beds feet lower probably gneiss rocks coal himalayan series range
INDIA GEOLOGY, for geological purposes India may be mapped out into the three geographical divisions of the Himalayan region, the Indo-Gangetic plain, and Peninsular India.
The Himalayan Region. - The geology of this district is far more complex and less fully known than that of the Peninsular area. Until the ground has been carefully gone over by the Geological Survey, many points must remain doubtful ; probably even then the problems will not be fully solved, as large areas of the Himalayas (Nepal and Bhutan) are at present inaccessible to Europeans. The oldest rock of the Himalayas is gneiss, but its age is quite unknown. It generally differs in character from the gneiss of the Peninsula, and also from that of Assam and Burmah. The Himalayan gneiss is usually white and grey, its felspars being orthoclase and albite ; it contains much mica and mica schist, and is generally much more uniform in character than the gneiss of the Peninsula. The latter is usually pink, its felspar being orthoclase and oligoclase ; it contains little mica schist, but often has quartzite and hornblendic rock. Hornblende occurs in the syenitic gneiss of the Northern Himalayan (or Ladakh) range. The Central Himalayan region may be roughly described as consisting of two gneissic axes, with a trough or synclinal valley between them, in which fossiliferous beds have been deposited and are now preserved. The gneiss of the southern or main axis (the "central gneiss " of Dr Stoliczka) is the oldest ; that of the northern or Ladakh axis conies next in age. The gneiss of the Ladakh axis is generally syenitic, or is that variety of the Himalayan gneiss already described as containing hornblende. It is probably an extremely altered condition of ordinary marine sediment. The gneiss of the central axis is the ordinary kind ; it is penetrated by granite, which ranges along some Of the highest peaks. Between these two gneissic axes occurs the basin-shaped valley, or the Ilundes and Zanskar synclinal. In this valley fossiliferous rocks are preserved, giving representatives of the Silurian, Carboniferous, Triassic, and Cretaceous formations. All these seem there to have followed each other without important breaks or unconformities; but after the deposition of the Cretaceous rocks of the Himalayan region, there appear to have been important changes in physical geography. The Nummulitic (Eocene) strata were laid down on the eroded edges of some of the older beds, and in a long trough within the Silurian gneiss of the Ladakh axis. On the south of this true 'Himalayan region there is a band of country known as the Lower Himalaya, in which the beds are often greatly LI disturbed, and even completely inverted, over great areas, H the old gneiss apparently overlying the sedimentary rocks. la This Lower Himalayan region is about 50 miles wide, and consists of irregular ridges, varying from 5000 to 8000 feet in height, and sometimes reaching 12,000 feet.
Resting upon the gneiss, but often through inversion apparently underlying it, in the neighbourhood of Simla, is a series of unfossiliferous beds (schists, quartzites, sandstones, shales, limestones, &-e.) known in descending order as the Kr61, Infra-Kr61, Blaini, and Infra-Blaini beds. In the Krol beds is a massive limestone (KrOl limestone) probably representing the limestone of the Pir Panjal range, which is most likely of Carboniferous age. The Blaini and Infra-Blaini beds are probably Silurian. The Lower Himalayan range ends at the Sutlej valley, west of which the continuation of the central range is followed immediately by the third or Sub-Himalayan s range. This occurs almost always on the south of the 1 Lower Himalayas ; it is composed of later Tertiary rocks (Siwaliks, &e.), which range parallel with the main chain. Generally the Sub-Himalayas consist of two ranges, separated by a broad flat valley (" dun " or " doon ") the southern slope, overlooking the great Indo-Gangetic plain, is usually the steepest. Below Naini Tal and Darjiling (Darjeeling), the sub-Himalayan range is wanting ; on the Bhutan frontier the whole range is occasionally absent, and then the great plain slopes up to the base of the Lower Himalayan region. It is within the Sub-Himalayan range that the famous Siwalik beds occur, long since known for their vast stores of extinct mammalia. Of about the same age are the Manchhar beds of Sind, which also contain a rich mammalian fauna. The Lower Manchhars probably correspond to the Nahan beds, the lowest of the Siwaliks ; they rest upon the Gaj beds, which are probably Upper Miocene. Front this it would seem that the lowest Siwaliks are not older than Upper Miocene. The higher Siwalik beds are considered by Mr W. T. Blanford to be Pliocene, and to this later period he also refers the mammalian beds of Pikermi in Greece. These have a large number of fossils in common with the Siwaliks ; but they contain, at their base, a marine band with Pliocene shells, The Manchhar and Siwalik beds are chiefly of freshwater orhrin.
The Salt Range in the north-west of the Punjab has, in addition to its economic value, a special geological importance ; and from that point of view it is one of the most interesting districts in India. Representatives of most of the great European formations of Silurian and later epochs are found there ; and throughout all the vast length of time represented by these formations there is here no direct evidence of any important break in succession, or uneonformity. The lowest beds (salt marl, probably Silurian) and the highest (Siwaliks) are found throughout the range. But the others cannot all be traced continuously throughout; some occur well developed in one place, some in another. All the principal fossiliferous beds of the Jurassic, Triassic, and Carboniferous formations are confined to the western part of the range.
The Judo-Gangetic P/aiii, covers an area of about 300,000 square miles, and varies in width from 90 to nearly 300 € miles. It rises very gradually from the sea at either end ; the lowest point of the watershed between the Punjab rivers and the Ganges is about 921 feet above the sea. This point, by a line measured down the valley, but not fob lowing the winding of the river, is about 1050 miles from the mouth of the Ganges and 850 miles from the mouth of the Indus, so that the average inclination of the plain, from the central watershed to the sea, is only about 1 foot per mile. It is less near the sea, where for long distances there is no fall at all. It is generally more near the watershed ; but there is here no ridge of high ground between the Indus and the Ganges, and a very trifling change of level would often turn the upper waters of one river into the other. It is not unlikely that such changes have in past time occurred ; and if so an explanation is afforded of the occurrence of allied forms of freshwater dolphins (Platanista) and of many other animals in the two rivers and in the Brahmaputra.
There is no evidence that the Indo-Gangetic plain existed as such in Pre-Tertiary times. It is highly probable that the Jurassic and Cretaceous coast-line ran across the northern part of the Bay of Bengal, and that most of the area now occupied by the Gangetic plain was then above the sea. Probably the Jurassic traps of the llajmahal hills, west of the delta of the Ganges, were continuous with those of Sylhet, east of the delta. Marine Jurassic and Cretaceous beds are absent from the margins of the true Gangetic plain ; so too are marine Eocene beds. In Eocene times the sea spread up the Punjab ; but that too was land only in Miocene times.
The alluvial deposits of the plain, as made known by the boring at Calcutta, prove a gradual depression of the area through the later Tertiary times. There are peat and forest beds, which must have grown quietly at the surface, alternating with deposits of gravel, sand, and clay. The thickness of the delta deposit is unknown ; 481 feet was proved at the bore hole, but probably this represents only a very small part of the deposit. Outside the delta, in the Bay of Bengal, is a deep depression known as the "swatch of no ground " ; all around it the soundings are only of 5 to 10 fathoms, but they very rapidly deepen to over 300 fathoms. Mr J. Ferguson has shown that. the sediment is carried away from this area by the set of the currents ; probably then it has remained free from sediment whilst the neighbouring sea bottom has gradually been filled up. If so, the thickness of the alluvium is at least 1800 feet, and may be much more.
The Indo-Gangetic plain dates back to Eocene times ; the origin of the Himalayas may be referred to the same period. Numerous minor disturbances occurred in the area which is now northern India during Palaeozoic and Secondary times, but the great disturbance which has resulted in the formation of the existing chain of the Himalayas took place after the deposition of the Eocene beds. Disturbances even greater in amount occurred after the deposition of the Pliocene beds. The Eocenes of the Sub-Himalayan range were deposited upon uncontorted Palaeozoic rocks, but the whole has since been violently contorted and disturbed. There are some indications that the disturbing forces were more severe to the eastward during middle Tertiary times, and that the main action to the westward was of later date. It seems highly probable that the elevation of the mountain ranges and the depression of the Inclo-Gangetic plain were closely related. This view gains some support from a glance at the map, where we see that the curves of the great mountain chains are strictly followed by those of the great alluvial plain. Probably both are due to almost contemporary movements of the earth's crust ; these movements, though now of vastly diminished intensity, have not wholly ceased. 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.
Peninsular I ndia. - The oldest rocks of this area consist of gneiss, which occurs in three districts : - a very p large part of Bengal and Madras, extending to Ceylon ; la the Aravalli ; and Bundelkhand. Of these formations, the gneiss of Bundelkhand is known to be the oldest, because the oldest Transition rocks rest upon it ; whereas the same Transition rocks are altered and intersected by granitic dykes which proceed from the gneiss of the other districts. The Transition rocks are of great but unknown age. The Vindhyan rocks which succeed them are of very old Palaeozoic age, perhaps Pre-Silurian. But r( long before the earliest Vindhyan rocks were laid down the Transition rocks had been altered and contorted. The great movements of the earth's crust which produced that contortion are the latest which have taken place to any great extent in the Indian Peninsula. In more recent times there have been local disturbances, and large faults have in places been found ; but the greater part of the Peninsula rocks are only slightly disturbed, and the most recent of the great and wide-spread earth movements of this region date back to Pre-Vindhyan times. The Vincihyan series are generally sharply marked off from older rocks ; but in the Godavari valley there is no well-defined line between these and the Transition rocks. The Vindhyan beds are divided into two groups. The lower, with an estimated thickness of only 2000 feet, or slightly snore, cover a large area, - extending, with but little change of character, from the Son valley in one direction to Cuddapah, and in a diverging line to near Bijapur - in each case a distance of over 700 miles. The upper Vindhyan cover a much smaller area, but attain a thickness of about 12,000 feet. 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. So far as the evidence goes, it appears probable that they are of very ancient Palaeozoic age, perhaps Pre-Silurian. 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 s subdivisions have been made, but here we need only note the main division into two great groups :--Lower Condwanas, 13,000 feet thick ; Upper Gondwanas, 11,000 feet thick. The series is mainly confined to the area of country between the Narbada and the Son on the north and the Krishna 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 numbers 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. So vast was the time occupied by the deposition of the Gondwana beds that great changes in physical geography and in the vegetation repeatedly occurred. The plants of the Lower Gondwanas consist chiefly of acrogens (Equisetarece and ferns) and gymnogens (cycads and conifers), the former being the more abundant. The same classes of plants occur in the Upper Gond wanes ; but there the proportions are reversed, the conifers, and still more the cycads, being more numerous than the ferns, whilst the Equisetacem 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.
One of the most interesting facts in the history of the Gondwana series is the occurrence near the base (in the Talcher group) of large striated boulders in a fine mud or silt, the boulders in one place resting upon rock (of Yindhyan age) which is also striated. There seems good reason for believing that these beds are the result of ice-action. They probably nearly coincide in age with the Permian beds of Western Europe, in which Professor Ramsay long since discovered evidence of glaciation. But the remarkable fact is that this old ice-action occurred within the tropics, and probably at no very great height above the sea.
r The Damodar 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 Damodars are 8100 feet thick in the Tlaniganj coal-field, and about 10,000 feet 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. So far as is yet known, it covers an area of about 500 square miles, extending about 18 miles from north to south and about 39 miles from east to west ; but it extends further to the east under the laterite and alluvium. It is traversed by the Damodar 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. Much has been learnt concerning it since the last examination by the Geological Survey, and our remarks are in great part based on recent reports by Mr II. Bauerman. The upper or Ranfganj series (stated by the Geological Survey to be 5000 feet thick) contains eleven seams, having a total thickness of 120 feet, in the eastern district, and thirteen seams, 100 feet thick, in the western district. The average thickness of the seams worked i-s from 12 to 18 feet, but occasionally a seam acquires a great thickness-20 to 80 feet. The lower or Barakhar series (2000 feet thick) contains four seams, of a total thickness of 69 feet. 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 Barakhar river, are fairly good for coke and gas. The best coal in India is in the small coal-field at Karharbasi. The beds there are lower in the series than those of the Raniganj field ; they belong to the upper part of the Talcher group, the lowest of the Gondwana series. The coal-bearing beds cover an area of only about 11 square miles ; there are three seams, varying from 9 to 33 feet thick. The lowest scam is the best, and this is as good as English steam coal. This coalfield, 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.
r The Damodar 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. It generally contains phosphorus, which prevents its use in the preparation of the finer qualities of steel. A similar difficulty attends the use of the Cleveland ore of North Yorkshire. Experiments have been in progress for years in search of a process which shall, in an economical manner, obtain iron from Cleveland ore free from phosphorus, latterly, it is hoped, with some success. If this be so, India will be a great gainer. Excellent iron-ore occurs in the Metamorphic rocks south of the Damodar river. Laterite (see below) is sometimes used as ore. It is very earthy and of a low percentage ; but it contains only a comparatively small proportion of phosphorus.
The want of limestone for flux, within easy reach, is generally a great drawback as regards iron-smelting in India. Kankar or 91'11626 (concretionary carbonate of Line) is collected for this purpose from the river beds and alluvial deposits. It sometimes contains as much as 70 per cent. of carbonate of lime ; but generally the amount is much less and the fluxing value proportionally diminished. The real difficulty in India is to find the ore, the fuel, and the flux in sufficiently close proximity to yield a profit.
The enormous mass of basaltic rock known as the Deccan trap is of great importance in the geological structure of the Indian Peninsula. It now covers an area of about 200,000 square miles, and probably formerly extended over a much wider area. Where thickest, the traps are at least 6000 feet thick. They form the most striking physical features of the country, many of the most prominent bill ranges being the denuded edges of the basaltic flows. The great volcanic outbursts which produced this trap commenced in the Cretaceous period and lasted on into the Eocene period.
Laterite is a ferruginous and argillaceous rock, varying from 30 to 200 feet thick, which often occurs over the trap area, but is also found in other districts. As a rule it makes rather barren land ; it is highly porous, and the rain rapidly sinks into it. Laterite may be roughly divided into two kinds, high-level and low-level laterites. The former, which covers a large area of the high basaltic plains, is believed by Mr lt. B. Foote to be very frequently the product of decomposition of the trap, and to have been thus formed in the place in which it is now found. Sometimes the high-level laterite overlies gneiss or other rocks ; and in these cases it has probably been transported. The low-level laterite is generally more sandy in character, and is often associated with gravels. In most cases this has clearly been carried down to its present position, probably largely by subaerial action, aided by rains and streams. Possibly in some cases it has been spread out along the coasts by marine action. The low-level laterite fringes the coast of the Peninsular more or less from near Bombay on the west and Orissa on the cast to Cape Comorin. It is not continuous throughout these districts ; and it is of very varying width and elevation. The age of the high-level laterite is unknown. Its formation probably extended throughout a long period of time, much of which must be of very ancient date ; for the laterite, together with the underlying basalt, has suffered extensive denudation.
The mercantile aspects of the coal, iron, and other mineral products of India will be fully treated of under a subsequent section (pp. 761-66). The geologist comes in this matter to the same conclusion as the economist, viz., that the mineral wealth of India, as represented by its precious stones, was the product of forced labour, and that the search for them in our days can scarcely repay the working expenses.