, and south- 
 ward across Denmark and the low grounds of Northern Ger- 
 many. The basin of the Baltic was completely choked up 
 with ice; so also was that of the North Sea as far south as the 
 neighbourhood of London. From the same evidence we know 
 that the ice which streamed off the British Islands moved 
 eastward from the slopes of Scotland into the hollow of the 
 North Sea, part of it turning to the left to join the south-western 
 margin of the Scandinavian sheet^ and move with it northwards 
 
366 GEOLOGY 
 
 and westwards across the Orkney and Shetland Islands into 
 the Atlantic, and another branch bending southwards and mov- 
 ing with the southerly expansion of the Scandinavian ice along 
 the floor of the North Sea and the low grounds of the east of 
 England; and that on the west side of Scotland the ice filled 
 up and crept down all the fjords, burying the Western Islands 
 under its mantle and marching out into the Atlantic. The 
 western margin of the ice-fields, from the south-west of Ireland 
 to the North Cape of Norway, must have presented a vast wall 
 of ice some 2000 miles long, and probably several hundred feet 
 high, breaking off into icebergs which floated away with the 
 prevailing currents and winds. The Irish Sea was likewise 
 filled with ice, moving in a general southerly direction. 
 
 Northern Europe must thus have presented the aspect of 
 North Greenland at the present time. The evidence of rock- 
 striae and ice-borne blocks enable us to determine approx- 
 imately the southern limit to which the great ice-cap reached. 
 As even the southern coast of Ireland is intensely ice-worn, the 
 edge of the ice must have extended some distance beyond Cape 
 Clear, rising out of the sea with a precipitous, front that faced 
 to the south. Thence the ice-cliff swung eastwards, passing 
 probably along the line of the Bristol Channel and keeping to 
 the north of the valley of the Thames. 
 
 That the northern ice moved down the bed of the North 
 Sea is shown by the boulder-clays and transported stones of the 
 eastern counties of England, among which fragments of well- 
 known Norwegian rocks are recognisable. Its southern margin 
 ran across what is now Holland, and skirted the high grounds 
 of Westphalia, Hanover, and the,Harz, which probably there 
 arrested its southward extension. There is evidence that the 
 ice swept round into the Lowlands of Saxony up to the chain 
 of the Erz, Eiesen, and Sudeten Mountains, whence its southern 
 limit turned eastward across Silesia, Poland, and Galicia, and 
 then swung round to the north, passing across Eussia by way of 
 Kieff and Nijni Novgorod to the Arctic Ocean. 
 
 In Europe no distinct topographical feature appears to mark 
 the southern limit reached by the ice-sheet; this limit can only 
 be approximately fixed by the most southerly localities, . where 
 striated rocks and transported blocks Tiave been observed. In 
 North America, however, the margin of the great ice-cap is 
 
POST-TERTIARY PERIODS 3G7 
 
 prominently defined by a mound or series of mounds of detritus 
 which seem to have been pushed in front of the ice. These 
 mounds,, beginning on the coast of Massachusetts, run across 
 the Continent with a wonderful persistence for more than 3000 
 miles. They form what American geologists call the " terminal 
 moraine/' 
 
 The detritus left by the ice-sheet consists of earthy, sandy, 
 or clayey material (Boulder-Clay, Till) more or less charged 
 with stones of all sizes up to blocks weighing many tons. For 
 the most part it is unstratified, and bears witness to the irregu- 
 lar way in which it was tumbled down by the ice. In some 
 districts, it has been more or less arranged in water, and then 
 assumes a stratified character. The stones in the detritus, more 
 especially where they are hard and are imbedded in a clayey 
 matrix, present smooth striated surfaces, the striae usually run- 
 ning along the length of the stone, but not infrequently crossing 
 each other, the older being partially effaced by a newer set (Fig. 
 24). This characteristic striation points unmistakably to the 
 slow creeping motion of land-ice. 
 
 But the boulder-clays, earths, and gravels left by the great 
 ice-sheet are not simply one continuous deposit. On the con- 
 trary, they contain intercalations of stratified sand, clay, and 
 even peat. In these included strata organic remains occur, for 
 the most part those of terrestrial plants and animals, showing 
 that the ice again and again retreated, leaving the country to 
 be covered with vegetation, and to be tenanted by land animals ; 
 but that after longer or shorter periods of diminution it once 
 more advanced southward over its former area. These intervals 
 of retreat are known as " interglacial periods/' Probably they 
 were of prolonged duration, the climate becoming comparatively 
 mild and equable while they lasted. The occurrence of boulder- 
 clays above the interglacial deposits shows a subsequent lowering 
 of the temperature, with a consequent renewal of glacial con- 
 ditions. 
 
 The Pleistocene deposits thus reveal to us a prolonged period 
 of cold broken up by shorter intervals of milder climate. The 
 fossils which they contain throw curious and interesting light 
 on these oscillations of temperature. Among the plants, leaves 
 of Arctic species of birch and willow are found far to the south 
 of their present limits; on the other hand, remains of plants 
 
3G8 
 
 GEOLOGY 
 
 now confined to temperate latitudes are found fossil in Siberia^ 
 and others, now living in more genial climates than those of 
 Central Europe, are associated in interglacial deposits, with the 
 remains of the still indigenous vegetation. 
 
 To the same effect, but still more striking, is the testimony 
 of the Pleistocene fauna, with its strange mingling of northern 
 
 Fig. 204. Pleistocene or Glacial Shells, (a) Pecten islandicus (J) ; 
 (&) Leda truncata () ; (c) Leda lanceolata (!) ; (d) Tellina lata 
 (!) ; (e) Satvicava rugosa (j) ; (/) Natica clausa (%) ; (g) Trophon 
 scalari forme ( \ ) . 
 
 and southern forms. The marine shells imbedded in the glacial 
 clays of Scotland, though chiefly belonging to species that still 
 live in the adjoining seas, include a few that are now restricted 
 to more northern latitudes (Pecten islandicus, Leda lanceolata, 
 Tellina lata, etc., Fig. 204) . Turning to the terrestrial mam- 
 mals, we find among the Pleistocene deposits the remains of 
 the last of the huge pachyderms which, through Tertiary time, 
 had been so striking a feature of the animal population of 
 Europe. The hairy mammoth (ElepJias primigenius, Fig. 205) 
 
POST-TERTIARY PERIODS 
 
 369 
 
 and the woolly rhinoceros (R. tichorhmus) now roamed all over 
 the Continent and across Britain, which had not yet become an 
 island. During the retreat of the snow and ice, they found 
 
 Fig. 205. Mammoth (Elephas primigenius) from the skeleton in the 
 Musee Royal, Brussels. 
 
 their way into the forests and pastures of Northern Siberia. 
 Driven southwards when the cold increased, they were accom- 
 panied by numerous Arctic animals which have not yet become 
 extinct. Herds of reindeer (Cervus tarandus) sought the pas- 
 tures of Central France and Switzerland; the glutton (Gulo 
 luscus) came to the South of Eng- 
 land and to Auvergne ; the 
 musk-sheep (Ovibos moschatus, 
 Fig. 206) and Arctic fox (Cam's 
 lag opus) wandered southward 
 to the Pyrenees. But as each 
 oscillation of climate slowly 
 brought in a milder temperature, 
 and pushed the snow and ice 
 northward, animals of southern 
 types made their way into 
 Southern and Central Europe. 
 Among these immigrants were the porcupine (Hystrix) , leopard 
 (Felis pardus), African lynx (Felis pardina), lion (Felis leo), 
 hya3na, elephant, and hippopotamus, the bones of which have 
 been found in the Pleistocene deposits. 
 
 Fig. 206. Back view of skull 
 of Musk-sheep (Ovibos mos- 
 chatus, ), Brickearth, Cray- 
 ford, Kent. 
 
370 GEOLOGY 
 
 After the height of the cold period or Ice Age had been 
 reached and the general temperature of the northern hemisphere 
 began to rise again, the ice retreated from the low grounds, 
 but still continued among the mountains. The existing snow- 
 fields and glaciers of the Alps, the Pyrenees, and Scandinavia 
 are the lineal descendants of those vaster ice-sheets which for- 
 merly overspread so much of Europe. The glaciers of the Alps, 
 large though they are, can be shown to be merely the relics of 
 their former size. The glacier of the Ehone, for example, as is 
 proved by rock-striae and transported blocks, once extended 170 
 miles in direct distance from its modern termination, and rose 
 hundreds of feet above its present surface, burying the valleys 
 and overflowing considerable ridges of hills. The glacier of 
 the Aar stretched once as far as Berne a distance of about 
 70 miles from its present termination; and, judging from the 
 marks it has left on the mountains, it must have been not less 
 than 4000 feet thick at the Lake of Brienz. 
 
 Though elsewhere in Europe the glaciers have long ago van- 
 ished from most of the high grounds, they have left unmistak- 
 able traces of their former presence. Thus in hundreds of 
 valleys among the Highlands of Scotland, in the Lake District, 
 and North Wales, admirably ice-worn bosses of rock and beauti- 
 fully perfect moraines may be seen. We can even trace, in 
 the succession of moraines that become smaller as they approach 
 the head of a valley, the stages of retreat of the original glacier 
 as it shrank before the increasing warmth, till at last it dis- 
 appeared together with the snow-basin that fed it. 
 
 Other relics of the retirement of the ice-sheet are supplied 
 by the long mounds and heaps of gravel and sand, so abundantly 
 strewn over many Lowlands of Northern Europe. These some- 
 times form ridges, rising 20 or 30 feet above the ground on 
 either side of them, and running for a number of miles. Else- 
 where they are heaped together irregularly, often enclosing pools 
 of water. They are known as Osar in Sweden, Kames in Scot- 
 land, and Eslcers in Ireland. 
 
 During the later stages of the Ice Age the level of the land 
 in Western Europe was lower than it is now. When elevation 
 began, the upward movement continued with long intervals of 
 rest until the land reached its present position. These pauses 
 during the prolonged upheaval are marked by lines of raised 
 
POST-TERTIARY PERIODS 371 
 
 beach which are well seen along both sides of Scotland, and also 
 along the sea margin of Norway. 
 
 So slowly and gradually did the great cold disappear that the 
 Ice Age insensibly passed into the Recent or existing period. 
 There can be no doubt that man appeared in Europe before the 
 climate had become as mild as it now is, for his flint-flakes 
 and bone implements are found associated with the bones of 
 Arctic animals in Central France, and traces of his presence 
 in rudely chipped stone instruments occur in deposits which 
 point to frozen rivers. Indeed, in a certain sense, it may be 
 said that the Ice Age still exists among the snow-fields and 
 glaciers of Europe. 
 
 Arranged in chronological order, the evidence from which 
 the history of the Pleistocene period is determined may be given 
 as follows: 
 
 Last traces of local glaciers; terminal and lateral moraines. 
 
 Marine terraces or raised beaches, sometimes with moraines resting 
 upon them ; rock-shelves cut probably by waves and floating ice, 
 and marking former levels of the sea. These beaches and shelves 
 indicate pauses during the last upheaval of the land. Marine 
 clays with Arctic shells. 
 
 Erratic blocks chiefly transported by the great ice-sheet, but partly 
 also by floating ice during the rise of the land, and by valley- 
 glaciers. 
 
 Sands and gravels (kames) arranged in heaps, mounds, and ridges, and 
 due in some way to the melting of the edges of the ice-sheet, often 
 associated with lacustrine deposits formed in their hollows, and 
 containing lake-shells and terrestrial plants and animals. 
 
 Boulder-clay, till, or bottom-moraine of the great ice-sheet; the upper 
 part sometimes rudely stratified, and in some regions separated 
 from the lower part by a series of " middle sands and gravels " ; 
 the lower part quite unstratified and full of transported stones 
 and boulders. Finely laminated clays, sands, layers of peat, and 
 traces of terrestrial surfaces occur at different levels in the boulder- 
 clay, and mark " interglacial periods " of milder climate. 
 
 Polished and striated surfaces of rock, ground down by the movement 
 of the ice-sheet. 
 
 RECENT. 
 
 The insensible gradation of what is termed the Pleistocene 
 into the Recent series of deposits affords a good illustration of 
 the true relations of the successive geological formations to each 
 
372 GEOLOGY 
 
 other. We can trace this gradual passage because it is so recent 
 that there has not yet been time for those geological revolutions, 
 which in the past have so often removed or concealed the evi- 
 dence that would otherwise have been available to show that 
 one period or group of formations merged insensibly into that 
 which followed it. 
 
 The recent formations are those which have been accumulated 
 since the present general arrangement of land and sea, the 
 present distribution of climate, and the present floras and faunas 
 of the globe were established. They are particularly distin- 
 guished by traces of the existence of man. Hence the geological 
 age to which they belong is spoken of as the Human Period. 
 But, as has already been pointed out, there is good evidence that 
 man had already appeared in Europe during Pleistocene time, 
 so that the discovery of human relics does not afford certain 
 evidence that the deposit containing them belongs to the Kecent 
 series. Nevertheless, it is in this series that vestiges of man 
 become abundant, and that the proofs oi his advancing civilisa- 
 tion are contained. 
 
 Man differs in one notable respect from the other mammals 
 whose remains occur in a fossil state. Comparatively seldom 
 are any of his bones discovered as fossils ; but he has left behind 
 him other more enduring monuments of his presence in the 
 form of implements of stone, metal, bone, or horn. These relics 
 are in a sense more valuable than his bones would have been, 
 for while they afford us certain testimony to his existence, they 
 give at the same time some indication of his degree of civilisa- 
 tion and his employments. His handiwork thus comes, to possess 
 much geological value; his stone-hatchets, flint-flakes, bone- 
 needles, and other pieces of workmanship are to be regarded as 
 true fossils, from which much regarding his early history has 
 to be determined. 
 
 In the river-valleys of the north-west of France and south- 
 east of England human implements have been found in the 
 higher alluvial terraces. After careful exploration, it has been 
 ascertained that these objects have not been buried there sub- 
 sequently, but must have been covered up at the time the gravel 
 was being formed. The higher terraces are of course the older 
 deposits of the rivers, which have since deepened their valleys 
 until they now flow at a much lower level (Ch. Ill) . The excava- 
 
POST-TERTIARY PERIODS 
 
 373 
 
 tion of valleys must have been a slow process. Within a human 
 lifetime it is impossible to detect any appreciable lowering of 
 the ground from this cause. Even during the many centuries 
 of which we have authentic human records, we can hardly any- 
 where detect proof of such a change. How vast then must have 
 been the interval between the time when the rivers flowed at 
 the level of the upper terraces and the present day! Other 
 evidence of the great age of these higher alluvia is to be found 
 in the number of extinct animals whose remains are buried in 
 them. The human implements likewise bear their testimony 
 in support of the antiquity of the terraces, for they are ex- 
 
 Fig. 207. Palaeolithic Implements, (a) Flint implement, Reculver (), 
 chipped out of a rounded pebble; (6) Flint implement (J) from old 
 river-gravel at' Biddenham, Bedford, where remains of cave-bear, rein- 
 deer, mammoth, bison, hippopotamus, rhinoceros, and other mammalia 
 have been found: (c) Bone harpoon-head (1) from the red cave-earth 
 underlying the stalagmite floor of Kent's Cavern (a and & reduced from 
 Mr. Evans's "Ancient Stone Implements"). 
 
 tremely rude in design and construction, indicative of a race 
 not yet advanced beyond the early stages of barbarism. In the 
 lower and therefore younger terraces, and in other deposits which 
 may also be regarded as belonging to a later date, the articles 
 of human fabrication exhibit evidence of much higher skill 
 and more tasteful design, whence they have been inferred to 
 be the workmanship of a subsequent period when men had 
 
374 GEOLOGY 
 
 made considerable progress in the arts of life. Accordingly, 
 a classification has been adopted, based upon the amount of 
 finish in the -stone weapons and implements, the ruder work- 
 manship being assumed to mark the higher antiquity. The 
 older deposits, with coarsely chipped and roughly finished 
 human stone implements, are termed Paleolithic, and the 
 younger deposits with more artistically finished works in stone, 
 bone, or metal are known as Neolithic. It will be understood 
 that this arrangement is one rather for convenience of descrip- 
 tion than for a determination of true chronological sequence. 
 It is quite probable, for example, that some of the palaeolithic 
 gravels date back to the Pleistocene Ice Age, while other deposits 
 containing similar weapons and a similar assemblage of extinct 
 mammals may belong to a much later time, when the ice had 
 long retreated to the north. It is obvious, too, that we know 
 nothing of the relative progress made in the arts of life by 
 the early races of man. One race may have continued fashioning 
 the paleolithic -type of implement long after another race had 
 already learnt to make use of the neolithic type. Even at the 
 present day we see some barbarous races employing rude weapons 
 of stone not unlike those of the paleolithic gravels, while others 
 fabricate stone arrow-heads and implements of bone exactly re- 
 sembling those of the neolithic deposits. It would hardly be 
 incorrect to say that in some respects certain tribes of mankind 
 are still in the paleolithic or neolithic condition of human 
 progress. 
 
 i. PALEOLITHIC. 
 
 The formations included under this term are distinguished 
 by containing the rudest shapes of human stone implements, 
 associated with the remains of mammals, some sort of which are 
 entirely extinct, while others have disappeared from the districts 
 where their remains have been found. These deposits may be 
 conveniently classed under the heads of alluvium, brick-earth, 
 cavern-beds, calcareous tufas, and loess. 
 
 Alluvium. Reference has just been made to the upper river- 
 terraces, which, rising sometimes 80 or 100 feet above the 
 present level of the rivers, belong to a very ancient period in 
 the history of the excavation of the valleys, and yet contain rude 
 
POST-TERTIARY PERIODS 375 
 
 human implements. The mammalian bones, found in the sands, 
 loams, and gravels of these terraces, include extinct species, of 
 elephant, rhinoceros, hippopotamus, and other animals. The 
 human tools are roughly chipped pieces of flint or other hard 
 stone, and their abundance in some river-gravels has suggested 
 the belief that they were employed when the rivers were frozen 
 over, for breaking the ice and other operations connected with 
 fishing. The high river-gravels of the Somme and of the 
 valleys in the south-east of England have been specially prolific 
 in these traces of early man. 
 
 Brick-earths. On gentle slopes and on plains, .the slow 
 drifting action of wind and rain transports the finer particles 
 of soil and accumulates them as a superficial layer of loam or 
 brick-earth. In the south-east of England, considerable tracts 
 of country have been covered with a deposit of this nature. 
 It is still in course of accumulation, but, as already stated 
 herein, its lower parts must date back to a high antiquity, 
 for they contain the bones of extinct mammals, together with 
 human implements of palaeolithic type. 
 
 Cave-earth and stalagmite. The origin of caverns in lime- 
 stone districts was described in Chapter V, and reference was 
 made to the formation of stalagmite on their floors, and to the 
 remarkably perfect preservation of animal remains in and be- 
 neath that deposit. Many of these caves were dens tenanted 
 by hyaenas or other savage beasts of prey. Some of them were 
 inhabited by man. In certain cases, they have communicated 
 with the ground above by openings in their roofs, through 
 which the bodies of animals have fallen or been washed by 
 floods. The stalagmite, by covering over the bones left on the 
 floor of the caverns, or in the earth deposited there by water, 
 has preserved them as a singularly interesting record of the 
 life of the time. 
 
 Calcareous Tufa. Here and there, the incrustation of tufa 
 formed round the outflow of calcareous springs has preserved 
 the remains of the vegetation and of the land-animals of the 
 palaeolithic time. 
 
 Loess. This is the name given to a remarkable accumula- 
 tion of pale yellowish calcareous sandy earth which occurs in 
 some of the larger river valleys of Central Europe, especially 
 in those of the Ehine and the Danube; it likewise covers vast 
 
376 GEOLOGY 
 
 regions of China, and is found well developed in the basin of 
 the Mississippi. It is unstratified and tolerably compact, so 
 that it presents steep slopes or vertical walls along some parts 
 of the valleys, and can be excavated into chambers and passages. 
 In China subterranean villages have been dug out of it, along 
 the sides of the valleys which it has filled up. It contains 
 remains of terrestrial plants and snail-shells, also occasional 
 bones of land-animals. It bears little or no relation to the levels 
 of the ground, for it crosses over from one valley to another, 
 and even mounts up to heights several thousand feet above the 
 sea and far above the surrounding valleys. Its origin has been 
 
 Fig. 208. Antler of Reindeer (^ T ) found at Bilney Moor, East Dere- 
 
 ham, Norfolk. 
 
 the subject of much discussion among geologists and travellers. 
 But the result of much careful investigation bestowed upon it 
 goes to show that the loess is probably a sub-aerial deposit 
 formed by the long-continued drifting of fine dust by the wind. 
 It was probably accumulated during a comparatively dry period 
 when the climate of Central Europe, after the disappearance 
 of the ice-sheet, resembled that of the steppes of the south-east 
 of Russia. The assemblage of animals whose bones have been 
 
POST-TERTIARY PERIODS 377 
 
 found in it closely resembles that of these steppes at the present 
 time; for it includes species of jerboa, porcupine, wild horses, 
 antelopes, etc. Among its fossils, however, there occur also the 
 bones of the mammoth, woolly rhinoceros, musk-sheep, hare, 
 wolf, stoat, etc., together with palaeolithic stone implements. 
 
 Thus the association of animals in the palaeolithic formations 
 shows a commingling of the denizens of warmer and colder 
 climates, like that already noticed as characteristic of the Ice 
 Age, and hence the inference above alluded to that the paleo- 
 lithic gravels may themselves be interglacial. Among the animals 
 distinctively of more southern type mention may be made of 
 the lion, hyaena, hippopotamus, lynx, leopard, Caffer cat; while 
 among the northern forms are the glutton, Arctic fox, reindeer, 
 Alpine hare (Lepus varidbilis), Norwegian lemming (My odes 
 torquatus), and musk-sheep. The animals which then roamed 
 over Europe, but are now wholly extinct, included the mammoth, 
 woolly rhinoceros, and other species of the genus, Irish elk 
 (Megaceros hibernicus), and cave-bear (Ursus speloeus). The 
 traces of man consist almost entirely of pieces of his handiwork ; 
 only rarely are any of his bones to be seen. Besides the rude 
 chipped flints, he has left behind him, on tusks of the mammoth 
 and horns and bones of the reindeer and other animals, pre- 
 served in the stalagmite of cavern-floors, vigorous incised out- 
 line-sketches and carvings representing the species of animals 
 with which he was familiar, and some of which have long died 
 out. He was evidently a hunter and fisher, living in caves and 
 rock-shelters, and pursuing with flint-tipped arrow and javelin 
 the bison, reindeer, horse, mammoth, rhinoceros, cave-bear, and 
 other wild beasts of his time. 
 
 2. NEOLITHIC. 
 
 In this division, the human implements indicate a consider- 
 able advance in the arts of life, and the remains of the mam- 
 moth, rhinoceros, and other prevalent extinct forms of the 
 palaeolithic series are absent. The deposits here included consist 
 of river-gravels, cave-floors, peat-bogs, lake-bottoms, raised 
 beaches, sand-hills, pile-dwellings, shell-mounds, and other 
 superficial accumulations in which the traces of human occupa- 
 tion have been preserved. 
 
378 
 
 GEOLOGY 
 
 After the extinction of the huge pachyderms, the European 
 fauna assumed the general character which it now presents, but 
 with the presence of at least one animal, the Irish elk, that has 
 since become extinct, and of others, such as the reindeer, elk, 
 wild ox or urus, grizzly bear, brown bear, wolf, wild boar, and 
 beaver, which, though still living, have long been extirpated from 
 many districts wherein they were once plentiful. This local ex- 
 tinction has no doubt, in many if not in most cases, been the 
 result, directly or indirectly, of human interference. But man 
 not only drove out or annihilated the old native animals. As 
 
 Fig. 209. Neolithic Implements, (o) Stone axe-head (J) ; (6) Barbed 
 flint arrow-head (natural size) ; (c) Roughly-chipped flint celt (J) ; 
 (d) Polished celt (i), with part of its original wooden hand still 
 attached, found in a peat-bog, Cumberland; (e) Bone-needle (natural 
 size), Swiss Lake Dwellings; a, 6, c, d, reduced from Mr. Evans's 
 " Ancient Stone Implements." 
 
 tribe after tribe of human population migrated into Europe 
 from some region in Asia, they carried with them the animals 
 they had domesticated the hog, horse, sheep, goat, shorthorn, 
 and dog. The remains of these creatures never occur among the 
 palaeolithic deposits; they make their appearance for the first 
 time in the neolithic accumulations, whence the inference has 
 been drawn that they never formed part of the aboriginal fauna 
 
POST-TERTIARY PERIODS 379 
 
 of Europe, but were introduced by the human races of the 
 neolithic period. 
 
 The stone articles of human workmanship found in neolithic 
 deposits consist of polished celts and other weapons, hammers, 
 knives, and many other implements of domestic use. Knives, 
 needles, pins, and other objects were made out of bone or horn. 
 There is evidence also that the arts of spinning, weaving, and 
 pottery-making were not unknown. The discovery of several 
 kinds of grain shows that the neolithic folk were also farmers. 
 Vast numbers of these various relics have been found at the pile- 
 dwellings of Switzerland and other countries. For purposes of 
 security these people were in the habit of constructing their 
 wooden dwellings in lakes on foundations of beams, wattled- 
 work, stones, and earth. Sometimes these erections were apt 
 to be destroyed by fire, as well as to decay by age. And their 
 places were taken by new constructions of a similar kind built 
 on their site. Hence, as generation after generation lived there, 
 all kinds of articles dropped into the lakes were covered up in 
 the silt that slowly gathered on the bottom. And now, when 
 the lakes are drained, or when their level is lowered by pro- 
 longed drought, these accumulated droppings are laid open for 
 the researches of antiquaries and geologists. Many important 
 relics of neolithic man have likewise been obtained from the 
 floors of caverns and rock-shelters places that from their con- 
 venience would continue to be used as in palaeolithic time. In- 
 teresting evidence, also, of the successive stages of civilisation 
 reached by early man in Europe, is supplied by the older Danish 
 peat-bogs, in the lower parts of which remains of the Scotch fir 
 (Pinus sylvestris), a tree that had become extinct in that country 
 before the historic period, are associated with neolithic imple- 
 ments. In a higher layer of the peat, trunks of common oak are 
 found, together with bronze implements, while in the uppermost 
 portion, the beech-tree and iron weapons take their place. 
 
 Between the neolithic and the present period no line can be 
 drawn. They shade insensibly into each other, and the ma- 
 terials which reveal the history of their geographical and climatal 
 vicissitudes, their changes of fauna and flora, and their human 
 migrations and development, form a common ground for the 
 labours of the archaeologist, the historian, and the geologist. 
 
 During the Kecent period the same agencies have been and 
 
380 GEOLOGY 
 
 are at work as those which have been in progress during the 
 vast succession of previous periods. In the foregoing pages we 
 have followed in brief outline each of these great periods, and 
 after this survey we are led back again to the world of to-day 
 with which the first chapters of this book began. In this circle 
 of observation no trace can anywhere be detected of a break in 
 the continuity of the evolution through which our globe has 
 passed. Everywhere in the rocks beneath our feet, as on the 
 surface of the earth, we see proofs of the operations of the same 
 laws and the working of the same processes. 
 
 Such, however, have been the disturbances of the terrestrial 
 crust that, although undoubtedly there has been no general 
 interruption of the Geological Eecord, local interruptions have 
 almost everywhere taken place. The sea-floor of one period has 
 been raised into the dry land of another, and again, the dry- 
 land, with its chronicles of river and lake, has been submerged 
 beneath the se^. Each hill and ridge thus comes to possess its 
 own special history, which it will readily reveal if questioned 
 in the right way. 
 
 We are surrounded with monuments of the geological past. 
 But these monuments are being slowly destroyed by the very 
 same processes to which they owed their origin. Air, rain, 
 frost, springs, rivers, glaciers, waves, and all the other con- 
 nected agents of demolition, are ceaselessly at work wherever 
 land rises above the sea. It is in the course of this demolition 
 that the characteristic features of the scenery of the land are 
 carved out. The higher and harder parts are left as mountains 
 and hills, the softer parts are hollowed out into valleys, and the 
 materials worn away from them are strewn over plains. And 
 as it is now, so doubtless has it been through the long ages of 
 geological history. Decay and renovation in never-ending cycles 
 have followed each other since the beginning of time. 
 
 But amid these cycles there has been a marvellous upward 
 progress of organic being. It is undoubtedly the greatest tri- 
 umph of geological science to have demonstrated that the present 
 plants and animals of the globe were not the first inhabitants 
 of the earth, but that they have appeared only as the de- 
 scendants of a vast ancestr} 7 ", as the latest comers in a majestic 
 procession which has been marching through an unknown series 
 of ages. At the head of this procession we ourselves stand^ 
 
POST-TERTIARY PERIODS 381 
 
 heirs of all the progress of the past and moving forward into the 
 future wherein progress towards something higher and nobler 
 must still be for us, as it has been for all creation, the guid- 
 ing law. 
 
 THE END. 
 
INDEX. 
 
 Accumulations formed by the sea, 
 
 77 
 
 Adelsberg caverns, 55 
 Age of reptiles, the Jurassic, 316 
 Albian stage of the Jurassic period, 
 
 334 
 
 Alluvium, how formed, 35 
 Aluminium, 125 
 Ammonites in the triassic period, 
 
 305 
 
 in the Jurassic period, 313 
 Amphibians of the carboniferous 
 
 period, 284, 286 
 Amphibolites, 176 
 Anchor-ice, 63 
 Anhydrite, 141 
 Animals as causes of change on 
 
 earth's surface, 18 
 durable parts of, 223 
 geological records made by, 
 
 83-95 
 deposits formed by remains of, 
 
 87 
 
 remains in sedimentary depos- 
 its, 91 
 
 of the tertiary periods, 339 
 Annelids in cambrian period, 253 
 Apatile, 141 
 Aragonite, 140 
 Archaean rocks, 242 
 Archaeopteryx, 318, 319 
 Atlantosaurus, 318 
 Atmosphere, influence of on 
 
 changes of earth's surface, 
 
 10-23 
 
 Augite, 138 
 Avalanches, 64 
 Axmouth landslip, 52 
 
 Bajocian stage of Jurassic period, 
 
 321 
 Bathonian stage of the Jurassic 
 
 period, 321 
 Barytes, 141 
 Barium, 127 
 Basalt-rocks, 171 
 Birds first appear in the Jurassic 
 
 period, 318, 319 
 in the cretaceous period, 332 
 of the eocene period, 342 
 
 383 
 
 Blow-holes, 73 
 
 Bosses, in eruptive rocks, 209 
 
 Bone-beds, 167 
 
 Boulders transported by ice, 63 
 
 Brachiopods of the devonian pe- 
 riod, 274 
 in the carboniferous period, 
 
 290 
 
 of the permian period, 298 
 of the Silurian period, 265 
 
 Breccia, 155 
 
 Brick-earth, 20 
 
 Brontosaurus, 318 
 
 Cainozoic rocks. See Tertiary 
 
 Periods. 
 
 Calcareous springs, 57 
 Calcite, 139 
 Calcium, 125, 127 
 Calcareous tufa, 59 
 Gale-sinter, 59 
 Cambrian period, 246-257 
 Carbon, 122 
 Carbonates, 139 
 Carboniferous period, 277-294 
 Caverns formed by springs, 55 
 Cenomian stage of the Jurassic 
 
 period, 334 
 Cephalopods in cambrian period, 
 
 256 
 in the carboniferous period, 
 
 291 
 characteristic of the triassic 
 
 period, 305 
 
 in the cretaceous period, 329 
 in the cretaceous period, 330 
 Chalk, 164 
 
 Chalybeate springs, 61 
 Chemical action of running water, 
 
 25-27 
 
 Chemical action of springs, 54 
 Chlorides, 142 
 Chloride-schist, 176 
 Chlorine, 124 
 Chlorite, 138 
 Concretions, 143 
 Chronology, geographic shown by 
 
 fossils, 228 
 Clay, 158 
 
384 
 
 INDEX 
 
 Clay-slate, 175 
 
 Cleavage of rocks, 201 
 
 Cliff-debris, 154 
 
 Climatic changes shown by fossils, 
 
 227 
 
 Coal, 165 
 
 Coal seams as indicative of sub- 
 sidence, 279, 280 
 Coal, how formed, 280 
 Concretions, 186 
 Concretionary rocks, 147 
 Conglomerate, 156 
 Conifers in the permian period, 
 
 298 
 
 Consolidation of sedimentary de- 
 posits, 193 
 Continents formed in tertiary 
 
 periods, 339 
 
 Coral-reefs, how formed, 89-90 
 Coral-rock, 164 
 
 Corals of Silurian period, 261 
 of the devonian period, 273 
 of the devonian period, 275 
 in the carboniferous period, 
 
 287 
 
 in the j"^assic period, 311 
 Corralian stage of the Jurassic 
 
 period, 322 
 
 Cretaceous period, 324-337 
 Crinoids or sea-lilies, 253 
 of silurian period, 261 
 in the carboniferous period, 
 
 288 
 
 in the triassic, 305 
 in the Jurassic period, 311 
 Crustaceans of the devonian period, 
 
 272 
 in the carboniferous period, 
 
 289 
 
 in the Jurassic period, 314 
 Crystal forms, 129-131 
 Crystalline, 148 
 Cteosaurus, 317 
 Curvature of strata, 198 
 Cuttle fish in the Jurassic period, 
 
 313 
 
 Cycads in the permian period, 298 
 in the Jurassic period, 310 
 age of, 304 
 
 Danian stage of the Jurassic period, 
 
 336 
 
 Dead Sea, 50 
 Dendrites, 135 
 Deposition by running water, 35- 
 
 43 
 
 Deposition by springs, 56 
 Devonian period, 268-276 
 Diabase, 173 
 Diatoms, 86 
 Dinosaurs first appear in the trias- 
 
 sio period, 307 
 in the Jurassic period, 317 
 
 Diorite, 173 
 
 Dip of strata, 195 
 
 Dislocation of strata, 203 
 
 Dolomite, 140, 161 
 
 Dust, changes wrought by, 20 
 
 Earth, hot interior of, 7 
 
 interior, condition of, 97 
 crust, structure of, 179-234 
 
 Earthquakes, 114-118 
 
 Earthworms, effect of on soil, 18 
 
 Echinodermata, in Cambrian pe- 
 riod, 253 
 of silurian period, 262 
 
 Elements and minerals, 119 
 
 Erosion, 30-35 
 
 Erosion, geological effect of, G8 
 
 Erosive power of running water, 25 
 
 Erratic boulders, 66 
 
 Eruptive rocks, 148 
 
 Eruptive rocks, 167-180 
 
 Eruptive rocks, 208 
 
 Eocene period of the tertiary, 341- 
 346 
 
 Eophyton, 251 
 
 Eozoon, 243 
 
 Fairy stones, 143 
 
 Faults in strata, 203 
 
 Fauna of silurian period, 259 
 of the permian, 297 
 of the cretaceous period, 324 
 
 Felsite, 169 
 
 Felspars, 136 
 
 Ferns of the carboniferous period, 
 282 
 
 Fire-clay, 158 
 
 Fishes of the silurian period, 265 
 of the devonian period, 
 in the carboniferous period, 
 
 291 
 
 of the permian period, 299 
 of the triassic period, 306 
 in the Jurassic period, 315 
 of the cretaceous period, 331 
 
 Flint, 106 
 
 Flood-plain of rivers, 40 
 
 Flora of the triassic period, 304 
 of the Jurassic period, 310 
 
 Flourine, 125 
 
 Flourides, 142 
 
 Foot-prints, 184 
 
 Forests, submerged, 11C 
 
 Fragmental rocks, 146 
 
 Fresh-water lakes, how formed, 44 
 
 Frost, changes on earth's surface 
 by, 13 
 
 Fossil remains, 220 
 
 Fossilisation, 223 
 
 Fossils, how indicative of geo- 
 graphic change, 226 
 
INDEX 
 
 385 
 
 Gasteropods in cambrian period, 
 
 255 
 
 of the silurian period, 264 
 in the carboniferous period, 
 291 
 
 Geographic changes indicated by 
 fossils, 220 
 
 Geologic chronology, shown by 
 fossils, 228 
 
 Geological record, table of, 240, 241 
 
 Geology, nature of its inquiries, 5 
 the study of earth's history, 9 
 
 Glacial period, 364 
 
 Glaciers and ice-sheets, 65-71 
 
 Glassy rocks, 149 
 
 Glen Roy, parallel roads of, 46 
 
 Globe, earliest condition of, 234 
 
 Globigerina ooze, 88 
 
 Gneiss, 176 
 
 Granite, 170 
 
 Gravel, 155 
 
 Graywacke, 157 
 
 Great Salt Lake, 50 
 
 Greenland, fossils of, 326, 327 
 
 Ground-ice, 63 
 
 Guano, 166 
 
 Gypsum in lakes, 50 
 
 Gypsum, 140, 162 
 
 Hematite or specular iron, 133 
 Hornblende or amphibole, 137 
 Horse, ancestors of appear in the 
 
 eocene period, 344 
 Hydrogen, 124 
 
 Hydrozoa of silurian period, 260 
 Hydrozoon, 252 
 
 Ice-records, 63-71 
 
 Ice of rivers and lakes, changes 
 
 wrought by, 63 
 Iceland spar, 128 
 Ichthyosaurus or sea-lizard, 316 
 Iguanodon, last of the dinosaurs, 
 
 331 
 
 Infusorial earth, 85 
 Insects of the carboniferous period, 
 
 282, 285 
 
 Iron deposits in lakes, 49 
 Iron, 127 
 Ironstone, 163, 166 
 
 Joints in sedimentary rocks, 193 
 Jurassic period, 310-323 
 
 Kaolin, 158 
 
 Kimmeridgian stage of the Juras- 
 sic period, 322 
 
 Lamellibranchs, in cambrian pe- 
 riod, 255 
 in the carboniferous period, 
 
 290 
 
 in the triassic period. 305 
 in the Jurassic period, 313 
 
 Lakes, memorials left by, 44-50 
 
 filling up of, 45 
 Lake deposits, 47 
 Land, surface of, changing, 1-5 
 
 demolished by the sea, 72 
 Landslips, 52 
 Lava, varieties of, 100 
 Lias rocks, 320 
 Lignite or brown coal, 165 
 Limestone, 55, 160, 164 
 Limestone in the carboniferous 
 
 period, 278 
 
 Limonite, or brown\ iron ore, 134 
 Lizards first appear in the per- 
 
 mian period, 300 
 
 Lizards in the triassic period, 3o6 
 Loess, 159 
 Loess, 375 
 London, Roman remains in, 3 
 
 Magnesium, 126 
 
 Magnetite, 134 
 
 Mammals first appear in the trias- 
 sic period, 307 
 of the eocene period, 343 
 in the Jurassic period, 319 
 
 Mammoth, 369 
 
 Mammoth Cave, 56 
 
 Manganese, 127 
 
 Manganese oxides, 135 
 
 Mangrove-swamps, 85 
 
 Marble, 177 
 
 Marl in lakes, 48 
 
 Marcasite, 143 
 
 Mastodon, 352 
 
 Mechanical action of running 
 
 water, 27-30 
 of springs, 51 
 
 Megalosaurus, 317 
 
 Metals, 120, 125 
 
 Metamorphism in geology, 206 
 
 Metaloids, 120 
 
 Mesozoic periods, 302-337 
 
 Mica, 137 
 
 Mica-schist, 176 
 
 Minerals and elements, 119 
 
 Minerals, importance of in earth's 
 crust, 128-144 
 
 Mineral veins, 215 
 
 Miocene period, 350^-356 
 
 Mollusca of the silurian period, 
 
 264 
 
 of the devonian period, 275 
 in the carboniferous period. 
 
 285 
 of the eocene period, 342 
 
 Moraines, how formed, 65 
 
 Mosasaurus, 332 
 
 Mylonitic rocks, 152 
 
 Nature and use of fossils, 220, 233 
 Nebular hypothesis stated, 235 
 Necks, or volcanic vents, 215 
 
386 
 
 INDEX 
 
 Neocomian stage of the Jurassic 
 period, 334 
 
 Neolithic period, 377 
 
 Niagara Falls, show power of ero- 
 sion, 34 
 
 Nitrogen, 125 
 
 Obsidian, 168 
 
 Old red sandstone period. See 
 
 Devonian Period. 
 Oligocene period, 346-349 
 Olivine, and serpentine rocks, 173 
 Olivine, 138 
 Oolitic rocks, 148 
 Ooze, 88 
 Organic remains, how preserved, 
 
 221 
 
 Orthoclase, 136 
 Orthoclase rocks, 168 
 Outcrop of strata, 197 
 Overlap, in geology, 190 
 Oxfordian stage of the Jurassic 
 
 period, 321 
 
 Oxides, abundance of, 132 
 Oxygen, importance of, 121 
 Oysters abuMant in the Jurassic 
 
 period, 313 
 
 Palaeolithic period, 374 
 
 Palaeozoic periods, 246-301 
 
 Peat, 165 
 
 Peat-bogs, 84 
 
 Peak cavern, 55 
 
 Peridot, 138 
 
 Perlitic rocks, 150 
 
 Permian rocks, how formed, 295- 
 
 301 
 Petrifaction of organic structure, 
 
 225 
 
 Petrifying springs, 60 
 Phosphorus, 124 
 Phosphates, 141 
 Phyllopod crustaceans of the silu- 
 
 rian period, 263 
 Pines appear in the Jurassic period, 
 
 311 
 
 Plagioclase, 136 
 Plants, geological records made by, 
 
 83-95 
 
 durable parts of, 222 
 of the devonian period, 269 
 of the carboniferous period, 
 
 282-284 
 
 of the permian period, 297 
 of the cretaceous period, 325 
 of the tertiary period, 339 
 of the eocene period, 341 
 Plesiosaurs, first appear in the tri- 
 
 assic period, 307 
 Plesiosaurus, 316 
 Pliocene period, 356-362 
 Pleistocene group, 363 
 Polyzoa in the carboniferous per- 
 iod, 289 
 
 Portlandian stage of the juraasic 
 
 period, 322 
 
 Post-tertiary periods, 363-381 
 Potassium, 126 
 Pot-holes, how made, 30 
 Porphyritic rocks, 150 
 Pre-cambrian period, 241, 242 
 Pterodactyl, 317 
 Purberkian stage of the Jurassic 
 
 period, 322 
 Pyrite, 143 
 Pyroxene, 138 
 
 Quarries, geological evidence of, 4 
 Quartz crystals, 122, 132 
 Quartzite, 177 
 
 Rain, a cause of change on earth's 
 
 surface, 14, 18, 20 
 Rain-prints, 184 
 Raised beaches, 116 
 Recent period, 371 
 Reptiles in the triassic period, 306 
 development of characteristic 
 
 of the Jurassic period, 31(5 
 of the cretaceous period, 331 
 River-action, records of, 35 
 River deltas, 78 
 River-terraces, 39 
 Rock-salt in lakes, 50 
 Rock-salt, 163 
 
 Rocks, geological evidence of, 119 
 Roman remains in London, 3 
 Rotation of earth in early ages, 
 
 237 
 Running water, in geological 
 
 changes, 24-43 
 Ripple marks, 183 
 
 Salt-lakes, formation of, 49 
 
 Sand, 156 
 
 Sand dunes, changes of, 21 
 
 Sandstone, 157 
 
 Saturation and dessication, influ- 
 ence of, 13 
 
 Schistose grit, 177 
 
 Schistose rocks, 174 
 
 Sea, geological records made by, 
 72-82 
 
 Sea-serpents of the cretaceous pe- 
 riod, 332 
 
 Sea-weeds, accumulations of, 86 
 as land makers, 87 
 
 Secondary era, see Mesozoic Pe- 
 riods. 
 
 Sedimentary rocks, 153, 159, 146, 
 179-207 
 
 Senonian stage of the Jurassic 
 period, 336 
 
 Serpentine, 139 
 
 Serpentine and olivine rocks, 173 
 
 Shale, 158 
 
 Shearing of strata, 199 
 
INDEX 
 
 387 
 
 Sheets, of eruptive rocks, 211 
 
 Shell-marl, 164 
 
 Shell-banks, 87 
 
 Siderite, 146 
 
 Siliceous springs, 61 
 
 Silicates, 135 
 
 Silicon, importance of, 122 
 
 Silurian period, 258-268 
 
 Snow, geological effects of, 64 
 
 Sodium, 126 
 
 Soil beneath cities, nature of, 3 
 
 Soil and subsoil, changes in, 16-22 
 
 Sphagnum, or bog moss, growth of, 
 85 
 
 Spherulitic rocks, 150 
 
 Springs, geological records of, 51- 
 62 
 
 Stalactites, how formed, 57 
 
 Stalagmites, how formed, 57 
 
 Stegosaurus, 318 
 
 Stone implements found under 
 London, 3 
 
 Stones transported by rivers, 38 
 
 Storm beaches, 79 
 
 Stratification, 180 
 
 Strike of strata, 196 
 
 Strata, alternation of, 187 
 
 chronological value of, 188 
 conformability of, 190 
 
 Subsidence of land, 278-280 
 
 Subsidence, proofs of, 189 
 
 Sulphates, 140 
 
 Sulphides, 143 
 
 Sulphur, 124 
 
 Sun-cracks, 184 
 
 Syenite, 169 
 
 Temperature, influence of changes 
 in, 13 
 
 Tertiary periods, 338-362 
 
 Titanic iron, 135 
 
 Trachyte, 169 
 
 Transporting power of running 
 water, 25 
 
 Transportation by glaciers, 65 
 
 Travertine, 59 
 
 Trilobites in cambrian period, 254 
 of the silurian period, 263 
 in the carboniferous period. 
 
 288 
 of the devonian period, 273 
 
 Triassic period, 302-310 
 
 Turonian stage of the Jurassic pe- 
 riod, 335 
 
 Upheaval and subsidence, 115 
 
 Veins and dykes, 214 
 
 Vesicular rocks, 151 
 
 Volcanic ash, 159 
 dykes, 112 
 products, 100-107 
 vents and fissures, 107 
 
 Volcanoes, importance of their 
 study, 7 
 
 Volcanoes and earthquakes, 96-114 
 
 Water, running, in geological 
 changes, 24-43 
 
 Weathering, influence of, 11-14 
 
 Wind, effect of on earth's sur- 
 face, 18 
 
 Winding of streams, how formed. 
 32 
 
 Talus, how formed, 19 
 
 Zeolites, 136