GEOLOGY 
 
 CHAPTERS OF : ./ : 7H HISTO 
 
 .EORGE HICKLING M.Sc. 
 
 UC-NRLF 
 
 SB 277 
 
 XXth CENTURY SCIENCE SER1 
 
GEOLOGY 
 
 Chapters of Earth History 
 
WHIRLPOOL" NEBULA. 
 
 [Keeler. 
 M.51 Canum Venaticorum. 
 
 Reproduced by permission of Professor Campbell from 
 Publications of the Lick Observatory. 
 
 Plate I.} 
 
 [Geologv, frontispiece. 
 
GEOLOGY 
 
 Chapters of Earth Hiftory 
 
 GEORGE HICKLING, M.Sc., 
 
 N\ 
 
 Ledturer in Palaeontology and Assistant Ledlurer in Geology 
 in the University of Manchester. 
 
 NEW YORK 
 
 FREDERICK A. STOKES COMPANY, 
 PUBLISHERS. 
 
CONTENTS 
 
 PAGE 
 
 INTRODUCTION ... ... ... ... 1 
 
 CHAPTER I. THE ORIGIN OF THE EARTH ... 5 
 
 II. VOLCANOES AND EARTHQUAKES... 25 
 
 III. THE SOLID ROCKS ... ... 42 
 
 IV. EARTH SCULPTURE ... ... 55 
 
 V. THE SEA-FLOOR ... ... 70 
 
 VI. MOUNTAIN BUILDING ... ... 81 
 
 VII. THE PHYSICAL HISTORY OF 
 
 BRITAIN ... ... ... 97 
 
 VIII. THE HISTORY OF LIFE ON THE 
 
 EARTH 116 
 
 280864 
 
The Publishers are indebted to the following for per- 
 mission to reproduce illustrations in this volume : 
 
 Professor Campbell - - " The Whirlpool Nebula." 
 Dr Tempest Anderson - " Eruption of Alt. Pelee." 
 
 Mr. R. Geiler - - " Atemir, near Settle." 
 
 Dr. Bartholomew - - " Section of the Grampians." 
 Trustees of British Museum " Skeleton of Brontosaurus," 
 
 from the sketch by the late Prof. O. C. Marsh ; 
 
 " Development of the Skull of the Elephant." 
 Messrs. Macmillan & Co., Ltd. 11 Development of the 
 
 Limbs and Teeth of the Horse," from Huxley's 
 
 American Addresses. 
 
LIST OF ILLUSTRATIONS 
 
 Plate I. The " Whirlpool " Nebula. M.51 Canum 
 
 Venaticorum ... ... ... Frontis. 
 
 I 1. Eruption of Mt. Pelee, Martinique, 1902 
 
 facing p. 28 
 III. Atemir, near Settle ... ... ... 59 
 
 IV. Typical Fossils. 1, Trilobite crustacean ; 
 2, Brachiopod ; 3, Cephalopod ; 4, Echi- 
 noid ; 5, Gastropod ; 6, Pelecypod ... ,, 116 
 
 Fig. 1 Ideal Section of a Volcano ... ... ... 31 
 
 2 Aughros Head, Co. Sligo ... ... ... 41 
 
 3(A) Thin Slice of Granite from Dalbeattie, 
 Scotland. (B) Thin Slice of Rhyolite from 
 Elfdalen, Sweden ... ... ... 45 
 
 4 Lion Rock, Isle of Cumbrae, Clyde ... ... 51 
 
 5 (A) Portion of course of meandering river. 
 
 (B) Course of same river at later period, 
 changed by erosion of banks and deposit of 
 sediment ... ... ... ... 68 
 
 6 Fold in rocks forming cliff west of Little Hang- 
 
 man Hill, near Ilfracombe ... ... 89 
 
 7 Section illustrating the gentle folding of the 
 
 rocks in the South-east of England ... 91 
 
 8 Section illustrating the intense folding of the 
 
 rocks in the Grampians ... ... ... 91 
 
 9 Rocks on shore at Stccar Point, Berwickshire 101 
 10 Geological Map of the British Isles ... ... 104 
 
 II Skeleton of Brontosaurus, a Deinosaur, from 
 
 the Upper Jurassic of Wyoming .., ... 125 
 
 12 (A) Development of the skull of the Elephant. 
 (B) Development of the limbs and teeth of the 
 
 Horse ... ... ... ... ... 130 
 
 vii 
 
INTRODUCTION 
 
 THE tale has gone the rounds of the aged Abbe" 
 who, being accosted wandering alone among the 
 mountain wilds, answered the inquiry how he came 
 to be there with the story of a dream he had had 
 during the course of a fever which he had believed 
 to be mortal. In his vision he was asked by his 
 Maker what he thought of the beautiful world in 
 which he had been permitted to live. The question 
 was a revelation to him. He, who had spent his life 
 exhorting his fellows to look to the beauties of a 
 future world, had never thought to look around him. 
 He could make no answer. But, when he awoke, he 
 made a vow that if he should still be permitted to 
 live he would devote his grey years to an inspection 
 of the world. Health returned, and'so he was found 
 upon his pilgrimage. 
 
 The story has become a favourite, because few can 
 read it without sympathy. How many could give 
 account of the beauties of the world in which they 
 live ? It is recorded here because it expresses 
 eloquently, if unconventionally, the import of that 
 much-misunderstood word, Science. For it is the 
 whole object of science to know the beauties of the 
 world around us ; not only the world as it is to-day 
 
 1 
 
INTRODUCTION 
 
 but as it has been in the past, also. It is the special 
 province of Geology to recover the story of the 
 earth's past, to show through what vicissitudes it 
 has attained its present condition. In all ages and 
 all climes, from the earliest dawn of human civilisa- 
 tion, man has sought to know how the world came 
 to be. Not only had the Jews their story of Creation, 
 but similar legends are found among the oldest 
 writings and traditions of many nations of antiquity. 
 Whether Geology or Astronomy is the older science 
 can never be known, both having their roots deep in 
 the lost grounds of early human history. Indeed, in 
 the earlier ages, they were not two sciences, but one. 
 Cosmogony was alike the foundation of them both. 
 Various causes during the chequered history of 
 science led to their gradual separation, notably the 
 recognition of the true place of the earth in the 
 universe, and in modern times, the intense reaction 
 against the wild speculations of the Middle Ages, 
 which led geology to repudiate all concern with the 
 origin of the earth, and to confine her attentions to 
 its history since it became a globe such as we now 
 know it. But the step was a mistaken one, and 
 geology is rapidly returning to her old alliance. She 
 has been true long enough to her old watchword that 
 the Present is the key to the Past, and is beginning 
 to realise again that it is even more profoundly true 
 /that the Past is the key to the Present. Yet the 
 exigencies of our position compel us always to work 
 from the present to the past; and while the 
 
INTRODUCTION 3 
 
 astronomer, by observing the present condition of 
 other worlds throws light on the past of our own, it 
 is by carefully noting how changes proceed around 
 us to-day that we are able to interpret the changes 
 which have clearly occurred in the past. Thus, 
 occupying an intermediate position between 
 Astronomy, with the unfathomable universe for its 
 fields, on the one hand, and the many sciences which 
 seek to investigate the varied manifestations of life 
 and matter in our own world on the other, Geology 
 seeks to apply the truths gathered by them all to the 
 interpretation of the past history of the earth record- 
 ed by Nature herself in the rocks at our feet. It is, 
 therefore, the most comprehensive of all the sciences. 
 To the geologist, the world of to-day, with all its 
 varied aspects of being and doing, is but a moment- 
 ary glimpse of Nature in her grand progress of 
 evolution through illimitable time. 
 
 Tlae foregoing indication of the wide scope of our 
 subject should sufficiently indicate why we have 
 chosen to entitle this little volume " Chapters of 
 Earth History." So manifold and so diverse are the 
 branches into which the study of geology divides 
 itself that no volume, however large, can give a com- 
 plete account of the whole, even in outline. Nor is 
 a systematic account our intention; but rather to 
 glance at the subject from various points of view, 
 so that, without detaining ourselves for a minute 
 inspection of any aspect, we may gain a general 
 impression of the whole. As we have seen that 
 
4 INTRODUCTION 
 
 speculations concerning the origin of the world 
 attracted attention ages before any other geological 
 theme, we may believe that the same question will 
 still afford the surest foothold for the interest of the 
 general reader. Following a brief account of that 
 subject, we must see how the rocks around us may 
 be made to disclose their history, watch them 
 crumbling under the beat of wind and rain, see their 
 remains buried under the sea and raised again to 
 form new lands. We shall see in the incessant, 
 though imperceptible, heavings of the earth's crust, 
 the power which reproduces the lands which the 
 elements destroy ; in the volcano and earthquake its 
 visible manifestation. In the concluding chapters we 
 will endeavour to trace something of the history of 
 our own country and of that strange succession of 
 extinct creatures which peopled the world in past ages. 
 The chief hope of the writer is that this brief 
 sketch may do something to add to the reader's 
 interests by enabling him to find greater meaning in 
 the scenes around him. The great merit of geology 
 is its power to attract its followers out into the 
 country, to find food for the mind as well as exercise 
 for the body among the sea-cliffs and on the 
 mountain-side. However delightful the works of 
 the great pioneers of the science may be to occupy 
 the leisure hours at home and they form no mean 
 contribution to the literature of the past century 
 it is in the field that they acquire their full meaning 
 and their fit surroundings. Probably no other 
 science is so well calculated to form a healthy and 
 manly hobby; none, certainly, is better fitted to 
 enlarge one's perception of the beauties of Nature, 
 to lead one to a truer realisation of the littleness of 
 human concerns. 
 
GEOLOGY 
 
 Chapters of Earth History 
 CHAPTER I 
 
 THE ORIGIN OF THE EARTH 
 
 OUR knowledge of the true place of the earth in 
 the universe has been acquired only after many 
 centuries of patient search. Even so far back as 
 the time of Hipparchus perhaps long before men 
 had realised that the earth was a spherical body. 
 The constantly circular horizon at sea, the circular 
 shadow of the earth whenever it was seen projected 
 on the moon during an eclipse, did not hide their 
 meaning from the ancients. Some fair approach to 
 accuracy was attained in the measurement of the 
 diameter of the earth nearly two thousand years 
 before it was destined to be circumnavigated. 
 Measurement of the apparent height of the sun 
 above the horizon in different parts of Egypt at the 
 same time provided the necessary data. For the 
 apparent elevation depends, of course, on the 
 latitude; hence if it is found, for example, that two 
 places (one directly north of the other) are 1 degree 
 apart, and the distance in miles be measured, mere 
 multiplication of the latter distance by 360 is required 
 to give the circumference of the earth, assuming it 
 to be really a sphere. Even the distance of the 
 moon was approximately known to the Greeks, and 
 
 5 
 
6 GEOLOGY 
 
 consequently something of the size of that body. 
 Hipparchus himself devised a method for estimating 
 the distance of the sun. The difficulty of the 
 necessary observations (on the length of the various 
 phases of the moon) defeated his aims, though he 
 was enabled to conclude that the distance could not 
 be under 3,000,000 miles, and the sun, therefore, a 
 body many times larger than the earth. Ages earlier, 
 the ancient star-watchers of the East had picked out 
 those five " stars " which are distinguished from all 
 the hosts of the sky by their wandering habits, 
 changing their places night after night among the 
 true or " fixed " stars. Observation even permitted 
 them to conclude that two of these "planets" (Greek 
 a wanderer), viz, Venus and Mercury, were bodies 
 which circled round the sun, and were therefore at 
 times nearer than that body, while the remaining 
 three, Mars, Jupiter and Saturn, were more distant; 
 and that furthest of all were the stars. But, having 
 gone so far, human prejudice barred the way to 
 further progress; no mind was yet free enough to 
 escape the conviction that the earth must be the 
 centre of the universe. Whatever advance had been 
 made towards a true estimate of the relative dimen- 
 sions of the earth and the space and bodies around 
 it, the essential belief still remained that this globe 
 was in the fullest sense the centre of all creation. 
 Hence arose those complicated schemes which 
 sought to explain the observed motions of the sun, 
 moon and planets on the assumption that they all 
 moved round the earth. It can scarcely be necessary 
 to record how this " Ptolemaic " system of astronomy 
 was implicitly believed for upwards of fifteen 
 hundred years, until Copernicus, in 1543, announced 
 his discovery that all the observed phenomena were 
 
THE ORIGIN OF THE EARTH 7 
 
 explained quite simply if it were assumed that the 
 earth itself is a planet revolving annually round the 
 sun. The story of the persecution of the followers 
 of Copernicus for this appalling "heresy" is only 
 too well known, but it is worth recalling in order to 
 emphasise the real magnitude of this step towards 
 the realisation of the earth's true place. The earth 
 was " degraded "to be a mere satellite of the sun, 
 and but one of six, at that. 
 
 Now this great barrier was passed, progress was 
 rapid. It was soon possible to draw a map of the 
 " Solar System," showing the paths of the various 
 planets round the sun, and their relative distances, 
 with great accuracy. Kepler's discovery of the laws 
 of planetary motion, and Newton's of gravitation 
 followed quickly. The invention of the telescope at 
 once led to the recognition of the fact that the other 
 planets were spherical bodies like the earth. But 
 still the scale of the system was only vaguely known. 
 The distance from us to any other planet, once 
 determined, would give all the information required 
 the other distances would be known at once. Yet 
 so great is the space separating us from our nearest 
 planetary neighbour that only within the last century 
 and a half have even approximate measurements 
 been attained, while not more than thirty years have 
 elapsed since measurements worthy to be called 
 accurate have been available. Even now there 
 remains a substantial margin of uncertainty. Thus, 
 while our mean distance from the sun is certainly 
 very near 92,900,000 miles, we cannot be sure it may 
 not be two or three hundred thousand miles more or 
 less. In other words, there may be a doubt of about 
 one half per cent on the whole distance. Happily 
 this small uncertainty is of no moment from our 
 
8 
 
 GEOLOGY 
 
 present point of view. We now know the scale of 
 the Solar System with ample accuracy to allow us 
 to view the earth in its true relations to the other 
 members. It will materially assist our subsequent 
 discussion if we here append a table giving the 
 relative sizes of the various planets and their distances 
 from the sun. 
 
 TABLE OF THE SOLAR SYSTEM.* 
 
 
 Mean distance 
 from Sun 
 Miles 
 
 Mass 
 Earth=i 
 
 Der 
 
 Water=i 
 
 sity 
 Earth i 
 
 SUN 
 
 
 329,390 
 
 1-40 
 
 0-25 
 
 MERCURY 
 
 36,000,000 
 
 0'055? 
 
 5'56? 
 
 i-oo? 
 
 VENUS .. 
 
 67,200,000 
 
 0'807 
 
 5-14 
 
 0-93 
 
 EARTH .. 
 
 92,900,000 
 
 rooo 
 
 5-56 
 
 i-oo 
 
 MARS .. 
 
 141,600,000 
 
 0-106 
 
 3-92 
 
 0-71 
 
 MINOR PLANETS 
 
 
 
 
 
 JUPITER.. 
 
 483,300,000 
 
 314-50 
 
 1'37 
 
 0-25 
 
 SATURN .. 
 
 886,200,000 
 
 94'07 
 
 0'64 
 
 0-12 
 
 URANUS.. 
 
 1,782,800,000 
 
 14-40 
 
 1-35 
 
 0-24 
 
 NEPTUNE 
 
 2,793,500,000 
 
 16-72 
 
 1-29 
 
 0-23 
 
 Two large planets now appear in the system which 
 were unknown to the ancients, being very remote 
 and consequently too faint to be seen without a 
 telescope. Uranus was added upwards of a century 
 ago (1781) by Herschell, Neptune in 1846 as one of 
 the greatest triumphs of mathematical astronomy, 
 its existence and position being predicted simultan- 
 eously and independently by Adams and Leverrier, 
 from a study of the disturbing effects of its attrac- 
 tion on the movements of Uranus. In addition, a 
 host of " minor " planets swarm between the orbits 
 of Mars and Jupiter. They already number near six 
 hundred, and year by year more fall into the traps 
 set for them. 
 
 * From Ba.tt,<Spherical Astronomy. 
 
THE ORIGIN OF THE EARTH 9 
 
 Looking over the preceding table, certain facts 
 stand out prominently. All the bodies in the system 
 even the sun itself are minute compared with the 
 distances between them. Should we construct a 
 model in which the sun was represented by a 2-foot 
 globe, the earth would figure as a pea about 70 yards 
 away. Neptune would be distant nearly a mile and 
 a quarter ! Again, the planets fall readily into two 
 groups as regards size; near the sun are the four 
 small planets, among which we have to number 
 the earth. The outer planets are far grander in 
 their proportions. In the same model which has a 
 pea to represent the earth, Jupiter would be a good- 
 sized orange. Yet all the planets rolled together 
 would be quite insignificant compared with the sun. 
 As to the six hundred minor planets, they could not 
 build up the earth between them. 
 
 We must now compare the earth in certain other 
 respects with its neighbours. Mars and Jupiter will 
 be selected, since they may be taken as typical of 
 the rest, and are much better known than the others. 
 Mars is a globe whose diameter is a little more than 
 half that of the earth. Viewed with a first-class 
 telescope at a time when it is nearest to the earth, it 
 presents an appearance not unlike that of the full 
 moon as seen with the naked eye or with an opera- 
 glass. The light and dark areas are, of course, 
 different in form ; they have perhaps more of a rude 
 resemblance to the form of the continents and oceans 
 on the earth. And land and water they have long 
 been supposed to represent, with the more plausibility, 
 in that the darker areas have a greenish tinge while 
 the brighter ones are distinctly reddish, giving to the 
 planet its well-known ruddy light, the light which it 
 reflects from the sun. It is not possible as yet either 
 
10 GEOLOGY 
 
 to confirm or deny this old interpretation of the 
 appearance, though recent observations, as well as 
 certain theoretical considerations, throw considerable 
 doubt on its correctness. It may be well to recollect 
 that in the case of the moon the dark areas were 
 always held to be oceans until our telescopes clearly 
 demonstrated that there is not a particle of water on 
 its. surface. On Mars, however, we observe some- 
 thing certainly not to be paralleled on the moon. 
 Over each of its poles there is a snow-white cap, 
 reminding us irresistibly of the snowy coverings 
 around the terrestrial poles. In this matter we are 
 able to make an observation which gives us greater 
 confidence. Mars has its seasons like our own, and 
 it is regularly observed that as each Martian pole in 
 turn approaches its summer, its white cap diminishes 
 in area, while the winter is marked by its increase. 
 Here we seem to have a clear parallel to the annual 
 retreat and advance of our own polar ice. That we 
 are really dealing with ice and snow seems to admit 
 of little doubt. It has been suggested that the ice 
 may not be frozen water, but some other substance. 
 There is, however, no evidence that such is the case, 
 and it is not probable. 
 
 The so-called " canals " of Mars are so familiar 
 that they cannot be entirely passed over. Let us 
 begin by saying that any marking on that planet less 
 than ten miles in width would almost cetainly be 
 invisible with the best telescopes under the finest 
 conditions. That some long, straight markings exist 
 on its surface is undoubted. Whether the multitude 
 of finer " canals " which make up the wonderful net- 
 work reported by some observers have any real 
 existence cannot yet be settled. If they have, they 
 enable us to draw at least one useful conclusion, for 
 
THE ORIGIN OF THE EARTH 11 
 
 some of them cross the " seas ." Should this obser- 
 vation be confirmed we shall have destroyed the 
 oceans of Mars, though the nature of the " canals" 
 will be as problematical as before. 
 
 The question of an atmosphere on Mars is likewise 
 involved in uncertainty. It seems fairly evident that 
 if the planet were supplied with air as abundant and 
 moist as that round the earth, it would make itself 
 more conspicuous than it does. On the other hand, 
 some observers have believed that they have observed 
 slight changes of appearance to be attributed to 
 masses of cloud. Certainly slight changes do occur 
 in the aspect of the planet from time to time, which 
 are not easily interpreted apart from an atmosphere 
 of some kind. If the conclusion favoured by the 
 balance of evidence is correct, that the atmosphere 
 is much more rarified than our own, then that is a 
 further reason for doubting the existence of true 
 seas on the planet, for the average temperature 
 would probably be so low that any water present 
 would normally exist as ice. 
 
 It is much to be regretted that we cannot at 
 present speak with more certainty of Mars, for it is 
 a body of unusual interest from our present point of 
 view. But let us mark the fact that so far as the 
 evidence goes, it points to a state of affairs inter- 
 mediate between those of the Earth and the Moon. 
 While its atmosphere and surface-water are probably 
 less abundant than on the Earth, they do not appear 
 to be entirely absent as in the case of the Moon. 
 Mars clearly shows a solid surface with well-marked 
 permanent features, by the observation of which it 
 may be seen to rotate on its axis in a period only 
 half an hour longer than our own day. 
 
 When we turn to Jupiter, the evidence is more 
 
12 GEOLOGY 
 
 satisfactory, in spite of the much greater distance of 
 that body. In every way there is the most marked 
 contrast with Mars ; in its gigantic size its 
 equatorial diameter being eleven times that of the 
 earth in its great polar flattening, but most of all 
 in its physical condition. The most moderate tele- 
 scope shows the familiar dusky belts of Jupiter, 
 parallel with his equator. With a good instrument 
 the picture is superb. The true cloud-like character 
 of the bands then becomes evident, with their beauti- 
 ful curling contours and intertwinings displayed as 
 in a most delicately toned engraving. And on Jupiter 
 the scene is always new. The broad outlines of the 
 bands may last for months or years, but the more 
 delicate cloud-tracery is in constant change. No 
 feature on the surface of the whole planet is constant. 
 We see merely a sea of clouds. 
 
 Now this constant and universal cloud-envelope of 
 Jupiter is more remarkable than at first appears. 
 True, our own skies are commonly enough filled with 
 cloud, and were our atmosphere more dense we 
 might never see the sun at all. But our clouds 
 depend for their existence entirely on our proximity 
 to the sun, whereby the surface of our planet is kept 
 at a temperature which causes the watery vapours 
 to rise. Jupiter is five times more remote from that 
 source of heat, and must receive a correspondingly 
 small supply. If, therefore, his condition resembled 
 that of the Earth or Mars, any water on his surface 
 ought to be eternally solid, and no cloud should ever 
 dim his skies. Yet we rarely, if ever, see a rift in 
 his cloudy shell. Again, all the winds and storms of 
 our own atmosphere result from its disturbance by 
 solar heat. Jupiter should be comparatively calm, and 
 still most violent storms occur. Only one conclusion 
 
THE ORIGIN OF THE EARTH 13 
 
 can be drawn ; the heat which is not supplied from 
 without must come from within : Jupiter must be a 
 hot planet. Certain other observations support this 
 conclusion. The markings on the belts enable us to 
 mark the rotation, and to make the noteworthy dis- 
 covery that all parts of the planet do not rotate with 
 the same period. The equatorial regions rotate in a 
 shorter time than those nearer the poles. Clearly 
 the atmosphere of Jupiter does not form a thin cover- 
 ing over a solid globe, or this would be impossible. 
 The matter below the visible surface must be gaseous 
 to a great depth, if not, indeed (as is not improb- 
 able) to the very centre. In further confirmation we 
 have the remarkable fact that the density (i.e., the 
 weight relatively to the size) of Jupiter is but one 
 quarter of that of the earth. He is but little heavier 
 than a similar-sized globe of water. The sun has 
 almost exactly the same density. Either this means 
 that Jupiter is made of extraordinarily light materials 
 (which there are the very strongest reasons for 
 doubting) or that the greater part, if not the whole, 
 of the great globe is in a gaseous condition, which is 
 almost tantamount to saying that it is at a high 
 temperature. 
 
 The question naturally arises, if Jupiter is highly 
 heated, whether he should not be self-luminous. 
 That would obviously be a matter of surface temper- 
 ature of the visible surface. Direct observation 
 can give no information as to the interior. Even 
 regarding the surface, no definite answer can be 
 given. Undoubtedly most of the light which reaches 
 us from that surface is reflected sunlight. When 
 one of the four larger satellites of Jupiter passes 
 between the planet and the sun it casts a shadow on 
 the surface, and that shadow looks intensely black. 
 
14 GEOLOGY 
 
 Yet even this by no means proves that the surface 
 is not luminous. The spots on the surface of the 
 sun itself look as intensely black, and yet are certainly 
 as bright as an electric arc. So little can the eye be 
 trusted. On the other hand some observers have 
 strenuously maintained that signs of inherent light 
 are visible on Jupiter. However that may be, the 
 evidence previously considered gives us much safer 
 ground for judgment on the physical condition of the 
 planet, and we may sum it up with little hesitation 
 by saying that, as Mars shows us a condition inter- 
 mediate between that of the earth and the moon, so 
 Jupiter divides its resemblances between the earth 
 and the sun. 
 
 Little more can be gained from a study of the 
 other planets. Venus and Mercury, as far as we 
 know them, are closely similar to the Earth and 
 Mars respectively essentially small, cold, heavy 
 earth-like planets. Saturn, Uranus, and Neptune 
 are light giant planets, probably like Jupiter, hot and 
 sun-like. 
 
 To complete this brief survey of the Solar System 
 we have to enquire what is known of the physical 
 condition of the sun itself. The vast dimensions of 
 that globe are difficult to realise, its diameter being 
 about 108 times that of the earth, or about 3i times 
 the distance which separates us from the moon. 
 Though we have seen it is relatively only one quarter 
 as heavy as our planet its actual weight is 330,000 
 times greater. The vastness of the heat and light it 
 radiates must be in some measure evident to all. 
 Though the problem of determining the real tempera- 
 ture of its surface is beset with very great difficulty, 
 it has been solved with sufficient approximation for 
 our purpose. The mere statement that it is probably 
 
THE ORIGIN OF THE EARTH 15 
 
 6000 or 8000 centigrade conveys little. It is double 
 the temperature of the electric arc. It is a tempera- 
 ture at which the most refractory substances we know 
 can exist only in a state of vapour. And that is the 
 temperature of the surface only ; the interior must 
 be hotter still. 
 
 The state of things inferred from the evidence of 
 temperature is confirmed by the telescope. The 
 appearance of the solar surface has been well likened 
 to a sheet of light grey cloth, almost hidden under a 
 layer of freshly fallen snow. The " snow-flakes " are 
 dazzlingly brilliant clouds, each some hundreds of 
 miles across. It is doubtful if these are true clouds, 
 consisting of liquid droplets ; if they are, the droplets 
 are of iron and the more refractory metals. Possibly 
 they are purely gaseous. Whatever their precise 
 nature, they are constantly under violent agitation. 
 Mighty cyclones sweep them about with perfectly 
 incredible velocities. From time to time they are 
 completely cleared away from some area or sucked 
 down towards the interior, when relatively cool 
 gases from above fill the depression, and, cutting off 
 much of the light from below, create a false impres- 
 sion that one is looking through a hole in the brilliant 
 surface to a dark interior a mistake to be carefully 
 avoided. These "sun-spots" are among the most 
 beautiful objects the telescope can show. When we 
 have the brilliant disc of the sun hidden behind the 
 moon during a total eclipse, we may witness another 
 striking manifestation of the endless turmoil on his 
 surface. Shot out in every direction are the so-called 
 red-flames, in reality vast jets of glowing hydrogen 
 and calcium vapour, tens or hundreds of thousands 
 of miles in height. Fortunately, with the aid of the 
 spectroscope, we are now able to cut out artificially 
 
16 GEOLOGY 
 
 the brilliant light of the disc, which ordinarily over- 
 powers the light from these " flames," and watch them 
 actually thrown out from the surface, rising at times 
 with the stupendous velocity of 300 miles a second. 
 The most violent terrestrial hurricane is perfect calm 
 by comparison. 
 
 In its rotation, the sun displays in a more marked 
 degree the peculiarity we observed on Jupiter: the 
 equatorial regions turn more quickly than the polar 
 ones. While a point on the equator is carried com- 
 pletely round in twenty-five days, the period of 
 revolution gradually increases to upwards of thirty 
 days near the poles, again testifying that we have to 
 do with a gaseous, not a solid globe. In the case of 
 the Sun, the known temperature leaves no doubt that 
 it is gaseous throughout. But perhaps the term is 
 misleading. We have described it as a relatively 
 light globe ; but it is still somewhat heavier, bulk for 
 bulk, than water. And that is merely its average 
 density. While its superficial layers must be much 
 lighter, its density must gradually increase towards 
 the centre, the materials at any depth being com- 
 pressed under the weight of all the overlying matter, 
 so that the central portions may be denser, and far 
 more rigid than steel. That the Sun as a whole is an 
 intensely rigid body is amply shown by the absence 
 of any perceptible polar flattening. Such conditions 
 are scarcely consistent with our ordinary notions of 
 a gas; but that is merely because we are only 
 familiar with gases under low pressures. This 
 general density and rigidity, nevertheless, does not 
 prevent a very evident and effective circulation of 
 the gases from the interior to the surface and back 
 an important fact to note, since it must keep the 
 materials well mixed, if we may so phrase it. 
 
THE ORIGIN OF THE EARTH 17 
 
 We can learn from the Sun what will never be 
 possible from the planets the nature of the materials 
 composing it. For the theory of that wonderful 
 weapon of modern science, the spectroscope, we 
 must reluctantly refer the reader to some work on 
 astronomy or physics. Suffice it to say that every 
 substance, when in a hot gaseous condition, and not 
 under great pressure, shines with its own peculiar 
 type of light : strikes, as it were, its own character- 
 istic notes on the rainbow scale of colour. Every 
 glowing solid, or highly compressed gas, on the con- 
 trary, gives out the entire range of colours, which 
 blend to give the impression of white light : and from 
 such light any ordinary gas will pick out and absorb 
 the colours which it would show if shining itself, 
 producing gaps, or dark lines, across the rainbow 
 band. Precisely this condkion we have in the sun ; 
 the light from the dense gases of his brilliant surface 
 contains all the spectral colours. But as it leaves 
 the surface it has to run the gauntlet through the 
 relatively cool gases of his atmosphere. Each gas 
 robs it of certain-coloured beams, so that when the 
 light, as it reaches us, is analysed by the spectro- 
 scope, many thousands of dark lines are found across 
 the rainbow-coloured band, representing the lost 
 colours. The study of these lines results in a dis- 
 covery of the most profound significance: of the 
 eighty or so elements which the chemist has recog- 
 nised as composing the multitude of substances he 
 has been able to analyse, about two-thirds have 
 thereby been found to be constituents of the Solar 
 atmosphere. Copper, Zinc, Iron, Calcium and 
 Magnesium are a few of the gases of this relatively 
 cool atmosphere of the Sun. No more striking testi- 
 mony could be found to the intense heat of that 
 
 B 
 
18 GEOLOGY 
 
 body. But of far greater importance is the indica- 
 tion that the sun, however different in other respects, 
 is composed of essentially the same materials as the 
 earth. To anyone familiar with the difficulties of 
 spectroscopic investigation, the wonder is not that 
 we should not yet have found all the known terres- 
 trial elements in the sun, but rather that we should 
 have detected so many. The violent circulation of 
 solar gases, already referred to, assures us that the 
 composition of the low-lying layers of atmosphere in 
 which the absorption of light chiefly occurs may be 
 taken as fairly representative of that of the sun as a 
 whole. And with that remark we must leave the 
 many other wonders of this fascinating globe to the 
 astronomer. 
 
 The various bodies of the Solar System are clearly 
 very fundamentally related. Though we can never 
 directly know the material of which the other planets 
 are made, the example of the sun confirms what 
 would be otherwise probable that it would prove to 
 be like that of the earth itself. And many likenesses 
 pervade the system besides those we have already 
 noted. All the planets revolve round the sun in the 
 same direction, and in much the same plane. They 
 all, together with the sun himself, rotate on their axes 
 again in the same direction. We cannot but believe 
 that the history of each must be intertwined with that 
 of the others. The differences among them are differ- 
 ences of physical condition. The Sun is intensely hot, 
 Jupiter still hot enough to be largely or wholly 
 gaseous, the Earth cold and solid externally but with 
 abundant evidence of internal heat, Mars certainly 
 cold on its surface, while the Moon shows a rugged 
 exterior, covered with the scars of volcanic fires long 
 since extinct. As far as we can judge, it is now cold 
 
THE ORIGIN OF THE EARTH 19 
 
 to the core. The present state of the various bodies 
 appears, therefore, to depend on size. 
 
 What relationship does the Solar System bear to 
 the rest of the universe ? So soon as the distance 
 separating us from the Sun became known, it opened 
 the way for an attack on the larger problem of the 
 distance of the stars, and we are now in a position to 
 state the remoteness of some of the nearer ones 
 with tolerable certainty. But miles, or even millions 
 of miles, are quite useless as units in which to express 
 these depths of space. The movement of light pro- 
 vides the readiest means of comparison. A light- 
 beam darts through space with a velocity of over 
 eleven million miles per minute. A little more than 
 eight minutes suffices for it to bridge the gap between 
 us and the sun ; about four hours and a quarter will 
 take it to Neptune. But it must rush on through 
 space month after month for 3\ years before it 
 reaches the nearest star, and it is likely to travel as 
 far again before it strikes another. Such is space. 
 The sun himself from such a depth must appear a 
 star of small importance in the heavens. 
 
 Clearly the stars themselves must be sun-like 
 bodies. Our telescopes, were they a thousand times 
 more powerful, could never enable us to see them 
 other than as mere points of light. Yet, knowing 
 something of their distances, it requires no elaborate 
 calculation to show that many a star must give out 
 far more light than the sun himself. And while the 
 telescope is rendered impotent by distance, the 
 spectroscope can still analyse the light of the star, 
 and learn something of its composition and physical 
 condition. The result is to add enormously to the 
 significance of what was learned regarding the sun. 
 Turn where we may throughout the universe, the 
 
20 GEOLOGY 
 
 same familiar substances make their presence known. 
 Some may fail to appear occasionally nearly all 
 but we look in vain for anything entirely novel. 
 To be more precise, while some stars, like Capella, 
 shine with a light which can scarcely be distinguished 
 from that of the sun, others, like Sirius or Vega, 
 have such vast atmospheres of hydrogen that the 
 effects of other substances are almost masked. 
 Others again seem to show the presence of certain 
 chemical compounds, which would betoken a lower 
 temperature, while we must add to the list those 
 stars which no longer shine at all, and whose exist- 
 ence we should not be aware of but for their proxim- 
 ity to other stars whose movements they disturb. 
 Some of these differences among the stars appear to 
 be attributable directly to variations in size, others 
 are certainly due to different temperatures. 
 
 Whether the stars, or any of them, have attendant 
 planets we can, of course, never directly ascertain. 
 Yet the study of their probable history renders it 
 practically certain that they have. Besides the stars, 
 the telescope reveals in the sky large numbers of 
 faint cloud-like bodies the nebulae. They appear 
 of every size from masses which, despite their un- 
 fathomable remoteness, cover areas of sky larger 
 than the moon (invisible to the naked eye because 
 too faint) down to mere hazy specks only visible with 
 the best telescopes. The smallest of them must be 
 larger than the whole Solar System. The larger 
 ones must occupy the most inconceivable regions of 
 space ; yet so flimsy are they that the faintest stars 
 are visible through all but their densest central por- 
 tions. The smaller are usually more compact. 
 They pass, indeed, insensibly into nebulous stars. 
 What, then, are these airy bodies, and what are their 
 
THE ORIGIN OF THE EARTH 21 
 
 relations to the stars ? Tested again by the quality 
 of their light, some of them show, like the stars, the 
 presence of many of our familiar elements in a 
 gaseous condition. Others, however, show nothing 
 but two or three substances which are gaseous at 
 ordinary temperature notably hydrogen. And 
 these nebulae send us no other light than that pro- 
 duced by the glowing of these few gases. But 
 further information may be gleaned from certain 
 bodies which partake in some degree of the nature 
 of nebulae and which we are able to examine at much 
 closer quarters,- namely, the comets which from time 
 to time appear. Some of these are regular members 
 of the Solar System, others visit us from remote 
 regions of space. They occasionally pass very close 
 to one of the planets, or even collide. Then there is 
 a disaster to the comet. The space within the 
 solar system is literally strewn with their wreckage, 
 and the Earth, as it dashes through, attracts the 
 scattered fragments to itself in millions. When a 
 fragment falls, the friction created by its headlong 
 rush through our atmosphere raises it to incande- 
 scence, or drives it off in vapour. We witness the 
 familiar spectacle of a shooting-star. Of all the 
 thousands of these which fall on the earth every day, 
 one rarely escapes complete dissolution in the air. 
 It does occasionally happen, however, and our 
 museums contain not a few of these justly treasured 
 meteorites the only samples of the outer worlds we 
 can ever take in our hands to examine. 
 
 The careful examination of many hundreds of 
 these meteorites has not resulted in the recognition 
 of a single substance with which we were not already 
 familiar. And there is every reason to believe that 
 a comet is nothing more or less than a swarm of 
 
22 GEOLOGY 
 
 myriads of these bodies the dust of space. The 
 comet glows because the meteorites clash together 
 as the swarm rushes on its course, some being there- 
 by made hot, others possibly vapourised, though the 
 vast majority are cold and black. What is true of 
 the comet, is in all probability true of the nebula. 
 There is every reason to believe that it is likewise 
 an immense scattered meteoric swarm. When it is 
 very widely scattered, collisions among the particles 
 are few and the general temperature consequently 
 low. Only a few gases therefore shine out. But it 
 is readily demonstrated on purely mathematical 
 principles that such a vast swarm will gradually con- 
 tract, in consequence of the mutual attraction of all 
 the particles. As it becomes more closely packed, 
 collisions will be more frequent, and the general 
 temperature will rise. The light given out will 
 gradually become more like that of a star until the 
 swarm becomes so compacted and *he temperature 
 so high that no substance can longer remain solid, 
 but the whole becomes a globe of dense intensely hot 
 gas. It has ceased to be a nebula and become a star. 
 Unable to contract further, its accumulated heat will 
 be gradually radiated away, the successive stages of 
 cooling giving rise to some of the types of stars 
 already referred to. Cooling will continue until it 
 rolls through space a dead black globe, unless, as is 
 very probable, some passing star comes so close as 
 to cause its disruption and a return to its primitive 
 meteoric condition, when the whole story must begin 
 afresh. 
 
 We have assumed our swarm to contract abso- 
 lutely uniformly to the centre. But that can rarely 
 happen. Unless it is uniformly distributed to begin 
 with, minor centres of condensation must be set up, 
 
THE ORIGIN OF THE EARTH 23 
 
 and if, as is usually the case, the whole nebula has a 
 rotatory motion, these minor masses will never join 
 the central sun, but will continue to revolve round it, 
 while each contracts and collects up the meteorites 
 in its neighbourhood as it gradually resolves itself 
 into a planet. Such, in its broad outlines, we believe 
 to have been the origin of the Earth. Thus in the 
 thousands of beautiful spiral nebulae in the heavens 
 we may watch the evolution of new Solar systems. 
 In the central condensations we see the future suns ; 
 in the knots on the spiral nebulous folds which sur- 
 round them, the embryos of their planets. And in 
 the stars and the greater planets we may witness 
 something of the stages of development through 
 which the Earth itself may have passed. Precisely 
 how far it has passed through them it will be the 
 business of the future geologists to discover. 
 
24 GEOLOGY 
 
 CHAPTER II 
 
 VOLCANOES AND EARTHQUAKES 
 
 IN the preceding chapter we have considered the 
 Earth in relation to the other worlds of space, and 
 gathered from them some suggestions as to its early 
 history. It now becomes our duty to examine a little 
 more closely the Earth itself, and to see what manner 
 of world it is on which we dwell. Our direct 
 acquaintance with it is, after all, very feeble. By 
 united effort we have explored most of its surface, 
 and here and there have crept a mile or so towards 
 the interior. As we shall find in the sequel, we may 
 even examine materials which have been perhaps 
 ten miles deep. But what of that? The skin of an 
 apple bears a larger proportion to the whole than a 
 ten-mile layer to the earth. Two questions we chiefly 
 want answered: Is the material which forms the 
 visible surface of the globe a fair sample of the 
 whole ? And what is the physical condition of the 
 great interior mass ? 
 
 Indirect evidence alone can furnish us the answer. 
 Something may be learned from the weight of the 
 globe, and something, too, from its behaviour in 
 relation to its fellow-worlds ; but, most directly, the 
 evidence of volcanoes and earthquakes, with their 
 allied phenomena, comes to our aid. 
 
 The problem of weighing the earth is a highly 
 interesting and, in theory, a very simple one. Every- 
 one knows that the weight of a body is merely the 
 
VOLCANOES AND EARTHQUAKES 25 
 
 force with which it is attracted to some other body 
 in ordinary cases, to the Earth. The strength of the 
 attraction in other words, the weight depends 
 on three things : the amount of matter in the body, 
 the amount in the earth, and the distance between 
 them (or, rather, between their centres). In ordin- 
 ary instances, it is only the first of these which is 
 varied. Hence, if we find that one body is attracted 
 with twice as much force as another (i.e., if it is twice 
 as heavy) we are justified in concluding that the 
 former contains twice as much matter as the latter. 
 But this would no longer be true if we varied the 
 other factors. For example, if we could transport 
 a pound of sugar to the surface of the Sun, we should 
 find that it there weighed nearly two stones but there 
 would be no more sugar. The increase of weight 
 would be due to the enormous mass of the Sun. If 
 these simple facts are comprehended, there is no 
 difficulty in understanding how the earth itself may 
 be weighed. Suppose we take a large ball of lead, 
 weighing 1 cwt, and place near it a small ball of 
 some kind. Here we have, in miniature, a model of 
 the earth and a body near its surface. The differ- 
 ence is one of scale alone. The small ball is attracted 
 towards the large one (more accurately, of course, 
 they attract each other). The attraction is all but 
 infinitesimal, yet with sufficiently delicate apparatus it 
 is measurable. We can measure, that is, the weight 
 of the small ball, due to the attraction of the large 
 one. But we also know its weight (its ordinary 
 weight) due to the attraction of the Earth. The 
 difference between these two weights is clearly due 
 to the difference between the amount of matter in 
 the lead ball, in the one case, and the Earth in the 
 other, allowing for the difference in their respective 
 
26 GEOLOGY 
 
 distances from the smaller ball, also. Hence, know- 
 ing the amount of matter in the lead ball, we can 
 calculate by proportion the amount in the earth, or, 
 in more popular language, the weight of the earth. 
 Various methods have been used for the solution of 
 this fascinating and important problem, but the 
 fundamental principle is the same in all the com- 
 parison of the forces with which some small body is 
 attracted by the earth, on the one hand, and some 
 other body whose weight is known on the other. 
 
 The statement of the weight of the earth in tons 
 would be, of course, utterly meaningless. What is 
 significant is its relative weight, which may best be 
 stated as almost exactly 5i times that of an equal- 
 sized globe of water, or more than two-thirds that of 
 one of solid iron. As a whole, therefore, it is a dis- 
 tinctly heavy globe. This knowledge enables us to 
 answer, in some degree, the first of the questions 
 proposed; for, as everyone knows, the ordinary 
 rocks accessible to us are comparatively light sub- 
 stances. They vary, naturally, but the great major- 
 ity of common rocks are relatively from 2j to 3 times 
 heavier than water; and we may say with perfect 
 confidence that the average density of the outer 
 crust of the earth is about 2| times that of water, or 
 almost exactly one half that of the entire globe. 
 Whatever the reason, it is clear that the lighter 
 materials preponderate on the surface, and the 
 heavier inside. For the present, we will be content 
 with noting the fact. 
 
 Volcanoes form by far the most impressive testi- 
 mony to a heated condition of the interior. But we 
 must note in the first place a much more general 
 indication of the same fact. In these days of deep 
 mining, everyone is familiar with the fact that the 
 
VOLCANOES AND EARTHQUAKES 27 
 
 rocks become hotter as we descend below the sur- 
 face, this being one of the most serious obstacles to 
 the further increase in the depth of our coal-mines. 
 Every deep boring or sinking which has been made, 
 in every part of the world, has told the same story 
 of a more or less uniform rise of temperature with 
 increase of depth. The rate of increase, or tem- 
 perature gradient, as it is called, varies considerably 
 in different places, so that even a fair average 
 cannot easily be obtained. The most generally 
 accepted figure is about 1 Farenheit for every 60 
 feet of descent. From our present standpoint the 
 exact figure is of no moment. Consider what it 
 means. The figure given is equal to a rise of 88 
 degrees to the mile 1 If this continues, everything 
 at a depth of a few miles must be red-hot; at 
 twenty miles, the temperature will be above the 
 melting point of the most refractory substances 
 known ! Here, indeed, is an astounding conclusion. 
 Can we consider ourselves justified in extending the 
 purport of our observations so far? 
 
 Corroborative evidence is forthcoming in the hot 
 springs found in every part of the world, and 
 especially in those remarkable springs the Geysers 
 from which super-heated water is belched forth ; 
 but most of all the volcanoes support the conclusion. 
 
 The crude notions once prevalent concerning 
 volcanoes have been largely cleared away, yet few 
 natural phenomena, perhaps, are more vaguely 
 apprehended. Some definite conception may prob- 
 ably best be attained by the briefest history of an 
 actual eruption. Few natural disasters in modern 
 times have compelled more sympathetic attention 
 than the devastation of Martinique in 1902. The 
 West Indies are encompassed by a great volcanic 
 
28 GEOLOGY 
 
 zone, but the island of Martinique showed no 
 symptom of volcanic activity for more than two 
 hundred years after its discovery. First in 1792, 
 violent earthquakes disturbed its unbroken peace, 
 and a slight eruption took place on Mt. Pele. Quiet 
 was soon regained, and lasted until 1851, during the 
 summer of which year a sulphurous odour was 
 observed to hang round the mountain. On the 5th 
 of August violent explosions were heard, and a 
 covering of dust was observed over the summit on 
 the following day. The whole disturbance resulted 
 from explosions of gas and steam. At intervals 
 until the end of the year these outbreaks were con- 
 tinued, after which quiescence reigned again for 
 nearly another half -century. Already in 1889, how- 
 ever, the portents of coming disaster began to 
 appear. Little clouds of steam began to curl up 
 from the mountain, and the sulphurous gases re- 
 appeared. In 1900 the emissions increased. Earth- 
 quakes became frequent. By March, 1902, the 
 odour of the gases had reached St. Pierre. By the 
 22nd of April the earthquakes were so severe as to 
 break the submarine cable to Guadeloupe. Two 
 days later the column of vapour and dust from the 
 explosions rose two thousand feet above the moun- 
 tain, and the next day began to fall in the sur- 
 rounding villages. Explosion followed explosion 
 every two or three minutes. A comparative lull on 
 the 26th was but the calm before the next, and in 
 one respect most frightful storm. Until the 8th of 
 May things grew steadily worse. Earthquakes 
 became most frightful and almost continuous. 
 Ashes began to fall over the whole island. The pall 
 over the mountain grew in size and denseness, 
 putting all the region below it in blackest darkness. 
 
ERUPTION OF MT. PELEE, MARTINIQUE, 1902. 
 
 Reproduced by the kind permission of Dr. Tempest Anderson. 
 
 Upper photograph slightly modified by retouching. 
 Plate II.] [Geology, 28 
 
VOLCANOES AND EARTHQUAKES 29 
 
 The streams burst their banks from the torrents 
 which fell. Lightning and thunder in the cloud 
 mingled with the roar of explosion and earthquake. 
 First the cable to Dominique, then that to St. Lucie, 
 broke under the strain of repeated shocks. On the 
 fatal 8th, the great cloud rose higher than ever 
 before, every violence was at its worst, when the 
 huge curling pillar rolled down to the sea, and in a 
 few minutes blotted out St. Pierre with its twenty- 
 eight thousand souls; fire, started by the red-hot 
 blocks of lava, destroying anything which was not 
 crushed under the weight of falling stone and dust. 
 Blocks of stone a hundred cubic yards in size fell 
 mixed with smaller stones and the finest dust. 
 From the human standpoint the eruption had done 
 its worst ; but it yet waxed more terrible. The 20th 
 and 26th of May and the 9th of June were each 
 marked by accessions of violence equal to that of 
 the 8th, while the climax was not reached until the 
 30th of August, when all previous efforts were sur- 
 passed by the production of a cloud several miles in 
 height, which buried half the island in dust and took 
 toll of another thousand lives. When it rolled away 
 the island looked like a country buried in snow. 
 
 In the preceding description, reference to molten 
 lava has been purposely omitted, because it plays 
 much too large a part in the popular idea of an 
 eruption. Lava streams were ejected, the first 
 being seen two days before the destruction of St. 
 Pierre, or about the twelfth day of the outbreak. 
 But in this, as in most eruptions of the more violent 
 type, the great mass of the ejection consisted of 
 solid material blown out and shattered by the 
 explosions, much of it reduced to the most impalp- 
 able powder. 
 
30 GEOLOGY 
 
 This eruption was remarkably similar to the 
 classical outburst of Vesuvius in the year 79, of 
 which we have so circumstantial an account in the 
 description given by the younger Pliny to Tacitus. 
 All the essential features were exactly repeated. 
 One sentence of the narrative deserves quotation. 
 Speaking of the scene presented after the three 
 days of perfect darkness we are told : " Every 
 object which presented itself to our eyes which 
 were extremely weakened seemed changed, being 
 covered over with white ashes, as with a deep 
 snow." Everyone knows that the same story was 
 repeated again in the recent great eruption of 
 1906. 
 
 One more great outburst may be referred to by 
 way of giving a more complete idea of the magni- 
 tude which volcanic forces may attain. The small 
 island of Krakatoa, in the strait between Java and 
 Sumatra, showed no sign of volcanic nature undl 
 1883. In May of that year a comparatively slight 
 eruption took place, but only in the following 
 August was there any indication that a violent 
 disturbance was about to occur. Then in two days 
 (the 26th and 27th), two-thirds of the whole island 
 was blown away, the original island being six miles 
 by three. New islands were formed in the neigh- 
 bourhood, and the map of the region rendered 
 completely useless. The cloud of steam and ashes 
 rose to a height of 17 miles, and darkness extended 
 for 150 miles from the volcano. The fine ash was 
 subsequently carried by the winds to every part of 
 the earth. So fine was some of the powder that it 
 was estimated that about twenty-thousand particles 
 would go to a single grain. Though we may be 
 thankful indeed that the event occurred in a sparsely 
 
VOLCANOES AND EARTHQUAKES 31 
 
 populated portion of the globe, no less than 36,380 
 lives were lost. The sound of the explosions was 
 heard so far away as Rodriguez, distant 2,500 miles, 
 while the great sea-waves caused by the accom- 
 panying earthquakes devastated many hundreds 
 of square miles of country, and in one case carried 
 a man-of-war three miles inland, and left it stranded 
 thirty feet above the sea. The foregoing facts are 
 
 FIG. 1. IDEAL SECTION OF A VOLCANO. 
 
 Lava black; ash dotted. The original cone has been partially 
 
 destroyed, and a new one constructed within. On the right, 
 
 a small "parasitic" cone has been formed. Dykes of lava are 
 
 seen penetrating the various rocks. 
 
 indeed a mighty tribute to the forces within the 
 earth. And they are no mere fancy of over-excited 
 narrators, but the result of evidence collected by a 
 special commission appointed by our Royal Society 
 to investigate the circumstances of the eruption. 
 Very different is the story of such a volcano as 
 Stromboli, whose constancy has well deserved its 
 popular appellation the "lighthouse of the Medi- 
 terranean." Year in and year out the puffs of steam 
 lazily curl up from the crater, the molten lava gently 
 
32 GEOLOGY 
 
 heaves and falls within, carrying with it the slaggy 
 floor, and rarely rising high enough to cause a little 
 to well over the side. Violence is almost unknown. 
 
 It needs little reflection to see the meaning of this 
 difference of behaviour. In the case of Stromboli, 
 and others like it, the volcanic forces the steam 
 and other gases are continually escaping, and the 
 energy is thereby slowly dissipated. But only seal 
 up the volcanic pipe, let the lavas solidify within, 
 make your volcano sleep, and then, under this 
 outward aspect of repose, the forces must gather 
 within till they burst all their bonds, and a great and 
 disastrous eruption is the result. 
 
 Equally evident should be the reason why a 
 volcano has commonly the aspect of a mountain of 
 more or less comical form. Any misconception of 
 this point may be removed by the history of a small 
 hill near the Bay of Baia?, north of Vesuvius. It 
 bears the significant name of Monte Nuovo (New 
 Hill) because its existence dates only from 1538. 
 Of the rise of this hill we possess the account of an 
 eye-witness. Before the year mentioned the site 
 was a low-lying tract of ground. Volcanic activity, 
 as usual, was announced by earthquakes for two 
 years before the event. On the 29th of September, 
 1538, twenty shocks occurred in the one day. The 
 following night a slight sinking of the ground took 
 place, followed by gushing out of water, first cold, 
 then hot. Later, flames were seen, and then the 
 ground was torn open, and an eruption of ashes 
 began, which in two days built up the present hill, 
 440 feet in height. By the end of a week the 
 eruption ceased, and it has not since been renewed. 
 Here, in miniature, we have the history of all 
 volcanoes, their cones being built up of the ashes 
 
VOLCANOES AND EARTHQUAKES 33 
 
 and lava shot out from the vent, whether they be 
 small hills like Monte Nuovo, or vast mountains like 
 Etna. Yet the same force which builds up the cone 
 not infrequently leads to its destruction. Many 
 volcanoes are mere wrecks of their former selves. 
 The old Vesuvius Monte Somma was largely 
 blown away by the historic eruption of A.D. 79, the 
 present cone being a new one, while in such a 
 volcano as the well-known Kilauea we have a mere 
 low wall left round a great lake of lava, practically 
 the entire cone having been destroyed. 
 
 The realisation of the true nature of the cone 
 prepares one for the appeciation of a fact, the 
 significance of which may otherwise be lost the 
 circumstance, namely, that volcanoes are usually to 
 be found along the great mountain ranges of the 
 world, and like them, therefore, chiefly by the 
 borders of the great oceans. This association of 
 volcanoes with mountains is readily understood, for, 
 as we shall see in the sequel, the mountain chains 
 are the great lines of weakness of the earth's crust, 
 along which all the pent-up energies of the interior 
 exhibit their most profound effects. But why they 
 should be found bordering the oceans is a problem 
 much more profound and much less readily solved. 
 As regards the volcanoes, at any rate, the associa- 
 tion has given rise to a hypothesis, which, true or 
 not, serves to emphasise one of the most noteworthy 
 of volcanic phenomena. No fact stands out more 
 prominently in the accounts which have been given 
 above than the important role played by steam in 
 every eruption. It is the motive power of the 
 entire action. The explosions which hurl solid rock 
 and molten lava alike into the air are almost 
 entirely explosions of steam. The earthquakes 
 
 c 
 
34 GEOLOGY 
 
 themselves, which are so inseparable from all 
 violent eruptions, are not improbably due in large 
 measure to more deep-seated explosions of the 
 same character. The actual amount of water given 
 off in the form of vapour from a great volcano, even 
 in a brief space of time, baffles imagination. It is 
 not without reason, therefore, that the view has 
 long been put forward that the whole existence of 
 volcanoes is due to water which has percolated 
 down from the oceans through fissures to the heated 
 rock below, the fluidity of which has been increased 
 by its solvent powers, till the mobile mass has 
 yielded under the pressure of the overlying rock 
 and sought its way in turn through some fissure to 
 the surface. The one point in dispute is the 
 ultimate source of the water. But whether it is 
 derived, as described above, from the oceans, or, as 
 others suppose, it has been imprisoned in the rocks 
 since the earth became a planet, its vital importance 
 is undoubted. 
 
 Nevertheless, the water is, of course, for the most 
 part, merely the agent through which the real cause 
 of the whole phenomenon is made manifest. The 
 internal heat of the earth is the real source of 
 energy which gives the water its explosive power, 
 which shatters the rocks, and hurls millions of tons 
 of them miles through the air. Looked at in this 
 light, the vast clouds of steam and solid fragments, 
 the explosions and the earthquakes, are no less 
 eloquent of the vast stores of internal heat than the 
 molten rocks themselves. And though we have 
 referred to certain limitations in the distribution of 
 existing volcanoes, they are still very widely spread 
 over the face of the globe. The entire west coast 
 of the Americas is one long line of them from Alaska 
 
VOLCANOES AND EARTHQUAKES 35 
 
 to Cape Horn. Across the Pacific, another line 
 stretches from the north-east of Asia through 
 Japan, the East Indies, and New Zealand, on to the 
 South Polar Continent. And a third great belt 
 runs across the whole of southern Europe and Asia, 
 practically joining the other two, to omit all refer- 
 ence to scattered volcanoes. These are merely the 
 volcanoes of the present day ; were we to include all 
 those of past geological ages, scarcely a spot on the 
 globe would be free. Little wonder, then, that the 
 early geologists imagined that the whole earth, 
 except quite a few miles down from the surface, 
 must be one molten mass. To them, the " crust " of 
 the earth was a perfectly literal crust. 
 
 What are we to say of these ideas to-day ? The 
 evidence of internal heat is not one whit less 
 convincing than formerly. Indeed, the more ad- 
 vanced state of physical science probably justifies 
 us in placing more reliance on the general indica- 
 tions than was proper for the older geologists. 
 There seems to be no escape from the conclusion 
 indicated at the outset, that at a depth of twenty or 
 thirty miles the temperature must be above the 
 melting point not merely of the ordinary rocks but 
 of every substance with which we are acquainted. 
 Below the volcanoes, as we shall see later, reser- 
 voirs of molten rock exist at much smaller depths. 
 Is the interior, then, liquid? Perhaps we ought 
 rather to ask, is it gaseous ? For most substances 
 remain in the liquid condition through a compara- 
 tively small range of temperature; after they are 
 liquified, they are soon vapourised. 
 
 Until recently, we have been under the necessity 
 of appealing again to the astronomer for evidence 
 on this question. In principle, the method by which 
 
36 GEOLOGY 
 
 Sir George Darwin has attacked the problem is 
 readily understood. Everyone knows that the tides 
 are caused by the attraction of the Moon. Owing 
 to that body being comparatively close to us (its 
 average distance is rather less than 60 times the 
 earth's own diameter) the attraction is appreciably 
 greater on that side of the earth which is, for the 
 moment, nearer to the moon, than on the remoter 
 side. As a result of this unequal attraction, the 
 moon constantly tends to pull the earth out of 
 shape; and in the case of the oceans it succeeds, 
 tides being the result. The solid globe does not 
 appreciably yield, and by mathematical analysis it is 
 possible to calculate what its rigidity as a whole 
 must be to withstand the enormous strain which the 
 moon puts upon it. Darwin concludes that it must 
 be more rigid than a sphere of steel. Everyone, at 
 least, will readily understand that if the interior 
 were liquid in the sense once supposed, tides like 
 those of the ocean would be created in this fluid 
 mass, which the feeble solid crust would be utterly 
 unable to withstand. Without paying attention to 
 the other astronomical methods of approaching the 
 problem, let us turn to a more strictly geological 
 field which offers great promise. 
 
 Earthquakes, after the recent disasters of San 
 Francisco and Messina, are only too familiar in 
 their general manifestations. The heaving of the 
 ground, which in the most severe shocks may even 
 cause the earth to gape open with the strain, the 
 overthrowing of pillars, chimneys, and even houses, 
 are well-known phenomena. After a great earth- 
 quake, various permanent changes are usually 
 observable in the district affected. Streets, railway 
 lines, and similar features which were previously 
 
VOLCANOES AND EARTHQUAKES 37 
 
 straight have become more or less crooked. Fields 
 which were rectangular have their sides no longer 
 square. The ground in some areas may be per- 
 manently raised, and in others depressed. In some 
 instances, as in the great earthquake which affected 
 the Mino-Owari districts of Japan in 1891, a great 
 line of fracture may be developed for miles across 
 the country, along which it may be raised on one 
 side or depressed on the other to the extent of 
 even twenty or thirty feet. Lateral displacement 
 of the ground of still greater magnitude occurred 
 along the line of fracture in the San Francisco 
 earthquake. These displacements are, indeed, the 
 very essence of the earthquakes. There is little 
 doubt that the majority of shocks are due to just 
 such sudden slippings of the earth's crust along 
 some line of fracture. We shall see later that these 
 lines of fracture and movement are among the 
 commonest of geological phenomena in every part 
 of the world ; and not one can have arisen without 
 giving rise to an earthquake. Most, indeed, can 
 only have resulted from a long succession of slips, 
 each giving rise to a more or less severe shock. 
 The difficult question of the ultimate cause of these 
 sudden snappings and slidings must be postponed 
 awhile. Meantime, let us fix our attention on the 
 nature of the shock itself. Could we free ourselves 
 from the ground during an earthquake and atten- 
 tively watch any point on it, we should see it moving 
 up and down, backwards and forwards, side to side, 
 in a most complicated dance. But it is just the 
 misfortune that we cannot free ourselves which 
 makes reliable observation difficult, and greatly 
 exaggerated statements the general rule. However, 
 it needs no refinement of observation to ascertain 
 
38 GEOLOGY 
 
 the essential fact that the earthquake proper is a 
 vibration of the ground. As an iron bar struck on 
 one end with a hammer is set into vibration which 
 travels in rapid waves along the bar, so the earth, 
 shaken by a sudden snap, vibrates in like manner, 
 and the vibrations spread out in waves in every 
 direction from the place of the shock. The waves 
 travel rapidly. A minute suffices for them to cover 
 several hundred miles ; but as they spread out they 
 are correspondingly weakened, so that they soon 
 cease to affect our senses. The study of these 
 waves is the essential feature of the science of 
 Seismology, a science yet in its earliest infancy, but 
 one which is already shedding light on the state of 
 the interior of our globe, and which seems destined 
 in the near future to place our knowledge of that 
 subject on a surer footing than it has ever been 
 before. The first requisite of the study is an 
 accurate record of the movements which any point 
 of the ground undergoes during a shock. Clearly 
 this can only be obtained by referring the move- 
 ments to some body which is at rest, and here is the 
 problem: how to obtain a " steady point"? No 
 body, of course, can be completely detached from 
 the ground ; yet we may have it detached in certain 
 respects. Take an ordinary pendulum for example. 
 If the support to which it is attached is moved up 
 and down, the whole pendulum must be carried with 
 it ; but if the support is moved sideways, the weight 
 of the pendulum will tend to remain at rest where 
 it was. If now the support remains in its new 
 position the pendulum will begin to swing. If, on 
 the other hand, it moves quickly back to its old 
 place, little disturbance of the weight may occur. 
 Unfortunately, no matter how quickly the move- 
 
VOLCANOES AND EARTHQUAKES 39 
 
 ments are performed, some swinging will occur; 
 and, moreover, the actual movements in an earth- 
 quake are comparatively slow. By a variety of 
 mechanical contrivances, however, this tendency to 
 swing may be more or less completely counter- 
 balanced, and so a modified pendulum may be used 
 to record the horizontal movements of the ground 
 during an earthquake, a pointer attached to the 
 pendulum writing on a recording roll of paper which 
 moves with the ground. A similar pendulum may 
 be constructed which swings vertically instead of 
 horizontally, and used to obtain a tracing of the 
 vertical movements. 
 
 The first fact we learn from such instruments 
 seismographas, as they are called is that the actual 
 movements in an ordinary earthquake are small. 
 Any shock in which the ground actually rises and 
 falls more than an inch is a severe one. In the 
 great majority, the movement is much smaller. 
 We are, further, able to confirm another important 
 and significant observation. An earthquake, in the 
 region where the shock originates, is not heralded 
 by any previous trembling of the ground. The full 
 force of the shock falls at once, without warning, 
 Now this is not the case when a shock is felt at a 
 considerable distance say one or two thousand 
 miles from the point of its origin. In such a case 
 the instrument shows a gentle trembling for some 
 minutes before, quite suddenly, the main earth- 
 waves arrive. At such a distance, of course, even 
 the principal shock is scarcely, if at all, perceptible 
 without the instrument. Now what is the meaning 
 of these " preliminary tremors " ? Clearly they are 
 vibrations which in some way have raced the main 
 waves. Those latter waves, we know, travel over 
 
40 GEOLOGY 
 
 the surface of the earth, much as common ocean 
 waves travel over the sea. Can it be that the 
 preliminary tremors are waves which have taken a 
 short cut through the globe? Two observations 
 suffice to settle this point. Firstly, the further we 
 go from the centre of disturbance, the longer is the 
 interval Jby which the main shock lags behind the 
 preliminary tremors ; and clearly, the further we go, 
 the greater would be the advantage gained by waves 
 cutting through the earth. Secondly, when we time 
 the waves accurately, we find that the intervals 
 taken by the main waves to pass from place to place 
 are proportional to the distances measured over the 
 surface, while the time taken by the preliminary 
 tremors to reach any point is nearly proportional to 
 its distance from the point of origin measured 
 through the earth. 
 
 These preliminary tremors now possess a most 
 profound interest. They reach us after having 
 passed through the innermost recesses of the globe, 
 and we may be sure that, can we but interpret it 
 aright, they bring with them a record of their 
 adventures within. One, and that the most im- 
 portant, point is readily gathered. The speed with 
 which a wave travels through a body depends 
 mainly on the rigidity of that body : the more rigid, 
 the greater the speed. Clearly then, if the interior 
 of the globe be liquid, or diminished in rigidity by 
 the intense heat towards the centre, those waves 
 which have traversed the deeper regions will be 
 retarded. What do we find ? That the preliminary 
 tremors which have passed most deeply through the 
 globe, to emerge near the antipodes of the point 
 from which they started, have travelled a little 
 faster than those whose paths have not been so 
 
VOLCANOES AND EARTHQUAKES 41 
 
 deep. Here, then, we have a striking confirmation 
 of what Darwin has told us from his studies of the 
 tides. The interior of the globe is at least as rigid 
 as the crust, and possibly more so. 
 
 What are we to make of the seeming contradic- 
 tion? On the one hand we have the clearest 
 evidence of high temperatures at quite small depths, 
 and the strong probability of truly sun-like tempera- 
 tures towards the centre. On the other we have 
 unimpeachable testimony to the fact that the globe 
 is a most rigid body to the core. If you consult a 
 text-book of geology for the answer, you are likely 
 to find the statement that the interior is extremely 
 hot, but is kept in the solid condition by the 
 enormous pressure of the overlying portions. It is 
 a familiar fact that when a body is kept under great 
 pressure its melting point and boiling point are 
 usually raised. Unfortunately the physicist is likely 
 to point out that there is a limit to this process, that 
 there is a certain definite temperature for each 
 substance its " critical temperature " above which 
 it will pass into the gaseous state no matter how 
 great the pressure to which it is subjected; and he 
 may add that there is good reason to believe that 
 the temperature within the earth may be above 
 the critical points of most of the substances 
 composing it. 
 
 Yet, in face of what we have seen, can we 
 believe the interior to be gaseous ? We must leave 
 the responsibility for an answer with the physicist. 
 Of this we may feel certain, that before the heat 
 may compel the rocks to pass into a gaseous con- 
 dition, the pressure will be so enormous that a gas 
 under it will have no resemblance to anything of 
 which we have experimental knowledge. The truth 
 
42 GEOLOGY 
 
 is that we have to do with a state of things al- 
 together foreign to our experience, and we cannot 
 postulate what the properties of matter may be 
 under such conditions. Perchance the tremor of 
 the earthquake may yet enable us to learn. 
 
 CHAPTER III 
 
 THE SOLID ROCKS 
 
 IT is now time to interrogate the rocks around us. 
 Some of them readily yield their story; others 
 divulge their secrets only after patient investigation. 
 On the one hand, let us consider the rocks which 
 are so admirably displayed along the coast from 
 Whitby to Flamborough. The most casual glance 
 at those magnificent cliffs suffices to show that the 
 rocks are arranged in layers, sheet laid over sheet, 
 through thousands of feet of rock; now a layer of 
 sandstone, now of shale, now of clay, and now of 
 limestone. They are stratified rocks (compare 
 Fig. 2). We take a piece of the sandstone and 
 scrutinise it more closely. The grains of which 
 it is made are not to be distinguished from the 
 sand-grains on the beach; each is rounded and 
 smoothed, as those are being made in the dash of 
 the waves. We crush the stone, and sand it 
 becomes. Or we pick a fragment of shale or clay. 
 While dry it is hard and compact ; but only place it 
 in water and it crumbles to mud. We take another 
 block, break it open, and find within the beautiful 
 form of a coiled shell with all its delicate ornament ; 
 but not the form merely it is a shell, too real to 
 
THE SOLID ROCKS 
 
 43 
 
44 GEOLOGY 
 
 deceive (see Plate IV.). Fully aroused now, we 
 find the rocks on every hand filled with these fossil 
 remains of vanished life ; shells of every form and 
 size, corals, sea-urchins, fish scales, and even the 
 bones of some animal. The next layer of rock may 
 yield us the most beautiful fern fronds, or fragments 
 of wood; at least the form of them, for the sub- 
 stance is changed. Or we chance on a spot at the 
 foot of the cliff where the storm has dislodged a 
 great slab of sandstone, and see the surface of the 
 bed below marked with ripples as perfect as those 
 left yonder on the sands by the retreating tide. 
 
 What does it all mean ? If the facts around you 
 do not proclaim the answer loud enough, move on 
 to the H umber, and there watch the sandbanks and 
 mudbanks growing, layer upon layer, out of the 
 burden brought down by the rivers, burying the 
 cast-off shells left upon them, and only awaiting 
 consolidation to become the new rocks of future 
 ages. Thus readily the stratified rocks tell their 
 story, and show that where is now dry land the 
 ocean rolled, and buried on its sandy bed the 
 remains of creatures whose very kind has long since 
 passed away. They are built up out of the wreck- 
 age of pre-existing lands for sand and mud result 
 only from the disintegration of the rocks. What 
 then, are the primitive rocks, if, indeed, any may 
 still exist which have not been through nature's 
 grinding mill ? 
 
 Let us turn to one of the granite hills of the 
 Scottish highlands. Here is no trace of stratifica- 
 tion, no alternation of bed with bed, no sign of 
 fossils. The whole mountain is one uniform mass 
 of granite, whose even texture is reflected in the 
 smooth, monotonous grandeur of the hill. We 
 
THE SOLID ROCKS 45 
 
 come closer and take a sample of the rock. No 
 rounded grains betoken the rolling of the sea. 
 Instead we catch the glitter of crystals whose 
 polished faces have never yet been dimmed. Still 
 closer scrutiny reveals the fact that the whole rock 
 is crystalline. Here a bright metallic-looking flake 
 of olive-brown mica is embedded in a fleshy-pink 
 crystal of felspar. There an irregular space is filled 
 with clear glassy quartz. Do you marvel at the 
 
 A. FIG. 3. 
 
 A Thin slice of Granite from Dalbeattie, Scotland, magnified 
 12 diameters. The clear crystals are quartz, the cloudy 
 ones felspar, those with numerous parallel cracks mica. 
 
 B Thin slice of Rhyolite from Elfdalen, Sweden, magnified 
 12 diameters. The glassy ground-mass shows "flow- 
 structure," and contains multitudes of minute 
 " crystallites " which cause the cloudy appearance. The 
 large crystals are much broken and corroded. 
 
 wonderful accuracy and intricacy with which the 
 crystal grains are fitted together and interlocked? 
 You may search the rock with lens and microscope ; 
 not the smallest space is left unfilled. The truth 
 proclaims itself irresistibly that the minerals have 
 grown together (see Fig. SA). How can such a struc- 
 ture have been attained? We can conceive of only 
 
46 GEOLOGY 
 
 two possible answers. Either the minerals must have 
 crystallized out from some solution, or the whole 
 rock may have been in a molten condition and have 
 crystallized as it slowly cooled and solidified. 
 
 It is indeed not easy to imagine how such minerals 
 as felspar, quartz, and mica could be dissolved in 
 quantities so vast. Water, we know, has wonderful 
 powers of solution. Even such minerals as these 
 are dissolved, though in excessively minute propor- 
 tions ; and in the early days of geology her followers 
 were sometimes less disposed to bridle the imagina- 
 tion with the sordid considerations of mere physical 
 facts than this matter-of-fact age demands. More- 
 over, there was ample justification for the belief 
 that the primitive oceans of the young earth might 
 have been highly heated, and thereby greatly 
 enhanced in solvent power. So arose, during the 
 latter half of the eighteenth century, the hypothesis 
 that rocks of the granitic class represented the 
 earliest deposits from the primaeval oceans, a view 
 taught by Werner, of Freyberg, and spread by his 
 pupils over the length and breadth of Europe. 
 When the waters cooled, they were no longer able 
 to hold these minerals in solution, and their forma- 
 tion ceased, giving place to the deposits of mechanical 
 sediment which form the rocks of later ages. But 
 at the same period minds were not wanting more 
 willing to abide by ascertained physical facts, and to 
 observe more closely the phenomena of the rocks. 
 Our countryman, Hutton, steadily opposed the 
 Wernerian teaching, as being supported neither by 
 chemical and physical considerations, nor by the 
 facts observable in the field. A controversy was 
 carried on for many years with more than academic 
 ardour between the " Neptunists," as the followers 
 
THE SOLID ROCKS 47 
 
 of Werner were dubbed, and the " Plutonists " who, 
 with Hutton, believed the granitic rocks to have 
 resulted from the slow cooling of molten material in 
 deep reservoirs below the surface in the regions 
 which mythology had consecrated to Pluto. Hutton, 
 with keen insight, perceived a crucial test. If 
 Werner were right, then the stratified rocks must 
 lie evenly upon those of the granitic class; no 
 confusion could occur at the line of junction. On 
 the contrary, the Plutonist might expect to find his 
 molten granite injected into all the cracks and 
 crevices of the overlying stratified rocks. It was a 
 momentous journey when Hutton set out to observe 
 the edge of the granite mass in Glen Tilt, and we 
 may well credit the story that his exultation was so 
 great on seeing the veins and strings of granite 
 penetrating into the adjacent rocks that his guides 
 believed he had discovered gold. For him, the 
 great controversy was ended, and he had won. 
 
 The reader may be disposed to think that the 
 controversy was unnecessary to say that volcanoes 
 clearly show the existence of reservoirs of molten 
 rock below the surface, and thereby settle the 
 question at once. But not so; far from it. The 
 lavas were one of the surest strongholds of the 
 Neptunists. And if the reader will take a piece of 
 lava from Vesuvius in one hand, and a fragment of 
 granite in the other, the force of the argument will 
 appear. Indeed, could any two rocks be more 
 unlike? On a casual inspection, assuredly not. 
 The latter is a pale grey or pink glittering crystalline 
 rock, the former a nearly black dull stony mass. 
 True, a closer inspection of most lavas shows a 
 good sprinkling of well-developed crystals scattered 
 through them, but, nevertheless, the ground-mass of 
 
48 GEOLOGY 
 
 the rock remains, to the eye, compact and stony. 
 Hutton's penetrating sagacity enabled him to foresee 
 the explanation of this striking difference, but much 
 patient investigation was needed before his views 
 could be clearly demonstrated. 
 
 In order to attack the difficulties one at a time, 
 let us reject the Vesuvian lava for the moment, and 
 take a lighter coloured specimen from one of the 
 volcanoes of the Lipari Islands. Here again we see 
 the stony-ground mass with embedded crystals, 
 among which we recognise colourless quartz and 
 felspar and blackish glistening micas. To make 
 further progress we must appeal to the microscope, 
 and examine with it a thin transparent slice of the 
 rock. The picture presented as we look down the 
 tube reminds us strongly of the surface of some 
 sluggish stream covered with a light brown scum 
 which winds in curling eddies round floating logs 
 and weeds (see Fig. SB). The " logs " are the 
 crystals we have already seen, the "scum" is the 
 ground-mass of the rock. But the eddies are no 
 make-believe ; they are frozen solid, but they carry 
 us back irresistibly to the day when, in a fiery 
 stream, the rock flowed down the mountain-side. 
 To disclose the secret of the ground-mass we must 
 use a high magnifying power. We now find that 
 this owes its turbid appearance to a host of minute 
 crystals, some just recognisable as quartz or felspar, 
 others mere rudiments of crystals distinguishable 
 only as minute rods or beads ; and all are embedded 
 in a structureless, glassy matrix, which is, indeed, a 
 natural glass. 
 
 Experience will soon teach us that different lavas 
 vary considerably in character. The large crystals 
 are an almost constant feature ; but in the stony 
 
THE SOLID ROCKS 49 
 
 groundwork we discover the most wonderful variety. 
 In some cases we find nothing but pure glass, or 
 glass containing only swarms of the little rod-like 
 and bead-like bodies; in others, the whole is 
 minutely crystalline. Nay, the same lava is not 
 uncommonly glassy on the edge of the stream and 
 crystalline inside. And here we have a little 
 suggestion which may guide our enquiry. Everyone 
 knows that if we aim at obtaining good crystals 
 from a solution, the more slowly we carry out the 
 process the better. Can it be that the slowness or 
 rapidity of the cooling of a lava has something to 
 do with its becoming crystalline or glassy? The 
 observation just noted suggests that it may. We 
 may experiment by completely melting up a lava 
 and cooling it artificially. The result completely 
 confirms our suspicion. Only by the slowest possible 
 cooling can we get any crystallization to take place, 
 and, at best, we can get only minute crystals. We 
 have solved the problem of the varying degree of 
 crystallisation among volcanic rocks; but a moment's 
 reflection will show that we have raised another. 
 How comes it that the large crystals occur so 
 profusely among them ? We can get nothing in the 
 remotest degree comparable to them experimentally, 
 though we may reproduce the structure of the 
 ground-mass of the rock. Is it that the great mass 
 of lava on the mountain side cools so slowly that 
 we cannot imitate the conditions ? If so, why does 
 not the whole rock consist of large crystals ? But 
 the suggestion is altogether crushed if we examine 
 one of the fragments of lava which has been blown 
 out by an explosion and sent in a molten state 
 hurtling through the air one of the so-called 
 volcanic bombs. This must have cooled and 
 
50 GEOLOGY 
 
 solidified in a few minutes merely; yet the big 
 crystals appear in as much force as ever. It is 
 clearly out of the question that they could have been 
 developed in so short a space of time, and the 
 conclusion is inevitable: they were there already 
 when the stony matrix in which they are embedded 
 was still in a molten condition, while the lava was 
 still heaving in the crater. The mind is now carried 
 back to the deep recesses below the volcano where 
 for untold ages the molten lava has been hidden 
 from view. We see it now in imagination, not as a 
 wholly liquid mass, but with slowly growing crystals 
 floating in it. How slowly the cooling and crys- 
 tallization must proceed under the vast covering of 
 the overlying rocks the imagination is baffled to 
 conceive ; but of this we may be sure that had it 
 been allowed to continue, each crystal would have 
 grown until no drop of liquid remained, and the 
 whole would have become one mass of coarsely 
 crystalline rock, such a rock precisely as granite is. 
 
 Here we have the explanation which Hutton 
 already foresaw. In their deep reservoirs, lavas 
 cool with excessive slowness, and so give rise to 
 coarsely crystalline rock; but the same molten mass, 
 brought to the surface, will solidify with compara- 
 tive rapidity, and in a glassy or minutely crystalline 
 condition which gives it a dull and stony appearance. 
 The former is the " plutonic," the latter the 
 " volcanic " phase of the same rock. 
 
 To complete the enquiry, the British Isles will 
 serve us better than Italy. Britain abounds in 
 volcanoes on every hand. We do not recognise 
 them, because even the youngest of them have been 
 too long extinct, and their cones have suffered too 
 much from exposure through countless ages to the 
 
THE SOLID ROCKS 51 
 
 elements. But, for our purpose, this is so much to 
 our advantage. The cones are presented for our 
 inspection in every stage of decay, dissected for us 
 by Nature herself so that we may examine in the 
 light of day every detail of their inner structure. 
 Here we have exposed to view the old lava flow 
 which had been buried for ages under later flows 
 and sheets of ash. There is the solid plug of lava 
 filling the old pipe through which it was rising to 
 the crater. Running across the hillside like a 
 ruined stone wall stands out the edge of a vertical 
 
 FIG. 4. LION ROCK, ISLE OF CUMBRAE, CLYDE. 
 
 A weathered-out " dyke " of lava, standing as a wall-like 
 mass of rock. 
 
 sheet of lava, which had been driven up through 
 some rent in the rocks (see Fig. 4). Perhaps the 
 next volcano we examine has had its entire cone 
 swept away, leaving nothing to tell of its former 
 existence but the old " neck " of lava in the pipe, 
 and the " dykes " in the fissures of the surrounding 
 rocks. In other cases even these last remnants 
 have been removed, and the deep reservoirs of 
 lava, now frozen into granite, exposed to view. 
 We have, then, every opportunity of comparing 
 lavas which have undergone their final cooling in 
 
52 GEOLOGY 
 
 every possible variety of position, and of realising 
 to the fullest extent the large number of distinct 
 types of rock which are produced by this one 
 varying circumstance. 
 
 The typical Vesuvian lava was rejected at the 
 beginning of this discussion, because it differs from 
 granite, not only in being a " volcanic " instead of a 
 " plutonic " rock, but also in respect of the minerals 
 of which it is composed. Quartz is absent. The 
 felspar is largely replaced by beautiful crystals of 
 colourless leucite, brownish nepheline, pale blue 
 sodalite, or bright blue haiiyne. In place of mica, 
 augite chiefly occurs, while honey-yellow olivine is 
 frequently present in addition. In petrological 
 language these facts are expressed by describing the 
 Vesuvian lavas as basalts (of a peculiar type), while 
 the volcanic equivalents of granite are spoken of as 
 rhyolites. Basalts likewise have their plutonic 
 equivalents, of which perhaps the most familiar 
 British examples are the rocks forming the rugged 
 Cuilin Hills of Skye. These latter rocks are known 
 to the petrologists as gabbros. The difference 
 between granite and rhyolite on the one hand, and 
 gabbro and basalt on the other, is therefore a 
 difference of composition. The relative proportions 
 of the various chemical constituents is changed, 
 with the result that when the liquid mass begins to 
 crystallize, different minerals are formed. 
 
 The reader is now in a position to realise how 
 great is the variety of rocks of this character. The 
 composition is capable of almost indefinite variation, 
 while each rock-type so produced is again capable 
 of multiplication according to the conditions under 
 which its solidification takes place. We see before 
 us an immense field for study, which considerations 
 
THE SOLID ROCKS 53 
 
 of space forbid us now to enter. Not only have the 
 innumerable varieties to be compared and classified, 
 and all their component minerals studied individu- 
 ally and in relation to one another, but we have also 
 to discover how far the peculiarities of each rock 
 are due to the varying conditions of pressure and 
 temperature under which it solidified. We have to 
 seek for the laws which determine what minerals shall 
 be formed out of the homogeneous molten magma 
 from which the whole rock arises. And, lastly, we 
 have to account for the great diversity of composi- 
 tion which is found. We have, in fact, to discover 
 the complete natural history of igneous rocks for 
 so they are collectively termed, in allusion to the 
 " fiery " state through which they have all passed. 
 
 We have now seen what are the two fundamental 
 types of rock which the geologist is able to recognise 
 the sedimentary and the igneous. In other words, 
 we arrive at the generalisation, after hunting the 
 world over, that every rock which is not more or 
 less clearly built up of the worn fragments of pre- 
 existing rocks has attained its present condition 
 after having been'in a molten state. And, since this 
 leaves us with nothing but igneous rocks from which 
 ultimately to derive the materials which form the 
 sedimentaries, we finally attain the still broader 
 generalisation that there is not a rock on the earth's 
 surface whose materials have not at some time 
 passed through the molten condition. Here, indeed, 
 is a surprising conclusion, and one which might 
 have seemed incredible but for the known facts 
 which have been detailed in the preceding chapter. 
 Remembering, however, that the interior of our 
 globe is certainly in a highly heated condition at 
 the present time ; seeing that the sun is certainly, 
 
54 GEOLOGY 
 
 and the larger planets are probably in a gaseous 
 condition ; noting further that the moon shows clear 
 proof of its former possession of great internal 
 heat which has now vanished; and at the same 
 time recollecting that all these bodies have had 
 their histories closely involved and largely common, 
 the conclusion suggested by the facts now to hand 
 is clearly that the entire earth was formerly in a 
 molten state. It is only just to say that the con- 
 clusion is perhaps not so inevitable as might at first 
 sight appear, because the action of the elements in 
 destroying the rocks is so powerful that it is in the 
 highest degree improbable that any portion of the 
 original surface of the globe now remains in its 
 primitive condition ; and owing to the alternate 
 exposure of rocks once deeply covered, and the 
 burial of other areas under vast accumulations of 
 sediment, perhaps no rock is now to be found which 
 has not, at some period or other, been more or less 
 deeply secluded below the surface. Yet, notwith- 
 standing that these considerations compel us to 
 admit the bare possibility of an alternative explana- 
 tion, the supposition that the now solid surface was 
 once completely molten explains so simply the 
 cardinal fact indicated above, and is so probable on 
 other grounds, that little doubt need remain as to 
 its truth. 
 
EARTH SCULPTURE 55 
 
 CHAPTER IV 
 
 EARTH SCULPTURE 
 
 WE turn now to a new aspect of our subject. In 
 the preceding chapters we have been seeking to 
 gain an impression necessarily a crude one of the 
 earth as it may be supposed to have resulted by 
 simple cooling from its original nebulous condition. 
 It has not been possible to do this without referring 
 more than once to the subsequent changes which 
 have been brought about on its surface ; the 
 problems of geology are too intricate and involved 
 to be treated independently. Now these changes 
 become themselves the main subject of enquiry. 
 
 The brief span of a human lifetime permits us to 
 witness very little change in the scenes around us. 
 Centuries are but the minutes in the life of the 
 earth. Yet even this short time suffices to effect 
 much. Here the storm has torn out a new gully 
 on the slope of the hill ; there the river is ever 
 undermining its bank and slowly removing the field 
 by its side. The ravages of the sea are evident to 
 all. If one stands again by some estuary when the 
 tide is out, the scene speaks volumes. There are 
 the broad banks of sand and mud, surrounded by 
 the brown waters of the river ; out to sea, the water 
 is clear and blue, the mud having all dropped to 
 swell the banks. Even more striking is the case 
 when a river flows through a lake in its course. At 
 the upper end it may enter as a turbid stream ; it 
 
56 GEOLOGY 
 
 leaves again with sparkling water, the mud sinking 
 as soon as the current is checked and adding to the 
 delta which forms the meadow through which the 
 river enters at the head of the lake. And so the 
 lake is slowly filled till meadow alone remains to 
 mark its site. The longer one contemplates this 
 burden of solid matter carried by every river, the 
 more one becomes impressed with the enormous 
 waste which the land in the course of ages must 
 suffer. It is estimated, for example, that the Danube 
 carries every year into the Black Sea enough sand 
 and mud to make a sheet of rock a square mile in 
 extent, and over thirty-three feet thick, or about 67f 
 million tons ! The total discharge of the Mississippi 
 may be eight times that amount ! 
 
 Everyone is aware of the source of much of this 
 material. Every shower of rain produces its 
 thousand tiny streams, each busy hurrying the finer 
 particles of soil to the nearest brook. Even dry 
 weather cannot prevent the waste. Scorched by 
 the sun, the soil becomes a prey to the wind wher- 
 ever protecting vegetation is absent, and driving 
 hither and thither in dusty clouds much of it still 
 finds its way to the river, and so to the sea. It 
 needs little imagination to see that if the loss were 
 not in some way made good, all soil must ages ago 
 have disappeared. 
 
 Where the underlying strata consists of clay or 
 soft sandy rock, nothing beyond the moistening by 
 rain and the penetration by the roots of the plants 
 above is needed to convert them into soil. But 
 what of .regions where underlying rocks are hard 
 and compact ? No one can have failed to observe, 
 in quarry or road-cutting, the actual condition of 
 affairs in such a case. Below is the solid rock, with 
 
EARTH SCULPTURE 57 
 
 only a barely perceptible joint or crack here and 
 there. Higher up, the cracks become more 
 numerous and obvious the rock is divided into 
 smaller blocks. Higher still, no fair-sized fragment 
 of sound rock is to be seen ; it is completely smashed 
 into small angular pieces, most of which have been 
 obviously displaced. Then this "subsoil" passes up 
 imperceptibly by the gradually decreasing size of 
 the fragments, and discolouration by organic matter 
 into the true soil. Such facts speak for themselves. 
 The solid rock below is gradually breaking up into soil, 
 and making good the waste from above. But how ? 
 It is a matter of common knowledge how the roots 
 of trees seek their way into the smallest cracks and, 
 growing there, exert great force in widening the 
 crevices. This must clearly help in the process, 
 yet it goes on equally where trees are absent. The 
 organic acids, again, secreted by plants into the soil, 
 aid in the later stages of disintegration, but are in 
 no wise competent to begin it. Neither will the 
 percolating rain explain the phenomena. There 
 still remains a power as irresistible as it is gentle 
 and elusive the power which resides in the rays of 
 the sun. As the rocks are gently warmed, they, like 
 all other solid bodies when heated, slightly expand. 
 On cooling, they shrink again. The daily changes 
 of temperature are, of course, not felt through more 
 than a foot or two of soil, but the influence of the 
 seasonal variations is felt for many feet down. The 
 result is clear and inevitable. In summer, the 
 temperature of the superficial portions of the rock 
 gradually rises, and the rock expands, while the 
 more deep-seated parts are unaffected. The effect 
 is that with which everyone is familiar when a piece 
 of glass or porcelain is unequally heated the 
 
58 GEOLOGY 
 
 material cracks. Here, then, we have the primary 
 cause of the numerous cracks, which naturally 
 become more numerous towards the surface, where 
 the temperature variations are more marked. But 
 the story does not end here. Later, and more 
 indirectly, the sun makes its influence felt still more 
 strongly. As winter approaches, the rocks shrink 
 and the cracks slightly gape : they become filled 
 with water, and then, the temperature falling 
 further, the water freezes. The sudden expansion 
 of water when converted to ice is familiar to all. 
 The cracks are thereby widened, only to be filled up 
 again with water on the next thaw and the process 
 repeated ; and so on indefinitely. Should the rock 
 itself be at all porous, the freezing of water in its 
 minute pores may add greatly to the process of 
 gradual disintegration. Everyone knows how 
 effective is a frost in " breaking up " the soil itself ; 
 and it is but the same process on a greater scale 
 which gives rise to the great "tundras" of Northern 
 Russia firm ground when frozen in winter, but vast 
 death-traps in summer, because of the enormous 
 depth of the soft soil in which man and beast may 
 sink out of sight. 
 
 Thus it is solar heat, acting directly in the alter- 
 nate heating and cooling of the rocks, and indirectly 
 through the medium of water which freezes when 
 that heat is partially removed, which is the real 
 cause of all this disintegration. Yet, so far, we have 
 been considering its action under the most unfavour- 
 able circumstances in the lowlands, namely, where 
 the soil is largely allowed to accumulate and so to 
 act in the meantime as a very efficient protection to 
 the rocks below, screening them from all the more 
 violent changes of temperature. But let us turn to 
 
W!J^ <U 
 SH CO j? 
 
 s . J 
 
 Sa a 
 
 - ,: 
 
 r.S'S --S 
 
 o.BPa. 3 
 
 .e H cu 
 
EARTH SCULPTURE 59 
 
 the hills. On the mountain top no soil is allowed to 
 gather; and those who have been in these lofty 
 regions will have no difficulty in discovering the 
 reason. There the battle of the elements proceeds 
 with a vigour unknown in the lowlands. The 
 fiercest lowland gale is almost a daily feature of 
 the mountain. The rain is more abundant, and is 
 driven with almost inconceivable fury against the 
 rocks. The gale sweeps before it not merely dust 
 and sand, but coarse gravel and even large slabs of 
 rock should they chance to present a wide surface. 
 Soil, therefore, can never gather. The bare rock 
 itself is exposed to the fierce heat of the sun. 
 Neither need we dwell on the greatly increased 
 power of the sun himself at these altitudes. The 
 heavy moisture-laden air below has not yet robbed 
 the solar beams of their power. Hence, as every 
 climber knows, under the mid-day sun the rocks 
 become so hot as to be almost painful to touch ; yet 
 few nights pass without a frost. In winter, the 
 bitter cold of every night, coupled with the abundant 
 moisture from snows melted the preceding day, works 
 still greater havoc. If one stands alone in the awe- 
 some solitude among the mountain crags on a clear 
 winter day, the deathly silence is broken only by the 
 sudden crash of a falling block, finally dislodged 
 from its place by the last night's frost. Crash after 
 crash tells us we are listening to the mountains 
 falling to ruin. But our eyes reveal even more 
 eloquent testimony, for there, on every hand, are the 
 vast piles of boulders which one by one have fallen 
 from the peaks above. These great " screes " are 
 the most familiar features of mountain scenery ; 
 every gully has its fan of boulders at the bottom, 
 every cliff has its long boulder-slope at the base. 
 
60 GEOLOGY 
 
 Many a mountain, indeed, is all but buried under its 
 own debris. These are the " everlasting hills " ! 
 One turns away with a deeper sense of human 
 littleness. 
 
 In our own islands the mountain tops are largely 
 protected from the attack of the winter frosts by 
 their mantle of snow. Only the steepest crags, 
 where the snow cannot lodge, are constantly 
 exposed; and there, consequently, destruction is 
 most rapid. The same partial protection is afforded 
 all the year round to the peaks of the Alps, whose 
 glittering mantle never completely melts away. But 
 there the snow itself turns destructor. Year after 
 year the snowfall exceeds the quantity melted, and 
 the snow tends to accumulate in ever-increasing 
 mass upon the hills. On the steeper slopes its 
 growing weight sooner or later drags it down. A 
 mighty mass rushes headlong down the hill, sweep- 
 ing along all the loose boulders in its path, foaming 
 through the forest or dragging the trees down in its 
 wild career. The avalanche is no small instrument 
 of destruction. But far more effective is the quiet 
 and almost imperceptible descent of the snow which 
 is left. Growing thicker until its increasing weight 
 compresses the lower layers into compact blue ice, 
 we then witness the strange fact that where the soft 
 snow held firm, the solid ice begins to flow, and as a 
 glacier, seeks out the valleys, and creeps slowly 
 down till the increasing warmth melts it away as 
 rapidly as it advances. Glaciers are the rivers 
 which drain the snowfields ; and the flow is curiously 
 alike in the two cases. When the river comes to a 
 steeper part of its bed it breaks into a cataract. 
 The ice, under like circumstances, becomes cracked 
 and broken up. The great crevasses gape open, to 
 
EARTH SCULPTURE 61 
 
 close again only months later when the declivity is 
 passed. Like the waters of the river, too, the ice 
 carries along its burden of wasted rock. From the 
 frowning crags by its sides the sun and frost splinter 
 off blocks which are hurled down on to the sides of 
 the ice to be carried along in bouldery piles the 
 lateral moraines and perhaps to be tipped off in a 
 great terminal moraine at the foot of the glacier. 
 But, should crevasses open, another fate may await 
 them. Under the summer sun much of the ice on 
 the surface of the blocks left between the crevasses 
 may melt, and the boulders may slip one by one to 
 the bottom of the yawning chasms. There they are 
 firmly gripped by the icy mass, which closes over 
 them and drags them forward with it over its rocky 
 bed, crushing them down on to it at the same time 
 with all the weight of the ice above. Armed thus 
 with thousands of engulfed boulders, the glacier 
 grinds over its bed like a mighty rasp. Rocks and 
 boulders alike are ground to the finest powder, and 
 thus, when the ice finally melts, it issues from the 
 glacier as a milky stream, carrying with it the 
 powdered rock from its bed. Where the glacier has 
 retreated from its former channel, the rocks are 
 seen smoothed and scored. No rough crags remain. 
 In their places are the most wonderfully smooth 
 and rounded knolls of naked rocks the roches 
 moutonnees (sheep-backs) of the French. And by 
 these characteristic effects, as well as in other ways, 
 the work of glaciers may be recognised in many 
 regions where they are not now to be found. The 
 smooth outlines which are so characteristic of our 
 own hills we shall later find reason to attribute 
 largely to this action. 
 While the lower mountain-regions of tropical and 
 
62 GEOLOGY 
 
 sub-tropical countries are not subject to erosion by 
 glaciers, it is precisely in those parts, and especially 
 where the air is dry and rain nearly absent, that the 
 action of solar heat is most pronounced. In the 
 drier regions of Africa, mid-day temperatures of 
 140F. are no uncommon thing, while in exposed 
 places the thermometer may fall a complete hundred 
 degrees in the night. Exposed to extreme changes 
 such as these, and robbed of any protective covering 
 of vegetation by the continual drought, the rocks 
 are shattered to fragments. Cracked into smaller 
 and smaller pieces, the fragments are finally caught 
 up by the gale and driven along till they are at 
 length reduced to sand. So the mountains crumble 
 down till they are surrounded by a shifting sea of 
 sand formed from their own debris a typical 
 desert. 
 
 We have now seen some of the principal factors 
 in the breaking down of the solid rocks of the earth's 
 surface. Watched only for a day, their effect may 
 seem trifling. In a lifetime even the change pro- 
 duced may be relatively small. But when we 
 remember that these forces have been ceaselessly 
 active not only for thousands of years but for many 
 millions, then we may begin to realise the vastness 
 of the changes they must have accomplished. And 
 we shall find in the sequel ample evidence of changes 
 vastly greater than our unaided imaginations would 
 ever have been likely to conceive. 
 
 It remains to ask how all this mass of debris we 
 have seen accumulating on the land is ultimately 
 transferred to its burial-place on the bed of the sea. 
 We already know that the rain and the rivers are 
 the agents of transport. But, though the rain may 
 sweep the finer soil from the lowlands into the 
 
EARTH SCULPTURE 63 
 
 brooks, it is powerless to move the boulders on the 
 mountain side. It does ultimately move them, not 
 in a mass, but particle by particle. Everyone is 
 aware that the weather attacks all exposed stone, 
 with varying rapidity according to its nature. The 
 picturesque character of an old building largely 
 depends on the removal of the marks of artificial 
 treatment from the stone, which exposure effects. 
 Nowhere can the effect of " weathering " be better 
 observed than in an old cemetery. Nothing could 
 be more striking than the difference in the resistance 
 to decay exhibited by the various stones, whose 
 inscribed dates at once tell the length of their 
 exposure. The marble stones usually crumble most 
 rapidly. In half a century their inscriptions are 
 likely to be entirely corroded away unless protected. 
 Granite proves itself far more durable. Its polish 
 may dim after ten or twenty years, but any incised 
 lettering will usually be quite distinct at the end of 
 a century, and it may be two or three times as long 
 before it is completely erased. The greatest endur- 
 ance, however, is exhibited by really good sandstone. 
 Soft sandstones may crumble very rapidly, because 
 they are porous and become a prey thereby to the 
 frost. But provided that it is really compact and 
 hard, the centuries pass lightly over it. One or two 
 hundred years may produce scarcely a noticeable 
 effect, while its records may remain legible even a 
 thousand years. 
 
 The most familiar observations show that it is 
 exposure to rain which causes this decay. Why the 
 great differences in the effect produced on the 
 different stones ? In their general physical charac- 
 ters the three rocks considered do not greatly differ ; 
 the distinctions are mainly chemical. Marble wholly 
 
64 GEOLOGY 
 
 consists of crystals of carbonate of lime ; granite, as 
 we know, of the three minerals quartz, felspar, and 
 mica; sandstone almost entirely of grains of quartz. 
 None of these substances are greatly affected by 
 pure water ; but rain-water is not pure. It contains 
 dissolved air. The addition of oxygen and nitrogen 
 cannot greatly affect its powers, but the small 
 quantity of carbonic acid gas in the air makes the 
 water acid a very weak acid, it is true. Now 
 carbonate of lime is soluble in carbonic acid ; and, 
 though the amount of this acid in a gallon of rain- 
 water is very minute, this fact is compensated for 
 by the large quantity of rain which in a few years 
 flows over the stone. Each shower removes its 
 mite of dissolved material. Even so, the process 
 would be very slow, but for the fact that the rain 
 eats in between the tiny crystals, and thereby loosens 
 them so that they may be washed bodily away. If 
 we turn now to a long-exposed granite we find the 
 surface very rough. The quartz crystals stand out, 
 while the felspars are all more or less eaten away. 
 In time, of course, the particles of quartz must 
 tumble out, but it is almost entirely to the corrosion 
 of the felspar that the gradual decay of the rock is 
 due. And so the sandstone, consisting entirely of 
 quartz, might endure almost for ever so far as the 
 chemical corrosion by the rain is concerned. Most 
 varieties of this last rock, however, are very 
 porous, and thereby yield readily to the freezing 
 of water in their pores, or the solution of the 
 cementing material which holds the grains 
 together. The great importance of carbonic acid 
 in the final disintegration of rocks is strikingly 
 shown in the much more speedy decay of most 
 building stones in large towns, where the quantity 
 
EARTH SCULPTURE 65 
 
 of this gas in the air is considerably in excess of 
 its normal amount. 
 
 By the chemical corrosion of the rain, then, or by 
 the continued action of some of the other processes 
 of destruction we have previously considered, the 
 larger debris of the rocks is gradually crumbled 
 down, and swept away particle by particle into the 
 rivers and thence to the sea. 
 
 Let us turn now to the rivers themselves. Among 
 the mountain crags, their work is far from being 
 confined to the carrying of sand and mud. Not a 
 little of the bouldery debris from the peaks above 
 falls more or less directly into the torrents. Those 
 who have never stood and watched the mountain 
 torrent when swollen by the storm can have little 
 conception of the power of the waters. In fine 
 weather, only the gentle splash of the water is heard, 
 but in spate, when the volume of the stream is 
 suddenly increased tenfold, and its speed is corres- 
 pondingly greater, the sound of the water, increased 
 to a roar, cannot muffle the clash of the pebbles and 
 boulders as they are rolled over or dashed along the 
 bed. The stream is converted into a great mill in 
 which boulders are smashed to pebbles and ground 
 to sand. While large angular blocks are shot in at 
 the head of the stream, only small and beautifully 
 rounded fragments are to be found when the lower 
 valley is reached. But not only are the boulders 
 ground on one another. They are equally ground 
 on the rocky bed over which the water rushes. 
 When the spate is over and the stream reduced to 
 its normal size, the beautiful rounded smoothness of 
 its rocky banks testifies to the grinding they have 
 undergone. So, age after age, the torrent runs on, 
 ever cutting deeper and deeper into the rocks, 
 
 B 
 
66 GEOLOGY 
 
 carving out one of those dark winding gorges which 
 are the most exquisite feature of highland river 
 scenery. 
 
 In the middle reaches of its course to the sea only 
 an occasional waterfall revives the youthful vigour 
 of the stream. Here, leaping over the edge, the 
 waters dash on the bed below, and hurl the pebbles 
 there collected again and again at the foot of the 
 rock over which it is foaming. The incessant battery 
 slowly undermines the fall until a great block crashes 
 down from the top, only to be smashed into frag- 
 ments which will be used in their turn to continue 
 the constant undermining. So the fall gradually 
 retreats up the river, and the valley is slowly cut 
 deeper. 
 
 The final course through the true lowland regions 
 has its quiet flow unbroken even by these episodes. 
 Very occasionally an unusually severe flood may 
 lead to the sudden carrying away of large quantities 
 of material from its banks, but as a rule the stream 
 meanders quietly through its meadows, carrying 
 with it a certain amount of mud, and sweeping the 
 sand along its bed. At each great bend the current 
 impinges with some force on the outer bank, which 
 is thereby slowly undermined and its materials 
 carried away. But the immediate journey of this 
 material is usually short. At the next bend in the 
 reverse direction the waters of that side of the river 
 are on the inner side of the curve, and consequently 
 become "slack." The checking of the current at 
 this point causes most of the material which has 
 been picked up from the bank above to be dropped 
 here, so that a " spit " of sand and gravel grows out 
 at this point, to balance the loss of material which 
 the stream is now cutting away from the other side. 
 
EARTH SCULPTURE 
 
 67 
 
 And so at each bend, while the outer side is gradually 
 cut away, the inner side grows by the deposit of 
 material derived from the erosion at the bend above. 
 Reference to the accompanying diagram should 
 clear away any difficulty on this point. A little 
 reflection will now show that the position of the 
 river channel must be continually, though slowly, 
 shifting ; and a little extra patience will discover that 
 
 FIG. 5. 
 
 FlO. SA Portion of course of meandering river. The arrows 
 indicate the course of the main current. 
 
 FlO. SB Course of same river at later period, changed by 
 erosion of banks and deposit of sediment. The areas 
 over which the meadow has been worn away are line- 
 shaded. The growth of meadow due to deposit is 
 indicated by dotting. Note that the curves also become 
 constantly exaggerated, so that adjacent bends, by 
 breach or the narrow isthmus between, may join, as 
 would shortly take place at the lower bend here shown. 
 
68 GEOLOGY 
 
 the ultimate effect is of the nature indicated in the 
 next diagram. The bends of the river, as it were, 
 creep bodily down the valley. Hence the deposit of 
 material on the inner sides of the curves is only 
 temporary. As the next bend advances down the 
 valley it is picked up again and carried on another 
 stage. And at each removal more grinding takes 
 place, and some of the finer material each time will 
 be swept by the current right on to the sea. 
 
 So rivers fulfil their great function of transporting 
 the waste of the rocks to the sea, while performing 
 themselves not a little work in the way of earth- 
 carving. The making and deepening of the valleys 
 belongs entirely to the rivers ; the widening of them 
 is the work of heat and cold, rain, and sometimes 
 of ice. 
 
 Even yet we have not exhausted the roll of forces 
 which conspire to destroy the dry land of the globe. 
 The attack of the sea is so familiar that it is un- 
 necessary to dwell on it at length. Where the coast 
 consists of soft rocks, the toll of the waves may be 
 measured by feet per annum. The site of more than 
 one old town on the East Anglian coast now lies 
 below the sea. In Bridlington Bay, the loss is said 
 to have averaged about 2i yards yearly along 36 
 miles of shore. Yet it is not so much in the removal 
 of these soft materials as in its battles with the 
 sterner rocks that the waves show their power. 
 Few scenes can rival in grandeur the wild rocky 
 coast in a storm. When the roar drowns every other 
 sound and the crash seems to shake the very founda- 
 tion of the rocks, wave after wave retires without 
 appearing to have made the slightest impression. 
 And indeed they would be almost powerless against 
 the solid phalanx of the rocks but for two circum- 
 
EARTH SCULPTURE 69 
 
 stances. In the first place, they are aided by the 
 boulders which lie at the foot of the cliff. These 
 are caught up and flung with the greatest violence 
 against the rocks, till their foundations are cut away 
 and block after block falls a prey to the waves. But 
 even more aid is derived from the fact that the rocks 
 themselves do not present an unbroken face. The 
 joints are the lines of weakness. Into these crevices 
 the water is forced with all the pressure of the wave. 
 The air is driven in and compressed before it, and 
 when the wave retires it expands again with almost 
 explosive violence and drives the water back. When 
 it is known that the pressure of the waves, even in 
 the North Sea, may be one and a half tons to the 
 square foot, and on the Atlantic coast twice that 
 amount, one is in some position to realise the power 
 this action represents. The joint is soon widened to 
 a cave, and the cave enlarged till its roof breaks 
 down, and likely enough it joins with an adjacent 
 cave. So the sea bores into the cliff all along the 
 line, and gradually cuts back the coast. In this 
 process, large pillars of cliff are frequently isolated 
 and left standing out in the sea while the shore re- 
 treats behind them fit monuments to mark the 
 conquests of the waves. These outlying "stacks" 
 are familiar objects all round our more rocky shores, 
 and they beautifully illustrate the fact that the 
 progress of the sea, no matter how slow, is none the 
 less sure. 
 
 Here we must conclude this very brief and partial 
 account of the agents which carry on the great 
 process of rock-destruction which geologists refer to 
 as denudation. When we reflect on the number of 
 the forces which combine in the attack, when we 
 remember the unceasing activity of every one of 
 
70 GEOLOGY 
 
 them ; when we know, too, that each has been busy 
 through all the unnumbered ages since the earth 
 became a planet, then we may well wonder how it 
 comes that any land still remains above the ocean. 
 And assuredly it would not be so, but for the fact 
 that there are forces counteracting the tendency of 
 denudation. These, however, belong to another 
 chapter. 
 
 CHAPTER V 
 
 THE SEA FLOOR 
 
 WE have now traced the fate of the materials of 
 the wasted rocks as far as the sea. It is time, 
 therefore, to enquire into their later history; and 
 this must begin, clearly, with an investigation into 
 the manner in which they are deposited on the sea 
 floor. The very extensive dredging and sounding 
 which has been carried out during the last forty 
 years over the whole world, and especially about the 
 great highways of shipping, has put us in possession 
 of an ample mass of the facts of the case, The 
 whole bed of the North Sea, for example, we know 
 to be strewn with shingle, sand, and mud. Here 
 and there, of course, is a patch of bare rock, but 
 these are not sufficient to affect the truth of the 
 general statement. The same is true, again, of the 
 English Channel, the Irish Sea, and, indeed, the 
 whole of that submerged plateau of which the 
 
THE SEA FLOOR 71 
 
 British Isles are only the hills. For an elevation of 
 only six hundred feet would drain all these seas, and 
 cause the Atlantic coast to run from Norway, out- 
 side the Hebrides and Ireland, down to the Bay of 
 Biscay. Beyond this line the existing sea rapidly 
 deepens into the great trough of the Atlantic. In 
 places off the west of Ireland, only about twenty-five 
 miles separates points where the depth is 100 fathoms 
 (600ft.) and 1000 fathoms respectively. As the deeper 
 waters are reached, the coarse shingle and sand are 
 soon left behind; the sediments become finer and 
 finer, until, at about 400 fathoms, the last traces of 
 land-mud are found. This depth, commonly spoken 
 of as the mud-line, marks the boundary of the 
 terrigenous deposits, as these debris of the land are 
 appropriately termed. It may be a matter for some 
 surprise that the distribution of these deposits 
 should be regulated by depth, not by distance from 
 the shore. Where the waters remain shallow, the 
 land sediments may be carried out several hundred 
 miles; on a rapidly deepening sea bed they are 
 limited to a very narnr:: belt. But let us consider 
 how they come to be distributed at all, why they do 
 not wholly accumulate at the mouths of the rivers 
 or the foot of the cliffs. We have seen that when 
 a river enters a lake, its sediment is dropped as soon 
 as the water is brought nearly to a standstill. So, 
 again, the stoppage of the current on entering the 
 sea leads to the deposit of material at the mouth of 
 the estuary. The power of water to carry sediment 
 depends entirely on its own motion. What move- 
 ment, then, carries the sediments about in the seas? 
 Everyone is aware that there are currents in the 
 ocean, the most familiar, perhaps, being the " Gulf 
 Stream " across the North Atlantic, These are set 
 
72 GEOLOGY 
 
 up mainly by the unequal heating of different parts 
 of the ocean by the sun, in the same way that the 
 irregular heating of the atmosphere causes the winds. 
 But these currents are much too feeble to carry 
 sediment. Around the coasts, however, very much 
 stronger currents run. The strongest swimmer may 
 be unable to make headway against these, and they 
 are amply strong enough, therefore, to sweep before 
 them even coarse shingle. These are tidal currents > 
 and it is worth a moment's consideration to under- 
 stand how they arise. The tides themselves are not 
 currents; they are waves, in the open ocean only 
 two or three feet high but several thousand miles 
 broad. Now everyone knows that in an ordinary 
 wave the water merely moves up and down. Away 
 from the shore, a boat may dance up and down on 
 the waves all day without being moved from its 
 original position, so far as they are concerned. The 
 wave travels along, but not the water. But this is 
 no longer true near the beach. There the water 
 rushes with great force along with the wave. The 
 shallowness of the water in the latter case is the 
 cause of its different behaviour. The actual up-and- 
 down movement in an ordinary wave extends down- 
 wards in the water only a very few feet, and so long 
 as it is not interfered with no other movement 
 occurs. But as the shore is reached, the bottom 
 does interfere with it, and a forward and backward 
 rush of the water is the result. So with the great 
 tidal waves. Even at depths of several hundred 
 fathoms the natural rise of the wave entails the 
 bodily movement of large masses of water. 
 Hence we see that these tidal currents are 
 necessarily confined to the comparatively shallow 
 waters, As the waters become deeper they become 
 
THE SEA FLOOR 73 
 
 weaker, and herein we have the explanation of why 
 the distribution of terrigenous deposits is regulated 
 by depth and not by distance. 
 
 Sand and mud is not the only material, however, 
 which the tidal currents have to distribute on the 
 sea bed. No one can have failed to observe that in 
 places the shore is largely or entirely made of sea 
 shells. Such is the quantity of cast-off shells in the 
 sea that it bears no inconsiderable proportion to the 
 sand and mud. Here and there whole banks of 
 shells or patches of shell-mud are scattered about. 
 More impressive still are the great reefs of coral 
 to be found round the coasts in the warmer seas, 
 where the exquisite beauty of the corals themselves 
 and the brilliant colouring of the profuse sea-life 
 among them adds so greatly to the interest. In the 
 Great Barrier Reef of Australia alone we have many 
 thousands of square miles of coral, which must be 
 at least hundreds of feet in thickness it may be 
 thousands. Coral and shell alike, of course, consist 
 of carbonate of lime ; nor is the supply of this sub- 
 stance limited to their productions. Many others 
 among the more lowly inhabitants of the sea, besides 
 shell-fish and coral-polyps, make themselves limy 
 coats which on their death go to swell the calcareous 
 deposits. So all these creatures toil to form the 
 limestones of future ages. 
 
 Whence comes all the lime? The organisms 
 themselves obtain it from the water of the sea, in 
 which it is dissolved in small quantity; and so their 
 ancestors have done for ages before, as the great 
 masses of ancient limestones testify. Yet there is 
 no evidence that the creatures of to-day have any 
 greater difficulty in obtaining a supply than their 
 early progenitors. The explanation lies in the fact 
 
74 GEOLOGY 
 
 that the supply in the sea is restored by the rivers. 
 Rain-water is pure, except for the gases it has 
 dissolved out of the air; but the moment it reaches 
 the ground it begins to dissolve the rocks, as we 
 have already seen. Hence, when it finally flows 
 back in the rivers to the sea, it bears with it not 
 only the rock debris in suspension, but also a con- 
 siderable amount in solution. Among this latter 
 material, the salts lime, soda, magnesia, and potash 
 usually form the main constituents. To the igneous 
 rocks, then, we must ultimately trace the supply of 
 lime which forms the shells of the sea and the beds 
 of limestone. 
 
 We are now in a position to see, in broad outline 
 at least, the relationship of the various sedimentary 
 rocks to their igneous parentage. Among the long 
 and varied list of minerals which compose those 
 latter rocks, there is a single one which is at once 
 abundant, hard enough to survive the eventful 
 journey from mountain to sea without being ground 
 to impalpable powder, and incapable of decomposi- 
 tion. Quartz holds this record alone ; and it is for 
 this reason that a handful of sand picked up at the 
 river mouth consists almost to a grain of quartz. 
 The most abundant of all the minerals of the igneous 
 rocks felspar we have already noted as yielding 
 readily to the corrosion of the rain. Its dissolved 
 portion is the most noteworthy source of the lime, 
 soda, and potash in the sea, while the insoluble 
 residue is washed away as a fine mud to form the 
 future clays. The remaining minerals for the most 
 part are likewise decomposed or ground up, and so 
 add their quota to the salts of the sea or the muds 
 on its bed. Among the salts, magnesia and iron are 
 their principal contributions. 
 
THE SEA FLOOR 75 
 
 In our sandstones, therefore, we get back the 
 quartz of the igneous rocks; the shales and clays 
 represent the insoluble residues of their other 
 minerals ; through the agency of the organisms of 
 the sea the dissolved lime is extracted from its 
 waters, and reappears in due course as limestone. 
 This statement is, of course, incomplete, but it 
 exhibits in a few words the main features of the 
 grand process of the reconstruction of rocks. 
 
 The remaining dissolved materials are slowly 
 precipitated from the waters of the sea, with one 
 exception. The magnesia from time to time dis- 
 places some of the lime from the limestone, forming 
 magnesian limestone or dolomite. The iron is 
 deposited among the sediments as their great 
 colouring agent. The beautiful red, green and 
 yellow tints of the sedimentary rocks are almost 
 entirely due to iron in various states of combination 
 with oxygen and water. Soda alone remains dissolved 
 in large quantities ; owing to its high solubility it is 
 not deposited, but constantly accumulates in the sea, 
 and hence, as everyone knows, chloride of sodium is 
 now the salt of the ocean. 
 
 It remains for us to consider that most fascinating 
 region of the sea the floor of the deep ocean. This 
 was an unknown world until, about forty years ago, 
 the problem of deep-sea sounding began to be seri- 
 ously attacked. Previously, it had only been vaguely 
 known that the depth of the great oceans is to be 
 measured in miles. Nothing was known as to the 
 real conditions at those depths, or as to the state of 
 the bottom. It had been confidently predicted that 
 life of any kind must be impossible from the 
 enormous pressure of water and the total darkness. 
 How far wide of the mark this prediction has proved 
 
76 GEOLOGY 
 
 is now familiar knowledge. It is difficult to resist 
 the temptation to stray from our proper path to 
 describe the wonderful creatures which the deep-sea 
 dredge has brought up to the light of day, to remark 
 on their extraordinary forms and the phosphorescent 
 organs which serve to light them in the " dark 
 unfathomed caves." One observation, at least, we 
 may legitimately make. It is the tendency of these 
 deep-sea creatures to develop extraordinarily long 
 and delicate feelers of all descriptions. Some of 
 the prawns, for example, have all their legs many 
 times longer than the body, and slender to a degree. 
 No prettier testimony could be found to the 
 unbroken stillness of these depths ; such creatures 
 would be utterly helpless in a current. 
 
 The sediments from the land, as we have seen, 
 are all dropped by the weakening currents by the 
 time the 400 fathom line is reached. Beyond this, 
 the dredge in most places brings up fine, usually 
 light-coloured mud or " ooze " of very different 
 nature. On examination, fragments of the delicate 
 shells of some of the shell-fish which pass a floating 
 existence on the high-seas may usually be found 
 sometimes they may even form a considerable pro- 
 portion of the material but usually the eye recog- 
 nises little but a fine powder. How different when 
 a little of the mud is placed under the microscope ! 
 The grains become minute shells. Only the largest 
 are as big as a pin's head, yet all are of the most 
 exquisite workmanship. The most astonishing 
 variety of form prevails ; shells of one chamber 
 and of many; straight, curved or coiled; flat or 
 globular. Some are chalky and opaque, others like 
 crystal. One feature characterises them all they 
 are perforated with numerous minute pores or 
 
THE SEA FLOOR 77 
 
 foramina ; whence the name given to the group of 
 creatures whose productions they are, the Fora- 
 minifera. But all the shells, we observe, are 
 empty. If we would gather them with their living 
 inhabitants we must change the dredge for the 
 tow-net, and drag the surface waters of the ocean. 
 In almost every part of the world they abound. 
 The animals themselves are mere specks of almost 
 formless and structureless living jelly, and one 
 cannot but marvel that so lowly a creature, belong- 
 ing to the lowliest of all the great branches of the 
 animal world, should build itself so beautiful a home. 
 But how much more strange does it seem that these 
 most weakly of builders should be chosen by Nature 
 to provide the mantle of sediment for two-thirds of 
 the earth's surface the greater part of its ocean 
 floor! 
 
 Certain noteworthy differences are observable 
 between the minute shells as gathered at the 
 surface, and as dredged from the greater depths 
 say from 2000 fathoms. Those from the surface 
 have all their ornament sharply defined; they 
 resemble cut glass. They are, moreover, in many 
 cases provided with numerous long and delicate 
 spines, which help to keep the creature afloat. On 
 the other hand, the shells from below have lost the 
 clear-cut appearance, and their spines are gone. In 
 view of the great stillness of the waters at these 
 depths this cannot be attributed to the rolling of 
 the shells on the bottom. It must be referred to 
 the corrosive action of the sea-water. Besides a 
 small quantity of carbonic acid dissolved directly 
 out of the air, the sea is constantly supplied with 
 this substance by the respiration of all the teeming 
 life it contains. In consequence it is able to dissolve 
 
78 GEOLOGY 
 
 very slowly indeed carbonate of lime. The 
 delicate shells of the foraminifera, while occupied, 
 are protected by the living substance which sur- 
 rounds as well as fills them ; but when the animal 
 dies, or vacates the shell, this protection is lost, and 
 as the shell slowly sinks to the bottom it is exposed 
 during the whole time to corrosion. The minute 
 size of the object causes its fall to be very slow it 
 may be only a few yards per day s6 that as much 
 as a couple of years may be occupied in the journey. 
 Ere this time, many of the more delicate shells may 
 be completely dissolved ; and, clearly, there will be a 
 limiting depth beyond which none will remain. It is 
 found at nearly 3000 fathoms. This rain of foram- 
 inifera on to the bed of the ocean may be aptly 
 compared to a shower of snow falling through warm 
 air to uneven land. The snow reaches the high 
 grounds before it is melted, and they become white 
 with the fall, but in the valleys the snow is melted 
 before it reaches the ground. 
 
 At least one great deposit of foraminiferal ooze 
 was familiar long before the depths of the oceans, 
 or the oceans themselves had been explored. The 
 white cliffs of Albion are formed of just such a 
 deposit as the ooze of the deep Atlantic. If a piece 
 of chalk is gently crushed and the finest powder 
 gently washed away, there will remain large 
 quantities of foraminiferal shells almost identical 
 with those of the present seas. The whole mass of 
 the chalk is almost completely composed of them or 
 their broken remains, and one may contemplate 
 Beechy Head or Flamborough with the thought 
 that the vast thickness of chalk which is there par- 
 tially exposed to view, and which extended over 
 some hundreds of thousands of square miles of 
 
THE SEA FLOOR 79 
 
 Europe, was the production of creatures barely 
 visible to the eye. 
 
 These calcareous oozes of the ocean bed are not 
 without admixture of inorganic mineral matter, as 
 well as some organic material which is not cal- 
 careous. Those wonderful microscopic plants, the 
 diatoms, abound in some parts of the ocean, and 
 contribute their glassy box-like shells which they 
 make of silica. The Radiolaria are equally minute 
 members of the animal kingdom, somewhat closely 
 related to the Foraminifera; they also form siliceous 
 skeletons of the most delicate lace-like ^character 
 which in places form almost pure siliceous deposits. 
 The purely mineral matter is mostly of such a 
 nature as to indicate that it is mostly derived from 
 volcanic dust blown over the oceans, or from the 
 decomposition of fragments of pumice floating over 
 the surface. 
 
 In the deepest basins of the ocean these non- 
 calcareous deposits alone are found. The proportion 
 of limy matter in the oozes steadily diminishes 
 with increasing depth, until, as we have seen, 
 scarcely any remains below 3000 fathoms. A fine 
 red clay almost universally occurs in the great 
 deeps, composed of such materials as we have seen. 
 How slowly this red clay accumulates is strikingly 
 shown by the great quantity of remains which lies 
 unburied on its surface. Almost every haul of the 
 dredge brings up numbers of sharks' teeth, the ear- 
 bones of whales and the like. How many thousands 
 of years must it have taken for such quantities of 
 these objects to accumulate 1 Yet so slow is the 
 formation of this mud that they lie unburied still. 
 In view of these facts it is indeed not improbable 
 that two other sources of supply very materially 
 
80 GEOLOGY 
 
 contribute to the growth of this deposit. When the 
 shells and skeletons of the various marine creatures 
 dissolve there will be in most cases a little insoluble 
 residue, a kind of " ash " left over, which may still 
 be added to the clays. And, lastly, some supply 
 may be obtained from without the earth altogether. 
 The shooting-stars which shower down on to the 
 earth in thousands every day probably are not for 
 the most part larger than grains of sand, and 
 they nearly all reach the surface ultimately as fine 
 dust; yet even this all but imperceptible fall of dust 
 may add appreciably to the growth of the red clay. 
 Thus the fineness of the sediment on the ocean floor 
 increases regularly from the gravel on the shore to 
 the impalpable clay in the deepest hollows. 
 
MOUNTAIN BUILDING 81 
 
 CHAPTER VI 
 
 MOUNTAIN BUILDING 
 
 THE last stage in the cycle of changes through 
 which the rocks pass yet remains to be considered. 
 We have seen the primitive igneous rocks crumbling 
 under the action of the sun and the weather, we 
 have watched the fragments on their journey to the 
 sea, and we have discovered the manner of their 
 distribution over the ocean floor. We know, further, 
 that the deposits of former days have been elevated 
 above the sea, so that our existing lands consist very 
 largely of rocks of this sedimentary character. This 
 elevation completes the cycle ; and a cycle it really 
 is, for no sooner do the new deposits peer above the 
 waves than they become a prey again to the agents 
 of destruction, so that the whole series of events 
 may begin anew. 
 
 The mere existence of sedimentary rocks (with 
 often the old sea-shells in them) now high among 
 the hills, is proof enough that the sea-level did not 
 always have the same relation to those lands that it 
 has to-day. But the observation gives no indication 
 whether it is the land that has risen or the sea which 
 has subsided. It may seem more natural to con- 
 clude that the sea has changed its level, and that 
 view seems to have been adopted by the ancients. 
 A fuller consideration of the facts, however, leads to 
 the opposite conclusion. Let us note, firstly, that 
 the positions of the sedimentary rocks are by no 
 means the only indications of change of level. Few 
 
82 GEOLOGY 
 
 persons can have failed to notice that many of our 
 seaside towns are built on remarkably flat, low-lying 
 stretches of ground, while the country immediately 
 behind is frequently hilly, the hills rising abruptly 
 from the flat. The North Wales coast furnishes 
 perhaps the most striking examples. Almost every- 
 where, however, these low-lying terraces are to be 
 found ; here, it may be, only a few yards wide, there 
 several miles. Not uncommonly, especially round 
 the estuaries of the Forth and Tay, behind the first 
 low terrace may be found a second at a higher level, 
 with an abrupt step from one to the other. There 
 may even be several of these steps one above 
 another. A little investigation soon reveals the 
 meaning of these terraces. Where the rocks are 
 hard, the terrace is commonly backed by regular 
 cliffs, on which the marks of the sea are not to be 
 mistaken. The sea-caves are there as perfect as 
 those on the adjacent shore. If, again, one digs 
 below the soil of the terrace, one finds a layer of 
 typical sea sand and gravel spread over the solid 
 rock, out of which one may pick the shells of 
 limpets, periwinkles, cockles, and oysters, as freely 
 as on the sands of the existing shore. Clearly, then, 
 we are standing on an old beach, an old terrace cut 
 out by the sea. But when it was cut the level of the 
 sea was evidently higher than now, or the land 
 lower. Where we see terrace above terrace, we are 
 evidently marking the steps by which the land rose, 
 or the sea fell. These " raised beaches," therefore, 
 testify to the changes of relative level of the land 
 and sea. 
 
 The reader may be disposed to ask whether it 
 never appears that the sea has relatively risen. Un- 
 fortunately the answer must generally be sought 
 
MOUNTAIN BUILDING 83 
 
 below the sea. As we find on the dry land clear 
 proofs of the former presence of the ocean, so we 
 must search below the sea for the characteristic 
 features of a land-surface. For movements of small 
 extent in this direction there is very clear evidence. 
 In the estuary of the Mersey, for example, at 
 Leasowe, and at many other places round our 
 coasts, a dead-low tide exposes a profusion of tree 
 stumps and roots on the shore. The stumps, for 
 the most part, are in an upright position, and a little 
 digging reveals the fact that they are naturally 
 rooted in an old clayey soil. At the locality named, 
 also, the stumps themselves are surrounded by peat ; 
 this, in turn, is covered by another soil, at about the 
 level of the highest tides, in which Roman remains 
 may be found, and that soil is itself buried under the 
 sand-dunes. Here, then, the proof is conclusive. 
 The forest could never have grown had the older 
 soil always been, as it is now, permanently covered 
 by the sea. The land has there sunk, or the sea has 
 risen. For evidence of more extensive movements 
 in the same direction we may appeal again to the 
 sedimentary rocks themselves. Deposits which may 
 have been formed in deep water obviously cannot 
 assist us, and even the finding, at depths much 
 below sea-level, of such deposits as are normally 
 formed in shallow water, is not quite conclusive ; for 
 very coarse materials may gather in deep water 
 where the land dips steeply under the sea. But, 
 fortunately, the sedimentary series includes oc- 
 casional deposits which must have been formed 
 above water altogether. The most familiar and 
 obvious are our beds of coal. Everyone knows that 
 coal is merely fossilised wood and other plant 
 remains, and very commonly the bed of clay or 
 
84 GEOLOGY 
 
 sandstone below the layer of coal in the pit is 
 crowded with roots of all descriptions, including 
 frequently the roots of large trees in their natural 
 position. The clay is evidently the old soil in which 
 the plants grew, and at the time of their growth it 
 was evidently not below the sea (at least, not more 
 below than would convert it into a swamp, which 
 very probably was its true condition). Yet, in parts 
 of each of our great coal fields, these beds are now 
 found and worked at depths of two and three 
 thousand feet below present sea level. One con- 
 clusion is now amply demonstrated : the phenomena 
 are in no wise explained by the supposition that the 
 sea has gradually subsided and the land emerged 
 during past geological ages. While, for example, in 
 central Derbyshire we have limestones which ac- 
 cumulated under the sea, and largely, perhaps* in 
 deep water, now rising fourteen hundred feet above 
 sea-level, we have also, only a few miles away in 
 Lancashire, the beds of coal now three and four 
 thousand feet below it. One more piece of cor- 
 roborative evidence may be added to amplify the 
 case. The land-surface receives its characteristic 
 outlines at the hands of the various agents of 
 denudation, and it is naturally, therefore, far more 
 diversified in contour than the bed of the ocean. 
 Hence we may search for sunken lands by their 
 characteristic contours below the sea. River valleys 
 are the most characteristic feature. Nothing re- 
 sembling them could ever be produced below the 
 sea; yet the soundings not rarely show that the 
 actual valleys of the land do continue below the sea, 
 sometimes for considerable distances and to con- 
 siderable depths. So far as the true valley continues 
 it marks the former extension of the land, but the 
 
MOUNTAIN BUILDING 85 
 
 land has been submerged and the end of the valley 
 " drowned." 
 
 The only indication among the facts just considered 
 as to how these displacements have occurred is that 
 given by the series of raised beaches. Where these 
 are seen forming step above step it is evident that 
 after each alteration of level there was a pause in 
 the movement while the sea cut out a terrace. The 
 movement in these cases was intermittent, but we 
 are still left in doubt whether actual displacements 
 were sudden or slow. Some further light may be 
 obtained from another reference to the coal-bearing 
 strata. In Lancashire those rocks are altogether up- 
 wards of 5,000 feet in thickness, and include several 
 hundred beds of coal, most of which are much too thin 
 to be of any value. Most, if perhaps not all, of these 
 seams were formed at or about sea-level at any 
 rate not below it. The layers of sandstone, shale, 
 and clay between the seams are such as to indicate 
 that they were all formed in shallow water. Now, 
 seeing that the uppermost layers of rock and coal 
 were formed at or near sea-level, the lowest must 
 then have been over 5,000 feet below it ; but they, 
 too, had been formed at sea level. The land must 
 have sunk, or the water risen, 5,000 feet during the 
 interval ; but so slow and so gradual must have been 
 the sinking of the land that the slow-growing 
 sediments, on the whole, kept pace with it. There is 
 still no proof that the sinking might not have 
 proceeded by jumps of a few feet, or even fifty feet, 
 with long pauses between while the sediment 
 " caught up," but, averaging the whole movement of 
 five thousand feet, it must have been extremely slow. 
 An inch per annum would almost certainly be a very 
 excessive estimate for the rate at which the sediments 
 
\ 
 
 86 GEOLOGY 
 
 would be deposited, and the average rate of move- 
 ment must have kept below their rate of accumula- 
 tion. The story which we have just worked out for 
 the Coal Measures is only that which is repeated by 
 the majority of the sedimentary rocks. Again and 
 again they show that they have been formed during 
 periods of slow subsidence of the land relatively to 
 the sea. But again also we remember that the very 
 fact that the land of to-day consists largely of such 
 sediments shows that there have been periods when 
 the movements in those same localities have been in 
 the reverse direction. We have now reached two 
 important conclusions: the movements were not 
 constant in direction, and those, at least, whose 
 duration we can estimate, occupied enormous periods 
 in their performance. 
 
 We had occasion to remark, in the second chap- 
 ter, that earthquakes are accompanied occasionally 
 by permanent changes in the level of the ground. 
 These changes are sudden, but they are never large. 
 In minor earthquakes they are commonly absent or 
 unmeasurable. After a series of earthquakes in 
 Chili during 1822 and 1835 the coast is stated to 
 have been raised from two to four feet for a distance 
 of 1,200 miles. During the recent San Francisco 
 earthquake the railway along the coast was sub- 
 merged for three miles. A series of shocks affecting 
 the Indus delta in 1819 submerged an area of some 
 2,000 square miles, while an adjacent area was 
 elevated 10 feet. We have already seen that the 
 vertical displacement along the line of fracture in 
 the Mino-Owari earthquake in 1891 amounted in 
 places to as much as 20 feet. This last figure must 
 be regarded as very exceptional; even greater 
 displacement occurred during the disastrous earth- 
 
MOUNTAIN BUILDING 87 
 
 quake of 1692 in Jamaica, when parts of Port Royal 
 (at that time the capital) sank 20 to 40 feet, but it 
 has been suggested, with much probability, that this 
 was really due largely to the "settling" of loose 
 sand. 
 
 Among the phenomena of earthquakes, then, we 
 have to number small permanent changes in the 
 level of the land. Even the largest are minute 
 compared with the vast movements the rocks evince, 
 but if they be indefinitely repeated in the same 
 direction they may obviously mount up to ultimate 
 changes of any magnitude. We have already ob- 
 served that the periods of time involved must be 
 immense. The known rate at which denudation 
 takes place gives a basis for the estimation of that 
 of deposit, from which we may fairly conclude that 
 growth in thickness at the rate of one foot in 500 
 years may not be an unfair average, while one foot 
 per century may certainly be regarded as rapid. 
 Evidently ordinary earthquake movements will not 
 need to be repeated with undue frequency to keep 
 pace with deposits growing even at the latter rate, 
 while, as we have seen, the common occurrence of 
 vast thicknesses of sediment, all of which have been 
 deposited in shallow water, shows that the rate of 
 deposit has usually fully kept up with that of move- 
 ment. Such displacements as might accompany 
 very small earthquakes, happening a few times in a 
 century, would suffice. Indeed, we have the best 
 reasons for believing that the subsidences and 
 upheavals may in many cases have proceeded by 
 stages so gentle that no shocks at all marked their 
 occurrence. A tradition has for a very long period 
 prevailed among the inhabitants of the Swedish 
 coast that the sea is gradually receding from its 
 
88 GEOLOGY 
 
 shores. So far back as the early part of the 
 eighteenth century the celebrated Celsius brought 
 forward observations supporting the contention, and 
 after long and severe criticism it has been firmly 
 established that the Gulf of Bothnia is actually 
 subsiding relatively to the land, though not every- 
 where at the same rate. While at Stockholm the 
 fall appears to be less than six inches per century, 
 further north it may be more than two feet in the 
 same period. On the other hand, in the south of 
 the peninsula, it is the land which appears to be 
 subsiding relatively to the sea. The truth of the 
 matter, of course, is that in both cases it is the land 
 which is moving, not the sea. Were the level of the 
 water to alter, the change must be everywhere alike. 
 Here, then, we have elevation and subsidence going 
 on so gently that the region is singularly free from 
 earthquake shocks, and yet proceeding quite as 
 rapidly as the greatest movements appear to take 
 place. 
 
 If we turn again to the rocks we readily perceive 
 that irregularity of movement has been the rule, not 
 the exception. The evidence is of the simplest 
 description. Knowing that the sedimentary rocks 
 have been laid down, sheet upon sheet, under water, 
 we can have no hesitation in affirming the layers 
 were originally horizontal. Yet we rarely find them 
 so now. Instead, we find them inclined at every 
 angle ; in the language of the quarrymen the beds 
 " dip " in some direction. Obviously, if the rocks 
 have been elevated in one place and depressed in 
 another, such tilting must be the result. Almost 
 every section of the rocks in sea-cliff or quarry 
 exhibits such tilting, and that to a much greater 
 degree than might have been anticipated. A gentle 
 
MOUNTAIN BUILDING 89 
 
 dip might be expected, but what are we to say 
 when we find the rocks turned absolutely on end. 
 Nay, further, we may even find beds turned up 
 and forced over so as now to lie upside down. 
 The rocks are not merely tilted, but crumpled 
 and folded like a rucked- up carpet. The folds may 
 
 FIG. 6. FOLD IN ROCKS FORMING CLIFF WEST OP LITTLE 
 
 HANGMAN HILL, NEAR ILFRACOMBE. The rocks on the left 
 
 side of the fold have been completely overturned. 
 
 be of any size. Small folds can nowhere be better 
 seen than in the beautiful cliffs of the North Devon 
 coast. Nothing can be more impressive than to 
 walk along that coast, mile after mile, seeing the 
 rocks everywhere twisted into folds like a series of 
 great waves, folds often so sharp that the rocks on 
 one side, as in our illustration, are actually inverted. 
 
90 GEOLOGY 
 
 The mind is helpless indeed to conceive of the force 
 which can thus have crumpled the solid rocks like 
 sheets of paper ! 
 
 The greater folds of the rocks are naturally not to 
 be actually seen to the same effect. The Pennine 
 Chain affords a very perfect example. That range 
 of hills is nothing more nor less than the worn-down 
 remnant of a great arch of rock thirty miles across, 
 and which, if the material worn away from the top 
 could be replaced, would be some 10,000 feet higher 
 in the middle than at the sides. Here we have a 
 mountain range carved out of a single great fold of 
 the earth's crust. But the great ranges of the 
 world are of another nature. The Grampian Hills 
 belong to the class. Wherever we turn among those 
 mountains, we find the rocks folded and contorted 
 in a manner which would be perfectly incredible 
 were it not there before our very eyes ; folded not 
 only in vast curves miles in length, but all minutely 
 crumpled even down to microscopic puckerings. 
 The accompanying section across those hills only 
 illustrates the greater folds. A glance at it will 
 convince the reader that all this crushing and 
 crumpling cannot have taken place without causing 
 a general elevation of the whole tract. It is evident 
 that to produce all the folds the length of the tract 
 must have been shortened by some miles ; and what 
 has been lost in length must have been made up for 
 by increase in height. Of this nature are all the 
 greater mountain-ranges of the world. They are the 
 lines of weakness of the earth's crust, along which 
 the forces of deformation have been concentrated 
 to crumple up the rocks, and so to cause a great 
 wrinkled ridge on the surface. The whole line has 
 been elevated high above the surrounding tracts, to 
 
gsouuna I 
 
92 GEOLOGY 
 
 become a prey to the forces of denudation, and by 
 them to be carved out into mountain peak and 
 valley. In passing, we cannot but note the eloquent 
 testimony here offered to the power of denudation. 
 By following the foldings of the rocks, and restoring 
 the parts of the curves which have been worn away, 
 it is a simple matter to make an approximate 
 estimate of the amount of material which has been 
 removed. Even from the Pennine Chain, a thick- 
 ness of six or eight thousand feet of rock must have 
 been denuded from the top ; from the Alps, several 
 miles may have been stripped off. 
 
 We have already seen that it is not only in the 
 tilting of the rocks and the production of folds that 
 these earth-movements exhibit their effects. In the 
 greater earthquakes we generally find that the rocks 
 have snapped along some line and been displaced, 
 producing a "fault." The number of such faults 
 whose formation has thus been observed is rela- 
 tively small ; and the largest displacement we have 
 noted has been only about twenty feet. Yet, as has 
 already been remarked, such faults are among the 
 commonest phenomena of the rocks all the world 
 over. It may be doubted if there is a square mile 
 in the whole of Britain where the rocks have not 
 been displaced in this way. In many cases the 
 displacement is only a few feet, but in many others 
 it is to be measured by hundreds, and in not a few 
 by thousands. These fractures are not to be seen 
 at the surface, because denudation has long ago 
 smoothed away the irregularities they may have 
 created, but in cliff and quarry we see the lines of 
 fracture exposed, and we can estimate the displace- 
 ment. The greatest fracture in Britain cuts across 
 the whole of Scotland from Stonehaven to the Clyde, 
 
MOUNTAIN BUILDING 93 
 
 and separates the hard rocks of the Grampians from 
 the softer beds of the Central Lowlands. Along 
 this line, the displacement may well amount in 
 places to 10,000 feet. What shall we say of the 
 force which has caused this displacement ? 
 
 It must be self-evident to all that the formation of a 
 great fault must be a process spread over the same 
 vast intervals of time that are occupied in the slow 
 movements of elevation and depression, or the pro- 
 duction of great folds. Indeed, it is clear that these 
 phenomena are but diverse results of the same 
 movements. When one area is elevated or de- 
 pressed relatively to another the result may be a 
 general tilting of the rocks, or the production of a 
 more or less sharp bend at some point, or the 
 displacement may be confined entirely to a single 
 line of fracture. Nor is this true only of vertical 
 displacements. In speaking of the effects of earth- 
 quakes we remarked that the ground is frequently 
 distorted horizontally ; and we correspondingly find 
 that the rocks on the sides of a fault are commonly 
 displaced laterally as well as vertically, though, in 
 consequence of this movement being more difficult 
 to detect it is frequently quite overlooked. 
 
 The most stupendous and striking of all rock- 
 movements have been in a horizontal plane. Along 
 the sides of the great mountain-chains, where the 
 lateral crushing has been most intense, the rocks 
 have not uncommonly been torn across, and the 
 sides of the range bodily crushed in under the more 
 central portions. No finer example is known than 
 the North-west Highlands of Scotland. The whole 
 of what is now the coastal area has been thrust 
 under the mountains to the east. Along some of 
 these great " thrust-planes," where the rocks have 
 
94 GEOLOGY 
 
 been sheared across, the overlying mountain masses 
 have been forced over the rocks below for a distance 
 of not less than four miles. 
 
 We have now sufficiently illustrated the nature of 
 the movements by which the lands of the globe are 
 raise \ again from the ocean grave in which the 
 forces ol denudation strive incessantly to bury 
 them, and by which also the rocks are buckled up 
 along their lines of weakness to form the mountain 
 ranges, awaiting only the action of sun and rain to 
 carve them into peak and valley. It remains for us 
 briefly to mark the effects of this movement on the 
 rocks themselves, and to consider wherein these 
 stupendous forces may reside. 
 
 It can cause no one surprise to learn that the 
 rocks are profoundly altered by the enormous 
 pressure, crushing and shearing to which they are 
 subjected. The most familiar example of these 
 effects is in the conversion of clays and shales into 
 slates. Not merely are the rocks greatly hardened, 
 but an entirely new structure is developed in them, 
 which causes them to split or " cleave " readily into 
 thin plates. In other cases, the rocks have been 
 completely broken up into larger or smaller frag- 
 ments, which the pressure has subsequently com- 
 pacted together again. Along with these structural 
 changes there is usually found a considerable 
 chemical reconstruction of the rock. The old 
 minerals are largely destroyed, and new ones 
 are built up out of their materials. This latter 
 change cannot be due, to any large extent, to the 
 pressure itself, but is probably stimulated by it. The 
 ultimate cause of the change is to be found in the 
 fact that the constituents of the rock are not 
 (especially in the case of the sedimentary rocks) 
 
MOUNTAIN BUILDING 95 
 
 in chemical equilibrium, while its occurrence is 
 
 rendered possible by the heating to which the rock 
 is subjected when depressed far below the surface, 
 and by the presence of moisture, through which the 
 minerals may be brought particle by particle into 
 solution and subsequently deposited in their new 
 combinations. From the joint action of all these 
 processes there results a whole series of new rocks, 
 by the alteration of each of the various igneous and 
 sedimentary types. Granite is converted into 
 gneiss, sandstone into quartzite, limestone into 
 marble, and shales and clays into slates and schists. 
 These are the " metamorphic " rocks. 
 
 What is the force which exhibits itself in these 
 vast heavings and crumplings of the earth's solid 
 crust? The geologist to-day answers this question 
 much more cautiously than he would have done fifty 
 years ago. The comparison of the earth to a 
 shrivelled apple has become almost a tradition. 
 The outer crust of the globe is now cold, and has 
 been for many millions of years. The interior is 
 hot, but is slowly losing heat. The natural inference, 
 therefore, is that the interior is cooling, and if 
 cooling, shrinking. The cold crust cannot contract, 
 but must accomodate itself to the shrunken interior 
 by wrinkling, like the skin of the apple. Many 
 difficulties, however, present themselves when the 
 theory is rigorously tested. The amount of shrink- 
 age of the interior necessary to account for the 
 observed folding of the crust is very considerable, 
 and would indicate a great fall in temperature. On 
 the other hand, the amount of heat now escaping 
 annually can be fairly estimated, and it seems very 
 improbable that loss at such a rate could lead to 
 sufficient cooling in any period of time it would be 
 
96 GEOLOGY 
 
 reasonable to assume. Another element of un- 
 certainty is introduced by the enquiry whether the 
 lost heat may not be restored. We know now that 
 in various ways it may be ; whether it is, we cannot 
 tell. To the objection regarding the inadequacy of 
 the heat-loss, it may be very legitimately answered 
 that we really know little or nothing of the properties 
 of matter under such conditions of temperature and 
 pressure as probably exist within the earth ; but that 
 is merely a dignified way of saying we cannot prove 
 the case. The second objection throws doubt on 
 whether any case exists. For the present, the main 
 support for the theory must be derived from the 
 absence of any completely successful alternative- 
 While, however, the suggestions which have been 
 made are too complex to be considered here, there 
 remains one aspect of the question which cannot be 
 entirely passed over. Amid the endless alternation 
 of subsidence and elevation affecting all parts of the 
 globe, we find every indication that the great con- 
 tinents and oceans have retained their identity. In 
 spite of every effort of denudation to destroy them, 
 the continents are still pretty much where they were ; 
 which is tantamount to saying that through all the 
 vagary of rise and fall, there is a general tendency 
 for the continental areas to rise, while the ocean 
 floors sink. The very fact that the great land-masses 
 are able to exist at all, elevated above the ocean 
 floor, seems to imply that they must consist of 
 relatively light materials ; for the enormous weight 
 of the continental protrusions must be balanced by 
 something, and presumably it is by the heavier 
 character of the rocks below the ocean. Now 
 denudation tends to destroy this balance; by its 
 various agencies, enormous masses of material are 
 
THE PHYSICAL HISTORY OF BRITAIN fl 
 
 transported from the land and dumped on the bed 
 of the sea. This loss of balance puts a strain upon 
 the crust, and here, perhaps, we may see, if not one 
 of the chief forces which cause the great earth- 
 movements, at least one of the guiding principles 
 which direct their action. 
 
 CHAPTER VII 
 
 THE PHYSICAL HISTORY OF BRITAIN 
 
 WITHIN the narrow limits of this little volume 
 there is little opportunity to enlarge on the results 
 achieved by geology in the construction of a detailed 
 history of the earth. The barest indication only 
 can be given of what has been learned regarding 
 the past fortunes of the British Isles themselves ; 
 but this is perhaps not altogether to be regretted. 
 The person whose interest in the subject ceases 
 with the reading of geological history in books will 
 be but poorly repaid for his pains. In proportion as 
 the story is read in the rocks themselves it becomes 
 vivid and absorbing. 
 
 Let us consider, then, how the facts we have 
 learned are to be applied to the interpretation of 
 the geological record. A few very simple consider- 
 ations will supply the key. Since the sedimentary 
 rocks have accumulated layer upon layer at the 
 bottom of the sea, it is evident that the lowest 
 layer must be the oldest, and the uppermost the 
 youngest ; and herein we have an infallible guide to 
 the historical order of the nocks. So far as we can 
 see how the beds lie one above another we can read 
 the history at a glance. For instance, all along the 
 
 G 
 
98 GEOLOGY 
 
 eastern flanks of the Pennine hills we may see the 
 beds of blue limestone and shales, full of corals and 
 sea-shells, " dipping " to the east and passing in that 
 direction underneath the layers of coarse yellow grits 
 and black shales which succeed them, and in which 
 here a few plant remains, there a few shells, may be 
 found. The limestones take us back to a time when 
 that region was under the clear waters of an open 
 sea, in which corals could flourish. The grits above 
 tell us that at a later period the waters became 
 shallow, and great rivers swept into them vast 
 quantities of sand, in which were buried not only 
 such shells as could live there, but some of the 
 fallen trees borne out from the land. Above the 
 grits in turn are found those beds of sandstone and 
 clay among which the seams of coal occur, and in which 
 shells of fresh-water species abound, together with 
 the remains of whole forests of plants. We learn, 
 then, that later still the accumulating sands had 
 driven back the sea altogether, leaving broad swampy 
 tracts on which great forests sprang up a swampy 
 jungle. From time to time the land subsided, and 
 fresh-water lagoons gathered, till fresh sand and 
 mud filled them up and allowed a new forest to rise. 
 In the case just considered, the story is clear and 
 straightforward. But where the rocks have been 
 greatly folded, or where they are covered for long 
 distances by superficial beds of sand or clay, and 
 laid bare only at rare intervals, it is often by no 
 means so easy to discover what is the real order of 
 succession, or what is the relation of the particular 
 bed of rock seen to any other bed. For example, 
 in very many parts of Britain isolated patches of 
 limestone may be found which are scarcely to be 
 distinguished in general character from the lime- 
 
THE PHYSICAL HISTORY OF BRITAIN 99 
 
 stones of the Pennine Chain; but how are we 
 to know whether they are parts of those lime- 
 stones, or whether they belong to any of the 
 scores of other limestones which represent totally 
 distinct periods of the world's history? The 
 key to this problem one of the most important 
 discoveries in the history of geological science was 
 found by an engineer of Bath, William Smith, 
 upwards of a century ago. Having traced with 
 great care the distribution of all the various beds of 
 rock for many miles around that town, he was 
 enabled to make the all-important discovery that 
 each layer of rock everywhere contained the same 
 assemblage of fossil remains, while the fossils in 
 any group of rocks were always different from those 
 in the layers above or below. Thus he established 
 the proposition that strata may be identified by their 
 fossil remains. Applying it to the case of our 
 limestones/we see that it is only necessary to examine 
 the fossils from the various isolated patches to 
 decide which are, and which are not, part of the 
 same series as those in the Pennines. We see, too, 
 in this grand discovery, not only a most valuable 
 guide in the sorting-out and arrangement of the 
 rocks, without which the history of the world could 
 never have been read, but also a record of the fact 
 that the inhabitants of the earth have been gradually 
 and constantly changing, each period of its history 
 being marked by its own peculiar forms of life. 
 Fossils, then, enable us to piece together the various 
 isolated sections of the rocks which are actually 
 exposed to view, and to build up the whole grand 
 sequence of the stratified rocks in its proper order. 
 We find, when our British rocks are thus placed in 
 order, one above another, they form a pile some 
 
100 
 
 GEOLOGY 
 
 twenty miles in thickness. If the average rate of 
 accumulation has been about one foot in five hundred 
 years, the reader may form his own estimate of the 
 ages which have passed while these deposits grew. 
 In the following table, the greater groups of the 
 fossiliferous rocks are arranged in historical order, 
 the youngest at the top. 
 
 TABLE OF BRITISH STRATIFIED ROCKS. 
 
 Cainozoic 
 
 Mesozoic 
 
 I Pleistocene 
 Pliocene 
 Miocene 
 Oligocene 
 Eocene 
 
 Cretaceous 
 
 Oolitic 
 
 Liassic 
 Triassic 
 
 Permian 
 
 f Chalk 
 
 1 Greensand and Gault 
 
 1 Wealden 
 
 [_ Purbeckian 
 
 Portlandian 
 
 Kimmeridgian 
 
 Corallian 
 
 Oxford ian 
 
 Callovian 
 
 Great Oolites 
 
 Inferior Oolites 
 
 f Keuper 
 1 Bunter 
 
 [ Coal Measures 
 Carboniferous j Millstone Grit 
 
 ( Carboniferous Limestone 
 Old Red 
 Sandstone and 
 Devonian 
 
 j Ludlow beds 
 
 Palaeozoic 4 Silurian j Wenlock 
 
 I Llandovery 
 f Bala 
 
 Ordovician ] Llandeilo 
 I Arenig 
 
 {Tremadoc 
 Lingula 
 Menevian 
 Harlech 
 
 Pre-Cambrian unfossiliferous rocks. 
 
 Cambrian 
 
C PHYSICAL Hisfbfcv OF' BRITAIN 101 
 
102 GEOLOGY 
 
 The groups of rocks named in the foregoing 
 table are extremely unequal in thickness, those 
 nearer the base of the series being in general much 
 thicker than those higher up ; and, on the whole, they 
 doubtless represent correspondingly greater periods 
 of time. But, as in human history the records of the 
 later centuries are much more perfect than those of 
 earlier times, so it is with the records of the rocks. 
 The further down the series we go, the more im- 
 perfect as a rule the record becomes. Moreover, we 
 must remember that the geological record was of 
 necessity incomplete to begin with. The site of these 
 islands has certainly been for long ages below the 
 sea, while the various sedimentary strata accumula- 
 ted ; but for periods equally long, in all probability, 
 it has from time to time been raised above the waves. 
 And during the latter times, not only were no rocks 
 formed, but those which had already gathered were 
 largely destroyed again by denudation, till subsidence 
 once more carried them below the sea and the old 
 land surface was buried under newer sediments. In 
 the interval, the older rocks have not merely been 
 denuded, but commonly tilted or folded as well, so 
 that the newer strata have frequently been laid down 
 horizontally over the worn edges of their inclined 
 beds, as in the accompanying illustration. Every 
 such " unconformity," therefore, tells of a great gap 
 in the rocky series; of an interval during which the 
 lower rocks were raised above the sea, folded, and 
 denuded. The older rocks, too, have naturally 
 suffered most from the repeated earth movements. 
 Folded and folded again, they have often been 
 crushed almost out of recognition, and their fossils 
 have been damaged or completely obliterated. 
 From one cause or another, therefore, the geological 
 
THE PHYSICAL HISTORY OF BRITAIN 103 
 
 record is most fragmentary. In the expressive 
 language of Darwin, "we may compare it to a history 
 of which we possess only here and there a chapter; 
 of each chapter only a few pages ; and of each page 
 but a few lines." 
 
 With the foregoing considerations in mind, then 
 let us turn again to the rocks and trace some of the 
 grander features of this country's history. The 
 oldest rocks of these islands, the foundations, so to 
 speak, on which everything else rests, are to be 
 found in the extreme north-west, forming the rugged 
 coasts of North-western Scotland, and the whole 
 of the Outer Hebrides. That region is merely a 
 continuation of the great plateau of Scandinavia, a 
 remnant of the ancient axis, the old "back-bone" of 
 Europe. Along that line, the rocks have been more 
 intensely crushed and folded than, perhaps, anywhere 
 else in the Old World. And so our history begins, 
 as is inevitable, among its greatest difficulties. The 
 rocks are so altered that scarcely a trace of their 
 original character is to be found. They are so 
 folded that no order in their sequence remains. 
 It is nearly certain that they never had any fossils, 
 but if any were there no sign of them is left. We 
 believe them now to have been for the most part 
 igneous rocks of a volcanic nature, vast sheets of 
 lava, the products of immense primaeval eruptions. 
 The one fact which is abundantly evident is that 
 these "Lewisian" rocks (after the island of Lewis) 
 were very early subjected to the most powerful 
 earth movements, and folded and altered to almost 
 as great a degree as they are at present, long 
 before Cambrian times. After their folding, further 
 volcanic activity is evinced by innumerable "dykes" 
 of lava which penetrate them. In turn, we find 
 
104 
 
 GEOLOGY 
 
 FIG. 10, GEOLOGICAL MAP OF THE BRITISH ISLES. 
 
THE PHYSICAL HISTORY OF BRITAIN 105 
 
 evidence of a second period of folding, which 
 affected the dykes as well as the older rocks. 
 Then there must have followed an immense space 
 of time while all these rocks were exposed to the 
 ceaseless attack of the elements, by which the 
 great mountain range into which they had been 
 folded was worn down to its roots. There is some 
 evidence that almost desert conditions prevailed, 
 and the valleys were slowly filled with the debris 
 from the hills. So accumulated the vast piles of 
 sand and gravel, in places not less than 10,000 
 feet thick, which now form many of the drear 
 forbidding hills of the North-west Highlands. 
 Below these sheets of now solid rock we may 
 still follow the outlines of some of the old land 
 they buried, the oldest surface of this country of 
 which any trace remains. 
 
 What lapse of ages the events just narrated may 
 represent we have little means to judge. We only 
 know that they had all transpired, and the "Torri- 
 donian" sands and gravels (from Loch Torridon) 
 had become hard rock, and been themselves carved 
 out into hill and valley, before the beginning of 
 the Cambrian period. We know, too, that in other 
 parts of the country great changes had occurred. 
 Subsequently, in all probability, to the formation 
 of the Lewisian rocks, the whole area of these 
 islands, perhaps, had been below the sea, and in 
 that sea were deposited the sheets of sand and 
 mud, with bands of limestone which now form 
 almost the whole of the Grampian highlands, as 
 well as the ancient rocks which in many other 
 parts of the country peep out from below the 
 younger sediments. But these rocks, too, had 
 suffered so profoundly from the earth-movements 
 
106 GEOLOGY 
 
 that their primitive character is almost entirely 
 lost. The shales were altered to slates and glitter- 
 ing crystalline schists, the limestones to marbles, 
 the sandstones to hard quartzite. We cannot but 
 believe that they once enclosed the fossil remains 
 of the strange creatures of these early oceans. 
 Yet no trace of them is now to be found. 
 Volcanoes poured out their lavas over the sea-beds, 
 or intruded them among the sediments, while deeper 
 down, the intruded molten masses cooled to form 
 the granites of to-day. Among Britain's truly 
 ancient mountains we have the mutilated and 
 almost obliterated records of the early history of 
 the country. Here and there we may decipher a 
 fragment, but most of the story of those early 
 times must remain forever a part of the great un- 
 known. 
 
 From the lowest Cambrian rocks upwards, we 
 have the invaluable aid of the fossils to guide our 
 enquiries, and the main features of the subsequent 
 history are known with certainty. Before that 
 period began, it is clear that all the older rocks 
 had been greatly worn down by the elements. 
 Wherever the base of the Cambrian system can be 
 traced, it rests on an irregular surface of the rocks 
 below the second surface which we know this 
 country to have possessed. The Torridonian rocks, 
 for example, had been in many places completely 
 worn away again. The Cambrian rocks themselves 
 consist, in Wales, from whose classical name their 
 title is derived, of sandstones and slates (the latter 
 altered shales) containing a great variety of marine 
 fossils. In North-west Scotland, nearly two thou- 
 sand feet of limestone is included in the system. 
 On the whole, however, we may be confident that 
 
THE PHYSICAL HISTORY OF BRITAIN 107 
 
 these deposits were accumulated in fairly shallow 
 water, and as they reach in Merioneth a thickness 
 of upwards of twelve thousand feet, they therefore 
 clearly point to a gradual subsidence of the sea- 
 bed. And further, as these deposits occur in the 
 south and north of Wales, in Shropshire, and the 
 Malvern Hills, in parts of Ireland and north of 
 Scotland, it is fairly clear that the Cambrian sea 
 must have extended over the greater part of these 
 islands, though the rocks are now in most parts 
 either hidden under newer deposits, or completely 
 worn away again. 
 
 The Cambrian subsidence continued without 
 serious interruption throughout Ordovician and 
 Silurian times, and to such an extent that no less 
 than sixteen thousand feet of the former and five 
 thousand feet of the latter rocks were deposited. 
 The fineness of the mud which forms some of the 
 slates and shales of these systems would seem 
 even to indicate that the water over some areas 
 was at times of considerable depth. Yet the evidence 
 that some areas remained above the water, in 
 spite of the prolonged denudation to which they 
 must have been subjected, proves also that the 
 subsidence was not universal. In places, the country 
 must have been actually rising, and we must con- 
 clude, as usual, that what was really occurring was 
 not a bodily subsidence of the whole region, but 
 rather a slow folding of the earth's crust. There 
 is no doubt, however, that during this immensely 
 long period, the greater part of the British area 
 remained almost continually below the sea, with 
 only scattered islands peering above its waters; 
 by far the longest period of uninterrupted submer- 
 gence of which we have a certain record. The 
 
108 GEOLOGY 
 
 Ordovician rocks, which attain their greatest develop- 
 ment in Central Wales (and are named, therefore, 
 after the territory of the ancient Ordovices) consist, 
 like the Cambrians, mainly of sandstones and slates, 
 the famous slate-quarries of North Wales begin 
 mainly opened in them. Among the Silurian beds 
 (from the country of the Silures, to the east of 
 Wales) several thick bands of limestone occur, 
 often full of fossil corals. So abundant, indeed, 
 are the corals, that some of the beds have been 
 well described as fossil coral-reefs. 
 
 The advent of the Ordovician period was 
 marked by great and widespread outbursts of vol- 
 canic activity. All over North Wales and in the Lake 
 District volcanoes arose. Nearly the whole of the 
 Cader Idris range, and much of the Arenig and 
 Snowdon ranges, is composed of sheets of lava 
 and volcanic ash ejected at this period. In places, 
 the aggregate thickness of the volcanic materials 
 is not less than six thousand feet. The volcanoes 
 clearly, were of the first magnitude. Yet the 
 eruptions of the Lake District far exceeded those 
 of Wales. By far the greater part of the Cumbrian 
 group of hills is carved out of a continuous mass 
 of volcanic material which is estimated at from 
 ten to fourteen thousand feet in thickness. Even 
 the portion of the lavas now exposed extends over 
 an area about twenty miles by twenty-five, and this 
 is evidently but a small part of their original extent. 
 Thus, as one stands on the summit of Scawfell or 
 Helvellyn (both carved entirely out of this material) 
 and looks over the surrounding hills, one sees on 
 every hand the products of one of the greatest 
 volcanic outbursts this country has ever witnessed. 
 Throughout the Ordovician period local eruptions 
 
THE PHYSICAL HISTORY OF BRITAIN 109 
 
 continued, but the first great effort was never again 
 equalled. By the Silurian epoch, quiet was com- 
 pletely restored. 
 
 The prolonged subsidence of most parts of this 
 country, which had kept them for ages below the 
 open sea, at length ceased, or at least became slower 
 and less general. The waters became everywhere 
 shallow and perhaps land-locked, and the creatures 
 of the open sea were no longer able to live in them. 
 Under these conditions thick masses of coarse sand- 
 stone and marl accumulated, in those areas where 
 subsidence continued, to form the Old Red Sand- 
 stone system. The formation of these rocks may 
 have been comparatively rapid, seeing that the area 
 of land exposed to denudation in the immediate 
 neighbourhood was now much greater than during 
 the preceding periods. Yet, when we find these 
 sandstones sometimes upwards of 16,000 feet in 
 thickness, we must allow that they represent an 
 immense lapse of time. During this period, again, 
 the volcanic fires broke loose, this time principally 
 in Central Scotland ; and the Sidlaw and Ochil Hills, 
 among others, are mainly composed of their lavas 
 and ashes. One part of the country remained 
 throughout below the open sea the extreme south. 
 In that region beds of sand, mud and limestone, 
 full of marine shells and corals, continued to gather, 
 while elsewhere the Old Red Sandstone was form- 
 ing. Everyone must be familiar with the beautiful 
 so-called " marbles " of Devonshire, so largely 
 quarried and used in polished slabs for ornamental 
 purposes, which are the limestones of this 
 " Devonian " system. 
 
 The continued earth-movements at length raised 
 the entire country, except possibly the south, com- 
 
110 GEOLOGY 
 
 pletely above sea-level. For the third time, at least, 
 the British area became part of the great continent, 
 The Old Red and Devonian rocks, along with all the 
 older ones, were greatly folded, and exposed for a 
 prolonged period to denudation. In many parts, 
 thousands of feet of rock were worn away during 
 this " continental " period, and the whole region was 
 carved anew into hill and valley. Hence it was a 
 very uneven land-surface which at length subsided 
 again below the ocean, and over which in due course 
 the Carboniferous rocks were laid down. The sub- 
 mergence began in the south, and there the corals, 
 sea-lilies and shells began to build up the Carbon- 
 iferous limestone while the central and northern 
 parts of Britain were still above water. The shore- 
 line crept slowly northward, the advancing sea 
 converting the hills into islands, and ultimately 
 submerging them. At the foot of the -Grampians 
 the onward march of the sea was stopped, and 
 throughout the Carboniferous period great masses 
 of land persisted in that direction. The succession 
 of events during this age has already been noted 
 the shallowing of the sea, and consequent covering 
 of the limestone with great sandbanks, now forming 
 the Millstone Grit, followed by the conversion of the 
 whole tract into a swampy jungle which periodically 
 subsided, but was repeatedly brought above water 
 again by new accumulations of sand and mud, and 
 whose successive coverings of vegetation are now 
 preserved to us as beds of coal. Already, before 
 this formation of the Coal Measures was complete, 
 renewed " warping " of the country began, raising 
 some areas into high land, while others were 
 depressed and became covered with land-locked 
 arms of the sea. The Pennine Chain is one of the 
 
THE PHYSICAL HISTORY OF BRITAIN 111 
 
 ridges produced at this period. In the enclosed seas 
 the Permian rocks gathered. To the east of the 
 Pennine ridge some limestone was formed, but to 
 the west bright red sandstones and clays alone 
 gathered. Meanwhile, the ridged-up Carboniferous 
 rocks were extensively denuded, and the Permian 
 strata in many parts rest over their worn edges. 
 The waters of the enclosed seas became intensely 
 salt, supporting only a dwarfed assemblage of living 
 creatures and leaving deposits of salt and gypsum 
 here and there among the sediments. 
 
 Again Britain was raised completely beyond the 
 reach of the waves, in common with all Central and 
 Northern Europe, for its fourth known period of 
 continental existence. So far was the region 
 thoroughly continental at this time, that, for a 
 lengthy period, desert conditions appear to have 
 prevailed. In this desert, the "Bunter" ("mottled") 
 sandstones of the Triassic system were formed. 
 This soft sandrock, with its warm yellow and red 
 tints, is a well-known feature in the Midland counties 
 and elsewhere. Subsequently a great Y-shaped lake 
 gathered in the desert, its arms enclosing the Pen- 
 nine area, and its " tail " reaching at least as far as 
 our present south coast. Perhaps, like the present 
 Lake Chad in the Sudan, it was a mere film of water 
 which often entirely vanished. That it did so at 
 times is sufficiently proved by the fact that the 
 animals of the period roamed about and left their 
 footmarks all over its bed. Certainly it became 
 intensely salt, and the famous salt-beds of Cheshire 
 were deposited from its waters. Its chief relic is 
 the thick mass of bright red clay which forms the 
 upper portion of the Triassic system. 
 
 Throughout the remainder of the Mesozoic epoch, 
 
112 GEOLOGY 
 
 Britain was reduced to an essentially insular condi- 
 tion, though no doubt it was now and then connected 
 with such parts of the continent as happened to be 
 above water. Subsidence during this period took 
 place mainly in the eastern and southern parts of 
 England, while the rest of the British area was 
 doubtless, on the whole, rising ; only small parts of 
 it being, from time to time, submerged. In the 
 south-eastern sea, beds of sand, clay and limestone 
 gathered in the most varied succession, pointing to 
 an oscillating sea-level, and giving rise to the varied 
 series of rocks which now characterise that portion 
 of the country. 
 
 During the Liassic part of this period, the sea 
 spread widely over these islands, but later, in early 
 Oolitic times, the land-area became more extensive. 
 The north-east of England at this time was the site 
 of a great estuary, till subsidence once more gained 
 the upper hand in that area, and carried it below the 
 sea of the Upper Oolites. At the beginning of the 
 Cretaceous period the conditions were reversed. 
 The south-east of the country was now an estuary, 
 opening eastwards, while the more northern parts 
 were under deep water. This latter state of affairs 
 was shortly followed by the closing scene of the 
 Mesozoic epoch, when all the centre and south of 
 Europe, levelled down by the long-continued 
 denudation, sank deeply to be covered by a great 
 " epi-continental " ocean the ocean of which our 
 present Mediterranean is the last diminished 
 remnant. On the bed of this ocean there gathered 
 vast deposits of fine grey ooze, in essential char- 
 acters like the ooze which is gathering to-day on the 
 bed of the Atlantic, and formed, like it, mainfy of 
 countless myriads of the microscopic shells of 
 
THE PHYSICAL HISTORY OF BRITAIN 113 
 
 Foraminifera. That ooze now forms the white 
 chalk cliffs of Albion. 
 
 We come now to the thoroughly modern ages of 
 geological history. The general subsidence of the 
 Chalk period was closely followed by an equally 
 wide-spread elevation. Europe resumed its full 
 continental form, and included Great Britain within 
 its bounds. Very extensive geographical changes 
 clearly took place, which resulted in the invasion of 
 Europe by entirely new forms of life, both animals 
 and plants. For a very long period Britain remained 
 entirely continental. The Eocene rocks, however, 
 mark the encroachment of the sea over the south-east 
 of the country. At first, continuous shallow sea 
 stretched from the H umber to Portland, but later 
 it was divided by the elevation of a ridge which now 
 forms the watershed between the Thames basin and 
 the rivers of the south. The rising of this arch 
 above the waves was the first sign of a series of 
 great earth-movements which culminated in Miocene 
 times. In Britain itself, the chief result of these 
 movements was the formation of this fold (of which 
 the North and South Downs are, so to speak, the 
 worn edges) and the elevation of the entire country 
 above the sea before the Miocene period. But on 
 the continent, far more stupendous effects were 
 produced : nothing less, in fact, than the formation 
 of the Alps themselves, which were then raised to 
 their present eminence, or perhaps much higher. 
 Could we have been present, nevertheless, it would 
 not have been the raising of the Alps which would 
 have impressed us. Apart from earthquakes, that 
 would have been imperceptible. On our own 
 western shores much more striking phenomena 
 would have borne witness to the unquiet state of 
 
 H 
 
114 GEOLOGY 
 
 the earth. From Ireland to Iceland one long line 
 of volcanoes broke out, with all the vigour which 
 characterises a series of eruptions following a long 
 period of inactivity for the whole Mesozoic period 
 had been unmarked by a single outbreak. Indeed, 
 unable to escape rapidly enough through single 
 vents, the lava rose at times simultaneously through 
 countless fissures in the rocks, and buried thousands 
 of square miles of country under vast lava-floods. 
 Almost the whole of the county of Antrim is under- 
 lain by one of these lava-sheets, or rather by a small 
 remnant of one, while Arran, Mull, Skye, and many 
 other of the Western Isles of Scotland are largely 
 the worn-down stumps of the volcanoes. 
 
 We have noted that during Miocene times Britain 
 was entirely above the waters of the sea. When 
 East Anglia sank again under the early Pliocene 
 sea, considerable changes had occurred in the 
 marine life since Oligocene times. While at the 
 latter period the local sea-fauna had an almost 
 tropical aspect, it had now become temperate in 
 character. The climatic change continued through- 
 out the Pliocene period. At its close, the climate 
 was almost arctic, and became thoroughly so in the 
 early stages of Pleistocene history. We may imagine 
 the snow gathering on our mountain-tops, and the 
 glaciers forming and creeping ever further down 
 their sides. The valleys were filled with ice, which 
 united in vast sheets over the lowlands till the whole 
 country north of the Thames and Severn was buried 
 under one vast ice-field as completely as modern 
 Greenland. All Central and Northern Europe lay 
 under the same icy mantle, and the British ice 
 united with that of Scandinavia over the bed of the 
 North Sea. For many thousands of years the 
 
THE PHYSICAL HISTORY OF BRITAIN 115 
 
 surface of this country was ground and polished 
 under the slowly streaming field of ice, and the 
 smooth outlines which now characterise British 
 scenery are certainly due in no small measure to this 
 Great Ice Age. While the ice still persisted, a 
 general submergence of the lowlands appears to 
 have occurred. Into the water the ice dropped pell- 
 mell its burden of sand, mud and boulders, thus 
 covering the country with that superficial coating of 
 sand and gravel which to-day hides the solid rocks 
 almost everywhere except among the hills, and out 
 of which thousands of ice-scratched boulders may 
 be gathered. Slowly the country rose again, the 
 climate ameliorated, and the ice shrank back into 
 the hills. During their retreat, the glaciers left 
 many a moraine of debris behind them, still to be 
 recognised in hundreds among our highland valleys. 
 The sea left its mark in the raised beaches round 
 our coasts, and so the country reached the condition 
 in which we now find it. The story of Geology is 
 ended ; the work of the antiquarian and historian 
 begins. 
 
116 GEOLOGY 
 
 CHAPTER VIII 
 
 THE HISTORY OF LIFE ON THE EARTH 
 
 BRIEF and fragmentary as these sketches of 
 Earth-history necessarily are, we cannot conclude 
 them without a reference to the most fascinating 
 story which the rocks contain that of the life of 
 former ages. We saw in the preceding chapter that 
 each great stratum of the sedimentary rocks con- 
 tains its own peculiar assemblage of fossil remains, 
 which differ from those of any other stratum, but 
 are generally most similar to those in the rocks 
 immediately above or below. Thus, while the 
 organisms found fossilised in the uppermost (and 
 therefore newest) strata differ very slightly from the 
 creatures still living in the same region, those in the 
 successively lower (and therefore older) rocks 
 appear more and more unlike the existing forms, 
 until, in the oldest rocks, we find ourselves among 
 the most strangely unfamiliar and most primitive 
 creatures. Moreover, in our receding glance, we 
 lose sight of whole groups of animals one after 
 another ; and it is always the most highly developed 
 creatures which are the first to be lost in our 
 backward march. These few simple facts that the 
 inhabitants of the world have gradually changed, 
 that there has been a constant approximation to the 
 forms now living, and that the groups of animals 
 low down in the scale of life have always appeared 
 before those of a higher organisation are the grand 
 features of the support given by Palaeontology (as 
 

 TYPICAL FOSSILS. 
 
 1. Trilobite crustacean. Calymene Blumenbachii, 
 
 Wenlock Limestone. 
 
 2. Brachiopod. Spirifer striatus, Carboniferous Limestone. 
 
 3. Cephalopod. Ammonites (Aigoceras) capricornus, Lias. 
 
 4. Echinoid. Cidaris florigemma, Corallian. 
 
 5. Gastropod. Valuta luctatrix, Eocene. 
 
 6. Pelecypod. Cardita planicosta, Eocene. 
 
 Plate IV. \ [Geotogv, 116. 
 
THE HISTORY OF LIFE ON THE EARTH 117 
 
 the study of extinct creatures is called) to the 
 doctrine of Evolution. If life has been continuous 
 on the earth from the beginning, and if every new 
 form which has appeared has resulted from the slow 
 modification of some preceding form, then the signifi- 
 cance of those facts is clear ; if not, then the whole 
 of palaeontology and biology is unintelligible and 
 incapable of scientific explanation. With these few 
 introductory remarks, let us turn to the rocks to 
 learn a few of the facts they have for so many ages 
 preserved for us. 
 
 In the most ancient strata in which fossils are 
 still to be found in a recognisible condition, we meet 
 already with a large and varied assemblage of 
 organisms. Nearly all, that is to say, of the great 
 groups of animals lower in the scale of life than the 
 Fishes, are represented, though for the most part 
 by forms very different to those now living. The 
 question naturally arises, therefore, how all these 
 forms came to be evolved so early ? In reply, the 
 geologist points to those great masses of altered 
 rocks, for the most part crushed and twisted almost 
 out of recognition, of which we can only say that 
 they were formed before the Cambrian period. 
 That many of them once contained relics of still 
 earlier life, now destroyed, is practically certain, and 
 the high development of life in Cambrian times only 
 confirms what on other grounds seems probable 
 that many as must be the millions of years which 
 have elapsed since the Cambrian rocks were formed, 
 the period which preceded it must have been fully 
 as long. All, or nearly all, of the history of those 
 earlier times has been irretrievably lost by the 
 alteration of the rocks and consequent obliteration 
 of their records. 
 
118 GEOLOGY 
 
 The lowliest organisms have played no inconsider- 
 able part in the past history of the world. In the 
 oldest fossil-bearing rocks the almost microscopic 
 shells of the Foraminifera are found, while they 
 have contributed not a little towards the building of 
 the limestones of various ages. The Chalk, we have 
 already seen, consists not uncommonly mainly of 
 their remains, though some thousands of their shells 
 may be gathered in a thimble; and the Tertiary 
 limestones of Egypt (of which the pyramids are 
 built), and some of the Eocene limestones in the 
 neighbourhood of Paris and elsewhere in Europe 
 are formed largely of the shells of giant members of 
 the race. To the palaeontologist, however, the fossil 
 foraminifera are of interest mainly as illustrating 
 the persistence of lowly forms of life. While the 
 higher creatures come and go, changing more 
 rapidly in proportion to their complexity, these 
 simple creatures persist from age to age, almost 
 unchanged by the hand of Time. Even in the 
 Palaeozoic rocks, the foraminifera have a remarkably 
 modern aspect, some of the forms being scarcely 
 distinguishable from those now living, while the 
 higher creatures whose remains are preserved in 
 the same rocks are all unfamiliar. 
 
 The Corals, too, we have noted as rock-building 
 creatures in all ages. More highly organised than 
 the Foraminifera, they have undergone greater 
 changes during their history. The group which 
 includes the typical reef-builders of the present day 
 did not appear in the world till the early part of the 
 Mesozoic epoch. Their ancestors were already 
 living in Ordovician times, and certainly long before, 
 but they were in some respects different from the 
 modern forms. While the latter build skeletons 
 
THE HISTORY OF LIFE ON THE EARTH 119 
 
 which are usually radially symmetrical, their palaeo- 
 zoic ancestors formed skeletons of a bilateral 
 character, i.e., with distinct right and left halves. 
 In this change the corals exemplify a modification 
 which has nearly always come over animals which 
 have adopted a sedentary mode of life. Fossil 
 corals are among the most familiar of objects. Our 
 mantel-pieces and other objects of coloured "marble" 
 (really limestone) seldom fail to show the beautiful 
 lace-like forms of the corals they contain. Especially 
 is this the case with the well-known red marbles of 
 Devonshire. 
 
 The " precious corals " of to-day belong to a group 
 which has long since passed its prime. Though now 
 comparatively rare, their ancestors abounded in the 
 Palaeozoic seas, and contributed largely to the coral 
 reefs of those early ages. 
 
 Of shell-fish (apart from lobsters, shrimps, etc.) 
 there are two widely distinct classes. The Mollusca 
 include all the common sea-shells and land-shells of 
 the present day, and are now in the prime of their 
 existence. The Brachiopoda, or "lamp-shells," on 
 the other hand, are rarely seen by the ordinary 
 person outside a museum. Yet it is to the latter 
 group that by far the greater number of the countless 
 shells found in the Palaeozoic rocks belong. (See 
 Plate iv, fig. 2.) We have here another group of 
 animals whose day is long past ; and we may learn 
 from them an oft-repeated story. During their 
 prosperous days they gave rise to many varied and 
 complicated forms; yet the few which survive are 
 relatively simple. Again and again we are faced 
 with the same fact. Highly complex and specialised 
 organisms may succeed in the struggle for existence 
 for a time, but it is the relatively simple organisa- 
 tion which endures. 
 
120 GEOLOGY 
 
 The Mollusca are represented by their three chief 
 groups in the earliest fossiliferous rocks. The two 
 simpler groups, viz., the " Bivalves " (including 
 oysters, cockles, etc.) and the " Univalves " (or snail 
 group), have steadily increased in variety, complexity 
 and numbers throughout past ages (see Plate iv, 
 figs. 5 and 6). It is the highest group, again, 
 represented now by the cuttle-fish, octopus, and 
 nautilus, which ran through its development most 
 quickly, and is to-day in a very decadent condition. 
 In Palaeozoic and Mesozoic times these " Cephalo- 
 pods" were much more abundant. The great 
 family of Mesozoic cephalopods the Ammonites 
 are probably the most familiar and popularly prized 
 of all fossils. The beauty of their volute-like shells 
 is seldom excelled, while their exceeding abundance 
 ensures their familiarity. (See Plate iv, fig. 3.) 
 
 The history of the great group of animals to 
 which the star-fish and sea-urchins belong the 
 Echinodermata is very fully recorded in the rocks. 
 While the primitive stock consisted of more or less 
 globular or egg-shaped creatures, covered with an 
 irregular mosaic of calcareous plates, they developed 
 along a variety of diverging lines, after having first 
 become a typically sedentary group and acquired in 
 consequence a general radial symmetry. Some, 
 becoming first more definitely globular, and de- 
 veloping great regularity in the arrangement of 
 their plates, arming themselves all over at the same 
 time with an elaborate defence of spines, gave rise 
 to the existing group of the sea-urchins, which in 
 later times have in many cases lost their spherical 
 form, becoming more or less flattened or even 
 biscuit-shaped (see Plate iv, fig. 4). Another series, 
 having many of their covering-plates reduced to 
 
THE HISTORY OF LIFE ON THE EARTH 121 
 
 mere nodules in the skin, and developing also five 
 finger-like lobes or " arms " to the body, became the 
 starfishes. But a much more striking series of 
 modifications began with the attachment of some 
 forms to the rocks of the sea-bed by means of a 
 flexible jointed stalk. Subsequently, these attached 
 forms developed a set of five jointed, slender, and 
 often much-branched feathery arms, the whole 
 creature thus becoming very plant-like in appearance. 
 A few of these " sea-lilies " or Crinoids still exist, 
 mostly in the deep sea the refuge of many " living 
 fossils " but they reached the height of their glory 
 in Palaeozoic times, and not a little of the Carboni- 
 ferous limestone of the Pennine Chain is mainly 
 composed of their jointed stems. 
 
 Passing now over the remainder of the lower 
 forms of life, let us turn to the higher creatures, 
 whose geological history must ever be of the greater 
 interest to the palaeontologist and to the student of 
 evolution. This must be the case for two reasons : 
 in the first place, the history of the higher animals 
 is or will be known from the beginning: while the 
 great groups of the lower animals had all been 
 evolved before the period at which the geological 
 record begins to be legible, so that we can only 
 trace the details of their subsequent modification. 
 The starting point of each of the higher groups, 
 from the fishes onwards, lies within the range of 
 known geological history. Secondly, the structure 
 of these more advanced forms can be known with 
 much greater completeness. In the case of the 
 lower animals, the only part capable of preservation 
 in the fossil state is usually some form of shell, or 
 external covering, which as a rule conveys little 
 information as to the structure of the real living 
 
122 GEOLOGY 
 
 tissues of the animal. On the other hand, as every- 
 one knows, the higher forms possess an internal 
 skeleton, consisting of a skull, back - bone, and 
 supports for the limbs ; and the bones of this skeleton 
 are related so intimately to the structures around 
 them that it is possible, from their study, to recon- 
 struct almost every detail of the animal's body. 
 The muscles, nerves, and blood-vessels can all be 
 restored with more or less completeness, and the 
 dry bones made to live again. 
 
 The fishes present many points of interest in their 
 past history. The gradual elaboration of their 
 scales, teeth, fins, tails, and general skeleton can all 
 be more or less perfectly traced, though most of 
 these features require some technical knowledge 
 for their comprehension. Being much the most 
 primitive of the vertebrate animals, the fishes are 
 naturally the first to appear in order of time viz., 
 during the Silurian period. In the same connection 
 it is of interest to note that the simplest group of 
 the fishes is the earliest to be developed and 
 elaborated. The class which contains the sharks, 
 dog-fish, and skates is in many respects more 
 primitive than the great mass of living fish. While 
 the majority of fish, like all higher animals, have a 
 skeleton of bone, the shark class has only cartilage, 
 which never becomes replaced by bone as in the 
 higher forms; and it is to this cartilaginous or 
 "shark" class that nearly all the early fossil fish 
 belong. In the Palaeozoic period they formed the 
 dominant group, while it was only in the Mesozoic 
 period that the " bony " fish became abundant. Of 
 still greater interest is an even more primitive group 
 of so-called fishes for it may be doubted whether 
 they ought to be classed as fish at all the hags and 
 
THE HISTORY OF LIFE ON THE EARTH 123 
 
 lampreys. This little group is the sole remnant of 
 a class of primitive vertebrates which had a short 
 but brilliant career during the Silurian and Old Red 
 Sandstone periods, while fish life, if we may so put 
 it, was still in a very experimental stage. Seldom 
 has any group of creatures produced more truly 
 archaic-looking forms than these relatives of the 
 hag-fish in the Old Red Sandstone waters. Yet 
 many of the other contemporary fish were little less 
 remarkable, the entire Old Red Sandstone group 
 forming a most fascinating collection. Those fish 
 enticed Hugh Miller to geology, and the reader may 
 still turn to his " Old Red Sandstone " and " Foot- 
 prints of the Creator " for the most lively accounts 
 of them. 
 
 Geological history is still silent regarding perhaps 
 the most notable event in the history of life on the 
 earth. We can only surmise that sometime previous 
 to the Carboniferous period, probably during the 
 preceding Devonian times, some type of fish ac- 
 quired the power of breathing the oxygen in the air, 
 as well as that dissolved in the waters, and at the 
 same time had its paired fins modified into true 
 limbs which enabled it to crawl on to the land. 
 Meantime we must patiently wait the day when some 
 fortunate blow of the hammer shall disclose the 
 remains of these forerunners of the amphibia, 
 reptiles, birds, and mammals. The earliest Car- 
 boniferous and perhaps the highest Old Red 
 Sandstone strata contain the footprints of four- 
 footed creatures the first traces of them yet found. 
 Somewhat higher in the Carboniferous rocks the 
 first skeletons of the amphibia occur salamander- 
 like creatures, of very varied size. In many 
 respects these early amphibia were more primitive 
 
124 GEOLOGY 
 
 than their living descendants. In particular, many 
 of them appear to have breathed during youth by 
 means of external gills, like the tadpole of the frog. 
 They show also many other curious points of 
 resemblance to certain of the Old Red Sandstone fish 
 in the form of their teeth, the development of 
 certain protective plates in the eye, the arrangement 
 of the bones in the skull, and the presence of scaly 
 plates over a portion of the body resemblances which 
 can scarcely be altogether accidental. While some of 
 these early amphibia were no bigger than a modern 
 newt, others became very large and massive, one 
 having a skull no less than four feet in length. 
 
 The most highly developed of the Palaeozoic 
 amphibia grade almost imperceptibly into the most 
 primitive of the reptiles, which make their first 
 appearance in the Permian rocks. This transition 
 from the amphibia to the reptilia is made doubly 
 interesting by the fact that the reptiles themselves 
 almost immediately gave rise to forms which present 
 the most remarkable resemblances to the mammalia 
 in almost every part of their anatomy, resemblances 
 so strikingly complete that some of the best authori- 
 ties now believe that these primitive reptiles were 
 the direct ancestors of the mammals. To this point, 
 however, we must return later. 
 
 The history of the reptiles is undoubtedly the 
 most romantic of any group of organisms. The 
 person whose knowledge of the group was confined 
 to the living crocodiles and alligators, tortoises, 
 turtles, snakes, and lizards, would little guess that 
 the reptiles had once attained a dominance over the 
 world such as no other group of animals has ever 
 approached. Immediately on their appearance in 
 the world they evinced a most wonderful capacity 
 for modification and adaptation, and so gave rise to 
 forms suited to every kind of environment and mode 
 of life. Fleet, agile forms more delicate than the 
 cat or the deer were associated with powerful 
 massive creatures more reminiscent of the hippo- 
 
THE HISTORY OF LIFE ON THE EARTH 125 
 
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126 GEOLOGY 
 
 potamus or the elephant and the variation in size 
 was much greater. Several groups became modified 
 for amphibious, and ultimately for completely 
 aquatic life. On the other hand, one group, the 
 Pterodactyls, developed a bat-like form and gave 
 rise to a large number of flying reptiles. Two 
 groups of these creatures quite overshadow all the 
 others in the magnificence of their development. 
 Some of the Deinosauria have long been familiar to 
 English readers from the classic descriptions by Dr. 
 Mantell of the remains of Iguanodon, Hylaeosaurus, 
 and other forms from the rocks of Tilgate forest in 
 Sussex. But perhaps few persons would be more 
 surprised than Mantell, could he return to review 
 the Deinosaurs now known. The name, which 
 signifies "terrible reptiles," was appropriately ap- 
 plied to the forms first known reptiles ten or 
 fifteen feet in height, and much more in length. 
 How much more appropriate would it have seemed 
 had the American Brontosaurus, sixty feet in length, 
 been then known ; or the still larger Atlantosaurus ? 
 We need scarcely add that they were the largest 
 land animals which have ever existed. And yet the 
 same group includes the fleet little creatures no 
 larger than a rabbit, already referred to. It would 
 be quite impossible to do justice to this wonderful 
 group without lengthy descriptions and numerous 
 illustrations. They were the land animals par 
 excellence of the Mesozoic age. The great size of 
 many of them, the wonderful variety of horns and 
 scaly armour with which many were provided, have 
 well merited them the appellation " the dragons of 
 their prime." While the largest forms were adapted 
 to feed on the vegetation of the period, they were 
 kept in check by the scarcely less formidable 
 carnivorous species. 
 
 Little, if at all, inferior to the Deinosaurs in 
 abundance, variety, and size were the great marine 
 reptiles belonging to the groups of the long-necked 
 Plesiosauria and whale-like Ichthyosauria, which are 
 
THE HISTORY OP LIFE ON THE EARTH 127 
 
 probably the most familiar of all fossil vertebrates. 
 As the Deinosaurs dominated the land, so they must 
 have been supreme in the oceans. There is a 
 striking analogy between these great marine reptilia 
 and the whales among the mammals. Both attained 
 enormous size, the Plesiosaurs rivalling the whales 
 in this respect, both had their structure modified in 
 a similar manner for aquatic life, both were de- 
 scended from comparatively small land-dwelling 
 ancestors. 
 
 The ancestors of the true crocodiles and alligators 
 also existed in the Mesozoic seas or, rather, on the 
 shores, and were, on the whole, remarkably similar 
 to the modern forms in their structure. In general 
 appearance and in habits, they were probably 
 scarcely to be distinguished from their living 
 representatives. True tortoises also appeared as 
 early as the Triassic period, with their typical 
 shield-like armour. The snakes and lizards are 
 scarcely known among fossil forms; but in the 
 Cretaceous period there existed a group of marine 
 reptiles closely allied to them true " sea-serpents " 
 in fact, and some of these, again, attained enormous 
 size, probably sixty feet in length. 
 
 Far from the least remarkable feature in the 
 history of the reptiles is their sudden extinction at 
 the end of the Mesozoic era. In the Cretaceous 
 period the Plesiosaurs and Ichthyosaurs were, it is 
 true, already in a decadent condition, but they were 
 replaced by the no less formidable sea-snakes. The 
 Deinosaurs were represented by their largest forms, 
 and the flying reptiles likewise included species 
 measuring no less than twenty-seven feet across the 
 wing. The reptiles, in fact, were undisputed masters 
 of air, sea, and land. Yet, without any obvious 
 cause, all these dominant groups died out simultan- 
 eously, leaving, at the dawn of the Tertiary era, 
 only the same contemptible group of "creeping 
 things " which are the reptiles of to-day. 
 
128 GEOLOGY 
 
 The history of the birds is very lightly sketched 
 by their fossil remains. What there is of it, how- 
 ever, is of great interest. The earliest known bird, 
 Archaeopteryx, appears only in the middle of the 
 Mesozoic era in the Corallian rocks of Germany. 
 Outwardly a typical bird in the possession of a 
 perfectly normal coat of feathers, closer inspection 
 readily reveals a number of deeply significant 
 features. In place of a horny beak, the jaws are 
 armed with simple rows of conical teeth, set in 
 sockets in the normal manner. The wings possess 
 three (possibly four) well developed grasping fingers 
 armed with claws, as well as the one greatly en- 
 larged digit which carries the wing feathers. But 
 perhaps the most remarkable feature of all is 
 the long graceful tail, which, instead of consisting 
 of a simple bunch of feathers, as in all living birds, 
 was rather the tail of a reptile, an extensive flexible 
 prolongation of the back-bone, each joint of which 
 carried a pair of feathers; surely as incongruous a 
 combination of characters as nature ever produced. 
 Indeed, in all characters in which Archaeopteryx 
 differs from the living birds, it assimilates to the 
 reptiles, and points unmistakably to the reptilian 
 ancestry of the former. Curiously enough, how- 
 ever, it was not to the flying reptiles that the birds 
 were immediately related, but rather to the Deino- 
 saurs, many of which presented several bird-like 
 modifications. Up to the end of the Mesozoic era 
 the birds retained the teeth in their beaks, but the 
 other special marks of their reptilian descent 
 speedily disappeared. 
 
 The history of the mammals is peculiar. We 
 have already had occasion to refer to the quite 
 extraordinarily mammal-like character of some of 
 the early reptiles of the Permian and Triassic rocks 
 which lead to a strong suspicion that the mammals 
 are descended from them or their near relations. 
 Now the earliest mammalian remains are, in fact, 
 found in the highest part of the Triassic rocks ; yet 
 
THE HISTORY OF LIFE ON THE EARTH 129 
 
 they are, as far as the meagre evidence indicates, 
 far from being the kind of mammal we should have 
 expected to arise from the reptiles referred to. 
 They are puny creatures, of very primitive type, 
 about the size of a rat. And these little beasts are 
 the only known forms throughout the Mesozoic era. 
 During the early part of the Cainozoic era, in 
 Europe and in America, the principal types of the 
 mammalia appear in rapid succession, the majority 
 of forms showing no special relationship to the 
 little creatures already referred to, or any other 
 indication of their ancestry. These facts have been 
 explained on the assumption that we are dealing 
 with a group of immigrants from some unknown 
 region, whose advent was now made possible by the 
 extensive geographical changes which marked the 
 close of the Mesozoic and dawn of the Cainozoic 
 eras. If true, until that unknown region is dis- 
 covered, the history of the early evolution of the 
 mammalia must remain unread. 
 
 When the mammals, then, for all practical pur- 
 poses, first appeared in Europe and America, the 
 great families were already marked out. Neverthe- 
 less, as by way of compensation, the story of their 
 subsequent development is preserved with wonder- 
 ful completeness, and we already know in great 
 detail the history of the development of several 
 important types. 
 
 The story of the horse has become classic. That 
 familiar animal is one of the most specialised of the 
 living mammalia, at least, as regards the structure 
 of its limbs. The so-called " knee " of the horse is 
 really its ankle or wrist, the "shin" or "cannon" bone 
 being the first or " palm " joint of the single finger 
 or toe which each limb possesses, while the hoof is 
 of course the enlarged "nail" covering the last joint. 
 A greater contrast could scarcely be imagined than 
 that between this lower portion of the limb of a horse 
 and the hand 01 foot of a man which latter repre- 
 sent the normal mammalian type. Yet, as we trace 
 
 i 
 
130 
 
 GEOLOGY 
 
 FIG. 12A. DEVELOPMENT 
 OF THE SKULL OF THE 
 ELEPHANT. 
 
 After Andrews. 
 
 1. Moeritherium 
 
 Middle Eocene. 
 
 2. Palceomastodon 
 
 Upper Eocene. 
 
 3. Tetrabelodon 
 
 Miocene. 
 
 4. Tetrabelodon 
 
 Miocene and Pliocene. 
 
 5. Elephas 
 
 Pliocene and Living. 
 
 FIG. 12B. DEVELOPMENT OF 
 THE LIMBS AND TEETH OF 
 THE HORSE. 
 
 1. Orohippus Eocene. 
 
 2. Mesohippus Miocene. 
 
 3. Miohippus Miocene. 
 
 4. Protohippus Pliocene. 
 
 5. Pliohippus Pliocene. 
 
 6. Equus Living. 
 
 N.B. There should really be a 
 gradual increase in size from 1 to 6 
 as noted in text. 
 
THE HISTORY OF LIFE ON THE EARTH 131 
 
 the ancestors of the horse back stage by stage 
 through the successive stages of the Cainozoic rocks, 
 we find the specialisation of its limbs becoming 
 gradually less marked till an almost perfectly normal 
 type is reached, in a manner which is conveyed 
 much more clearly by the accompanying illustration 
 than it could be by any description. (See fig. 12b.) 
 Along with these modifications in the limbs other 
 changes proceeded ; the size of the animal, which, 
 to begin with was no larger than a collie, gradually 
 increased, and, more notably, the teeth gradually in- 
 creased in complexity from a very simple form to 
 their present highly specialised condition. 
 
 More recently the history of another very special- 
 ised group of mammals has become very completely 
 known that of the elephants. Until a few years 
 ago we only knew the latter part of their story, 
 because the earlier history was buried in the rocks 
 of Egypt. The elephant shows its specialised 
 character partly in its great size, but more particu- 
 larly in its highly modified teeth and jaws. Besides 
 the pair of enormously developed teeth which form 
 the tusks of the ordinary elephant, there is at any 
 one time on one side of each jaw only one other 
 large tooth to be seen, or at most a portion also of a 
 second one. Nevertheless the size of each tooth is 
 such that it almost completely fills the side of the 
 jaw; and the complexity of the teeth is commen- 
 surate with their size. The "crown " is enormously 
 high, and, instead of bearing merely a few tubercles 
 on the surface, as a normal tooth, these are modified 
 into large transverse vertical plates ranged com- 
 pactly one behind the other to the number of over 
 twenty, the whole forming the most efficient tooth for 
 grinding vegetable material that Nature has ever 
 devised. Again, though but one tooth at a time ap- 
 pears on each side of the jaw, the animal during its 
 life has several such teeth successively in the same 
 position. These successive teeth, however, do not 
 represent successive "sets" in the proper sense, but 
 
132 GEOLOGY 
 
 are all members of the one permanent set, which, 
 unable from their enormous size to appear 
 simultaneously, have been forced to become succes- 
 sional. Traced back to the Eocene rocks of Upper 
 Egypt, the elephants are found to have their 
 descent from a small animal which has been named 
 Moeritherium, a creature about the size of a pony, 
 with an almost perfectly typical set of low-crowned 
 tuberculate teeth, and no tusks more striking than 
 those of a modern boar. In the later Eocenes, 
 larger forms occur with more prominent tusks (in 
 both lower and upper jaws), with the "front" or 
 incisor teeth already obliterated, and the " chin " of 
 the lower jaw elongated in the direction of 
 the forwardly-projecting tusks, so as to form, 
 with them, a kind of " spade." Evolution con- 
 tinued in the same direction to the early Miocene 
 period, when the chin was extraordinarily elongated, 
 and the teeth larger and correspondingly less 
 numerous. Subsequently the chin became again 
 reduced, and the nose began to develop into the 
 familiar " trunk," while the line of descent leading 
 to the modern elephants branched off in forms 
 possessing tusks only in the upper jaw. All the 
 while the teeth grew larger and more complex, the 
 tusks in particular became enormously developed, 
 as also the trunk, till the line reached its maximum 
 development in forms including the recently extinct 
 Mammoth, some of which were much greater in 
 bulk than any living elephant. 
 
 Finally, one other line of mammalian descent 
 may be briefly referred to that of the deer. This 
 line presents many features in common with that of 
 the horse, to which it is somewhat closely allied. 
 The teeth present a practically identical evolution, 
 albeit it has never reached so advanced a stage. 
 The limbs also have specialised in a similar manner, 
 with, however, this important difference : that 
 instead of only one digit having persisted, two are 
 preserved; but this pair of digits has almost 
 
THE HISTORY OF LIFE ON THE EARTH 133 
 
 completely fused together, thus giving the same 
 effect as in the horse, and exemplifying the 
 oft-repeated truth that Nature may attain the 
 same end by quite diverse means. It is to the 
 history of the antlers, however, that we wish 
 to refer. The earliest known deer occur in the 
 upper Eocene, but are quite devoid of antlers ; and 
 not till the later Miocene period were these 
 developed. When they did appear they were quite 
 simple in form, with a single prong or " tine." As 
 we trace the deer through their later development 
 we find the number of tines gradually increasing, 
 and the whole antler becoming larger and more 
 complex, till it reached its maximum in the magnifi- 
 cent crown of the Deer found fossil in the Irish 
 peat-bogs, with antlers ten feet across. Now, as 
 everyone knows, this history of antler development 
 is repeated by every deer in the course of its own 
 life. Its first pair of antlers is simple, but each 
 successive pair becomes more complex till it attains 
 complete maturity and becomes a royal stag. Here 
 we have a striking illustration of the fact that an 
 animal, in the course of its development, repeats to 
 some degree the stages of evolution through which 
 its ancestors have passed it "climbs its family tree" 
 and the study of development, Embryology, 
 materially aids us, therefore, to supplement and 
 interpret the fragmental palaeontological history 
 of life. 
 
 Here we must conclude these brief " chapters " 
 out of the vast history of the earth, with the hope 
 that they may stimulate the reader who has come 
 thus far to go a little further. 
 
 THE END. 
 
INDEX 
 
 PAGE 
 . 57 
 120 
 123 
 124 
 133 
 114 
 . 128 
 . 114 
 . 125 
 
 91 
 12S 
 120 
 
 49 
 
 Acids, organic, in soil . . 
 
 Ammonites 
 
 Amphibia, first occurrence of 
 
 relation to fishes 
 
 Antters, history of, in deer 
 Antrim, volcanic rocks . . 
 
 Archaeopteryx 
 
 Arran, volcanic rocks 
 
 Atlantosaurus 
 
 Balance of continental masses 
 
 Basalt 
 
 Ben Lawers, section of . . 
 
 Birds, fossil 
 
 Bivalves (Pelecypoda) .. 
 Bombs, volcanic 
 
 Bothnia, Q-ulf of, movement of 
 
 land 8 
 
 Brachiopoda, fossil .. -.119 
 Bridlington Bay, encroachment 
 
 of sea .-68 
 
 British Isles, geological map of 104 
 
 submerged 
 
 plateau of 
 
 Brontosaurus . 125, 126 
 Caderldris, volcanic rocks of.. 108 
 Cainozoic 
 
 mammalia of . . 
 
 Cambrian rocks 
 
 Carbonic acid, action on rocks 
 
 in sea water . . 
 
 Carboniferous rocks 
 
 Cemeteries, weathering ex- 
 hibited in 
 
 Cephalopoda 
 
 Chalk period, geographical 
 conditions 
 
 Chalk, origin of 
 
 Circumference of Earth, early 
 measures 
 
 Climatic changes in Cainozoic 
 
 Coal Measures 
 
 as proof of 
 subsidence . . 
 
 Comets 
 
 Cone of volcano, origin of . . 
 
 "Continental" periods of 
 
 Britain . . . . 105, 110, 113 
 
 Cooling of lava and size of 
 crystals 
 
 Copernicus, discovery of true 
 nature of Solar System 
 
 Corals, fossil 
 
 "precious" 
 
 Coral-reefs 
 
 Cretaceous reptiles 
 
 Crevasses, engulfing moraines 
 
 Crinoids (Sea-lilies) 
 
 Crocodiles, fossil 
 
 Crystalline rocks 
 
 Crystallisation of lavas . . 
 
 Cumbrae Lion Rock . . 
 
 100 
 129 
 106 
 64 
 77 
 110 
 
 63 
 
 120 
 
 112 
 
 78 
 
 5 
 
 114 
 110 
 
 83 
 21 
 32 
 
 49 
 
 6 
 
 118 
 119 
 
 73 
 127 
 
 61 
 121 
 127 
 
 45 
 
 51 
 
 PAQB 
 .. 52 
 .. 71 
 . 56 
 
 Cullin Hills, Skye .. 
 Currents in oceans 
 Danube, sediment carried by 
 Darwin, Sir G., on tides and 
 
 rigidity of earth . . . . 36 
 Deep sea 75 
 
 life of 76 
 
 Deer, history of 132 
 
 Deinosaurs 126 
 
 Deltas, formed in lakes . . . . 55 
 Denudation 69 
 
 estimated from 
 
 folds 92 
 
 Depth of Sea, distribution of 
 
 sediments 71 
 
 Deserts, origin of 62 
 
 Devonian rocks 109 
 
 Dip of rocks 88 
 
 Disintegration of rocks . . . . 57 
 Dolomite, origin of . . . . 76 
 
 Dykes 51 
 
 Earth, weight of 24 
 
 compared with surface 
 
 rocks 26 
 
 Earth-movements, cause of . . 95 
 
 in N. W. 
 
 highlands.. 103 
 Earthquakes .. .. 36-40 
 
 accompanying 
 
 eruptions .. ..28 
 
 displacements of 
 
 ground in . . 37, 86 
 
 effects of . . . . 36 
 
 extent 9f move- 
 ments in . . . . 39 
 
 . recording instru- 
 ments .. ..38 
 
 Echinodermata 120 
 
 Elephants, history of . . . . 131 
 Elevation of land, proofs of . . 81 
 Eocene period, geographical 
 
 changes 113 
 
 Estuary, deposits formed in . . 65 
 
 of Lower Oolites . . 112 
 
 of Lower Cretaceous 112 
 
 Faults 92 
 
 rate of formation of . . 93 
 
 Fishes, history of the . . . . 122 
 
 Folding of rocks 89 
 
 Footprints, earliest known 
 
 fossil 123 
 
 Footprints in Triassic rocks . . Ill 
 Foraminifera, fossil . . . 118 
 
 in deep-sea ooze 76, 77 
 
 Fossils as guides in arrange- 
 ment of rocks 99 
 
 Fossils, distribution of . . . . 116 
 Frost, action on rocks and soil 58 
 
 Gabbros 52 
 
 Gastropoda (Univalves) ..120 
 
INDEX 
 
 135 
 
 PAGE 
 
 Geological Map of British Isles 104 
 
 Glacial period 114 
 
 Glaciers 60 
 
 erosion by . . . . 61 
 
 Glen Tilt, Button's observa- 
 tions in 47 
 
 Grampian hills, rocks of . . 105 
 
 structure of 90, 91 
 
 Granite 44 
 
 microscopic section of 45 
 
 weathering of . . . . 64 
 
 "Hag-fish," fossil.. .. 122,123 
 Heat as cause of volcanic action 34 
 
 of earth's interior . . . . 27 
 
 solar, in destruction of 
 
 rocks 57 
 
 Hipparchus, estimate of sun's 
 
 distance 6 
 
 Horse, history of . . . . 130, 131 
 Hutton, the igneous origin of 
 
 granites 
 
 Hyleeosaurus 
 
 Ice, action on rocks and soil 
 
 Ice-age 
 
 Ichthyosauria 
 
 Iguanodon 
 
 Internal heat of earth . . 
 Interior of earth, probable 
 
 condition 
 Joints, importance in marine 
 
 erosion 
 
 Jupiter 
 belts of .. 
 
 temperature of 
 
 varying rotation 
 
 Krakatoa, eruption of . . 
 Lakes, deltas formed in . . 
 Lake District, volcanoes of 
 Land surfaces sunk below sea 
 Land-vertebrates, unknown 
 
 origin 
 
 Lava, How-structure in . . 
 microscopic structure of 45,48 
 
 46 
 125 
 
 58 
 114 
 126 
 125 
 
 35 
 
 41 
 
 11-14 
 .. 12 
 
 12,13 
 
 . 13 
 
 .. 30 
 
 .. 55 
 
 108 
 
 84 
 
 123 
 
 Vesuvian, nature of . . 52 
 
 Lavas, varying composition of 52 
 Leasowe, submerged forest . . 83 
 Level of land, changes of . . 81 
 
 Lewisian rocks 103 
 
 Liassic Sea 112 
 
 Life in early rocks 117 
 
 Lime in ocean, source of . . 73 
 Limestone, origin of . . . . 73 
 Lion Rock, Cumbrae . . . . 51 
 London basin, section of . . 91 
 
 Magma 53 
 
 Magnesian limestone, origin of 75 
 Mammalia, origin of . . 124, 128 
 
 Mammoth 132 
 
 Mars 9-11 
 
 atmosphere of * . . , . 11 
 
 canals of 10 
 
 snow-cap of 10 
 
 Martinique, history of erup- 
 tions in 27 
 
 PAGE 
 
 Meanders, action of rivers in 66, 67 
 
 Mesozoic 100 
 
 reptiles 186 
 
 rocks .. .. 111,118 
 
 Metamorphism .. .. ..94 
 
 Meteorites 21 
 
 Miller, Hugh, on fossil fish . . 123 
 
 Millstone Grit 110 
 
 Miocene, earth-movements in 113 
 
 volcanic activity in . . 114 
 
 Mississippi sediment carried by 56 
 
 Moaritherium 132 
 
 Molten stage of all rocks . . 53 
 
 of earth's crust . . 54 
 
 Moraines 61 
 
 Mountains, disintegration of 
 
 rocks 59 
 
 soil absent from .. 59 
 
 variation of tem- 
 perature .. ..59 
 Mountain ranges, structure of 90 
 Movement of land, rate of . . 86 
 
 Mud, origin of 74 
 
 Mull, volcanic rocks .. ..114 
 
 Nebul 20 
 
 evolution of . . . . 22 
 
 relation to comets and 
 
 meteorites .. ..22 
 
 Neck, volcanic 61 
 
 Neptune, discovery of .. .. 8 
 
 "Neptunista" 46 
 
 North Sea, deposits in .. ..70 
 North Wales-raised beaches of 82 
 North- West Highlands, rocks of 103 
 Nuovo, Monte, history of . . 32 
 Old Red Sandstone . . . . 109 
 Old Red Sandstone, fish of . . 123 
 
 Ooe of deep sea 75 
 
 Ordovician period, land and 
 
 sea of 107 
 
 rocks 108 
 
 volcanoes .. .. 108 
 
 Organic acids in soil . . . . 57 
 Outer Hebrides, rocks of . . 103 
 Palaeontology, bearing on 
 
 evolution lie 
 
 Palaeozoic . . . . . . . . 100 
 
 Paris, t'oraminiferal limestones 118 
 Pelecypoda (Bivalves) . . . . 120 
 
 Pelee, Mount, eruptions of . . 28 
 
 Pennine Chain, origin of . . 110 
 
 structure of . . 90 
 
 Permian rocks in 
 
 Planets, distance from sun . . 8 
 
 masses 8 
 
 densities 8 
 
 minor 8 
 
 origin of .. .. 22,23 
 
 Pleistocene 114 
 
 Plesiosauria 126 
 
 Pliny, account of eruption of 
 
 Vesuvius 30 
 
 Pliocene 114 
 
 Plutonic rocks 50 
 
 "Plutoniste" 47 
 
136 
 
 INDEX 
 
 PAGE 
 
 Pre-Oambrian period, length of 117 
 Preliminary tremors of earth- 
 
 quakes ...... 39,40 
 
 - evidence of rigid 
 
 interior of earth . . . . 40 
 
 Pterodactyls (flying reptiles) 125,127 
 i'tolemaic system ...... 6 
 
 Radiolaria in deep-sea deposits 79 
 Raised beaches ...... 82 
 
 Recapitulation in development 133 
 Red Clay ....... 79 
 
 Reptiles, dominance of . . . 124 
 
 - early forms of . . . 124 
 
 - extinction of .. .127 
 Rhyolite ....... 52 
 
 - microscopic section of 45 
 Rivers, action of ..... 66 
 
 - changes of course . 67 
 
 - lowland . . . . . 66 
 Roches moutonnees . . .61 
 Roots, aid in formation of soil 57 
 Salts of sea, source and fate of 75 
 Salt in Permian and Triassic 
 
 rocks ........ Ill 
 
 Sand, origin of ..... 74 
 
 Schists, etc., of Grampians . 106 
 Screes .. .. ..... 59 
 
 Sea-bed, distribution of sedi- 
 
 ments on ....... 71 
 
 Sea, erosion of coast by . . . 68 
 Sea-lilies (Orinoids) .. .121 
 Sea-urchins ........ 120 
 
 Sedimentary rocks, derived 
 
 from igneous 53 
 
 - relation to igneous 74 
 Sediments, distribution of . . 71 
 Sediments prove slow subsi- 
 
 dence ........ 86 
 
 Seismographs . . . . 38, 39 
 
 Seismology ........ 38 
 
 Sharks, among earliest fish . . 122 
 Shooting-stars . . . . . 21 
 
 - in deep-sea deposits 80 
 
 Silurian rooks 
 
 Simple organisms survive . . 
 
 Skye, Cullin Hills ...... 
 
 - volcanic rocks . . . . 
 
 Slate, origin of ...... 
 
 Snow, protecting rocks . . . . 
 
 Snowdon, volcanic rocks of . . 
 
 Spectroscope, applied to study 
 
 of Sun ........ 
 
 Soil, origin of ...... 
 
 - waste of ...... 
 
 Solar heat, in deserts . . . . 
 
 - in destruction of 
 rocks ...... 
 
 Solar System, table of elements 
 
 - unity of . . . . 18 
 
 Solution of shells in sea. . . . 77 
 
 Somma, Monte ...... 33 
 
 South-east England, section of 91 
 St. Pierre, destruction of . . 29 
 Stacks, origin of ...... 69 
 
 Stars, distances of 
 
 108 
 
 119 
 62 
 
 114 
 94 
 60 
 
 108 
 
 17 
 66 
 56 
 62 
 
 57 
 
 19 
 
 PAQB. 
 Stars, evolution of .. ..28 
 
 types of 20 
 
 Star-fishes m 
 
 Steam in volcanic eruptions . . 3d 
 Stratified rocks 42 
 
 table of British 100 
 
 Stromboli 31 
 
 Submerged forests . . . . 8* 
 Subsidence of land, rate of 85, 87 
 
 Subsoil 57 
 
 Succession of rocks, historical 
 
 order 97 
 
 Sulphurous gases in eruptions 23 
 
 Sun 14-18 
 
 composition of .. ..17 
 
 distance of 7 
 
 gaseous condition . . . . 16 
 
 red flames (prominences) 16 
 
 rotation of 16 
 
 spots on, nature of . . . . 16 
 
 telescopic appearance . . 16 
 
 temperature of . . . . 14 
 
 Survival of simple organisms. . 119 
 Sweden, movement of land in . . 88- 
 Temperature of Earth's interior 27 
 
 gradient . . . . 27 
 
 variation affecting 
 
 rocks . . . . . . 57 
 
 Terrigenous deposits . . . . 71 
 
 Thrust planes 9$ 
 
 Tidal currents 72 
 
 Tides proving rigidity of earth 36 
 Torridonian sandstones . . 105 
 
 Transport by rivers . . 65-68 
 
 Triassic rocks Ill 
 
 Tundras, origin of . . . . 58 
 
 Unconformity . . . . 101, 102 
 Univalves (Gastropoda) .. 120 
 
 Uranus, discovery oy Herschell 8- 
 Valleys sunk below sea . . . . 84 
 Vertebrata, knowledge of fossil 121 
 Vertebrates, unknown origin 
 
 of land- 123 
 
 Vesuvian lava, nature of . . 52 
 
 Volcanic rocks 5O 
 
 of Old Red Sand- 
 stone . . . . 109 
 Volcano, ideal section of . . 31 
 
 origin of cone . . . . 32 
 
 Volcanoes 27-35 
 
 British 50- 
 
 dissected . . . . 51 
 
 distribution of.. 34, 3& 
 
 of Miocene period . . 114 
 
 of Ordovician period 10* 
 
 relation to mountain- 
 
 ranges 33 
 
 Water given out by volcanoes 34 
 Waterfalls, action of . . . . 66- 
 Waves. action on cliff . . . . 6* 
 Weathering of rocks . . . . 63 
 
 exhibited in cemeteries 63 
 Werner, the origin of Granites 46 
 Wales, rocks of . . . . 106-108 
 
 volcanoes of . . 108- 
 
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