a boring was driven through the Gangetic alluvium to a depth of
1336 ft. from the surface, or nearly 1000 ft. below sea-level. Even
at this depth there was no indication of an approach to the base of
the alluvial deposits.
The deposits of the Indo-Gangetic plain are of modern date and the formation of the depression which they fill is almost certainly connected with the elevation of the Himalayas. Both movements are probably still going on. The alluvial deposits prove depression in quite recent geological times; and within the Himalayan region earthquakes are still common, whilst in Peninsular India they are rare.
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| after Geological Survey of India. | Emery Walker sc. |
Peninsular India.—The oldest rocks of this region consist of gneiss, granite and other crystalline rocks. They cover a large area in Bengal and Madras and extend into Ceylon; and they are found also in Bundelkhand and in Gujarat. Upon them rest the unfossiliferous strata known to Indian geologists as the Transition and Vindhyan series. The Transition rocks are often violently folded and are frequently converted into schists. In the south, where they are known as the Dharwar series, they form long and narrow bands running from north-north-west to south-south-east across the ancient gneiss; and it is interesting to note that all the quartz-reefs which contain gold in paying quantities occur in the Dharwar series. The Transition rocks are of great but unknown age. The Vindhyan rocks which succeed them are also of ancient date. But long before the earliest Vindhyan rocks were laid down the Transition rocks had been altered and contorted. Occasionally the Vindhyan beds themselves are strongly folded, as in the east of the Cuddapah basin; but this was the last folding of any violence which has occurred in the Peninsula. In more recent times there have been local disturbances, and large faults have in places been formed; but the greater part of the Peninsula rocks are only slightly disturbed. The Vindhyan series is generally sharply marked off from older rocks; but in the Godavari valley there is no well-defined line between them and the Transition rocks. The Vindhyan beds are divided into two groups. The lower, with an estimated thickness of only 2000 ft., or slightly more, cover a large area—extending, with but little change of character, from the Sone valley in one direction to Cuddapah, and in a diverging line to near Bijapur—in each case a distance of over 700 m. The upper Vindhyans cover a much smaller area, but attain a thickness of about 12,000 ft. The Vindhyans are well-stratified beds of sandstone and shale, with some limestones. As yet they have yielded no trace of fossils, and their exact age is consequently unknown. They are however certainly Pre-Permian, and it is most probable that they belong to the early part of the Palaeozoic era. The total absence of fossils is a remarkable fact, and one for which it is difficult to account, as the beds are for the most part quite unaltered. Even if they are entirely of freshwater origin, we should expect that some traces of life from the waters or neighbouring land would be found.
The Gondwana series is in many respects the most interesting and important series of the Indian Peninsula. The beds are almost entirely of freshwater origin. Many subdivisions have been made, but here we need only note the main division into two great groups: Lower Gondwanas, 13,000 ft. thick; Upper Gondwanas, 11,000 ft. thick. The series is mainly confined to the area of country between the Nerbudda and the Sone on the north, and the Kistna on the south; but the western part of this region is in great part covered by newer beds. The lowest Gondwanas are very constant in character, wherever they are found; the upper members of the lower division show more variation, and this divergence of character in different districts becomes more marked in the Upper Gondwana series. Disturbances have occurred in the lower series before the formation of the upper.
The Gondwana beds contain fossils which are of very great interest. In large part these consist of plants which grew near the margins of the old rivers, and which were carried down by floods, and deposited in the alluvial plains, deltas and estuarine areas of the old Gondwana period. The plants of the Lower Gondwanas consist chiefly of acrogens (Equisetaceae and ferns) and gymnogens (cycads and conifers), the former being the more abundant. The same classes of plants occur in the Upper Gondwanas; but there the proportions are reversed, the conifers, and still more the cycads, being more numerous than the ferns, whilst the Equisetaceae are but sparingly found. But even within the limits of the Lower Gondwana series there are great diversities of vegetation, three distinct floras occurring in the three great divisions of that formation. In many respects the flora of the highest of these three divisions (the Panchet group) is more nearly related to that of the Upper Gondwanas than it is to the other Lower Gondwana floras. Although during the Gondwana period the flora of India differed greatly from that of Europe, it was strikingly similar to the contemporaneous floras of South America, South Africa and Australia. It is somewhat remarkable that this characteristically southern flora, known as the Glossopteris Flora (from the name of one of the most characteristic genera), has also been found in the north of Russia.
One of the most interesting facts in the history of the Gondwana series is the occurrence near the base (in the Talchir group) of large striated boulders in a fine mud or silt, the boulders in one place resting upon rock (of Vindhyan age) which is also striated. These beds are the result of ice-action, and it is interesting to note that a similar boulder bed is associated with the Glossopteris-bearing deposits of Australia, South Africa and probably South America.
The Damuda series, the middle division of the Lower Gondwanas, is the chief source of coal in Peninsular India, yielding more of that mineral than all other formations taken together. The Karharbari group is the only other coal-bearing formation of any value. The Damudas are 8400 ft. thick in the Raniganj coal-field, and about 10,000 ft. thick in the Satpura basin. They consist of three divisions; coal occurs in the upper and lower, ironstone (without coal) in the middle division. The Raniganj coal-field is the most important in India. It covers an area of about 500 sq. m. and is traversed by the Damuda river, along which run the road from Calcutta to Benares and the East Indian railway. From its situation and importance this coal-field is better known than any other in India. The upper or Raniganj series (stated by the Geological Survey to be 5000 ft. thick) contains eleven seams, having a total thickness of 120 ft., in the eastern district, and thirteen seams, 100 ft. thick, in the western district. The average thickness of the seams worked is from 12 to 18 ft., but occasionally a seam attains a great thickness—20 to 80 ft. The lower or Barakar series (2000 ft. thick) contains four seams, of a total thickness of 69 ft. Compared with English coals those of this coal-field are of but poor quality; they contain much ash, and are generally non-coking. The seams of the lower series are the best, and some of these at Sanktoria, near the Barakar river, are fairly good for coke and gas. The best coal in India is in the small coal-field at Karharbari. The beds there are lower in the series than those of the Raniganj field; they belong to the upper part of the Talchir group, the lowest of the Gondwana series. The coal-bearing beds cover an area of only about 11 sq. m.; there are three seams, varying from 9 to 33 ft. thick. The lowest seam is the best, and this is as good as English steam coal. This coal-field, now largely worked, is the property of the East Indian railway, which is thus supplied with fuel at a cheaper rate than any other railway in the world. Indian coal usually contains phosphoric acid, which greatly lessens its value for iron-smelting.
The Damuda series, which, as we have seen, is the chief source of coal in India, is also one of the most important sources of iron. The ore occurs in the middle division, coal in the highest and lowest. The ore is partly a clay ironstone, like that occurring in the Coal-measures of England, partly an oxide of iron or haematite, and it generally contains phosphorus. Excellent iron-ore occurs in the crystalline rocks south of the Damuda river as also in many other parts of India. Laterite (see below) is sometimes used as ore. It
