-NRLF EbE SOh THE EARTH IN PAST AGES VESUVIUS IN ERUPTION, 1872. STORIES of the UNIVERSE The Earth In Past Ages By H. G. SEELEY, F.R.S. PROFESSOR OF GEOGRAPHY AND LECTURER ON GEOLOGY AND MINERALOGY IN KING'S COLLEGE, LONDON I WITH FORTY ILLUSTRATIONS NEW YORK REVIEW of REVIEWS COMPANY 1910 COPYRIGHT, 1895, 1902, BY D. APPLETON AND COMPANY. PREFACE. I HAVE endeavoured to tell the story of the Earth so that its past history helps to explain its present condition. Explanations are given of the nature of the common materials which form rocks, of the ways in which they rest upon each other, and of the means by which they may be distinguished. The story of the Earth is divided into epochs by layers of rock which rest on each other and rise to the surface of the visible land, and to the floor of the ocean. Geological time cannot be defined in years. The time occupied by an existing river like the Rhine or the Niagara river, in excavating the gorge through which it flows, dates back beyond the antiquity imagined for man by historians. Yet this incident in sculpture of the Earth's sur- face is subsequent to the newest of the regular layers of rock. It is convenient to forget the human standard of time, and think of a period of geological time as the age when some rock, such as coal, accumulated, or when an extinct plant or animal was dominant on the, Earth. Fossils are the remains of plants and animals by which each period of by-gone time is distin- guished. 5 6 PREFACE. I. Many kinds of animals, which still live> date back to the beginning of the Earth's story> or to an early period. II. Many groups of animals, such as Trilo- bites or Ichthyosaurs, endured on the Earth for long geological ages, varied in form and struc- ture, and became extinct successively, leaving no survivor. The life which now exists on the Earth is a survival of ancient types of life known from fossils, which have undergone substantially no change since first they became known in the rocks. They are associated now with groups, like the Mammalia, which are changing rapidly. The diversity of mammal orders in structure of the skeleton, is not unlike that which the ancient Saurians put on before they became extinct. Animals' orders which vary rapidly last for a relatively short time. I have used some scientific names of these fossils in the story of the Earth, since names give the easiest identification for fossils as for our fellow-men. The characteristics or lives of fossil animals and of living men give interest to their names. Practical knowledge of fossils ensures this enduring interest, and is gained by collecting them in the sea-cliff, quarry, or pit, and by comparing such specimens with named examples in museums. H. G. SEELEY. KENSINGTON, W., 1895. CONTENTS. I. INTRODUCTION . . . .- ,'-.' .-. . 9 II. THE EARTH'S INTERNAL HEAT ... 12 III. MATERIALS OF MOUNTAIN CHAINS v ,,; ; . 18 IV. VOLCANIC ROCKS . . ); Y . . . 26 V. THE MATERIALS OF STRATA . '.", . 31 VI. THE SUCCESSION OF STRATA . . ,- . 51 VII. ORIGIN OF STRATIGRAPHICAL GEOLOGY . 58 VIII. FOSSILS . T. ; .< . ' ..' . . 62 IX. THE CLASSIFICATION OF WATER-FORMED ROCKS . . . ..... . . 74 X. THE ARCH^AN ROCKS . . . . .78 XI. CAMBRIAN AND ORDOVICIAN ROCKS . . 81 XII. OLD RED SANDSTONE AND DEVONIAN . . 92 XIII. CARBONIFEROUS STRATA . . . . .97 XIV. PERMIAN AND TRIAS 115 XV. LIAS . . . ' ;V . . . / . 125 XVI. OOLITES . . . . . . . .130 XVII. THE NEOCOMIAN PERIOD . . . . 142 XVIII. LOWER CRETACEOUS ROCKS .... 149 XIX. THE CHALK 156 XX. THE LOWER TERTIARY STRATA . . . 162 XXI. THE MIDDLE TERTIARY PERIOD . . . 173 XXII. THE CRAG ; v / 178 XXIII. GLACIAL PERIOD AND GRAVELS . . .182 LIST OF ILLUSTRATIONS. VIGURB Vesuvius in Eruption, 1872 Frontispiec* 1. Gneiss 89 2, 3. Hertfordshire Pudding Stone 35 4. Laminated Sand 39 5. Ripple-marked Sandstone . . 41 6. Lithographic Stone ....... 46 7. Carboniferous Limestone 47 8. The Chalk in Yorkshire 56 9. Snowdon to Flintshire 83 10. Inlier at Usk .... . ' . .87 11. Feet of the Trilobite. ...... 91 12. To the Forest of Dean ...... 93 13. East of the Pennine Chain loo 14. Productus . . . . . . . . IOI 15. Pleu rot om aria IO2 16. Sigillaria . \ Iio 17. Teniseopteris ..... Hi 18. Pareiasaurus 119 19. Lias Outlier 126 20. Gryphsea Incurva . ,127 21. Cardinia Listen 128 22. Ichthyosaurus .....*.. 131 23. Belemnites Oweni 136 24. Shotover Hill 137 25. Skeleton of Archseopteryx . . . . .139 26. Skull of Archseopteryx 140 27. Hythe to Folkestone 148 28. Ammonites Deshayesii . . . . . .150 29. Section at Hunstanton . . I 5 I 30. Ammonites Planulatus . .152 31. Poterocrinus ........ 153 32. Micraster Coranguinum 159 33. Galerites Subrotundus 160 34. East of Herne Bay 164 35. Ostrsea Bellovacina 165 36. Cyprina Morrisi 166 37. Strata in Alum Bay ....... 169 38. Planorbis Euomphalus . . '*'' 175 39. Cardita Senilis ........ 179 40. Fusus Antiquus reversed ...... 181 THE STORY OF THE EARTH. CHAPTER I. INTRODUCTION. I. THE building of the surface layers of the Earth is recorded in rock materials, which are accumu- lated upon each other. But there is no trace of a beginning to their story of the Earth's history. In the remotest period of past geological time of which evidence has been found, the earth was in- habited by types of animals, some of which still survive. There is no evidence that the most ancient animals which have, been discovered were the first that existed, or that the oldest rocks at present known mark the beginning of geological records. It is as unprofitable to enquire for evi- dences of the origin of the earth, as it is to ask for proofs of the mode of origin of the life which has flourished upon it. Because the earth is a planet we may assume that it had a similar history in its origin to some of the heavenly bodies. The light which comes to the earth from the most distant stars in the universe, proves, when analysed, to result from the incandescence of elements which are mostly identical with those found in the earth. The small masses of matter, termed meteorites, which fall from time to time to the earth's surface, con- 10 THE STORY OF THE EARTH. sist of iron and other metals, and of minerals like those which combine to form crystalline rocks. The forces which act on the earth are like those manifested in other heavenly bodies. If the Earth's surface is not incandescent, as in the luminous stars, its interior demonstrates in many ways an internal heat, which has played an im- portant part in its history. So that, with the mat- ter and force substantially the same, there is some justification for the old definition of geology as that department of astronomy which tells the story of the Earth. The geological story differs from that told by the astronomer in giving results of unceasing ac- tion of the forces of nature upon the rock materi- als of the globe. They have worked during a time which is immeasurably long, when estimated by such changes on the earth as have happened dur- ing human history. This time cannot be expressed in centuries. The work of rivers in carving chan- nels upon the existing surface of the earth has been computed at from 15,000 to 30,000 years, in the case of Niagara river, without reaching the age when the newer layers of the globe were de- posited from the sea. This stupendous duration of time has brought about revolutions in the po- sitions of oceans and continents ; in the types of life which were predominant on the earth, as well as in the distribution of life over the globe, and in the succession of different kinds of life in the same region in successive ages, which would be incredible but for the evidence of fossil animals and existing animals which are everywhere around us. These changes have come about, not as result of catastrophes which have destroyed the fair sur- face of the land and its life, but as parts of the INTRODUCTION. 1 1 order of nature, and as conditions of that stability of government of the world by which the cre- ations of earlier times have been preserved, and passed on from one geological age to another to survive at the present day. ii. On various parts of the globe, meteorites have been found which vary in weight from a few ounces to a few tons. Examples of 400 of them are pre- served in the British Museum. Some have been seen to fall. It may therefore be inferred that ever since the earth has been in existence it has probably received such additions of material. Meteorites however do not demonstrate that the earth has been built up of meteoric matter; but they are the only clue of a practical kind to the origin of the globe, which the geologist encoun- ters. The iron in meteorites is metallic, usually com- bined with nickel. In the earth iron is rarely me- tallic, and rarely crystallized with nickel. Minute particles of metallic iron are present in the vol- canic rock named Basalt, which has flowed over the north of Ireland. Iron is found combined with nickel in the Van mine in Denbighshire. The percentage of nickel in the iron varies in different localities. There is only one or two per cent, of nickel in the great masses of iron, some- times weighing 50,000 Ibs., embedded in Basalt at Ovifak in Disco Island, on the west of Greenland. An alloy of these metals found in New Zealand, yields 67 per cent, of nickel. Both are regarded as of terrestrial origin. Although the mineral quartz is one of the most 12 THE STORY OF THE EARTH. abundant constituents of surface rocks, no true quartz has been recognised in any meteorite. But a rare mineral asmanite with many of the proper- ties of quartz occurs, which somewhat resembles the variety of quartz found in some volcanic rocks, which has been distinguished under the name tridymite. About ten rare minerals are met with in me- teorites which have never been recognised in the rock materials of the globe. On the other hand, earthy meteorites have yielded many of the constituents of volcanic and crystalline rocks. Two kinds of felspar named labradorite and anorthite have been recorded in meteorites, and such minerals as Augite, Bronzite, Enstatite, Oli- vine, which upon the earth are often combined with the felspars in mineral union to form crystalline rocks. But the facts are too few and too obscure to do more than stimulate interest in the relation of the earth to the bodies among which it moves. CHAPTER II. THE EARTH'S INTERNAL HEAT. THE earth has an internal heat of its own, which is not derived from the sun. The temper- ature of the outer surface layer varies with sum- mer and winter. In Java and India at a depth of 12 feet the thermometer is constant all the year round. In London and Paris an unvarying tem- perature occurs at about 100 feet below the earth's surface. The earth's heat begins to increase be- THE EARTH'S INTERNAL HEAT. 13 low this variable surface layer, though the rate of increase differs with the kinds of rock passed through, and with the locality. It averages one degree Fahrenheit for every 55 feet of depth. In the famous Artesian well at Grenelle near Paris, the water rose from a depth of 1794 English feet, with a temperature of nearly 82 F. The deep boring at Sperenberg near Berlin appears to show an increase of i F. in 42 feet at the depth of TOCO feet; i F. in 57 feet, at 2000 feet; and i F. in 95 feet, at 3000 and 4000 feet. From these facts the inference has been made that tempera- ture does not augment appreciably below a mod- erate external thickness of rock. The difference between the surface tempera- ture and the interior temperature, results from the loss of the earth's internal heat by radiation. On this circumstance attempts have been made to es- timate the duration of geological time. By meas- uring the amount of heat which the earth radiates from its surface in a year, Lord Kelvin has con- cluded that in a period of 20,000 millions of years, more than enough heat would have been lost to melt the entire bulk of the earth, if the* rate of loss had been always what it is now, and if the earth had consisted throughout of the same ma- terials as its surface rocks. This is the time which the physicist conceives as possible for the earth's origin and history. Sir John Herschel had doubt- ed the primitive fluidity of the earth. It is per- haps possible that the heat which the earth loses may not be the original heat of an igneous fusion, but the result of strain due to its rigid state. It rotates so that its surface experiences the lifting influence of tidal attraction which reduces the pressure, although the amount is too small to dis- 14 THE STORY OF THE EARTH. 1 % turb the stability of its surface. By the conver- sion of this attraction of gravitation upon its outer layers into heat, at a depth from the surface sufficient to ensure that the heat so generated could not be radiated in a day, a store of heat might accumulate near to the surface of the globe. The most ancient rocks give no evidence of greater internal heat, or of greater refrigeration of the earth, or of tidal action upon its surface having been in any way different from what it is now. The greatest depth at which the fractures and dislocations, termed earthquakes, are known by actual measurement to originate, is about 30 miles. It has also been calculated that a heat sufficient to melt granite might occur at a depth of 20 or 30 miles. This is the maximum depth to which geological theory extends its inferences. Attempts have been made to calculate the pressure under which masses of granite in moun- tain chains have consolidated. In some cases the crystal structure appears to indicate a superin- cumbent pressure equal to no more than 15 miles' thickness of rock, though the pressure was proba- bly lateral. The materials ejected from volcanos give no indication of having ascended from more than very moderate depths. The molten matter of lava streams does not appear to be the primitive substance of the earth's interior. That heated material might be rendered liquid by fractures which penetrate downward so as to remove the pressure which keeps the heated rock solid. It is thus manifest that some cause generates heat near to the earth's surface, which is associated with the crumpling of the earth's outer layers, with the changed distribution of level of land from age to THE EARTH'S INTERNAL HEAT. 15 age, and with the phenomena of volcanic ac- tivity. This cause is believed to be the cooling of the earth ; by which the shrinkage of the deeper lay- ers crushes the upper layers together, crumpling them into folds which are directed alternately up- ward and downward. As these folds are crushed closer together, the mechanical energy of com- pression, resisted by the rock material, becomes converted into heat along the lines of most intense squeezing;. The directions of these folds change from age to age in geological time; for every land consists of masses of rock which extend through it in direc- tions which were once approximately parallel to its shores. The late Mr. Robert Mallet believed that the energy of volcanic eruptions was developed by these compressions of the crust. He also urged that the lateral pressure exerted by the sides of an arch of continental land upon its supports would result in crushing along the lines of greatest weakness; and calculated that the temperature may be raised locally in this way to a red heat, or even to the fusing point of the. rocky ^materials which are crushed. This heat, which is produced locally, he believed to be consumed locally, and to be the source of the explosive energy which ejects the materials of which volcanos are built up. Active volcanos are commonly met with in regions undergoing upheaval. This is attributed to the underground compression of the rocks which causes upheaval, generating heat. The water near the shore which penetrates to the heated region is raised by that heat to an explo- sive temperature. Volcanos have a linear exten- 1 6 THE STORY OF THE EARTH. sion ; sometimes in islands rising from the sea, sometimes in mountain chains formed of islands united together. The linear arrangement is at- tributed to the opening of fissures, which pen- etrate downward along lines, in which the rocks have been folded and fractured in the process of upheaval. When rain water, in a region so bent and strained, is held back upon the land and hindered from escaping by the pressure of the sea round its shores, the water descends through the minor joints and capillary interspaces between the particles of rock. Then it rises in temperature with the internal heat of the earth, so as to facil- itate the melting of rocks, with which it combines. Some of this water eventually ascends through the planes of fracture and displacement forming outlets for explosive energy, discharging steam, dust, and the rock matter, both solid and molten, which builds volcanic cones. The past periods of geological time abound in evidences of volcanic activity. From the imper- fect nature of the records which remain upon the earth their linear arrangement is not always evi- dent ; but they may be inferred to mark lines of upheaval which brought islands into existence, or united them into continental masses of land in successive epochs of geological time. But be- sides the volcanos which are marked by beds of ashes and lava-flows, and the throats up which the molten matter ascended, there are in many parts of the world extinct volcanos with their cones well preserved, as though the craters had been recently active. A little south of the Pyrenees, in the basin of the Ebro, there are fifteen cones about Olot in Catalonia, built of cinders, from each of which THE EARTH'S INTERNAL HEAT. 17 lava has flowed in streams still to be traced, yet so long since that the existing rivers have cut passages for themselves through the lava. The Auvergne is a granite platform in which some ancient rocks of the carboniferous period occur. This district appears to have been an is- land traversed by a line of fracture from N.W. to S.E., which corresponds to the uplifting of the crystalline rocks. A second fracture runs from N. to S. In this region are the ruins of the four grand volcanos known as Mont Dore, Cantal, Canton d'Aubrac, and Mezen. The lava flowed from Mont Dore for 20 miles. The minor cones, of which there are hundreds, range through the country in a broad band, from N. to S. Many have the craters burst down by the lava which ascended in them, and overflowed into the neigh- bouring valleys. Beautifully preserved volcanic cones are found to the north of the Moselle river in the district known as the lower Eifel. It may have been in this country that the eruption took place which is mentioned by Tacitus as having affected the country near Cologne, in the reign of the Roman Emperor Nero. For a long way up tho Rhine the rocks are volcanic ; and evidences of extinct volcanos are found west of the Rhine, in many parts of central Germany; and a series ranges through Hungary S.W. of the Carpathians into Servia. The latest volcanic outbursts in the British Isles were at the beginning of the Tertiary period in Skye, Rum, Mull and the adjacent mainland of Scotland, and in the north of Ireland, where streams of mud due to volcanic dust, washed down by rains, covered up the vegetation of the 1 8 THE STORY OF THE EARTH. country before it was deluged with the black lava named basalt. Branches of the conifer Sequoia, and of plane trees covered with leaves, are pre- served in the consolidated mud which underlies these lava-flows. CHAPTER III. THE MATERIALS OF MOUNTAIN CHAINS. THE same cause which produced the local heat and fractures which led to volcanic outbursts, has folded the earth's crust. Rocks many thousands of feet thick have been bent, folded and crumpled. This structure, which is shown in the succession of rocks on the surface of every country, in folds termed saddles and troughs, is most astounding in its intensity in mountain chains. The upheaving of the parallel ridges of limestone rock known as the Jura chain, forming the frontier between Switzerland and France, is a beautiful example of troughs which form valleys, parting the elevated ridges from each other. In that part of the Alps known as the Grisons, all the geological deposits, from the tertiary down to the oldest, have been turned upside down, in the process of folding by lateral displacement ; which is the sole cause which lifts mountain ranges. The curved form of the earth necessitates that every axis of elevation must be accompanied by spurs at right angles to itself, or by parallel ranges. The parallel system is exemplified in the chains of North America, which lie between the Rocky Mountains and the Pacific. THE MATERIALS OF MOUNTAIN CHAINS. 19 These folds once formed remain for all time. They may be raised higher, or depressed beneath the sea, and new rocks laid down upon them ; but as those ancient folds increase in intensity with the slow succession of geological ages, the newer rocks become folded with their folds, and the folds run in the same direction. In such puckered and crumpled rocks as moun- tain chains exhibit on their denuded heights, there is almost invariably evidence of a crystalline tex- ture. This may be attributed to the influence of the heat produced by the mechanical power, trans- formed by the resistance which the rock mass of- fered to compression. The rocks which form mountains are chiefly slaty rocks, and schists, with here and there some granite masses or sheets of volcanic rock. They have only been laid bare by the removal of vast thicknesses of water-formed rock which once ex- tended above them. If the crystalline materials are not the necessary products of the upward thrust of the mountain chain and adjacent land which supports it, it may be difficult to account for the uniform character of the rocks of which the durable central masses of mountain cnains are built. There are stages in this process of change. The flanks of a mountain range commonly show the fine microscopic crystalline texture of slate, while the central masses show the coarse crystal- line texture of schist, or granite. Slate. The part which slate plays in the forma- tion of mountain masses is well seen in the struc- ture of the mountainous regions of North and Central Wales, in parts of the Lake District in Westmoreland and Cumberland, and in the south of Scotland. It is certain that slate was originally 20 THE STORY OF THE EARTH. a water-formed rock, a mud which consolidated into clay. It often shows successive parallel beds marked by differences of colour. Welsh slates sometimes contain clay pebbles, such as occur at the present day on shores where the cliffs are of compact clay. Many slates contain fossil remains of animals which lived in the sea when the old mud was accumulating. Those fossils are often distorted and squeezed into half their original breadth or length, showing that the whole moun- tain mass has undergone compression and con- densation. The compression has bent the rocks into synclinal troughs and anticlinal saddles. The slaty texture is most developed in the troughs. The effect of this lateral pressure has been in the first place to turn the films of water contained between the particles of the old mud at right angles to the direction from which the pressure came. The resistance offered by the rock trans- formed a large part of the motion imparted to its particles into heat. That heat raised the temper- ature of the water contained in the rock, enabling each film, under the pressure, to dissolve some constituents of the mineral matter in which it was contained. These slaty rocks often give evidence of having been fractured through their thickness by minute dislocations, and subsequently re-united. Such breakage, relieving the pressure, would cause the temperature to fall, and the substances which had been dissolved then crystallize in minute films, parallel to each other, extending throughout the mountain mass, and having no relation to the original planes in which the mud was deposited. These microscopic crystal films resemble such min- erals as mica or chlorite. They impart to the rock the property termed slaty cleavage. This THE MATERIALS OF MOUNTAIN CHAINS. 21 cleavage causes it to split in layers which cut across the original folded or faulted planes of bedding. This microscopic crystalline change of texture imparts to the rock, now termed slate, a remarkable durability. Its particles are laced together by a network of parallel films of micro- scopic crystals. Slates may be of any antiquity. Nothing but folding and uplifting of mountainous masses is needed to form them. In England and America they belong chiefly to the ancient epochs of time distinguished as pre-Cambrian, Cambrian, Silurian, and Devonian. Schists. The transitions between slate and schist are common in mountain regions. Crystals of other minerals are sometimes developed on the cleavage planes of slate. Some slates are very micaceous; and it is sometimes difficult to say where mica slate ends, and mica schist begins. The original bedding is usually obliterated in schists ; so that the rocks give no evidence of having been deposited in water. Occasionally, as in the mica slates south of Bergen in Norway, beds of limestone, in which fossils are preserved, are found in such rocks. In the north of Scot- land fossil-bearing beds, known as the Durness limestone, occur between schists, where they are introduced by horizontal dislocations. A schist presents to the eye an arrangement of short irregular layers of crystals, which is similar to the appearance which a thin film of slate shows under the microscope, although schists differ from slates in having all their material crystalline. There is some reason for regarding them as re- sults of intenser action of such compression as imparted a slaty texture to ancient beds of very varied mineral character. 22 THE STORY OF THE EARTH. A schist thus foliated is typically an alternation of films of the mineral quartz with some other minerals. Each quartz film is made up of a num- ber of crystals matted together, and occasionally little plates of mica separate the individual crys- tals from each other. The mineral, which alter* FIG. i. Gneiss : showing foliated structure, from Gairloch in Ross-shire. nates with the quartz, gives its name to the schist, as mica schist, hornblende schist, chlorite schist. Some schists, such as gneiss, are identical with granite in mineral composition ; some are identical with slates in chemical composition. Schists are frequently contorted and crumpled, as in the cliffs round Holyhead, with a minuteness of folding which is not seen in slates. Like slates they can be inferred to have been crystallized by the trans- formation into heat of the pressure which elevated them. They have been exposed at the surface by removal under the denuding action of water, of the rocks which originally covered them. Schists often alternate with crystalline quartz- rock, wfrich appears to have been originally sand- stone, metamorphosed by partial solution and THE MATERIALS OF MOUNTAIN CHAINS. 23 crystallization which has blended the grains. In some localities, as on the west coast of Scotland, limestone occurs in schists and gneiss. Its tex- ture is frequently compact and crystalline, and sometimes saccharoid like statuary marble. It contains many minerals but no fossils. All lime- stones were originally deposited from water. Thus the three chief types of water-formed rock sandstone, clay and limestone appear to be represented among schists. The process which has rendered them crystalline is termed metamor- phism. Metamorphic rocks, which divide into layers by differences in the mineral character of their crystalline constituents, are said to be foli- ated. This foliation may be regarded as closely comparable with the cleavage of slates. Schists may be formed of quartz, felspar and mica in parallel layers, when the rock is termed gneiss. The crystals of a schist may be thrown out of their parallelism, as in Anglesea, so as to present a confused mixture, which has been termed granite. Some observers, however, take the converse view, and believe that the original texture of the rock was granite, and that the schistose texture has been acquired by shearing movement acting on a heated plastic rock. In the south of Cornwall a schistose texture has been imparted in the metamorphic region of Cornish schists, to rocks which were originally volcanic. Metamorphism is produced in several distinct ways. When the rocks of an elevated tract be- come changed in texture throughout their mass, the expression " regional metamorphism " has been used to distinguish such wide-spread trans- formations of rock texture, from the local altera- 24 THE STORY OF THE EARTH. tions of texture termed "contact metamorphism," which result from highly heated rocks acting upon the sediments over which or through which they flow. The changes produced by the action of the atmosphere and infiltrating water, which break up minerals originated by heat or pressure, and elaborate others in their place, give rise to " sub-aerial metamorphism." In the central regions of mountain chains, such as the Grampians and the central axis of Devon and Cornwall, schists sometimes pass into the condition termed granite ; so that there has some- times seemed to be a relation of cause and effect between the position in which the granite occurs, and the way in which its mineral matter is ar- ranged. Granites vary so much in the minerals they include that they form a family of rocks dis- tinguished by chemical and mineral composition and texture. The minerals depend upon the chemical constituents. The silica varies from 55 per cent, to 80 per cent. The alumina from 7 to 20 per cent. So that the quartz is commonly from a fifth to a third of the bulk of the granite, though occasionally nearly two-thirds. The mica may occasionally be only i per cent., though it is commonly between 5 and 25 per cent. The felspars form between 40 and 70 per cent, of the rock. Sometimes a green variety of hornblende gives rise to hornblendic granite. Granite may include angular fragments of schists, slate and limestone, often of immense size. These frag- ments appear to show that the granite is intru- sive, and that it tore them away from rocks through which it passed. Instances have been recorded of granite resting upon schist. Granite THE MATERIALS OF MOUNTAIN CHAINS. 25 is also intruded on a smaller scale, forming veins, which penetrate into other rocks, or sometimes cut through the granite itself. The only evidence of the condition and temperature at which the granite was intruded is afforded by its junction with slate. In Cornwall, where the slate near to it has acquired the texture of mica schist and gneiss, there is no evidence to show whether that metamorphism was due to the heat of the granite, or to the pressure which it exerted, or both com- bined. A few rocks which are found in mountain regions resemble granite in texture, but differ from it in mineral constituents, owing to the original chemical difference of the material out of which the crystals are formed. Syenite is well known in Charnwood Forest and in Guernsey. Syenite is a rock formed commonly of orthoclase felspar, hornblende and black mica. They are a variable group, including mica syenites, augite syenites, nepheline syenites, zircon syenites and many others. A third type of granite rock is named gabbro. It is familiarly known in the Cuchullin hills in Skye. Its crystals are as large as those of granite, and similarly arranged. It is formed of a plagio- clase felspar like labradorite, associated with some mineral of a brassy or metallic aspect like diallage, and often contains black mica and olivine ; and in some localities hornblende. These granitic rocks have been termed plu- tonic because they appear to originate in the re- gion which mythology assigns to Pluto, in the interior of the earth, consolidating slowly under great pressure. 26 THE STORY OF THE EARTH. CHAPTER IV. VOLCANIC ROCKS. No clear distinction can be drawn between plutonic rocks and coarsely crystalline forms of volcanic rocks. Both are extruded in some in- stances from deep-seated parts of the earth. In consequence of the rigid condition of the globe it is impossible that those rocks came to the sur- face from an unconsolidated interior by ascend- ing fissures. Many writers have assumed the existence of molten areas or lakes in the interior of the crust, as a source for lava streams, which sometimes flow on the surface for a hundred miles. Others, again, assume that the longitudinal fissures, along which volcanic cones have been built, penetrate down to different layers of the earth, each distinguished by having the mineral character of the different kinds of volcanic rocks. Such a fissure allows the atmosphere to penetrate downwards, and removes from the heated rock the pressure which had kept it solid. The rock then liquefies and ascends the fissure like fluid in a pump, until it comes in contact with water de- rived from the earth's surface, and so generates steam, which forms the explosive outbursts. The steam ascends miles high into the air, carrying up the rock in the form of dust. The dust from the volcano Krakatoa, in the Strait of Sunda, ejected in 1884, remained suspended for more than a year. On this hypothesis the difference between plutonic rocks and volcanic rocks is in the circum- VOLCANIC ROCKS. 27 stance that the plutonic rocks consolidate deep in the earth, while the volcanic rocks consolidate under the pressure of the atmosphere, or near to the surface. The principal types of volcanic rocks are named Rhyolites, Trachytes, Andesites and Ba- salts. The basalt has been supposed to be the last formed; and to have come from a greater depth than the others, being commonly the densest of the volcanic rocks. It frequently rests upon andesites and rhyolites. These rocks have been repeated several times in succession in the history of the earth. Rhyolites are found in the old pre-Cambrian rocks of Wales; andesites in the Cambrian rocks of the Lake district, and the Old Red Sandstone of Scotland ; while in the later Coal Measures there were countless out- bursts of basalt. The volcanic rocks of the Ter- tiary period in Britain are a repetition of those of the Primary period, basalts succeeding andesites and rhyolites. There is at the present day something like a geographical distribution of the different volcanic rocks. The volcanos of the Andes pour out the rock named andesite. The voleanos of Southern Italy give out varieties of basalt. Metals are very rarely associated with volcanic eruptions, though an appreciable quantity of silver has been found in volcanic ash of eruptions in Chili. Chemical and mineral composition alike sug- gest the closest relation between the deep-seated crystalline rocks and those which flow from vol- canos. The plutonic granite appears to become the volcanic rhyolite. Plutonic syenite and dio- rite on reaching the surface appear to become andesite. And the rock which cooling under 28 THE STORY OF THE EARTH. pressure becomes gabbro, after flowing on the earth's surface becomes basalt. Rhyolite. The name rhyolite indicates the fluidal structure of the cooled lava, which results from the movement of minute crystals about larger crystals in the flow of the molten stream. Some of the crystals are visible to the eye. The material between them is named the ground mass. Under the microscope, this ground mass is seen to be formed of microscopic crystals, with an un- crystalline material between them, distinguished as the base. The visible crystals are principally quartz, with the glassy variety of orthoclase fel- spar named sanidine. These minerals may form the entire mass of a granite rhyolite. But rhyolite may be free from crystals, forming a glass, such as obsidian; or be expanded into a' froth like pumice. Nearly all crystalline rhyolites are full of con- cretions with a radiating structure, or alternations of granular layers with spherulitic layers, and these are known as spherulites. Besides the common form of quartz, another variety named tridymite occurs, in hexagonal plates. A little mica and sometimes hornblende may be diffused in the rock. The oldest British volcanic rocks of St. David's, Bangor and the Wrekin, are rhyolites. Rhyolites and rhyolitic ashes often occur around granitic centres, as though they were mutually related. Andesite. Andesites contain 55 to 75 per cent. of silica. As the silica increases, the percentage of alumina decreases from 20 to about 12 per cent. The oxide of iron and lime also become less with the increase of silica. Typical andesites are formed of oligoclase felspar, and columnar VOLCANIC ROCKS. 29 hornblende, in a glassy ground mass, with a little mica and magnetic iron. The quartz hornblende andesites correspond to syenites in chemical com- position ; just as syenites correspond chemically to some Cambrian slates. The hornblende ande- sites, which are free from quartz, are closely re- lated to the rocks named diorites. Andesites are largely quarried on the Rhine, in the Siebenge- birge, near the Apollinaris spring at Remagen. Andesite abounds in black concretions rich in hornblende, like those found in the granite of Shap in Westmoreland. Phonolite is probably a volcanic representative of a syenite which con- tains the mineral nepheline. Basalt. This is the most familiar volcanic rock. Its silica is reduced to 35 to 55 per cent. Oxide of iron, lime, and magnesia are more abun- dant in it than in other volcanic rocks. It con- sists chiefly of the minerals labrador-felspar, and augite, or some similar substance, usually associ- ated with a little magnetite and olivine. It is dark in tint, grey-brown, blue-black, or greenish black when freshly broken. Cooled slowly, it gains a fine granular texture, and is known as dolerite. * In the most ancient basalts of Cambrian, Silurian and Devonian ages the olivine and augite have been partly decomposed, and converted into a green mineral like chlorite. The basalt or dolerite is then known as diabase. The less altered dolerites, of carboniferous age, have been termed melaphyres. Occasionally the felspar in basalt may be replaced by allied minerals. In Etna and Vesuvius leucite takes its place. There is also a nepheline basalt. Olivine may take the place of felspar. That 30 THE STORY OF THE EARTH. mineral then gives a name, peridotite, to the rocks in which it is an essential constituent. Those rocks are frequently converted by decomposition into serpentines. Volcanic rocks of the basalt family sometimes divide into beautifully regular six-sided columns, such as are familiar in the island of Staffa, and the Giant's Causeway in the north of Ireland. A lava flow sometimes cools from its floor and also from its upper surface; and two independent sets of vertical columns of different sizes may then be formed, separated by a crystalline part in the middle. Each of these kinds of lava may also be repre- sented by fragmental rocks, having the aspect of cinders, or dust. In past periods of geological time, beds of volcanic agglomerate, of ashes, and vesicular lavas are common in association with compact lavas. In North Wales, among the Arenig rocks, the ashes are enormously thick in Cader Idris, Aran Mowddwy and Arenig moun- tains, and there is little doubt that the ash was ejected from volcanic throats near Dolgelly and Arenig. In the Permian rocks near Exeter, the beds of volcanic ash at Pocombe are manifestly drifted by the wind. And at Spence Combe the lava flow is highly vesicular, with the vesicles filled with minerals, giving singular evidence of elongation of the steam cavities by flow in these old lavas of Devonshire. There is a close chemical resemblance between the several types of volcanic and plutonic rocks, and a marked similarity in their mineral composi- tion, which suggests a common origin. The evi- dence is not quite so complete that would tend to . THE MATERIALS OF STRATA. 3! establish a transition from the plutonic rocks through schists to water-formed deposits. It has not been fully collected, but deserves examina- tion, since the earth offers no indication of a be- ginning in its geological history. If metamor- phism such as is manifest in the older rocks were extended over the earth's surface it would oblit- erate records. And the wearing up of such metamorphosed rocks into new sediments would ensure a succession of similar rock materials. CHAPTER V. THE MATERIALS OF STRATA. Terrestrial Rocks. AROUND many parts of the coast, as in Lanca- shire and Norfolk, the winds blow up sands from the sea-bed, laid bare at low tide. These sands form low ranges of hills, known as sand dunes. They often show forms of hill contours as varied as are produced by the work of water and frost in carving hills out of solid material. These sand dunes are but an insignificant illustration of the work done by the wind, in heaping and rounding the grains of sand which form desert regions. There, every grain of quartz, which in a sandstone usually retains some of its angles of crystal form, is rounded by long continued motion, till it be- comes a miniature pebble. There is some evi- dence that desert conditions not altogether dis- similar to those of Arabia or the Sahara may have existed in Great Britain at the beginning of the Secondary period of time, when the rock salt- 32 THE STORY OF THE EARTH. was in process of accumulation by the evaporation of land-locked basins of the sea. In Lancashire and Cheshire, in the lower part of the Trias, there are some layers known as the " millet seed beds " because the separate grains of sand flow between the fingers like millet seed or shot. Those minute pebbles are not all of quartz but partly of felspar. They can only be compared to blown sands of deserts in their pebble-like forms. The less completely rounded sand grains in ordinary sandstones have probably acquired their character from long continued rolling, partly in rivers, partly on shores, as they have passed from one geological deposit to another in successive epochs of time, as a consequence of the construc- tion of new layers of rock out of the materials of ancient lands; a process repeated again and again, and still in progress. Another terrestrial rock which can scarcely be termed water-formed, because it is accumulated by vegetable growth, is seen in the peat, which cov- ers large parts of the earth's surface where the mean temperature falls belows 42 F. It is well known that peat frequently originates in the fall of forest trees, because they obstruct the surface drainage on level lands, until bog plants grow and form a sponge-like covering to the land which buries the fallen trees, and kills the adjacent for- est. Such accumulations in Cambridgeshire have been stated to attain a thickness of 40 feet. In the East of England, as in Ireland, there are two successive peats. The older has yew trees at the base ; and the newer peat covers forests of pine trees. In the Fens of the Isle of Ely these peats are often separated by a clay of marine origin, the " buttery clay " of the Fen-man, the Scrobicu- THE MATERIALS OF STRATA. 33 taria clay of science, so named from a bivalve shell found in it, which lives in the swampy inlets on the east coast of England. In that clay are occasionally found the remains of walrus and seal, whale and grampus ; showing that the inlet known as the Wash extended southward during the deposition of the clay, over the lower peat in much of the Isle of Ely. When peat becomes compressed by the deposition of superincumbent rock, it is consolidated like the rocks with which it alternates. There are important geological de- posits, which have grown in the same way, in the successive periods of time. At the beginning of the tertiary period, at Bovey Tracey in Devonshire, alternations of lignite and clay form a succession of layers which fill up a lake-basin in the older rocks : similar growths are seen in the Brack- lisham beds of the Isle of Wight. In the second- ary rocks there is a remarkable bed of vegetable matter five feet thick, at Brora in Sutherland, which is worked for coal. Thinner beds are found on the Yorkshire coast, which appear to have grown like the modern beds of peat, in the posi- tions in which they are found. Far more impor- tant are the beds of consolidated vegetable*matter found in the upper carboniferous rocks of the pri- mary period, which are commonly known as coal. They often give evidence of change in level of land during their accumulation; the same bed being thick in one place and divided up at a little distance by intervening sedimentary de- posits. These accumulations of sediments pre- serve indications of the plant life of the earth, and in the associated sediments are occasional- ly found remains of insects and other terrestrial animals which lived in the same epochs of time. 34 THE STORY OF THE EARTH. Pebble Beds. Any rock which is sufficiently durable to break into compact pieces may give rise to a pebble bed when the fragments are further reduced in dimen- sions by the action of frost, or the transporting movement of a flowing river, or the battering ac- tion of waves upon a shore or shoal. The harder rocks are not rounded into pebbles without long continued rolling. The term pebbles is applied to stones more than half an inch in diameter, so that they vary in size from Barcelona nuts to co- coanuts. Stones which are larger than these are termed boulders. Stones which are smaller are often termed grits. A river flowing two miles an hour transports stones as large as eggs, so that pebbles may be brought by such means from many kinds of rock which are exposed in the in- terior of a country. They are mixed and ac- cumulated either on shores, or where the stream leaves them behind owing to its slower move- ment. The pebble beds around shores are carried backwards and forwards with the daily movement of the tidal waters, and they serve to mark, when covered up by other sediments, ancient shores of seas which existed in bygone time. Pebbles which exist in the old geological deposits have been derived from granites and schists, from con- solidated quartz rock, and lava streams, from con- solidated sandstones, and veins of quartz which infiltrating waters have deposited in the cracks which upheaval has produced in ancient slates. Many beds of pebbles have been formed from con- cretions of flint, and similar substances, which THE MATERIALS OF STRATA. 35 FIG. 2 . Conglomerate of flint-pebbles, from the Hertfordshire puddingstone, showing the external surface of the pebbles. FIG. 3. Fracture through this conglomerate, showing sections of flint-pebbles imbedded in a siliceous cement. 36 THE STORY OF THE EARTH. prove more durable than the geological deposits in which they were contained. Pebble beds indicate the ceaseless action of water in wasting the earth's surface, which has gone on without intermission through all the periods of past time, because tides have never ceased to flow and ebb or rivers to run. The positions in which pebble beds accumulate have changed, because the outlines of the seas alter with the folding of the rocks during past ages; and they frequently come back again age after age with variation in level of land, at long intervals to be re deposited upon a shallow sea- bed, or shore-line in the same district. Occasionally the pebbles are scratched, and some in the Permian rocks of Worcestershire are regarded as ice-worn fragments. The pebbles are frequently bound together by a cement, which converts a loose aggregate of stones into a compact and durable rock. The most important of these cements is silica which is occasionally more durable than the pebbles which it binds together, as may be seen in the Hert- fordshire puddingstone. Such rocks, named con- glomerates, are also formed of pebbles bound together with a cement of oxide of iron, or of car- bonate of lime. Conglomerates and pebble beds are among the oldest known geological deposits in Britain. Among the primary rocks they are formed almost entirely of granite, schists, and lavas. Among the secondary rocks the pebbles which make up conglomerates are frequently derived from rocks that have more obviously had a water-formed origin. In the tertiary period of time most of the English pebble beds have been derived from the THE MATERIALS OF STRATA. 37 grinding up of flints, liberated from the destruc- tion of the chalk. Pebble beds frequently mark important divi- sions of geological time in the country in which they are found; because their existence implies that change of level of land which resulted in the tidal denudation which brought them into exist- ence. Among examples of great pebble beds and conglomerates may be mentioned the geological deposit known as the Llandovery beds, which in Wales forms the base of the true Silurian rocks, and extends successively over the upturned edges of the older Cambrian rocks, which had previously been planed level by the sea. * Sands. There is a rapid gradation between pebble- beds and sands. Beds of intermediate texture, with many grains as large as peas, are named grits, and are typically seen in some layers of the " Millstone grit," which underlies the coal. The grains are miniature pebbles, often angular, formed by rounding angular masses of quartz rock on a sea shore. On a sandy shore, like that 6f Hast- ings, Reculvers or Hunstanton, the sands now being deposited are derived from sandstones which form the cliffs, broken up first by joints, and afterwards separated into grains, similar to the material out of which such sandstones were originally consolidated. Cambridge green sand and Neocomian sand contain many rock frag- ments and fossils derived from ancient deposits. The particles of quartz are often crystalline, but are sometimes derived from uncrystalline forms of silica such as chalcedony, chert, opal, 38 THE STORY OF THE EARTH. and flint. Under the microscope the source of the grains can usually be recognised; for if the quartz was originally crystalline all the crystal faces are rarely lost, and the mineral may include hair-like crystals of other minerals, or minute cavities in which there may be fluid with a bubble of air. This is enough to show that the quartz came originally from the wearing away of schists or granitic rocks, in times when the level of the land caused those rocks to be exposed to the de- stroying influence of the waves. But although a sandstone is mainly formed of quartz, it rarely contains more than from 50 to 85 per cent, of silica the whole of which is not in the crystalline form of quartz. There are frequently in a sand grains of water-worn felspar. The felspar, which is a silicate of alumina combined with a silicate of soda or potash, may decompose, liberating the soluble silicate of soda or potash. Felspar crys- tals abound in the old Cambrian sandstones of Barmouth, in the Devonian sandstones of South Devon, and in the Trias sandstones of the South of England. Sandstones often contain scales of the mineral mica, as in the Yorkshire sandstones, used for paving. Sometimes the mica decays, and then the iron oxide which was one of its constitu- ents, gives a rusty colour to the sandstone. Many sandstones are mainly the grains of quartz, which were constituents of crystalline rocks, liber- ated by the decay of minerals with which they were associated, and left behind comparatively near to the source from which they were derived. The finer particles associated with them which resulted from the decomposition of other minerals have been carried to greater distances. A stream flowing three miles an hour at its bottom carries THE MATERIALS OF STRATA. 39 away sand ; but sand may be carried out to a dis- tance of more than 50 miles from the shore. The FIG. 4. Laminated vertical sand (Bagshot sand) of Alum Bay in the Isle of Wight, showing current bedding. * size of the particles of sand when coarse is -fo of an inch ; but the majority of the grains vary from j-J-g- of an inch to y^j-gr of an inch in diameter. When a sand is laid down it is incoherent. It often shows evidence of its shallow-water origin, in the manner in which currents have brought its materials from different directions. After a de- posit of sand is uplifted, and exposed to the action of rain-water flowing through it, it begins to be bound together by a cement which determines its 40 THE STORY OF THE EARTH. durability. There are three chief kinds of ce- ment, which were originally of vegetable or ani- mal origin. First, there is the iron sand which is commonly red, yellow, or brown ; and is rarely green. The iron was probably collected from the sea-water by the growth of marine plants, and liberated by their decay. Oxide of iron is liable to accumulate in the planes in which water has flowed underground through the rock, which are sometimes determined by strain. Examples, of sand so coloured are seen in the Bagshot beds of Alum Bay, in the Isle of Wight ; and the Wealden beds of Warbarrow Bay, in the Isle of Purbeck. The Wealden sands of Kent and Sussex are so rich in iron that they were for a long time the main source of the metal out of which English imple- ments of war and ornaments of art were made. Sands are often bound into calcareous sand- stones by a cement of carbonate of lime. It is sometimes derived originally from the evapora- tion of water, but more frequently from the fall- ing to the bottom of organisms which lived in the water, or by the accumulation of marine shells. The Kentish Rag, which forms the lower part of the Lower Greensand, is a familiar example of a cal- careous sandstone in the S. E. of England. The shells become dissolved by water flowing through the rock ; and the carbonate of lime of which they consisted is re-deposited, so as to fill the inter- spaces between the grains of quartz, and form crystals which bind the sand into a sandstone. The third kind of sandstone as modified by the cementing material is the siliceous sandstone, in which the grains of quartz are bound together by silica.. These sandstones are of all geological ages. The material of the cement appears to be THE MATERIALS OF STRATA. 41 in most cases, if not always, the minute particles of the siliceous skeletons of sponges, some of which resembled the Euplectella speciosa from eastern seas, sometimes known as Venus's flower FIG. 5. Ripple-marked sandstone from Permian Rocks in the Karoo, near Prince Albert, Cape Colony. basket. The spiculae which form the skeletons of such sponges, dissolved by the water draining through the rock, furnished a cement which is deposited in the same way as the cement of cal- careous sandstones. Infiltration of this silica frequently builds, within the sandstone, compact layers of a flinty rock, known as chert. Such layers are found in the Lower Greensand, the Upper Greensand, and especially in the Carbon- iferous Limestone. In the S. E. of England the sands and sandstones, secondary and tertiary, are often coloured green, with the mineral glaucon- ite. As a rule sands are red, yellow, or brown, 42 THE STORY OF THE EARTH. coloured with oxides of iron. They are shallow- water or shore deposits which show current bed- ding; due to deposition by changing currents, as well as ripple-marks of wave movement, and foot- prints of animals. Clay. Any substance which is taken up by moving water so as to cloud it, is popularly termed mud, and mud when deposited consolidates into clay. Mud banks abound on parts of the coast where clay forms the cliffs, because tidal movement of the water converts the clay into mud. It is chiefly composed of light flocculent particles of silicate of alumina, which frost tears apart from each other, and the lightest shower moves down a valley. The mud of rivers is carried into the sea, as far as the fresh water can float over the ocean. The Yellow Sea is yellow with the mud of the Hoang-Ho. When the Rhine at Bonn is turbid and full of water, -%-fa$ part of its weight is mud; but after continued dry weather the sedi- ment falls to -g-tf^nr part of the weight. Thus the alternation of seasons may give a laminated char- acter to deposits carried to the sea, marking the succession of years by changes in the deposit, like the rings of growth in the wood of a tree. When clay extends parallel to a shore, in con- sequence of denudation of the cliffs, it commonly has a definite relation to coarser sediments which are deposited nearer to land. This is seen in the large percentage of sand, sometimes amounting to 50 or 60 per cent., which may be separated from clays by washing. The particles of sand however are extremely fine, so that they are held in suspension for a long time by moving water. THE MATERIALS OF STRATA. 43 Under such circumstances, the chemical composi- tion of a clay is sometimes the same as that of a sandstone; and there maybe an unbroken tran- sition from one deposit to the other through an intervening loam. Almost all the minerals which enter into the composition of crystalline rocks, are occasionally found in clays. Often the origin of a clay is revealed in the abundance of the flakes of mica which are scattered through it, for the silicate of alumina corresponds chemically with decomposed felspar, and the presence of particles of quartz diffused in it, shows that the rock has been derived from an old crystalline material. The presence of mica renders it prob- able that the schists which were denuded were of the kind termed gneiss, if the crystalline rock was not granitic. Some ancient clays have been found full of needles of the mineral rutile. The purest clays are formed on land from the decom- position of white granite. Such a source would appear to be necessary for the beds of white pipe clay, found interstratified in the Bagshot sands of -Hampshire and Dorset. The colours of clay are due to oxides of iron. Occasionally, as in the Woolwich and Reading beds at Reading, current bedding is marked by alternate layers of red and green clay. Such bed- ding would be unsuspected, but for the colour. Often a blue clay passes into a brown or yellow tint. It is then sandy and porous, so as to per- mit the infiltration of water charged with atmos- pheric air, which oxidises the iron. Minor sources for clay are the small percent- age of silicate of alumina which is the insoluble residue left when limestones have been dissolved. This forms the red cave earth in limestone dis- 44 THE STORY OF THE EARTH. tricts; and the red soil on limestones. Beds of volcanic ash, exposed upon the surface, may be- come broken up by atmospheric decomposition and converted into clay. The bedding of clays is frequently marked by the occurrence of thin layers of earthy limestone, or of concretions termed septaria, which are dis- tributed in parallel layers, on zones where car- bonate of lime was abundant. These concretions probably mark near approach to the limit to which the ancient mud was carried in the sea, where the sediment was becoming replaced on the ocean floor by calcareous layers of organic origin. The septaria often contain upwards of 50 per cent, of carbonate of lime, and their occurrence appears to mark, either oscillations in level of the sea bed which varied the distance to which sediment was carried, or indicates that the land area which was being worn away to form the new deposit became more calcareous. One of the most interesting clay deposits in England is the inflammable clay, known as Kim- eridge clay, found in Lincolnshire and Cambridge- shire as well as on the Dorset coast. Some layers of this clay when distilled yield as much as 40 per cent, of paraffin, naphtha, tar and heavy oils, which are similar to products of coal tar. Those chemical substances derived from coal be- ing of vegetable origin, it is probable that the in- flammable character of the clay is due to the growth of marine algae, though no traces of plant remains are found among the remains of marine shells which crowd the deposit. . Almost all clays yield the minerals, iron pyrites, and selenite, which are closely dependent upon each other. Iron pyrites mineralizes fossils, and THE MATERIALS OF STRATA. 45 occurs in irregular masses. Both iron and sulphur may have been liberated in the decay of marine plants. As the iron pyrites decomposes in con- tact with the air, its sulphur is converted into an acid, which dissolves the substance of shells, ex- pelling the carbonic acid, and forming a hydrated sulphate of lime, known as selenite. Occasion- ally phosphate of lime form concretions in clays and mineralizes fossils. Clays are commoniy formed in deeper water than sands, and further from the shores which furnished the sediment. Limestone. Limestones differ from other water-formed rocks in not being sediments. Their particles have grown, as portions of organisms ; and have become rock substance, when the animals or plants died, which separated the carbonate of lime from water. Sometimes limestone is pre- cipitated by evaporation of water. The carbonate of lime which forms limestones is usually in the mineral condition of calcite. Beds of limestone may be deposited over the whole sea-bed, whether the water is shattow or deep. As a rule they are most noticeable in the open ocean, beyond the limits to which sediments are carried. Limestone may be formed near into shore ; and when the rock is dissolved away by acids, in some cases nothing remains but a vary- ing percentage of siliceous sand. Rocks of that kind are termed calcareous grits. Evidences of the shallow- water origin of some oolitic limestones in the west of England, are also seen in current bedding, which characterizes some oolitic rocks, and is as marked as in sandstones. 4 6 THE STORY OF THE EARTH. The limestones named oolites, are probably all formed in moderate depth of water ; since there is FIG. 6. Lithographic limestone from Solenhofen, showing circular staining at the intersection of rectangular joints ; and corru- gated fracture on the right side. some evidence to show that the oolitic grains may be derived from plants like the Nullipores, and larger grains, termed Pisolite, show a minute tubular structure, attributed to an organism named Girvanella. Beds of shell limestone are seen in process of formation on many shores. Shell-haven, in the Thames, takes its name from the manner in which shells are drifted together so as to form a deposit ; and a similar accumulation may be ob- THE MATERIALS OF STRATA. 47 served at Shell Ness which makes the eastern end of the Isle of Sheppey. Parts of the forest mar- ble in Oxfordshire, consist of accumulated growths of shells; and in Gloucestershire portions of the same deposit show ripple marks which indicate shallow conditions of deposition. Coral reefs are also to be classed as shallow- water limestones since the coral grows most vigor- ously where the water is aerated by the movement of the waves near to the surface of the sea.,, The FIG. 7. Carboniferous limestone, the surface dissolved by rain, showing the remains of Encrinite columns, of which it is part- ly formed. great brainstone coral Meandrina and the com- pact coral Forties, associated with Nullipores build buttresses which constitute the living foun- dation of the reef. Our English coral limestones 48 THE STORY OF THE EARTH. all give evidence of shallow-water conditions, exactly such as are seen in the growths of fring- ing reefs of coral at the present day. There is a second group of limestones which may be termed oceanic, or deep-sea limestones, made known by exploration of the floors of the great oceans. They are largely composed of the minute organisms termed foraminifera, such as cover so much of the Atlantic, Pacific and Indian oceans. Among geological deposits due to such organisms, may be placed the Chalk of Europe ; the Nummulitic limestone, which extends through Europe, Africa, and Asia ; and the Fusulina lime- stone, which extends from Russia to Japan. There are other animals which appear to form deep-water limestones, since they live in some depth of water at the present day, such as the group of shells termed Brachiopoda; and Encri- nites, which make up portions of the Carbonifer- ous Limestone in this country. An oceanic limestone is not necessarily built tip in deep water, although such rocks often attain a great thickness. It is only necessary that the sea in which the rock accumulates should be be- yond the limits to which sediments from land can be transported. Ocean basins often increase in depth as the deposit increases in thickness, when limestones may be formed as thick as are the Carboniferous limestone of Flint and Derbyshire. Fresh Water Deposits. Accumulations of sands, clays, and limestones are brought down from higher land wherever fresh waters accumulate upon the land surface If the pond or lake rests upon a limestone the THE MATERIALS OF STRATA. 49 deposits formed within the lake will be mainly calcareous. Fresh-water plants like the Chara precipitate carbonate of lime upon the stem by absorbing the carbonic acid gas from the water, so that the carbonate of lime is no longer soluble. This ensures an accumulation of granular lime- stone as the plants decay. Such a deposit covers up the remains of fresh water shells, and fre- quently the remains of animals derived from tand. The lakes in Cumberland and Westmoreland are found to have their beds covered with deposits which consist of volcanic minerals when they lie in regions occupied by old volcanic rocks ; while the deposits are ordinary sediments when the lakes are surrounded by rocks formed of such ma- terials. If the lake is sufficiently large, like the Lake of Geneva, the sediments may be completely sorted, successively deposited, and pass from the condition of coarse pebbles and boulders where the Rhone coming from the Valais enters the lake, to the finest sediment where its clear waters leave it, southward of Geneva. Hence a lake may con- tain an epitome of all known water-formed rocks pebble-beds, sands, clays, limestones as well as layers of plant remains consolidated into lignite. Examples of such fresh-water growth of sedi- ments alternating with lignite, has been already referred to in the layers known as Coal Measures. In the lacustrine deposits which are so important in the northern part of the Isle of Wight, fresh- water limestones are familiarly seen at Headon Hill and Bembridge, which were formed in fresh- water lakes, and give no evidence of sediments being mixed with the calcareous matter. Other fresh-water limestones alternating with terrestrial 50 THE STORY OF THE EARTH. surfaces, on which the remains of coniferous forest trees stand erect, are seen in the Purbeck beds of the Isle of Portland and the Isle of Purbeck in Dorsetshire. Fresh-water sediments, alternations of sands and clays are found with numerous rep- etitions in the Wealden beds of the Isle of Purbeck, and the Isle of Wight ; and they are associated into a few deposits, fairly well defined into sands and clays, in the Wealden strata of Kent and ; Sussex. Recognition of the fresh-water origin of all such rocks rests upon the presence in them of animals which lived in fresh water. When these are shells they are often matted together to form layers of some thickness. The types or genera are identical with those which live in every pond, lake and stream on the surface of the country at the present day. The bivalve shells are usually species of Cyclas, or Unio, or Anodonta. The uni- valve shells are either the pond shells Planorbis, Paludina and Limncza, or such river shells as Neri- tina, and the fresh-water limpet. There is probably no fresh-water limestone from which the seed-vessels of the plant Chara are absent. Sometimes the presence of the sili- ceous spiculae of the fresh-water sponge, Spongilla, has resulted in fossils being mineralized with silica, as in the Purbeck beds, or the formation of sili- ceous layers and concretions in fresh-water lime- stones, which may be compared to the veins and concretions of flint found in marine strata like the Chalk and Carboniferous Limestone. THE SUCCESSION OF STRATA. 51 CHAPTER VI. THE SUCCESSION OF STRATA. Contemporaneous origin of water-formed rocks. THE conditions under whjch sediments grad- ually become finer, as the distance from shore and depth of water increase, show that all known varieties of rock may be formed and deposited adjacent to each other at the same time. Not only are the beds of peat in Irish bogs contem- poraneous with the shell marls in the loughs, but these are contemporaneous with the sands, clays, and limestones which are forming at the present time in our seas. Any one type of mineral mat- ter may be represented by all the other types of which layers of rock can be formed, in a succes- sion of different localities. A geological period of time may be as accurately represented by ter- restrial lignite, or fresh-water sands, as by any kind of marine deposit. The chalky muds dredged a few hundred miles west of Ireland are accumu- lated in deeper water in association with different types of life, but manifestly formed contempo- raneously with the shell beds of Shell Ness, the muds carried out by the Thames, and the sands which are spread by the tides off Yarmouth. In all geological ages there has been the same contemporaneity of rocks of different mineral character. Marine rocks must have been laid down at the same time as the fresh-w^ter sands and clays of the Weald. An organic limestone like the chalk formed in the open ocean, necessi- tates shores where sediments were laid down. 52 THE STORY OF THE EARTH. And beyond those shores of the chalk sea were land surfaces of islands and continents on which plants and animals survived from age to age. Lands have never ceased to exist from the earliest ages. They have changed their forms. Their height of elevation above the sea has been altered ; they have been broken up into islands and re-united with other islands newly formed. The lands which exist at the present day are built up almost entirely of water-formed rocks, which have been spread out one upon another in the ocean. Every continent shows this history : a succession of ancient sea-beds, with the deposits formed upon them, alternating occasionally with old land surfaces which make known epochs when the sea-bed emerged from the ocean, and became land as it is now. The shores, with their pebble beds and other evidences of tidal movement of the waters, have persisted from the earliest times, changing their positions upon the globe, as the lands altered their forms, never entirely passing away through the long epochs of geological time, although they only occasionally come back again to the places in which shores had previously existed. The open ocean with its limestones has proba- bly been equally persistent and as variable in form. Very little is known of limestones which may have existed in the earliest geological ages. But from their thickness and importance in the time named Devonian and Carboniferous, and in all subse- quent times, it is inferred that the open ocean has persisted, Chough its depth has varied. There can have been no breaks in geological time, though there are breaks in the continuity of land surfaces, in the continuity of shore lines, and the THE SUCCESSION OF STRATA. 53 continuity of deposits of the open sea. These breaks are local, like the breaks which are made by the islands or lands which divide the sea, and by the waters which separate lands from each other. The Succession of super -ijnposed Rocks. A sediment may be followed round a shore line, so that it has everywhere the same general character, except in so far as the rocks of the cliffs vary, which give rise to pebble beds in some localities and scarcely any sandy particles on the shore in others. As a rule, tidal work sorts and sifts the products which the sea carries down to its depths, so that they are arranged in bands which are parallel to the coast. The particles vary in size in-those zones of deposit. The finer particles remain suspended longest; and are there- fore transported to the greatest distance by the moving water. Thus there is a horizontal succes- sion of rocks on successive parts of the same ocean floor, which may be roughly classed as sands, clays, and limestones. Sands formed near- est to shore sometimes pass into grits and pebble- beds. And the limestones, like the sands/ some- times alternate with clays in vertical succession, where they pass horizontally into each other. In- stances may occur where limestones extend con- tinuously from the shore to the open ocean, with- out intervening deposits. The horizontal sequence of water-formed rocks, observed at the present day, explains the meaning of the vertical succession of the layers of rock termed strata, which differ from each other in mineral character. By their superposi- tion they build up most of the visible land, as well 54 THE STORY OF THE EARTH. as of those parts which are hidden under the oceans. Their vertical succession depends upon successive changes in position of the area from which sedi- ments are brought into the sea. If the land sinks down so that it becomes smaller, and its shores recede, each kind of sedi- ment derived from it, being carried by the moving water the same distance as before its depression, is transported for a less distance out to sea as compared with the .deposit formed previously. Therefore the finer sediments on a sinking sea- bed rest upon the coarser sediments, which had been formed previously, when the source of sup- ply was nearer to the place of deposition. In other words, clays rest upon sands ; while the new sands rest upon areas which had previously been dry land. If this process of depression continues, then while the clays follow the new sands, and become super-imposed upon them, limestones are super-imposed upon clays. Sandstones occasionally give evidence that they were deposited between tide marks, in pre- serving the footprints of animals, as well as in the ripple marks, sun-cracks and rain-prints which were formed when the surfaces dried between suc- cessive tides. Such memorials are preserved in the Trias Sandstone of Cheshire, and the Hast- ings Sand. Clays occasionally, in the abundant remains of terrestrial plants which they yield, give evidence of estnarine origin, which may not be strictly comparable to the succession of con- ditions seen upon a land which is being sub- merged. On the other hand some deposits are formed upon shores which are rising, and advance at the expense of the sea, and then the deposits which THE SUCCESSION OF STRATA. 55 result from waste of the land, succeed each other vertically in reverse order. In the sea which borders the coast of South America, the sand de- rived from its cliffs may be carried out to a dis- tance of from 20 to 150* miles from the shore. The mud formed at the same time would be car- ried much further. If then such a land were to be enlarged by slow upheaval, so that the shore extended over the area which had previously been the shallow sea, two sediments which would still be formed would be carried as great a distance as they were carried previously, and the sand at its furthest limit from the shore would gradually ex- tend beyond the limit of the sand beneath it, and would thus be super-imposed in part at least upon clay. In like manner the mud sediment would be carried further than the mud had gone pre- viously, so that it would similarly rest upon lime- stone. Therefore, under the influence of continual depression, the geological deposits come to be accumulated in the vertical order of sand, clay and limestone, in the same place. While under the influence of continued upheaval the vertical succession comes to be limestone, clay, sandstone. There are constant oscillations of level of land which are evidenced by successions of sand and clay, or limestone and clay, which are local. And occasionally a sediment is derived simul- taneously from two different sources, as when ancient cliffs furnish sand, and an ancient river supplies mud which is deposited at the same time, over part of the same area. The layers of water-formed rock which form every land, succeed each other vertically in some such order as sand, clay limestone, limestone, clay 56 THE STORY OF THE EARTH. sand. Their order in Nature, as seen in the cliffs and on the surface of the land, is evidence of great upward and downward movements both of the floor of the ocean and the dry land, which have been brought about by foldings of the rocks. Usually these rocks, the strata of sand clay and limestone, rest evenly upon each other, for the upward and downward movements are com- monly so gradual, that while the rocks are dis- tinguished from each other by mineral character, and the planes of bedding, which change with FIG. 8. Geological map of part of Yorkshire, showing the west- ward extension from Flamboro' Head of the Chalk and Hun- stanton Limestone, so as to rest unconformably upon the Kimeridge Clay, Lower Oolites and Lias. depth of the sea-bed, there is no physical break in the succession of limestone on clay or clay on sand, and the beds in parallel planes of deposit, THE SUCCESSION OF STRATA. 57 are said to be conformable to each other. This is particularly true of an area undergoing depres- sion. Yet as depression extends, and an area which had previously been dry land is submerged so as to be covered with new deposits, worn from the higher land which is near to the now sub- merged area, such a new layer begins a new order of succession in that district, and rests upon the edges of many older deposits which had been tilted up and worn level, so that their edges be- came exposed before the old land was raised from the sea. Such a succession is said to be uncon- formable. There is an unrepresented interval of time between such unconformable strata and the layers on which they rest; but the unconformity is local, and does not imply any real break in the succession of rocks, for the break is a conse- quence of the submergence of the denuded land, which had interrupted the even spread of sedi- ments by the water, so long as it remained above the sea. On the other hand, when land is undergoing upheaval, and the shore deposits begin to be raised out of the water, it must happen that the newest formed deposits will be worn up and re- mov,ed before they emerge from the sea. There is a break formed in this way in the horizontal sequence, though there is no break in the vertical sequence of strata. Traced by their mineral character to the circumstances in which they originate, the whole succession of water-formed rocks which is known demonstrates no more than three or four great oscillations in level of the earth's surface, which have converted lands into seas, and seas into lands. It is obvious that land is disturbed in level in 58 THE STORY OF THE EARTH. some localities, while the level elsewhere is un- affected, so that the succession of rocks may vary in mineral character in the disturbed district, with no indication of change in the succession elsewhere. On this account, the history of every part of the earth needs to be told separately from the other parts, for too little is yet known of the detailed events which took place in the successive periods of time, in the different portions of the globe, to piece the parts of the story together into a complete history of the earth. CHAPTER VII. ORIGIN OF STRATIGRAPHICAL GEOLOGY. GEOLOGY originated in observation of the earth's surface, by which records were made of the order and arrangement in which different rocks occur in England and Wales. This knowl- edge is expressed in two laws. The first is the law of stratification. It affirms that the rocks which are anywhere exposed on the surface of the country are usually portions of layers, which rest successively upon each other. ThereTore they rise from beneath each other, in the order of their relative antiquity, whenever they are in- clined to the plane of the horizon. Every such layer is a stratum. Strata differ from each other in relative antiquity, in their mineral materials, thickness, extension, and degree of disturbance from the original condition of their horizontal deposition. The law of succession of the layers upon each other is named their superposition. ORIGIN OF STRATIGRAPHICAL GEOLOGY. 59 The second law is that every stratum may be identified by means of the included remains of plants and animals, termed fossils, which lived when its rock material was being accumulated in the parUof the earth in which it is found. By these fossils the exposed edge of every stratum may be traced as it extends through the country. Therefore the area occupied upon the surface of the country by each geological deposit may be drawn upon a map. A map made in this way, which defines the limits of strata, lava flows, crystalline and other rocks which form the coun- try, is a Geological map. It shows how the strata in a country may be distinguished and classified by the succession of groups of animals and plants which have followed each other in occupying the same portion of the Earth's surface. These laws were discovered about 1790 by William Smith. He applied them in travelling through the country, so as to make the first Geo- logical Map of England and Wales, which was completed in 1815. In 1816 his collection of fossils, which distinguish and identify the several British Strata, was placed in the British Museum. It became the foundation of the National^ Geo- logical Collection and the beginning of all Geo- logical Museums. Other observers had recorded the order of the strata in different localities, and in some cases had recorded the occurrence of fossils in a single stratum ; but without making the discovery that the strata may be identified by their organic re- mains. Dr. Lister, in 1684, proposed to the Royal Society to make a map of the soils in our coun- try. This was the first proposal to make a Geo- 60 THE STORY OF THE EARTH. logical Map. In one of his writings Dr. Lister gives a drawing of a small fossil, a Belemnite, and states correctly, that it is found in all the cliffs along the Yorkshire wolds, for a distance of more than 100 miles, by Speeton, Londesbro' and Caistor, but always in a red ferruginous earth [now known as the Hunstanton Limestone]. Mr. John Strachey in 1719 laid before the Royal Society evidence that the upturned and levelled edges of the Coal Strata in the Somerset- shire Coal Basin were covered by nearly hori- zontal beds of the Red Marl, Lias, and Oolite. The Rev. John Holloway in 1723 described to the Royal Society the parallelism of the Chalk of the Gog-Magog and Chiltern Hills, the Sand Hills of Woburn, and the Clay country drained by the Cam, Ouse, Nen, and Isis. The Rev. John Mitchell in 1760 stated to the Royal Society that " we ought to meet with the same kinds of earths, stones and minerals appear- ing at the surface in long narrow strips, and lying parallel to the greatest rise of any long ridges of mountains, and in fact we find them [thus ex- posed]." The ridge in England which influences the direction of the strata is said to run first north to south, and then from north-east to south-west. Travelling between the Chalk hills of Cambridgeshire and the Coal of Nottingham and Yorkshire, he observed the succession of the strata; and in 1788, gave to Smeaton the Engi- neer, a table of these strata, with their thick- nesses. They are enumerated in vertical se- quence as Chalk, Golt, Sand of Bedfordshire, Northampton lime and Portland lime in several strata, Lyas, Sand of Newark, Red Clay of Tux- ford, etc., Sherwood Forest pebbles and gravel ORIGIN OF STRATIGRAPHICAL GEOLOGY. 6 1 [fine white sand], Roche Abbey and Brotherton limes, Coal Strata of Yorkshire. This table was given to,.Henry Cavendish, who preserved it. Mr. John Whitehurst in 1778 gave an account of the Geological structure of Derbyshire ; and remarks : the strata follow each other in a regu- lar succession, both as to thickness and quality, insomuch that by knowing the incumbent stra- tum, together with the arrangement thereof in any particular part of the earth, we come to a perfect knowledge of all the inferior beds, so far as they have been previously discovered in the adjacent country. Smeaton in 1786 expressed his belief that the Lias extends from Watchet in Somersetshire to Barrow in Leicestershire, probably with few breaks in continuity, and through the vale of Belvoir into Nottinghamshire and Lincolnshire, beyond Grantham and Long Bennington. There is no reason to believe that William Smith had heard of any of these observations. He was born 23rd March 1769, at Churchhill in /Oxfordshire, upon the Oolites. He became a /land surveyor and engineer; and at the age of twenty-one had found out for himself the succes- sion of such rocks as he had seen, and had begun to compare the appearances at one locality with those observed at a distance. His work was dis- tinguished from that of all predecessors by his method of untiring persistence in observing facts of stratification ; activity in comparing, extending and establishing the conclusions to which those observations led ; and care in recording upon his map nothing but what he had seen and proved. This work caused him to be known through the country as Strata Smith ; recognised among geol- 62 THE STORY OF THE EARTH. ogists as the Father of Geology ; and honoured as a great original discoverer in science. CHAPTER VIII. FOSSILS. Geological and Zoological Aspects of Fossil Plants and Animals. IN the early days of geology fossils were re- garded with interest because some species were limited in their range in time and only found in certain rocks. Attention was given chiefly to extinct species which were most abundant in each of the geological deposits. A large amount of practical work, in mapping the distribution of the strata, was done with the aid of very slight knowl- edge of a few species of animals. It became possible also to group the rocks to- gether in a rough way by limitation to the vertical range of a few fossils. The oldest rocks were defined as those containing Trilobites; the mid- dle group as those containing Ammonites; and the newest group as containing Nummulites. The geologist, having to classify the rocks and iden- tify them, was influenced in making divisions of the strata occur wherever a difference in life of any kind would permit the separation of strata, or groups of strata, from each other. A dim idea prevailed that the change in life was in some way connected with the succession of geological periods of time. Hypotheses were put forward that the groups of strata correspond FOSSILS. 63 to about six successive epochs, during which the life gradually became higher in the details of its organization. This theory was not suggested by examination of the rocks and their contents, be- cause divisions which separate strata in Europe are differently placed in America. The hypothe- sis endeavoured to forestall results at which science might arrive. The species of fossils found in each stratum were supposed to have been created in the period of time when they were first met with ; and to have become extinct when they disappeared with the succession of newer strata. Naturalists found that existing life varies with elevation above the sea level, and that there is a relation between distribution in height and in horizontal area. While some of the plants found in Great Britain are identical with those of Ger- many, there are a few, living on high ground, which are Scandinavian types. In the south-west of Ireland there are a few Spanish and Portuguese types. The Scandinavian life was accounted for on the hypothesis that, in a recent period of geo- logical time, those plants spread over land which is now the North Sea, when the temperature was lower than it is now. When the German types of plants subsequently spread over England, the Scandinavian species, which could endure greater cold, survived upon the hills; much as the Celtic population may have receded to the high ground before the invading Saxon peoples. Considerations of this kind indicate two great laws. First, that the existing life which occupies the earth's surface is grouped in series of geo- graphical assemblages, each of which may be termed a life province ; and secondly, that these 64 THE STORY OF THE EARTH. provinces occupy changed areas of the earth's surface, with alterations in the level of land. In the same way it was found that life in the sea varies with the depth of the water. Sea-shells which live between tide-marks, and are adapted to exist more or less exposed in atmospheric air, are different in genera or species to those in the deeper water, where the great growths of sea plants are found. Marine life again changes its character with greater depth. The shells which would be indicative of a shore, travel along the shore ; and the shells which are found in clays, are rarely met with in sand. Marine life also varies geographically in the horizontal direction, because there are natural history provinces of life in the sea, which may also change their area, when the depth of water changes, so as to scatter or concentrate or combine the life. About the year 1864 it began to be urged that the differences found in the fossil life between two successive geological deposits, were not due to great denudations of intervening strata, which had removed the intervening transitional organ- isms, making breaks in the geological record, but were the results of geographical migrations of organisms, so that these animals and plants came into an area which they had not previously occu- pied, by moving away from one which had for- merly been their home. When fossilized, the remains of such a group indicate a different as- semblage of animals or plants to those which lived previously in the area, when the life in the underlying stratum accumulated and was fossilized in the same way. Thus it is intelligible that the distribution of life in the strata has been brought about in the FOSSILS. 65 same way as the distribution of life is varied upon the earth's surface now. And instead of fossils in geological formations representing a multitude of successive creations, there appears to be but one creation. These types of life sur- vived from the earliest time by undergoing more or less adaptation to altered conditions, as a necessary circumstance for their perpetuation through all the revolutions which the earth's sur- face has undergone. Thus it is known that the elephant, hippo- potamus, lion, hyaena, rhinoceros, which are now living in Africa, have been common animals in Europe and Britain since the time during which men have lived here ; that those animals have changed their habitation ; and that the area of the life province to which they belong is mani- festly altered. There are no animals more dis- tinctive of Africa at the present day than the hippopotamus and ostrich, but in a recent ter- tiary period of geological time, these animals left their remains in rocks of Northern India, in asso- ciation with extinct allies of the giraffe, a type which is now limited to Africa. And so another change in the area occupied by a natural history province of life is made known by remains of animals preserved in the rocks. All down the sequence of the geological ages the story is of the same kind. Wherever there is a change in the material of which rocks are formed there is a change in the distribution of life on the earth. The upheaval or depression which varies the distribution of the mineral mat- ter and produces the succession of strata is also the cause which varies the distribution of life. Therefore the fossils found in any geological 66 THE STORY OF THE EARTH. formation are a portion of a natural history province, which has been preserved in the condi- tion in which it existed on the earth's surface at that particular epoch of time. If we suppose land and sea at the present day to be occupied over their areas with natural history provinces of life, in the manner in which they have been marked out by naturalists, such provinces are manifestly the survival of the life which has existed in the several periods of the geological record. They have reached their present positions in consequence of the geolog- ical circumstances of rock folding in the earth's crust which have given the earth's surface its present form. This truth is the only explanation of the succession of life in the past ages of the earth's history. It is impossible to imagine any change in life between the oldest deposit known and the bed which succeeded it, unless the life was already different in an adjacent area of the ocean, so that a new natural history province could be superimposed upon that which had pre- viously occupied the ground. The fossils of the geological formations are therefore the records of the succession of the natural history provinces of life on the earth. Each province has been formed by geological changes. They have succeeded each other like the movements of chess-men upon the same square of the chess-board. In this pro- cess many of the old life provinces are broken up, and their constituent animals and plants scat- tered and intermixed with others, almost beyond recognition. Such survivals have not been accom- plished, however, without the earth losing many of the kinds of life with which the geological story begins, and which characterize its greater epochs. FOSSILS. 67 The Succession of Life. The oldest geological deposits in the Cam- brian period give no indication of a commence- ment of life on the earth. The assemblage of fossils, after eliminating the types which have be- come extinct, is comparable to such as might be found upon an existing sea-bed. The most an- cient groups of fossils in the stratified rocks lend no support to the hypothesis that they are stages of a process by which animals came successively into existence, in the order of their grades of organisation. There are already several groups of animals co-existing, associated with each other as they are upon an existing sea-bed. On many shores at the present day the variety in life is not greater than the geologist finds in a quarry or cliff after examining a few yards of rock. Each of the great divisions of the animal king- dom has representatives in very old rocks. Man is limited, so far as is at present known, to the newest deposits. But geological research has pushed further and further backward in time, the epoch in which each of the highest types mam- mals, birds, reptiles, fishes is first met v;ith. Sometimes the earlier rocks are fancifully spoken of as the age of fishes ; those of the middle period are named the age of reptiles ; and the latest rocks are termed the age of mammals. Each of those groups of animals puts on a con- siderable diversity of organisation in the epochs which it is supposed to characterize ; and each includes some extinct groups which are not met with at the present day ; or subsequent to the epoch which the group characterizes. On the other hand, mammals are not only not limited 68 THE STORY OF THE EARTH. to the tertiary strata, but have been recorded as extending to the Trias, the beginning of the secondary rocks. Indications of their existence occur in connection with each of the old land surfaces which the strata make known in the south of England. Birds have been found on two different horizons in the secondary rocks. The presence in those secondary rocks of animals so remarkable as Ichthyosaurs, Plesiosaurs, Or- nithosaurs, Dinosaurs, and Anomodonts, fully justifies the term, age of reptiles. The modern type of crocodiles, lizards, turtles, and snakes, which are the true reptiles of the present time, do not extend back to very early parts of the secondary epoch, so far as is known at present. Extinct groups of reptiles such as the Anomo- donts and Labyrinthodonts date back at least as far as the time in which the Permian and Carbon- iferous coal was accumulated. The great facts which life presents to us when examined by means of fossil remains, preserved in the succession of strata, are : first, that it has been always changing in the same locality, in the same way as a fauna or flora undergoes change at the present day. In the lifetime of individuals, plants and insects have disappeared from the fen district of Cambridge- shire under the influence of embanking and drain- ing, just as in historic times animals like the wolf, brown bear, beaver and roebuck, have disappeared from South Britain. In other parts of the world the existing fauna has become poorer by the ex- tinction of birds like the Dodo, and the Moa. This process of extinction has never ceased. Its evidences remain in the extinct species which characterize every geological deposit. The process of extinction has extended to FOSSILS. 69 some larger groups, such as in natural history are termed Orders of Animals. Thus in the old slaty rocks termed Cambrian and Silurian the entire group of animals termed Graptolites is extinct. In the primary rocks there is an extinct group of corals, termed Rugosa, which have the radiating shelly partitions termed septa, in multiples of four. There are small extinct groups of Echino- derms in the silurian and carboniferous rocks, allied to the sea urchins, named Cystoidea and Blastoidea. Among Crustacea there are the ex- tinct groups Trilobites, comprising more than fifty genera ; and the Merostomata, comprising animals which are allied to the king crabs and scorpions. Other groups of animals, though not entirely extinct, are much better represented in the fossil state than in existing nature. Most of the genera of the groups of lamp-shells named Brachiopoda, are extinct, and found only in the Primary rocks; and the larger number of allies of the Nautilus are found in a fossil state, partly in the primary, and partly in the secondary period of time. A number of the existing groups of animals date from very remote, if not from the, earliest known geological ages. The genera in which they are first met with, frequently appear to have persisted ever since, without undergoing any ap- preciable change, beyond those minor modifica- tions which distinguish species. Although the fruits of the Araucaria, of various pines, and of Pandanus, are found in the lower Secondary rocks, it is not until the latter part of the secondary period that any deposit yields enough fossil leaves to enable the vegetation of the earth to be com- pared as a group with living types. The common 70 THE STORY OF THE EARTH. genera of ferns of the present day are well repre- sented in strata older than the chalk, by such types as Pteris, Asplenium, Adiantum, Aspidium, and Gleichenia. Palms are represented by Nipa. There are numerous representations of the oak, willow, beech, fig, laurel, ebony, magnolia. Noth- ing is known of the origin of this ancient cretace- ous flora, but there is no ground for believing that it suddenly came into existence in widely separated parts of the world, where it is first met with. In the oldest group of rocks every class of animals is represented by many genera which .still live. Thus the Foraminifera, which fill so large a place in the life of the open ocean at the present day, are represented in the Silurian period by the genera Dentalina, Lagena, Nodosaria, Tex- tularia. The existing genera of Echinoderms are not known from so early a period ; but in the begin- ning of the secondary time Cidaris and many other genera are found, which abound at the pres- ent day. Among shells the Brachiopods Lingula and Crania, Discina, Rhynconella, Terebratula and oth- ers survive from the older primary time. The common bivalve shells, which have few representatives in the Primary rocks, include such familiar forms as Pecten, Pinna, Cardiun, Area, Avicula. The common Univalve shells begin with such forms as Patella, Pleurotomaria, Chiton, Natica, Trochus, Dentalium, which have never since been absent from the earth. The Nautilus dates from a very early period. Thus, although the history of life has left be- FOSSILS. 71 hind enough extinct entombed forms to enable every deposit to be recognised by their remains, the great lesson of fossil remains is not so much extinction, as survival and persistence upon the earth of the life which has once existed. The natural inference would be that the variety in kinds of life has been steadily diminishing from the earliest time, owing to the loss of the extinct groups of plants and animals. But with each group of newer strata, genera and families of animals are met with among the fossils, which were not known in the older sets of fossils. There is perhaps no proof that they were pre- viously absent from the earth ; and it is possible that some of them may have come into existence as modifications of the types which were already in existence. The variation which life undergoes as the condition of its existence. There is a principle which affects the history of life, which necessitates that new modifications of plants and animals should constantly come into existence, under the varying conditions whiph the earth's surface assumes. The different organic types are saved from extinction by manifesting some degree of adaptation to altered circum- stances. It is this property which enables the genus to survive from the earliest times. It undergoes a series of changes by which 'slight differences of form or ornament are perpetuated for a time, eventually giving place to another similar series of modifications; and these char- acters distinguish the species of which the genus consists. Even persons who are not trained to 72 THE STORY OF THE EARTH. recognise the technical characters by which ani- mals and plants are classified, are aware that there are different kinds of scallop shells, and different kinds of cockles. The change in form and ornament can often be seen to originate as a consequence of the home of the shells being a place where the water is still, or one where it is exceptionally disturbed, the ribs of shells being always stronger in rough water. The presence of fresh water in an estuary would appear to be a frequent cause of variation, not only in orna- ment, but in form. Such variations t of the com- mon periwinkle and purple shell are found in Crag-beds at Norwich, and seen in inlets on the coast at the present day. Many of these varia- tions are such as might characterize different genera if they were persistent and became per- manent characters, but they do not even consti- tute species, and are only regarded as local races due to local causes. If it were possible that after a local race had come into existence, another set of circumstances affected it so as to cause varia- tion to take place in some new direction, it may be that what was previously but a race character, would be perpetuated in all the new modifica- tions, and become the distinctive attribute of a species, or even of a genus. The capacity of an animal for variation is usually in the develop- ment of something new, which did not previ- ously exist ; but the most remarkable evidences of variation are in the loss of parts which had existed in animals in a previous period of time. The capacity for variation is strikingly seen in the manner in which the antlers of a deer become complicated year by year, by the development of new processes. In the present state of knowledge FOSSILS. 73 certain fossil deer appear to have had antlers which were less complicated ; and in others the antlers were more complicated. It is on such characters that species of deer are distinguished. All the higher forms of life which are dis- tinguished in classification, are records of loss. Thus there can be no doubt that the common horse is closely related to the fossil horse named Hipparion, which had three toes on each foot, and the existing horse still preserves rudiments of the lateral toes which have been lost, in the splint bones, which occur at the sides of each cannon bone. Attempts have been made to show that the three-toed horse had ancestors with five toes, so that by loss of the digits of the feet, which are consequences of the ways in which the toes are used, genera may come into existence. At pres- ent there is very little in the way of fact out of which such a history could be constructed. Sci- ence can only be carried on, on a basis of un- broken evidence, from facts, which are to the scientific man what capital is to the merchant. There is, however, no doubt that the mammals have lost the composite structure of the lower jaw, which is found in reptiles ; and that reptiles have lost the greater part of the arch of bones which in fishes intervenes between the brain case and the lower jaw, if their structures are inherited from one group to the other. 74 THE STORY OF THE EARTH. CHAPTER IX. THE CLASSIFICATION OF WATER-FORMED ROCKS. IN every country breaks exist in the con- tinuity of the strata. Such interruptions in se- quence are in progress at the present day by the upheaval of land of existing islands, and conti- nents, which intermits the deposition of marine strata. Such breaks are evidenced by want of conformity in the order of succession of the de- posits. This is one of the main grounds for di- viding geological deposits from each other. The breaks which exist in any one country are some- what limited in the area which they affect. They can never be world-wide divisions between strata. Strata are also divided according to their differ- ences in predominant mineral character. The changes which take place in the prevalent types of extinct life which they severally preserve, give grounds for division of deposits, so that the cessa- tion of an ancient group of animals, or the in- coming of a new group, makes a division possible between the strata. There is no necessary connection between the break in stratification, termed unconformity, and the break in life. Frequently there is a great change in fossils between two successive strata without an indication of unconformity. It is difficult to understand how the life can change appreciably without a change in the level of adjacent land, which causes the life of an adja- cent area to migrate. An unconformity is not of necessity any greater evidence of an unrepre- sented interval of time than conformity ; because CLASSIFICATION OF WATER-FORMED ROCKS. 75 the unconformable beds might always be traced to a district where they become conformable, so that there is no break in the geological record. The changes in life between conformable strata, are no more than the differences which zones of life assume with depth. As a pebble bed changes to a sandstone, its life alters from the fauna of the littoral zone between tide marks, to the fauna of the deeper laminarian zone. As the sand is succeeded by a clay the fauna alters to the life of the coralline zone. Therefore there is, from the point of view of the existing distribu- tion of plants and animals, a necessary change of life, with change in stratification, which has no important connection with imperfections in the geological record. The breaks in life and in stratification were stated by the late Sir Andrew Ramsay in the following series: Between the Lingula flags and the overlying Tremadoc slates there is a nearly complete break in genera and species ; and un- conformity is probable. There is the same condition between the Tremadoc slates and the Arenig rocks; and between the Bala and Caradoc beds below, and the lower Llandovery above. There is a great break in species in alf these examples, and probably unconformity as well, but the unconformity is not seen. Between the Lower Llandovery and Upper Llandovery beds a break in life occurs, and marked unconformity ; and between the Upper Llandovery beds and Wenlock beds, is similar evidence of a break in succession. The Old Red-sandstone however shows no sign of unconformity at the junction where it succeeds the Ludlow rocks at the top of the Silurian, and no break in life though both 76 THE STORY OF THE EARTH. might be expected. Nor does any break occur be.tween the upper limit of the fresh water old red sandstone and the marine carboniferous rocks in the Welsh country. The carboniferous rocks are usually conformable from top to bottom; but there is sometimes an unconformable succession of Millstone grit upon mountain limestone, in the Forest of Dean. There is a great unconformity between the Carboniferous rocks and the Per- mian. This makes a total of ten physical breaks which are evidenced during the primary portion of geological time. There is a complete stratigraphical break, or unconformity between the Trias and the Permian. Near Ormskirk the new Red Marl rests uncon- formably on the new Red Sandstone. There is no visible unconformity between the Rhaetic beds and the Lias, which rests upon them, but the change in life indicates a great break in the uniformity of previous conditions. There is no complete unconformity between the Lias and the overlying Inferior Oolite. But the change in life to this deposit, and through all the succeeding divisions of the Oolites, is such as may be asso- ciated with unconformity in adjacent areas. At the top of the Oolites there is an insensible pas- sage from the marine Portland limestone to the Purbeck beds. But since the Purbeck deposits include terrestrial surfaces, and are largely of fresh-water origin, an unconformity must exist in the south of England in this part of the suc- cession. The wealden beds may also be uncon- formable. But in the overlying cretaceous series of deposits, the apparent unconformity is an overlap on the older strata which gave increased geographical extension to the Hunstanton lime- CLASSIFICATION OF WATER-FORMED ROCKS. 77 stone and Chalk in the north, in Yorkshire; and to the Greensand and Chalk in the south-west of England. There is no other physical break in this coun- try till that between the chalk and the tertiary strata, which is partly bridged in Belgium, and perhaps entirely bridged over in North America. The upheaval of a succession of land surfaces in the tertiary period is evidence of remarkable breaks in the sequence of deposits, and then a great gap, unrepresented by strata in England, occurs in the middle tertiary period. The mani- fest physical breaks in the area in which the Brit- ish strata were deposited, are much fewer than the breaks in the succession of life, many of which are evidently due to the way in which life is dis- tributed in successive zones in depth. Therefore there has been no uniform principle in distinguish- ing the strata from each other by their fossils; and more attention has been paid to the differ- ences in life, than to the circumstances by which the differences were caused. In England the principal changes in marine life occur (i) between the Silurian and Devonian rocks, though the changes in types of life appear to be unimportant. (2) Between the Primary and Secondary rocks there is a great change in both the marine and terrestrial life. (3) Between the Oolites and Cretaceous rocks there is apparently an important change in the terrestrial life, though the change in the marine life is less marked. (4) The gap in the marine succession between the Secondary and Tertiary is very striking; but the gap in the terrestrial plant life appears to be small. 78 THE STORY OF THE EARTH. It is on evidence of this kind that geological time is divided into stages, ages, and epochs, which record a series of transitions and succes- sions, which the life of a limited part of the globe has undergone. Sometimes the diffusion of world- wide species appears to have been as remarkable in the seas of old geological periods, as any geo- graphical extension of living species which is known at the present day. CHAPTER X. THE ARCHEAN ROCKS. THE most ancient rocks are termed Archean. They consist chiefly of crystalline schists, and other crystalline substances, such as quartzite, limestone, graphite. Formerly they were gener- ally regarded as metamorphic. At the present day some writers do not believe that they are crystallized out of ancient strata, which were ac- cumulated in water. Nevertheless they show in many localities, and especially in the Laurentian rocks of Canada, two constituents which may in- dicate a stratified origin. One is the presence of layers of crystalline limestone, which is not known to originate in nature, except by metamorphism of limestone built up by organisms which lived in water. Secondly, these Laurentian rocks contain an amount of graphite, which has been stated to be equal in bulk, if it were all brought together, to the quantity of coal found in a coal-field. No source upon the earth for the carbon of which graphite consists is known, except the meta- THE ARCHEAN ROCKS. 79 morphism of vegetable matter such as forms coal. Existing coal-fields show, in the formation of an- thracite, what appears to be a transitional step between coal and graphite, for the percentage of carbon augments as the other gaseous constitu- ents of coal are lost under the distilling action of pressure and heat. If the Archean limestones and graphite are of organic origin, they would appear to have been originally beds of coal and limestone which con- sisted mainly, if not entirely, of fossils. There- fore, the other constituents of these rocks, the schists which form the great mass of the country north of the St. Lawrence, would appear once to have been sands and clays in which fossils may have been distributed as they are in more recent deposits. The estimated thickness of the Laurentian and overlying Huronian rocks is about 50,000 feet, and in that thickness no fossil is found, unless the structure named Eozoon canadense which has been described from Laurentian limestones is correctly identified as an encrusting reef-building foramini- fer ; which its mode of occurrence in the rocks makes not improbable, though the structure is paralleled in volcanic rocks. Such metamorphism of ancient sediments all over the globe must be inferred to have obliterated all records of the early history of the earth. The geological story commences at a com- paratively late period, compared with the unre- corded epochs which have gone before. Without such an obliteration of a past record of almost infinite duration as compared with known geo- logical time, it is inconceivable that processes of variation, such as are now known to be in opera- 8o THE STORY OF THE EARTH. tion, can have given rise to the diverse types of life, which the oldest fossiliferous rocks make known. The Archean rocks are widely spread on the surface of Scandinavia, Finland and North-West Russia, Saxony, and Bohemia, and in Bavaria as well as in the British Islands. Rocks of this class will probably be found all over the globe, wher- ever there is an opportunity of examining the ma- terial upon which the most ancient fossiliferous rocks rest. In the Western Highlands in Scotland, from Cape Wrath southward, the schists and funda- mental gneiss of that region are displaced by an incredible multitude of horizontal thrusts which break them up into parallel sheets, almost as well marked as planes of stratification, with which they were at one time confused. Among these crys- talline rocks are included great thicknesses of sandstones, and folded in among them occasion- ally are fossiliferous bands of limestone. Other archaean areas are exposed further southward. The most interesting are the crys- talline rocks about St. David's, in Pembroke- shire ; at the Longmynd, in Shropshire; in the central axis in Carnarvonshire ; and in Anglesey. In these most ancient British rocks, evidences appear to exist of contemporaneous activity of terrestrial volcanos, so that among the oldest British rocks, alternating with schists, are the rhyolite lavas of Bangor, Carnarvon, Llyn Pa- darn, associated in some of these localities with .agglomerates. The Wrekin and Ercal Hill make known rhyolites of pre-Cambrian age which are .associated with indurated volcanic ash. In the neighbourhood of St. David's the rhyolitic lavas CAMBRIAN AND ORDOVICIAN ROCKS. 8 1 are in the same way associated with volcanic ash, interstratified in schists, the whole being affected by compression to which they have since been subjected. The British Geological Record begins with conditions which indicate volcanic outbursts, and the shallow water accumulation of grits and pebble beds. As far as the evidence goes, similar conditions might exist at the present day ; but the rocks have been modified from their original state by slow chemical changes. CHAPTER XL CAMBRIAN AND ORDOVICIAN ROCKS. THERE is an unconformity between the pre- Cambrian and Cambrian rocks, which implies a long interval of time, unrepresented by deposits in the localities which have been examined. The Cambrian rocks are of enormous thickness; and in Britain are probably not less than 30,000 feet thick in Wales and the border counties of Eng- land. There is difference of opinion as to the use of the term Cambrian. Some writers make it include four groups of rocks, named Longmynd rocks, Menevian beds, Lingula flags, and Tre- madoc slates. Others carry the name higher and make it include the succeeding rocks named Arenig, Llanvirn, Lower Bala and Middle Bala and Upper Bala, which have also been grouped as Ordovician. The overlying strata, termed May Hill rocks, 82 THE STORY OF THE EARTH. the Denbighshire grits, Wenlock and Ludlow beds, and the Downton sandstone series, are combined to form the Silurian group. From the physical history of the deposits, there is ground for dividing the rocks in this way, but from consideration of the life they contain, the whole might well be combined, and grouped together as ten successive series, with ten distinct fauna, which more or less resemble the life of similar natural history provinces, superimposed on each other, and preserved suc- cessively in sediments in the same area. At first the old rocks which comprise the Longmynd groups, and the Harlech and Llan- beris slates, which rise 1600 feet above the sea, in the Longmynd Hills in Shropshire, consist of slates, sandstones, grits, and conglomerates; with very few fossils. The water-worn pebbles in them prove deposition under ancient shore conditions ; and they are associated with beds which show the ripples of waves, runnels of rills on the shore, in- terlacing cracks formed by the heat of the sun, prints of raindrops, and burrows of sea-worms closely allied to the living Arenicola. Few fossils have been found in the Longmynd. They are scarcely more numerous in the Bangor country of Carnarvonshire. There the rocks are represented by green and purple slates, which stretch from the banks of the Ogwen through the lake of Llanberis, and the Penrhyn slate quarries. In South Wales, in the section near St. David's, the interest of these rocks is greatest. The conglomerates and sandstones found there, with red, purple, and green slates, appear to be on the same geo- logical horizon as the Bethesda and Llanberis slate quarries. Towards the base of this group CAMBRIAN AND ORDOVICIAN ROCKS. 83 of rocks are found two genera of fossils named Lingulella and Discina, which may be regarded as having survived through all subsequent ages of geological time to the present day without under- going any notable change, although the surviving shell is named Lingula instead of Lingulella. The lowest beds in which they occur, the Caerfai group, are succeeded by the Solva group, in which genera of the extinct order of Trilobites appear in profu- sion. Here occurs the oldest known sponge named Protospongia, and a species is met with of the ex- tinct genus of Pteropod named Theca, which sur- vived in the ancient seas for a long time. This assemblage of life, the earliest as yet known in the earth's history, consists of types which are in no sense embryonic. It distinctly points to a line of similar ancestors which has yet to be dis- covered. With the succession of the overlying Menevian beds, and the succeeding divisions of the Lingula flags and Tremadoc slates, a fauna of 185 species of fossils becomes known in which FIG. 9. Section through North Wales from the Cambrian slates of the Snowdon district to the Cheshire Trias. some of the shells, species of the genera of Brachi- opoda named Lingula and Orthis, and Obolella pass up into the overlying Arenig rocks. With the, Menevian beds the most ancient Echinoderm appears. It is a far-off relative of the existing stone-lines and sea-eggs. It belongs to the ex- tinct group of Cystidea, and is named Protoeystites. 84 THE STORY OF THE EARTH. The most ancient bivalve shells known are found in the Tremadoc rocks of Wales. They can be closely paralleled at the present day. Modiolopsis N is probably nothing but Modiola, the horse mussel under another name ; and Glyptarca, Palaarca, and Ctenodonta are only forms of the living genus Area, in which the shell has not developed the habit of growing in depth along its hinge, as in most of its living representatives. Pteropods are well repre- sented : the univalve Gasteropods are represented by the extinct genus Bellerophon, which appears to be a symmetrical shell abundant in the primary rocks, probably allied to the living Pleurotomaria. The group of many-chambered shells named Ceph- alopoda is represented by allies of the Nautilus, one of them the straight horn Otthoceras, and an- other Cyrtoceras, the curved horn. In the lower Tremadoc rocks the oldest known starfish is found, in a species of the genus P alceasterina j and the oldest known Crinoid or Stone-lily, in a species of the genus Dendrocrinus. The great Arenig series rests conformably on the Tremadoc slates. It forms Cader Idris, the Festiniog Mountains, Aran and Arenig. It in- cludes a great group of roofing slates worked in the quarries of Festiniog, and an immense quan- tity of volcanic ash. The total thickness of the ashes, agglomerates and lavas seen in Cader Idris is between 5000 and 6000 feet. The throats of the ancient volcanos which contributed so largely to form the Arenig rocks in North Wales, were placed near Dolgelly, and Aran Mowddwy by the late Sir A. Ramsay. The Cambrian period was an epoch of vigor- ous volcanic action. The products of the vol- canos are seen in the Skiddaw slates of the Lake CAMBRIAN AND ORDOVICIAN ROCKS. 85 district, where about 12,000 feet of volcanic ash and Andesite lavas, of Honister Crag and Sea- thwaite, mark the beginning of volcanic action which continued through the accumulation of the Borrowdale series of rocks. In North Wales Rhyolitic lavas continued to be ejected in the Bala period which followed. They are seen about Bettws-y-coed and the Conway falls. Rhyolitic lavas are seen in the Glyders, on the north side of the Pass of Llanberis, near Bedd- Gelert, and about Snowdon. The most remarkable feature in the life of these upper Cambrian rocks, is the extraordinary abundance of the extinct group of Graptolites. They are found not only in South Wales and the Lake District, but in as great a diversity of forms and complexity of branching structure in North America. Trilobites increase in number of genera and species. The genus Pleurotomaria, a Gasteropod only known at the present day from living species in the West Indies and verging on extinction, ap- pears for the first time in the lower Arenig rocks at Llanvirn, near St. Davids. In this period an- other Gasteropod Euomphalus, which continues to be important during the primary period, is found for the first time. Several corals make their appearance in the Llandeilo rocks. They are the most ancient representatives of the group in Britain. Among them is the chain coral Halysites, and a species of Favosites, both of which are important genera in the primary rocks. The Crinoids increase in number. Several genera of Brachiopod shells appear, two of which, Rhynchonella and Crania, afterwards 86 THE STORY OF THE EARTH. become much more important and still survive; while the genus Leptaena, which now first ap- pears, lives on till the lower part of the second- ary epoch of time. The common genus Mytilus, the edible mussel, is first found with the close of the Cambrian period. Cephalopods become more numerous and varied, and Cystidians are repre- sented by a number of genera. The appearance and abundance of Graptolites, and the increase of Trilobites in number of genera and species, are the chief changes which occur in the life of the Cambrian period of time. The Silurian. The Silurian rocks extend unconformably over the Cambrian Strata. Between the Long- mynd and Wenlock Edge, they cover up the whole series of the Cambrian strata, resting upon their upturned and denuded edges. But when they are traced into North Wales in Denbigh- shire, the evidence of Silurian unconformity is less marked. The Silurian rocks typically include the May- hill sandstone, the Wenlock rocks and the Ludlow rocks. The May Hill series, so named from May Hill in Gloucestershire, consists chiefly of sand- stones and conglomerates, yellowish and brown with oxide of iron, about 1000 feet thick, covered by the Wenlock group, which in the south is formed of shales and limestones, and in Denbigh- shire chiefly of sandstones known as the Denbigh grits, which overlie the Tarannon shales. Above these rocks are the Ludlow beds, which also in- clude shales parted by the Aymestry limestones. The Silurian group of rocks is capped by the SILURIAN ROCKS. 87 Downton sandstone, which makes a transition in rock character to the lower beds of the old red sandstone. These Wenlock and Ludlow rocks are the oldest British strata which include con- siderable beds of limestone. The exposure of Wenlock limestone at the surface forms the hill range south-west from Coalbrookdale, known as Wenlock Edge. These beds indicate shallow-water conditions by the May Hill sandstones at their base. The shales, which are only hardened muds, were pre- ibC'. OUL Rexl S FlG - 35- Ostrea belloVkcina, from a long range m^time, the Thanet Sands. Theshellhas Some Occurring in the grown on a branch of a tree. Montian beds and sur- viving to the London clay. It is therefore shown by these Woolwich and Reading beds and their fossils that the dome of the Wealden district, be- tween the Thames and the English Channel, was raised into land; and that the chalk which once covered it furnished the flints which were rolled into completely rounded pebbles before they were 1 66 THE STORY OF THE EARTH. swept down into these tertiary beds, which are now exposed on the northern slope of the North Downs. With the oscillations in level a part of this land area at least supported the vegetation which furnished those beds of lignite which extend by Woolwich and Bromley, and the forest trees, which show by their leaves a marked resemblance to those of Gelinden. This indicates that the an- cient connection, by continuous land, between the south-east of what is now England and Bel- gium and Hanover, was maintained in the tertiary period just as it had been in the older epoch of the Weald. In one locality, near Rheims in France, the lower tertiary beds have yielded many remains of mammals which fore- shadow lemurs and ro- dents. Among these oc- curs the Neoplagiaulax which seems like a sur- vival of the Plagiaulax of the Purbeck beds. This is worth recalling on ac- count of the resemblance FIG. 3 6.-Cyprina Morrisi in f . ^ tertiary plants of Thanet Sand. this horizon with the cre- taceous flora. The Lon- don clay indicates depression which banished the shores of the tertiary land to some distance. There are oscillations in its level which varied both the mineral character of the stratum and the fossil life. At the base there is usually a bed of small rounded flint pebbles known as the basement bed, with sharks' teeth. The clay gives evi- LOWER TERTIARY. 167 dence at its base of terrestrial life, and near proximity to land, in the presence of a few mammals, some of which are allied to the exist- ing tapirs. With these are found crocodiles of the type now living, and fresh-water tortoises, such as frequent estuaries at the present day. The middle of the clay abounds in crabs and lobsters. While the top bed, about 50 feet thick, is rich in plants represented by the fruits of a large flora. The upper beds, like the basement beds, are sandy. At Bognor the sand at the base is calcareous and concretionary, with many marine fossil shells. The London clay is 500 feet thick in Essex and Sheppey. It thins to the west and south-west ; be- ing 400 feet under London, 300 at Southampton, 200 at Alum Bay and 100 at Studland Bay on the opposite coast. To the south-east it is thickened with the sands in its lower part. Its fossil shells, such as Cyprea, Murex, Conus, Pleurotoma, Fusus, are of types which abound in the seas to the south of Asia. The plants which occur in its uppermost part also have an Asiatic character. The conifers are well represented by Cypresses, the Sequoia, Pines, and the Yew Salisburia. The lilies include the so-called American aloe, Agave. A species of Smilaoc represents the Sarsaparilla tribe. Bananas are known from the genus Musa. The ginger order is represented by Amomum which yields Cardamoms. Nipa, a screw pine common on the banks of the Ganges and in the Malay peninsula, is by far the most abundant fruit. It is associated with many palms, among which the areca palm, the nutmeg type, the fan palm of the south of Europe Chamerops, and the great palm Sabal are conspicu- ous. The oak, hazel, walnut, liquid amber, laurel, 1 68 THE STORY OF THE EARTH. magnolia, ebony and spurge are present, as are representatives of some medicinal plants like StrychnoS) which yields nux vomica, and of Cin- chona, which yields quinine. There are represent- atives of the tomato and melon. Apples are represented by Cotoneaster, and associated with almonds and plums. The cocoa is represented by a species of Theobroma. There are several limes and maples. Water-lilies are represented by the Lotus, and the Victoria lily of tropical America. The London clay is well developed in Belgium and in the north of France. It does not reach the Paris basin in a recognisable form ; though it may be represented by the lignites and sands of the Soissonnais. But neither the lignites in France, nor the Ypresien beds which represent the London clay in Belgium, yield the abundance of fruits which is- met with in the Isle of Sheppey. Some of the Belgian specimens of Nipa are much better preserved than the macerated and compressed fruits of Sheppey, as though they were deposited without being so long in the water. Above the London clay are the sands which cap the hills at Harrow, Hampstead, Highgate, High Beech, Haveringate and many places in Essex, having formerly been spread continuously all over the London clay, as they still are between Egham and Aldershot. They form Ascot Heath and Bagshot Heath, and are known as the Bagshot Sands. The lower part, termed Lower Bagshot Sands, thickens in the Isle of Wight to about 800 feet, and forms the brilliantly coloured vertical sands of Alum Bay. They are laminated with films of clay at Woking, where the thickness is about LOWER TERTIARY. 169 ioo feet. Beds of pipeclay frequently occur; and occasionally, as at Newbury, the pipeclay contains fossil leaves like those at Bournemouth. In the Isle of Wight the bands of pipeclay are exceptionally pure, as though they had been de- rived from white felspar, but never extend far. They are usually only a few inches thick, though occasionally thick beds are found. From them a flora has been obtained, which, although known only from the leaves of plants, indicates many of the types at the top of the London clay which are HCADON HILL N. S. PIG. 37. Cliff- section, Alum Bay, Isle of Wight, showing the rela- tion between the vertical eocene beds of Alum Bay and the more horizontal oligocene strata of Headon Hill. , known only from fruits; so that they may well be regarded as a surviving part of the vegetation of the London clay. Among these Alum Bay plants are some of the Cypresses and Sequoia, Smilax and the palm Sabal is identified in both. There is the same arum named Aronium. The oak, walnut, laurel, cinchona, ebony, magnolia, maple, the soapworts Sapindlus and Cupania, the allspice and Eucalyptus, the almond and plum, and the 170 THE STORY OF THE EARTH. mimosas are common to both deposits. Besides these, the Alum Bay beds make known many new types, of which the London clay gives no evi- dence. Among them are the beech, elm, fig, bread fruit, willow, poplar, sandalwood, the Mezereon, Aristolochia, olive, ash, convolvulus, verbena, bil- berry, some heaths, the aralia, dogwood, white water-lily, custard apple, holly, buckthorn, vine, sumach and pistachio. These are among the more important new plants introduced in the Bagshot Sands ; and with them are a number of Proteacese, referred to genera now living in Australia, as well as a representative of the type genus Proteoides, the sugar bush of South Africa. One of the most striking features in the flora is the small number of palms, and the absence of the Nipa, which in Sheppey is the predominant fruit and is present in the newer beds at Bournemouth. The fruits and leaves, like the shells in the associated strata, indicate affinities with the life of far-off regions of the earth. It is a generation since Unger, a Vien- nese student of the fossil plant-life of the lower tertiaries, impressed with the occurrence of nu- merous genera in Austria, which live at the pres- ent day in Australia, regarded the eocene flora as indicating a migratory passage of the ancient plants from Europe into the Australian region. The plane of junction where the Bagshot Sands come to an end, and the succeeding marine Bracklesham beds begin is not easy to draw ; be- cause the Bracklesham beds contain locally thick layers of lignite, which have the aspect of a coal seam, and indicate the persistence of terrestrial conditions and some oscillations of that terres- trial surface in level. Low down in these beds is found the large LOWER TERTIARY. 171 foraminiferous shell named Nummulites Zczvigatus. It does not form a thick bed ; but probably marks the geological horizon of the Nummulitic Lime- stone, which is one of the most important lime- stones in the old world, and extends from the Alps and Carpathians into Thibet, and from Mo- rocco, Algeria and Egypt through Cabul and the Himalayas to China. In Great Britain the Bracklesham beds in Sus- sex and the Isle of Wight are alternations of green sands and sandy clays, which are separated from the overlying Barton clay by a conglomerate formed of rounded flint pebbles. At Bournemouth their character has changed. They are foxy-brown estuarine sands with beds of pipeclay, in which occurs another flora, with many ferns, palms, cac- tus, eucalyptus, figs, willows, beech, and nipa. The cactus is an American type. Subsequently the North American type of vegetation became more abundant in Europe in the middle Tertiary period, and better defined by many genera. The Barton clay succeeds the Bracklesham beds. It is a blue clay, about 300 feet thick, with an extraor- dinary number of fossil shells, many of which are similar in genera to those found in the London clay and Bracklesham beds, though tj^e clay is characterised by the abundance of individuals of the genera Chama, Crassatella, Fusus, and Valuta , and by the presence of some peculiar genera like Typhis, a univalve similar to Murex, except that the spines are tubular. There is no such fauna anywhere to be met with at the present day. It is not unlike a blending of the existing Malayan and New Zealand forms of marine life ; and many of the shells, like the Crassatella, Typhis, Chama, Pecten, Pectunculus, are very similar to 172 THE STORY OF THE EARTH. species now living in or about the New Zealand seas. The Bracklesham beds have generally been re- garded as represented by the Calcaire grossier of the Paris basin, while the Barton clay corresponds to the grits with the Nummulites variolarius, and some newer deposits, such as the Ores de Beau- champ. These beds are well represented in Bel- gium. The Bracklesham beds have yielded some interesting serpents of the genus Palaophis though not so large as those of the London clay. In the Barton clay occurs a marine mammal, of the genus Zeuglodon shown by its back bone to be a true whale, which has the teeth double-rooted and ser- rated in a way that is seen in no other animal, though resembling some seals. The Barton period comes to an end with a deposition of 200 feet of sand, in which fossils are rare. Theoretically, the Bracklesham and Barton beds together are an immense expansion of the middle 50 feet of the Bagshot sands at Aldershot, which contain, in some of the clayey layers, im- pressions of fossils which appear to be identical with those found at Barton in Hampshire, and Bracklesham in Sussex. On this hypothesis the Bracklesham and Barton beds indicate in the Hampshire area a depression of the old sea-bed, into which peculiar faunas successively moved. The upper Bagshot or Barton sands bring back again the conditions of a shoal, or shore, due to a general uprising of the land. The few shells which have been found in them are Barton species. The sea-bed continued to be elevated until it passed into a land surface, in the succeeding MIDDLE TERTIARY. 173 period of time termed Oligocene, which is the only part of the middle Tertiary represented in Great Britain. CHAPTER XXI. MIDDLE TERTIARY. THE Middle Tertiary period, usually termed Miocene, makes a more striking approximation in its life to the animals and plants which exist at the present day. In the European area it is a "record of terrestrial and lacustrine conditions, alternating with the deposits or shallow seas. In Great Britain only a portion of the earlier part of the Miocene period is represented by de- posits which now cover the northern part of the Isle of Wight, much of the New Forest, and are exposed in the cliffs at Hordwell, Tollands Bay, at Bembridge and Hempstead. These strata are grouped together as Oligocene. Headon Beds. The oldest oligocene beds, knowp as the Headon series, are 130 feet thick at Headon Hill, in the Isle of Wight, are in the main fresh-water strata. They comprise first, about 70 feet of brackish water marls, and fresh-water limestones, superimposed upon the marine sands above the Barton series. This proves that the shallow sea, with the Upper Bagshot Sands for its floor, had become converted into dry land, upon which lakes were formed by fresh waters draining into the bottom of the trough from a limestone region such as the chalk or the oolites. 174 THE STORY OF THE EARTH. The strata and their fossils show that the level of the land fluctuated. The lakes became sometimes occupied with brackish water, so that marine life divides up the fresh-water deposits. After the lower fresh-water beds were formed, the land was submerged, so as to give rise to the Middle Headon beds, which are essentially ma- rine. There are great banks of oysters with nu- merous marine shells, most of them similar to the types which had previously been known in" the Barton clay. These marine beds became tje?ter developed at Brockenhurst in the New Forest, where some corals are found, together with the vertebral column of Zeuglodon, a marine mammal of the whale type, with teeth like seals, already known from the Barton clay. These marine beds are widely spread in Germany. After they were deposited the land was raised once more, and the Upper Headon beds formed, which reach a con- siderable thickness. They are fresh-water de- posits, consisting of marls frequently green, full of the large Paludina lenta, the Cyrena obovata and the extinct Potomomya plana^ which alternate with thick limestones, commonly full of fresh-water shells of the genera Planorbis and Limncea. These limestones are almost entirely the product of the growth and decay of the fresh-water plant Chara which precipitates carbonate of lime upon its tissues by absorbing carbonic acid gas from the water charged with carbonate of lime. In these limestones remains are found of terrestrial mam- mals of the types present in the Gypseous beds at Paris, although they are not so numerous as in the Bembridge beds. When the Headon beds are followed to the coast of Hampshire, the limestones disappear, MIDDLE TERTIARY. 175 leading to the conclusion that the upheaval of the chalk, which now runs in a nearly vertical position through the Isle of Wight, had already begun to supply the calcareous matter, which the streams brought into the lakes of the Headon period. The beds which rest upon the Upper Headon strata are termed the Osborne or St. Helen s series. They are sandstones and marls, much thicker than any of. the sandstones and marls in the Headon beds, and therefore in contrast to them. Some of the sandstones become calcareous, and pass into concretion- ary limestones. The shells are all of fresh- water types. The sandstones are some- times ripple -marked, probably by the wind. The Osborne beds are about 80 feet thick, and divide the Headon from the Bern- bridge beds. The RoTHhn'Ja-p Ti*nfistni>tp\* FIG. 38. Planorbis euomphalus and other fossils in the fresh-water thick, very like the Headon limestone.' Headon limestones, rather creamy in colour, full of the same types of fresh-water shells, and containing many land shells, especially examples of the genera Helix, Bulimus, and Glandina. These land-shells have a marked affinity with species now living in North America, with which one or two may be identical. The Bembridge limestone abounds in seed-vessels of the plant Chara, which formed it. Remains occur in it of several species of the extinct mam- 1 76 THE STORY OF THE EARTH. mal P alaotherium which in some ways approxi- mated in structure to existing tapirs. Where this limestone caps Headon Hill it is about 15 feet thick. The Bembridge marls rest upon it successively in the section at Hempstead. They are grouped into a number of sandy beds and shaly clays, full of estuarine shells, among which are the genera Melania and Melanopsis, which alternate with beds containing Cyrena and other bands in which the shells are of fresh water species. The top of the marls is the remarkable thin deposit known as the Black Band which is usually grouped with the overlying Hempstead series. The Hempstead Beds. Hempstead Hill lies to the east of Yarmouth in the Isle of Wight, on the shore of the Solent. It is formed of about 170 feet of fresh-water and estuarine marls, capped by a marine stratum. The marls have a general resemblance to the Bembridge marls. The Black Band at the base is about two feet of clay, coloured with vegetable remains, among which Sequoia and water-lilies have been recognised, together with the teeth of P alaotherium and other mammals and remains of tortoises and crocodiles. It is an old terrestrial surface on which rest, first, the lower marls with Melania muricata ; secondly, the middle marls with Cerithium Sedgwicki; and thirdly, the upper marls with Cerithium plicatum. At the top are the Corbula beds which contain several marine shells in addition to the estuarine forms, among them Valuta Rathiera, Natica, Corbula, and a species of oyster. The characteristic mammal of these MIDDLE TERTIARY. 177 beds is the Hyopotamus. These are the newest British deposits in the Isle of Wight of oligocene age. The lignites alternating with clays which fill up the basin at Bovey Tr^cey in Devonshire are probably of the same age. They form deposits about 300 feet thick ; probably once thicker. With the exception of a single beetle, the remains found in them are about fifty species of plants. The lignite itself is chiefly the flattened trunks of the Sequoia Couttsia. About half the plants are regarded as of peculiar species and the remain- ing twenty-five occur in the Miocene of Germany and Switzerland. Among these trees are species of fig, oak, laurel, cinnamon, the sour gum tree Nyssa, a palm, vine, and some ferns such as Lastrea. On the Continent the Miocene beds attain singular importance. Not only from the part they take in forming the basins drained by so many rivers, and in the structure of the Alps, but also on account of the remarkable mammalian remains which they yield. The Dinotherium, which appears to have been a sort of Mastodon with tusks in its lower jaw, is one of these. The three-toed horse* named Hipparion, is even more interesting, while the fossils obtained at Pikermi, near Athens, include giraffes and many other animals which have long passed away from Europe. Perhaps the most extraordinary Miocene fauna is found fossil in the Siwalik Hills in India, which lie between the Jumna and the Ganges, and rise to a height of 2000 or 3000 feet. The species of Hippopotamus, and allies of the giraffe and other African types which are there found, testify that change of 12 178 THE STORY OF THE EARTH. area in the distribution of genera on land in the Tertiary period, continued as persistently as the migrations of marine life in the Primary period. CHAPTER XXII. THE CRAG. AFTER the great terrestrial epoch of the newer Miocene period had passed away entirely unrepre- sented by strata, in Great Britain, deposits, named the Crag, are found, which fringe the coast in Norfolk, Suffolk, and Essex, occur in a few places in Kent ; and in Belgium. The relative age of these beds was first deter- mined by the method of counting the number of existing species in each of the tertiary strata. On that basis the tertiary epoch had been divided into Eocene, or lower Tertiary, Miocene, or middle Tertiary, and Pliocene, or upper Tertiary. Sub- sequently the Lower Miocene was named Oligo- cene. In the Pliocene the fossils include more than 35 per cent, of living species. In this great period the Crag finds a place The older beds, named Coralline Crag, have 84 per cent, of the shells still living; and in the newer or Red Crag 92 per cent, of the shells still exist. The Coralline Crag rests unconformably upon the London clay. Its lower part consists of yel- low false bedded sands, which sometimes form a building stone about 30 feet thick. This sand appears to have been derived from denudation of the Bagshot sands which once extended over the THE CRAG. 179 whole area of the London clay. The shelly beds include a number of kinds of life which are now only represented in southern seas. As many as two hundred species are said to be found living in the Mediterranean, and a few are paralleled off the coasts of "Japan, Mexico and the West Indies. But while about sixty-five of the species are known only in southern seas, fourteen only in* northern seas, and seventeen have been met with in no other deposit, there are as many as 185 Coralline Crag shells still found in the British seas. .The rock is named Coralline Crag from the large extent to which its upper beds consist of the remains of Polyzoa which were formerly termed corallines. Of those Polyzoa which are still living, 26 out of 30 are met with in British seas, but the majority of the fossils belong to the two extinct types Alveolaria semiovata, and Fasicu- laria aurantium with which are some species of the genera Retepora Idmonea and Eschara. The fishes of this crag include the common cod, green cod, power cod, the pol- lack, whiting, and whiting pout, with which have been found the great teeth of the shark Carcharodon, and of Otodus. At the base of the Cor- alline Crag, in places where the shark's teeth are found FlG 39 ._ Cardita senilis> from IS a bed of nodules Of the Coralline Crag. phosphate of lime, in which bones of the dolphin Choneziphius occur with teeth of the whale Balcenodon, associated with teeth of 180 THE STORY OF THE EARTH. deer, rhinoceros and Mastodon which were obvi- ously derived from a land surface, and perhaps from an older deposit. In this bed are multitudes of fossils from the London clay, and a few croco- diles and Plesiosaurs derived from the older Sec- ondary rocks. The characteristic shells of the Coralline Crag, besides the comparatively rare species of Voluta, Cassidaria y Pyrula and Lingula, include many spe- cies of the genus Astarte. That genus which now characterises northern regions, is here repre- sented by multitudes of individuals. The Cypri- ana islandica, Terebratula grandis, Cardita senilis, Buccinum dalei are typical fossils. The Red Crag. After the Coralline Crag was formed in some tranquil depth of water, the shores appear to have been upheaved. And on the eroded surface about 20 feet of false bedded sands and comminuted shells were laid down, as shore deposits, which fringe the island-like masses of white or coralline crag. This newer deposit, named Red Crag, indi- cates three or four successive depositions. Each of its beds was planed level by denudations, which left thin layers of pebbles and nodules of phos- phate of lime at the junctions. It has been observed that the older Red Crag at Walton on the Naze, has fossils more like the species of the Coralline Crag than are found else- where. At Butley a zone abounds in northern species, and on this is a newer crag still. The Red Crag along the river Deben contains a larger number of terrestrial mammals than has been found at the base of the Coralline Crag. The THE CRAG. l8l additional types comprise species of tapir, the Siwalik Hycenarctos, hyaena, Hipparion^ besides deer, bear, and among marine animals a Halithe- FIG. 40. Fusus antiquus reversed variety, from the Red Crag. rium and a walrus with large tusks. The shells are interesting from the dominance of a few types, such as the reversed variety of the Fusus antiquus, which is associated with the common whelk, the European cowrie, the common purple shells, and species of the genera Nassa, Emargi- nula, Pectunculus, Mya, Lucina and Cardium. At Norwich the Red Crag becomes estuarine. The Forest Bed of the Norfolk coast may be a part of its land surface. A patch of Crag is found north of Penzance, at St. Erth, 98 feet above the sea. A more interesting deposit on the, summit of the Chalk descends into pipes in the Chalk at Lenham in Kent, indicating that denudation has removed the Crag from the surface of the country. All through the crag the temperature on the east coast was becoming colder. This is evinced by the presence of stones in the newer crag which appear to have been floated southward in ice; and it may be indicated by the increasing number of shells which at the present day characterise 1 82 THE STORY OF THE EARTH. northern and arctic seas. Eventually the crag land was covered with boulder clay, and the whole country experienced glacial conditions. The cold is attributed by some to change of form in the earth's orbit, by which the winters increased in length. Others attribute it to up- heaval of land. Upheaval of Scandinavia and the North Sea would displace the shells south- ward, and lead to a condensation of vapour, from which glaciers would result large enough to cross the plain of the North Sea and reach Britain. CHAPTER XXIII. GLACIAL PERIOD AND GRAVELS. So manifest a break in the succession occurs with the superposition of the Glacial deposits upon the Crag, that some geologists regard them as beginning a fourth great division of the strata which is named Quaternary or Post- tertiary. Others place them in a division of the Tertiary period, which is named Pleistocene. The singular feature of the formation which justifies a separate name is the wide spread of the glacial conditions over the Earth. In many countries where ice now is only a passing inci- dent of Winter, clays are found, blue, purple, or brown, full of fragments of rocks which are mostly local, though many have travelled from distant places. These boulders, which cause the deposit to be named Boulder Clay, are often smoothed and grooved or scratched on one side GLACIAL PERIOD AND GRAVELS. 183 like stones which have travelled in the sides or bed of a Glacier. The deposit is often indis- tinguishable from the clay found in Alpine val- leys, from which Glaciers have retired which once covered the country. The high ground in every land in which Boulder Clay is found supports this inference with evidences of the work of ice. The Mountains of Scotland, the Lake district of England, and the Snowdon district in Wales are smoothed and grooved by sheets of ice which have passed away. Small joints in the old slates have been widened and deepened in the valleys, until rounded structures have been produced like the backs of huddled sheep at rest. This condi- tion known as Roches moutonndes is sometimes exposed by a retreating glacier in the Alps, and is manifestly due to the work of frost and glacier ice. Above many a mountain valley, such as the Pass of Llanberis, angular stones are perched in positions where water could never have left them. They are regarded as having been the stones of moraines once carried on the surface of a glacier, and left behind in their present places when the ice melted beneath them. The great blocks of crystalline rock above Netichatel are of the same substance as the Mont Blanc chain, and could only have reached their present position upon the limestone chain of the Jura by crossing the central valley of Switzerland. On such evidence Glacial conditions for a country may be inferred even though boulder clay is not seen. In North America Sir J. W. Dawson has described the evidences of the Canadian ice-age as comprising, (i.) a Lower Boulder Clay, which 184 THE STORY OF THE EARTH. rests upon a glaciated and grooved surface of rock ; (ii.) The Lower and Upper Leda-clay with marine shells and drift plants ; and (iii.) an Upper Boulder Clay with the shell Saxicava, and gravel. The Lower Boulder Clay forms the basins of the great Canadian lakes. The boul- ders are mostly of Laurentian gneiss. Their striation is attributed to the grating of pebbles included in shore-ice upon the rocky floor be- neath, when moved by the tide. In Britain the glacial deposits are spread irregularly. They consist of Upper and Lower Boulder Clays on the east coast, divided by Middle Glacial Sands with marine shells. The granite of Criffel in Kirkcudbrightshire is found in the boulder clay over Lancashire and North Wales. Boulders of Volcanic rocks from Cumberland are scattered over Cheshire. Dis- tinct streams of glacial drift extended down both the east and west sides of Britain. The boulders of Westmoreland Shap granite found over the plain of York and between Whitby and Scar- borough on the coast, prove that boulders were also distributed eastward from local centres, not- withstanding the Scandinavian source of many rocks in the Boulder Clay on the Norfolk Coast at Cromer. The Boulder Clay found near Lon- don at Finchley, and at Hornchurch in Essex, is full of travelled ice-grooved rocks, with fossils from the secondary strata of Yorkshire and Lin- colnshire. Glacier ice transported the rock mat- ter, but probably shore-ice and icebergs were partly concerned in depositing it, so as to fill up the old valleys and leave the clay on the surface of the country. The denudation since the glacial period has been very great, and the glacial beds GLACIAL PERIOD AND GRAVELS. 185 are cut through by modern valleys which are ex- cavated in the underlying deposits. In many parts of the east of England a series of gravel beds occurs beneath the Boulder Clay, and in these pre-glacial gravels, chipped flint im- plements of the Palaeolithic type are said to be found. In the east of England the Boulder Clay which caps hills is itself capped by coarse hill gravel which has the aspect of being boulder clay from which the clay has been washed out. The gravels descend to lower and lower levels till they occupy the broad shallow troughs through which exist- ing rivers flow, from which we may infer that ele- vation of the land has gradually contracted the width of existing rivers. Chipped flint imple- ments are found in both the high and low level gravels, with remains of mammals, which are mostly African in their affinities, and mostly ex- tinct. They include species of Hippopotamus, Rhinoceros, Elephas, Lion, Bear. Deer, Horse, and Ox survive in Britain. Evidences of severer frosts are found in the broken rock fragments, and of periodic floods due to melting of the snows. The leaves of the dwarf birch and dwarf willow are preserved in clay seams, with the land and river shells which now exist. As hunter or as husbandman the rude fore- fathers of the British people left in rock shelters and caves simple works of art which show that people had gained a primitive civilization who lived when the post-glacial gravels were formed, and the influence of glacial conditions was still felt. The dominance of man over animal and plant 1 86 THE STORY OF THE EARTH. may mark the beginning of a new geological pe- riod; but there is no gap in time or change in life to announce the human period, or to distinguish it in kind from earlier epochs in the story of the Earth, INDEX. Age of earth, 9. Alethopteris, 112. Alps, 18. Ammonites, 62, 95, 123, 127, 128. Andesites, 27, 28. Annularia, 112. Anodonta, 50, 97. Anomodontia, 117. Anorthite in meteorites, 12. Anthracite, 107. Araucarites, 109. Archaopteryx, 138. Archean rocks, 78. Ash, volcanic, 26, 30, 85. Asmanite, 12. Asterophylites, 112. Astronomy, relation to geology, 10. Augite in meteorites, 12. B. Basalt, IT, 27; varieties of, 29; columnar structure in, 30. Bclemnites, 60, 127, 128, 142. Birds, fossil, 138. 154. Bituminous coal, 107. Blastoidea, 69. Boulders, 34. Brachiopoda, 69, 83, 85, 101. Branchiosaurus, 117. Bronzite in meteorites, 12. C. Catamites, 112. Calcareous sandstone, 40. Cambrian period, 67, 81. Carboniferous period, 97. Cave earth, 43. Cephalopoda, 94. Cerastes, 123. Chalk, formation of, 48. Chalk period, the, 156. Clay, 42, 53> 54; colouring mat- ter of, 43; inflammable, 44; Scrobicularia, 33. Cleavage, slaty, 20. Clymcnia, 94. Coal, 33, 98, 104. Conformity of strata, 56, 75 Conglomerates, 36. Coralline Crag, 178. Coral reefs, 47. Corals, 85, 87. Cordaitcs, 109. Crag, the, 178. Cretaceous rocks, 149. Crinoidal limestone, 47, 48. Curculoides, 114. Current-bedding in clay, 43; in sandstone, 39. Cyclas, 50. Cystoidea, 69. D. Dadoxylon, 109. Deserts, 31. Devonian period, 92. Diabase, 29. Dinotherium, 177. Diorite, 27, 29. Dodo, the, 68. Dolerite, 29. Dust, volcanic, 26, 30, 85. Earthquakes, 14. Enaliornis, 155. 187 iSS THE STORY OF THE EARTH. Encrinites* 87, 100. Enstatite in meteorites, 12. Eocene, or Lower Tertiary pe- riod, 162, 178. Eophrynus Prestwichi, 113. Eoscorptus, 114. Eozoon canadense* 79. Etna, lavas of, 27, 29. Euphoberia.) 113. Eurypterus^ 88, 89. F. Felspar In sandstone, 38. Fishes, fossil, 88, 94, 96, 102, 117, i25 } 179. Folding in rocks, 15, 18, 22. Foraminifera of chalk, 159. Fossils, v, 20, 59, 62. Fuller's Earth, 233. G. Gabbro, 25, 28. Gasteropoda^ 102. Giant's Causeway, 30. Glacial period, 182. Gneiss, 22, 23. Goniatites, 95, 106, 123. Granite, formation, 14, 27 ; meta- morphism, 23 ; composition, 24. Graphite, 78. Graptolites, 69, 85. Greensand, Lower, 41, 151. Greensand, Upper, 41, 152. Grisons, 18. Grits, 34, 37 53- GrypJuea* 126. H. Heat of the earth, 12. Hemiaspis, 88. Herschel, Sir John, on fluidity of the earth, 13. Hipparion, 73. Holloway, Rev. John, 60. Horse, fossil development of, 73. Hunstanton Limestone, 60. Huronian rocks, 79. Hypsiprymus, 123. I. Ichthyosaurus, 125, 128. Inguanodon, 146, 147. Insects, fossil, 114, 124, 133, 145. Iron, in meteorites, n ; in sand- stone, 38, 40. Iron pyrites in clay, 44. J. Jura mountains, 18. K. Kelvin, Lord, on geological time, 13. Kimeridge clay, 44. Krakatoa, eruption of, 26. Labradorite in meteorites, 12. LabyrintJwdontSi 114, 117, 120. Laurentian period, 78. Lava, formation of, 14. Lepiaodendron, 104, no. Leucite in basalt, 29. Lias period, 125. Life, distribution of, on the earth, 63. Lignite, 33, 49. Limestone, 45, 52, 53 ; Archean, 78 ; fresh-water, 49 ; occurrence with schists, 21, 23. Limnaa, 50. Lingula^ 83. Lister, Dr. ? 59. Lithomantis* 114. 1C. Mammals, earliest fossil, 123, Man, glacial, 185. Maps, geological, 59. Meandrina, 47. Megalosauria, 122, 134, 147. Melaphyre, 29. Merostomata, 69. Metals in volcanic rocks, 27. Metamorphism, 23. Meteorites, n. Mica in sandstone, 38. Microlestes, 123. Millepedes, fossil, 113. "Millet seed beds," 32. Millstone grit, 37, 95, 102. Miocene period, 173, 178. Mitchell, Rev. John, 60. Moa, the, 68. Mountain forming rocks, 18, INDEX 189 Mountain limestone, 100. Mud, 42. Myriapoda, 113. MynapO( Mytilus, 86. N. Nautilus, 69. Neocomian rocks, 142. Neoplagiaulax, 123, 166. Neritina, 50. Neuropteris, 112. Niagara river, v, 10. Nickel in meteorites, II. Nummulitcs, 62. O. Obsidian, 28. Odontopteris, 112. Old red sandstone, 92. Oligocene period, 178. Oliyine in meteorites, 12. Oolite period, 45, 130. Ordovician period, 81. Origin of earth, 9. P. Palaosaurus, 122. Palccotherium, 176. Paludina, 50. Pareiasauria, 118, 120. Peat, 32. Pebble beds, 34, 53. Pennystone, 106. Peridolite, 30. Permian period, 115. Phonolite, 29. Pipe clay, 42. Plagiaulax, 123, 166. Planorbis, 50. Plants, fossil, 69, 96, 108, 129, 144, 149, 167, 169, 177. Pleswsauna, 125, 128. Pleurosternon, 145. Pliocene period, 178. Plutonic rocks, 25, 26. Polyzoa, of Coralline Crag, 179. Porites, 47. Protocystites, 83. Protospongia, 83. Pterodactyl, 146. Pteropoda, 83, 84. Pterygotus, 88, 96. Puddingstone, 36. Pumice, 28. Q. Quartz in meteorites, 12. R. Rain-prints, 54. Ramsey, Sir Andrew, 75. Reptiles, fossil, 117, 125. Rhyolite, 27, 28. Ripple marks, 41, 54. Roches montonnees, 183. Rocky Mountains, 18. Rugosa, 69. Rynchosauria, 122. S. Sand, 37, 53- Sand dunes, 31. Sand grains, rounding of, 31. Sandstones, 54; colouring of, 38. 40; occurrence with schists, 22. Scelidosaurus, 126. Schists, 21. Scorpions, fossil, 114. Sea-eggs, 89. Selenite, 44. Septaria, 44. Sequoia, 18. Serpentine, 30. Shrinkage of earth's crust, 15. Sigillaria, 104, no. Siliceous sandstone, 40. Silurian period, 86. Slate, 19- Smeaton, 60, 61. Smith, William, 59, 61. Sphenophyllum, 112. SphenopteriSf 112. Spherulites, 28. Spiders, fossil, 113. Spongilla, 50. Stigmaria, in. Strachey, John, 60. Strata, 53; classification of, 74. Stratification, contemporaneous, 51; laws of, 58. Sun-cracks, 54. Superposition of strata, 58. Syenite, 25, 27. T. Temperature of the earth, 12. Terrestrial rocks, 31. Tertiary period, 160. 190 THE STORY OF THE EARTH. Theriodontia, 118. Time, geological, v, 10, 13. Trachytes, 27. Triar period, 120. Tridymite, 12, 28. Trilobitcs, 62, 69, 85, 90. Troxites, 114. U. Unconformity of strata, 57, 74. Unio t 50. V. Variation in species, 71. Vesuvius, lavas of, 27, 29. Volcanic ash, 26, 30, 85. Volcanic rocks, 26. Volcanoes, 14; extinct, 16. W. Water, agency of, in volcanic action, 16, 26. Wenlock limestone, 87. Whitehurst, John, 61 Wind as a geological agency, 31. Xylobius, 113. Zeuglodon, 174. X. Z. (10) THE END. TA 02372 SM- Rio