54 BORING IN FORT WILLIAM. [CHAP. tree stumps, which are there represented, were found on examination to be those of Sundri trees, such as are abundant at the present day in the forests of the Sundarbans. The level at which these tree stumps occur has once been the surface of the ground, and the earth now accumulated above them has since been deposited by the river waters. Occasionally the decayed shells of snail-like animals, such as may now be found living in any tank or jhíl, are met with in the clay, and prove therefore that it has been deposited from FIG. 5.-Section of Ground exposed in a Tank at Sealdah, Calcutta. fresh water. Many years ago, a well was sunk in Fort William at Calcutta to a depth of 481 feet; the whole distance through successive layers of sand and clay, peat and pebbles. At a depth of 350 feet, part of the bony shell of a mud tortoise, such as are abundant in the Ganges at the present day, was met with. Further down again, at 380 feet, was a layer of fresh-water shells, and then a bed of decayed wood, showing that there was once a forest growing on ground that is now 380 feet below the surface; but which Iv.] THE KHÁSI HILLS. 55 must at that time have been the surface, and as high up as the present surface of the Súndarbans. The old land surface, then, must have sunk at least 370 feet since the time when these trees grew; and all the enormous mass of earth, now covering them, must have been since deposited from the rivers. From observations like these, we learn that all these broad plains of Bengal have been formed of the sand and clay brought down by the waters of the rivers and such is the character of the great Gangetic plain of the North-West Provinces, as well as the similar plain of Orissa, that of the Godávarí and other great rivers. Let us now make an ideal journey to the Khasi Hills in Sylhet. Up to the foot of these hills, we find the plains are very much like the rest of Bengal, except that the jhíls are larger and more numerous; and if we make our journey in the rains, we shall probably find the country flooded; but when, leaving the swampy plain, we begin to ascend from Tharia Ghát by the road to Chera Púnji, we meet with hard stone and rocks of several kinds not seen before. A little way up the ascent the rock is like sand hardened into stone; and at one place, diligent search will enable us to discover embedded in this stone (called sandstone) shells of several kinds. They are not like those met with in the clay beds around Calcutta, but rather like shells of animals that live in the sea; not indeed identical with any that now live in the sea, but so much like them, that we can entertain no doubt that we have before us the remains of marine animals. Leaving this place and continuing our ascent, we traverse a great mass of sandstone all formed in layers,1 sometimes horizontal, sometimes highly inclined; and at the top, close to Chera Púnji, we see, on the left, a little hill containing a bed of coal, and consisting chiefly of another kind of hard rock which, when burnt, yields lime, and is therefore called limestone. This limestone too contains shells, but they are not like those found in the sandstone at the bottom of the hill; nor, with a few exceptions, exactly like any now living. Like the former, however, they resemble marine shells; and See woodcut, Fig. 7, page 62. 56 SEDIMENTARY ROCKS. [CHAP. we infer therefore that they were formed by animals that at one time lived in the sea. But if so, great changes must have taken place since these creatures lived, for they are now more than 4,000 feet above the sea. Thus, then, we learn from the shells they contain, that the rocks forming some of our hills have been formed in the sea, in the manner described in the last chapter; and that the old sea bottom, by some means or other, has been lifted up so as to form a lofty chain of hills. We have thus learned that all the rocky or earthy materials forming the plains of Bengal and the part of the Khasi Hills that I have just described, have been deposited from water, in some cases from fresh, in others from sea water, and that they are all formed in layers termed beds or strata. All such rocks (for the term rock is applied by geologists indiscriminately to sand and sandstone, loam, clay and limestone, in fact to all earthy or stony masses whether soft or hard,) are termed stratified rocks; and this bedded or stratified structure is an indication that such rocks have been formed in water. From the fact that they are formed of sediment, or that which had settled down from muddy water, they are also called sedimentary rocks. Next let us suppose we make a journey up to Rániganj to the north-west of Calcutta ; or, still better, to the Barákar station, some twenty miles further on. The surface here is no longer flat as in Lower Bengal, but undulating; and every stream and river flows at the bottom of a depression, the surface sloping down gently towards it from some distance on either side. This surface is generally covered with soil of a yellowish colour, unlike that of the rice fields of Jessore and Dacca, and not so fertile. But this soil is of no great thickness; wherever an excavation is made to the depth of a few feet, and everywhere in the banks of the smaller streams or nálás, where their channels are cut deeply into the ground, hard rocks are exposed, which we readily recognise as sandstones. In some places also we meet with layers of clay, or hard clay breaking up in thin slabs, termed shales, and occasionally beds of coal. iv.] METAMORPHIC ROCKS. 57 All these are stratified, like the rocks of the Khasi Hills; and (with the exception of the coal, which is the remains of an ancient forest,) have been formed in water. But if we go a few miles to the north beyond the Adjai, or to the south a little beyond the Dámúdá, or up the Great Trunk road for three or four miles beyond the Barákar river, we come upon rocks of quite a different character. These latter are very hard and compact, frequently containing little shining black or grey scales of a mineral called mica, and with no distinct stratification. This is a crystalline rock. It never contains the remains of animals, whether of land or fresh water; and sometimes it looks as if parts of it had at one time been actually liquid, or at all events soft, so that it has been squeezed and contorted in a very irregular manner. This rock is termed generally gneiss; and there is reason to believe, that though at some former time it may have been a stratified rock, it has since been heated and so acted upon by heated water under great pressure, that all the materials of which it consists have undergone a chemical change, and have been rearranged. Rocks like this are very common in India. Except the soil of the surface, no other rock than varieties of gneiss is to be met with over the greater part of Southern India, and the same is generally the case in the western part of Orissa (in the Garhját states) and in Chutia Nágpúr and Hazáribagh. The greater part of the Himálaya is made up of rocks more or less similar, and so is all the northern part of the Gáro and Khási Hills. Whenever a series of stratified rocks can be followed down to their lowest layer, as a rule the bottom layer is found to rest on this gneiss. From the fact that gneiss has probably at one time been stratified and subsequently greatly altered or metamorphosed, a term which means much the same thing, it is termed a metamorphic rock. These two classes of rocks, the stratified or sedimentary and the metamorphic, are by far the most common. But there are yet two other classes, termed respectively volcanic and granitic rocks, which have originated in a different way. In the Rájmahál Hills the former of these occur, apparently 58 VOLCANIC ROCKS. [CHAP. in layers, interstratified with true sedimentary rocks, but their inner structure is not stratified. The stone of which they consist is generally hard and heavy; and sometimes contains rounded cavities which have been afterwards filled with minerals of a different kind. At some former time, this rock issued in a liquid molten state from the interior of the earth and flowed over the surface, or perhaps over the bottom of some lake; after which, being cooled by exposure to the cold air or water, it became solid, and so formed a layer as we now see it, on the top of which layers of sedimentary rock were afterwards deposited in the way I have already described. Sometimes rocks of the same kind are met with in a different kind of arrangement. A very good instance may be seen between the Núnia nálá and the Assansol Dâk Bungalow, about half-way between Raniganj and the Barákar, on the Great Trunk road. At this place, a line of rounded masses of very hard dark-coloured stone, very different from the sandstones and other stratified rocks of which I have spoken, may be traced across the country for many miles, sometimes forming a rocky ridge above the general surface, at other times nearly covered by the soil and only indicated by rounded blocks partly protruding from it or quite loose and resting upon it. This marks the top (or out-crop as it is technically termed) of a dyke of volcanic rock. If all the sandstones and coal beds that lie on either side of it were cleared away, it would be seen to stand out like a gigantic wall, and however deeply the excavation might be carried, the bottom of it would never be reached. In fact, it reaches far down into the depths of the earth, where it is connected with some large deeplyseated mass of volcanic rock; and at some former time it must have issued in a molten state, filling a gigantic crack more than twenty miles long, and, where it crosses the Great Trunk road, 120 feet across. Many other similar dykes may be seen about Rániganj, though not so large as this. Wherever the sandstones and the coal are in contact with these dykes, the former are seen to be hardened and altered as if they had been baked; as |