University Research Library This book is DUE on the last date stamded below MAY 1 4 1934 FEB 5 1953 DEClSWf OEC5 ! /C i - ' . ' __ ^+ VESUVIUS IN ERUPTION, 1872 THE STORY OF 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 WITH FORTY ILLUSTRATIONS 44872 NEW YORK MCMXII COPYRIGHT, 1895, 1902. BY D. APPLETON AND COMPANY. Printed in the United States of America O e. 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. 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- gies 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. CHAPTER TAG* I. INTRODUCTION 9 II. THE EARTH'S INTERNAL HEAT ... 12 III. MATERIALS OF MOUNTAIN CHAINS . . 18 IV. VOLCANIC ROCKS 26 V. THE MATERIALS OF STRATA .... 31 VI. THE SUCCESSION OF STRATA . . . .51 VII. ORIGIN OF STRATIGRAPHICAL GEOLOGY . 58 VIII. FOSSILS 62 - IX. THE CLASSIFICATION OF WATER-FORMED ROCKS 74 X. THE ARCH/EAN 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 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 178 XXIII. GLACIAL PERIOD AND GRAVELS . . 182 LIST OF ILLUSTRATIONS. Vesuvius in Eruption, 1872 . . 1 Gn?iia ... 3 a, 3. Hertfordshire Pudding Stone . . . .35 4. Laminated band 39 . 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. Pleurotomaria . 102 16. Sigillaria . . . . . . , , . Iio 17. Teniaeopteris . . . . .Hi IS. Pareiasaurus . . . . . . . .119 19. Lias Outlier ........ 126 20. Gryphaea Incurva 127 21. Cardinia Listeri 128 22. Ichthyosaurus ........ 131 23. Belemnites Oweni ....... 136 24. Shotover Hill 137 25. Skeleton of Archaeopteryx 139 26. Skull of Archaeopteryx ...... 140 27. Hythe to Folkestone 148 28. Ammonites Deshayesii . . . . . .150 29. Section at Hunstanton . . . . . .151 30. Ammonites Planulatus 152 31. Poterocrinus . 153 32. Micraster Coranguinum 159 33. Galerites Subrotundus 160 34. East of Herne Bay '. 164 35. Ostraea 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. 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 stilt 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. II 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. 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 Crenelle 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 1000 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. 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 e\ ery 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 fhis 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 u^'.vn 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 the 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 th r 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 a 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 Orisons, 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 chains 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, which 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. 2^ 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 volcanos 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 become? 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 nornblende, 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. 31 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 3 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 FlG. 2. Conglomerate of flint-pebbles, from the Hertfordshire puddingston'e, showing the external surface of the pebbles. FIG. 3. Fracture through this conglomerate, showing sections of flint-pebbles imbedded in a siliceous cement. 3