iv.]

TRAP ROCKS OF THE DAKHAN.

59

indeed they have been, by the intense heat of the dyke when molten.

In Western India, rocks of this kind are met with, exceeding in extent and magnitude, as far as we know, any similar mass elsewhere in the world. All the country between Nágpúr and Bombay is covered with enormous sheets of dark-coloured rock, which, at some former time, have been poured out in a molten state over the country, from volcanos that now no longer exist. The hills formed by them have a very peculiar appearance, being frequently flat on the top and with very steep sides. Sometimes the

sides, when seen from a distance, appear to be broken by terraces, looking like gigantic steps, as shown in the annexed woodcut of the Bore Ghát near Bombay. From this peculiarity (which is owing to the manner in which this part of the country has been formed, viz., by successive outpourings of rock of unequal degrees of hardness, since worn away unequally by the action of rain-water, the atmosphere and rivers), such rocks are often termed Trap, a name Trappa, Swed.; Treppe, German.

I

60

GRANITIC ROCKS.

[CHAP.

derived from the Swedish word signifying a stair, or step. The term is a very useful one, and is applied to rocks of the kind when they form dykes, as well as when they present the step-like arrangement. Thus the dyke spoken of above would be called a trap-dyke.

There are other kinds of rocks having their origin in volcanos, but as they are of less importance than these trappean rocks, we may defer their description for the present.

The origin of granitic rocks is in so far similar to that of volcanic rocks, that, like these latter, they have been in a liquid state in the interior of the earth; and while in this condition, they have been forced into other rocks (not themselves liquified, but perhaps only softened,) breaking into them and through them, and afterwards becoming cooled and solidified very gradually. But there is this important distinction between them: while trappean rocks are often poured out over the surface, granitic rocks are never met with in such a situation; and there is reason to believe that they have in all cases cooled down very slowly, while yet. deep in the interior of the earth. Long afterwards, they have become exposed at the surface, when, by the action of seas and rivers, together with those movements of the earth's crust which we have yet to speak of, either the whole of the rocks originally covering them have been worn away, or they have been thrust up in some great mountain chain and exposed at its deep fissures. Hence it results that they are very frequently met with in mountain chains, such as the Himálaya, generally breaking into rocks of the metamorphic class. In appearance, and in the minerals that compose them, they much resemble these latter, except that they rarely show any trace of that layer-like arrangement of the minerals, termed foliation, that is very common in gneiss. Sometimes, indeed, it is almost impossible to tell one of these rocks from the other; for gneiss, when very highly metamorphosed, becomes to all intents and purposes a granite. But true gneiss is never intruded into other rocks, as are volcanic rocks and granite, and this fact,

IV.]

GEOLOGICAL SUCCESSION OF FOSSILS.

61

and generally the peculiarity of its mineral arrangement, are characters by which the last may be distinguished.

All rocks then, with insignificant exceptions, belong to one or other of these four classes, stratified or sedimentary, metamorphic, (which are stratified rocks altered by heat and the action of heated water in the interior of the earth,) volcanic, and granitic rocks. All sedimentary rocks have been formed on the bottoms of seas, lakes and rivers, while those of the last two classes have proceeded from great depths in the interior of the earth. We know a great deal more about the way in which sedimentary rocks are formed, and under what circumstances it results that, sometimes clay, sometimes sand, and sometimes limestone forms their material, than we do in the case of volcanic and granitic rocks; for we can, to some extent, watch what goes on in seas and rivers, and even obtain specimens of the bottoms; but we cannot penetrate to those depths in the interior of the earth at which rocks are melted and metamorphosed. But in some respects the former are the more interesting to us, because we find buried in them, and still recognisable, the remains of creatures that have lived in past times, and thus we learn a great deal of the wondrous changes that animals and plants have undergone, ages

before man existed.

Not only do we thus learn that the whole animal kingdom has continually been subject to changes as great as those of the plains and seas that they tenanted, but we learn also the order in which one kind of animal has succeeded another in past time. It is not difficult to understand how this can be discovered. The case of the limestone and the sandstone of the Khasi Hills, already described, will afford us an excellent illustration of the method. The annexed woodcut represents a geological section of the Khasi Hills, that is, what would be seen, if the hills at Chera Púnji were cut through from north to south so as to expose the arrangement of the rocks that form them; b represents the position of the sandstone containing marine shells, while a represents the limestone. The sandstone

62

GEOLOGICAL SUCCESSION OF FOSSILS. [CHAP.

here must clearly have been formed before the limestone, because the latter rests on the former; and therefore the creatures whose remains are found in the sandstone must have lived long before those, the shells of which are embedded in the limestone. In the interval, a mass of sand not less than 1,200 feet thick (since hardened into stone) must have been deposited on the spot. How long this would take to accumulate we cannot well tell, but it must have been a very long time indeed; for it had all to

FIG. 7.-Geological Section of a part of the Khasi Hills.

be worn away from the land of that day by the action of the seas and rivers, and this is far from being a rapid process. Moreover, it would appear that, in the interval, nearly the whole of the shelled animals and other creatures inhabiting the sea had died out, and had been replaced by others differing from them; and this we have every reason to believe is a process of extreme slowness. It is therefore beyond doubt that a very long period, certainly many thousands of years, elapsed, while these great beds of sandstone were being slowly piled up on the bottom of the sea.

By comparing together the animal remains, such as shells, bones and the like, that are met with in sedimentary rocks in different parts of the world, it is found that each kind of animal has continued to exist for a certain time, in some cases longer, in others shorter; and that when a kind has once died out and become extinct, it does not reappear at any later epoch. Consequently their remains, termed fossil remains or simply fossils, characterize definite periods of the earth's history; and when a considerable number of identical fossils are found associated together in sedimentary rocks in two widely distant countries, they indicate that

IV.

EVIDENCE OF GEOLOGICAL AGE.

63

these rocks were formed about the same time; at all events within such a limited period that the animals, whose remains are found in both, continued to live during the interval. Such rocks are said to belong to the same formation. Thus we have learned that the sandstones of the Khasi Hills belong to about the same period of the earth's history as the chalk which forms the hills and cliffs of the English counties Kent and Sussex, and which is imported into India for writing on the black boards of our schoolrooms and for many other purposes. This formation is called the cretaceous formation, from the Latin word signifying chalk. There are other deposits of the same age in Trichinopoly and South Arcot in Southern India, and again at Bágh, not far from Indore; but these are sands, gravels, clays or limestone, in no case chalk. The summits of certain of the mountains bordering the Spiti valley in Tibet are also composed of a limestone of the Cretaceous period; and since this rock was formed on the bottom of the sea, it is clear that these gigantic mountains have been lifted up or upheaved since the Cretaceous period. There are but very few creatures, still existing, of those that lived in the cretaceous seas; and these few are such as live in very deep water.

In like manner, the limestone containing fossils at the top of the Khasi Hills, close to Chera Púnji, is found to be of about the same age as a portion of the Hála range in Sind; as certain limestones that occur in the Pyrenees and the Alps of Europe, and as the gravels, clays, &c., on which both London and Paris are built. The coal beds lying to the west of the Barákar are known to be of the same age as those that occur in the Nerbudda valley, near Chánda in the Central Provinces, and in the Nizam's territory in Southern India; and lastly, the coal fields of Bengal and the Central Provinces generally are about of the same age as the coal of Newcastle in Australia, and a formation which covers a large part of South Africa. These are far more ancient than any of the formations previously spoken of (probably of Permian age).

The mineral character of a formation, such as chalk,