64 GEOLOGICAL FORMATIONS. [CHAP. limestone or sandstone, does not then, taken by itself, mark the age of the formation. A substance exactly like chalk is now forming on the bottom of the Atlantic, as we know from the specimens of the bottom that have been obtained in deep-sea soundings; and a sandbank of the present or any past age may in process of time become a sandstone, under the influence of pressure and those chemical changes that are always in progress in the rocks. The geological age of stratified rocks is ascertained in the first instance by the order in which the layers rest on one another, those which are below being necessarily older than those which rest on them; and secondly by their fossils, which enable us to identify rocks as being of the same age, or nearly so, when they occur far apart, and perhaps neither resting on nor covered by any other stratified rocks. By thus comparing the stratified rocks of different parts of the world, geologists have distinguished a number of great formations, and have determined the order in which they have succeeded one another in time. These are given in the following table, which begins with the most recent, the others following in the order of increasing antiquity : Since no stratified rocks are ever formed on dry land, it follows that any region that has been land during the whole period of one of these formations can have no rocks of Iv.] AGE OF VOLCANIC ROCKS. 65 that age. And since, as we have seen, every formation is formed of materials derived from the destruction of those which preceded it, and which have become land in the interval, it also follows that formations which once existed in certain regions have since been destroyed. Thus it happens that in India, as in other parts of the world, many of these formations are not now to be met with, and perhaps have never existed there. India, south of the Ganges, is peculiarly deficient in this respect; and the chief reason is, that the greater part of this region has been chiefly in the condition of dry land from very early times. England, on the other hand, and a part of the Himálaya and the Punjab, which have been occupied by the sea, in part at least, through very long periods, consist of rocks which represent the majority of the formations enumerated in the Table. It is not in general so easy to ascertain the age of a volcanic or granitic rock as that of a sedimentary rock; nor can we very well learn when any particular rock was metamorphosed. Still, in some cases, this may be discovered with considerable precision. Of the trap-dykes at Rániganj and Barákar, we know at least that they are of later age than the coal-bearing rocks that they have broken through, but it is as yet doubtful to what later period they belong. The great trap-flows of Western India partly cover rocks of Cretaceous age, and must therefore have been poured forth after these were formed. Moreover, near Broach and Surat, certain rocks rest upon them that are known by their fossil remains to be of Eocene age, and these contain broken fragments of the trap rocks: therefore the trap rocks are older than these. In this case the age of the trap rocks has been determined therefore with considerable exactitude. Summary. In this chapter I have described the principal classes of rocks that constitute the superficial part of the earth; of the interior we know very little, and that little will be more conveniently noticed later on. We have seen that most of the rocks have either been formed very slowly by deposition at the bottom of the sea, or in lakes and B.G. F 66 SUMMARY. [CHAP. river inundations; the materials being the mud, sand, &c., that are worn away from the surface of the land; or that they have been softened or molten or otherwise rendered liquid in the interior of the earth, and, when liquid, have either been simply forced into other more solid rocks, or poured out over the surface, (sometimes indeed over the seabottom,) and have then gradually cooled and become solid. According to their mode of origin, they may be spoken of as aqueous or water-formed rocks, or as igneous rocks; by which terms is meant that they have been brought into their present condition by the agency of water and of heat respectively. Rocks which have been originally formed by water, but have been subsequently softened and altered by heat, without being actually liquified, are termed metamorphic. The stratified and metamorphic rocks form by far the greater part of the earth's surface, and indicate that every part of the earth has at some time or other been covered by water. Stratified rocks are distinguished by their bedded or stratified structure, igneous rocks by their being intruded into other rocks, or, in the case of certain volcanic rocks, poured out over the surface. They are also to be recognised by their mineral structure; but this is a matter which can only be studied in the rocks themselves, mere description being useless. For this reason, I have not attempted to describe the different varieties of each class of rocks, but have only referred to some of the commonest stratified rocks, such as clay, sand, sandstone, limestone, &c. Every opportunity should be taken by the student to obtain specimens of these, if he cannot visit and examine them in their native sites; but he must bear in mind that this, like all other parts of geology, can be properly studied only in the field. and In the sedimentary rocks are found the remains of creatures and plants that have existed in past times; by studying their forms, and comparing them with existing animals and plants, a wonderful history has been opened up to us. From them we have learned that man was not always a denizen of the earth; but is, on the contrary, only IV.] GEOLOGICAL TIME. 67 one of the most recent of its inhabitants; and while many of our most familiar animals are little, if at all, more ancient than he is, others have existed from very distant periods. How or when the earth began to be, geology can never tell us; but we know that everything on its surface has changed again and again. As one kind of creature has died out, others have appeared and replaced it, and this has gone on continuously and without a break, from the very earliest times of which we know anything. In this grand history, the notion of time presents itself under a new and unfamiliar aspect. Before the unmeasured past that geology reveals, our well-known divisions of months, years, and centuries sink into insignificance. We no longer speak of the life of an individual, but of that of a species or kind of animal; and an epoch, in geological language, means the duration of a whole creation of living things. As in astronomy, when we try to express in numbers the distance of the fixed stars, the figures are such as convey no distinct meaning to our minds; so in geology, the vast periods that have rolled away before man appeared, transcend all our powers of conceiving them. The feeling of the learner as he gazes for the first time on some old Silurian crag, and tries to realize the story told in its contorted strata and rugged surface, abraded by the breakers, and then half buried beneath the gravels of an only less ancient Devonian sea, is such as Playfair has recorded, when, standing on a rocky headland of the Berwick coast, Hutton traced out the meaning of the stony chronicle before them. "The mind," he says, "seemed to grow giddy by looking so far into the abyss of time; and while we listened with earnestness and admiration to the philosopher, who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination can venture to follow." CHAPTER V. THE INTERNAL HEAT OF THE EARTH AND ITS EFFECTS. THE facts recounted in the last chapter respecting igneous rocks have prepared us in some measure to understand how new land may be formed. While the action of water in the shape of rain, ice, rivers, and sea-waves is to wear away the surface of the land, and to carry down its waste and spread it abroad at the bottom of the ocean, we have learned that, deep in the interior of the earth, there is an antagonistic force at work, viz. heat, which softens and even renders liquid great masses of stony matter, and then forces them into and through the solid rocks above them; sometimes in such volume that they are poured forth in thick sheets over the surface. But if this were the only way in which new land is formed, it is clear that, with the exception of the alluvial plains of rivers, such as that on which we live in Bengal, and the low coral islets, the only visible rocks would be those of igneous origin, such as cover the greater part of the Bombay Presidency; and we should nowhere find sandstones, limestones, and the like, full of the remains of marine animals, such as enter into the structure of the Khasi Hills, the elevated land of Western Bengal, certain parts of Central India, and even the lofty peaks and mountain ranges of Tibet. The question therefore still remains to be answered,-how are old sea-bottoms lifted up, so as to become part of the hgihlinds and mountains of existing continents? |