1902 Encyclopedia > India > Geology

(Part 3)


Geology of India

For geological purposes India may be mapped out into the three geographical divisions of the Himálayan region, the Indo-Gangetic plain, and Peninsular India.

The Himálayan Region.—The geology of this districts 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 Himálayas (Nepál and Bhután) are at present inaccessible to Europeans. The oldest rock of the Himálayas 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 Himálayan 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 Himálayan (or Ladákh) range. The Central Himálayan 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 of main axis ( the "central gneiss" of Dr Stoliczka) is the oldest ; that of the northern of Ladákh axis is generally syenitic, or is that variety ofhte Himálayan gneiss already described as containing the hornblende. It is probably an extremely altered condition of ordinary marine sediment. He gneiss of the central axis is ordinary kind ; it is penetrated by granite, which ranges along some of the highest peaks. Between these two gneissic axes occurs the basin-shpaed valley, or the Hundes and Zanskar synclinal. In this valley fossiliferous rocks are preserved, giving representatives of the Silurian, Carboniferous, Triassic, Jurassic, and Cretaceous formations. All these seems there to have followed each other without important breaks or unconformities ; but after the deposition of the Cretaceous rocks of the Himálayan region, there 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 Ladákh axis. On the south of this true Himálayan region there is a band of country known as the Lower Himálaya, in which the beds are often greatly disturbed, and even completely inverted, over great areas, the old gneiss apparently overlying the sedimentary rocks. This lower Himálayan region is about 50 miles wide, and consists or irregular ridges, varying from 500 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, sandstone, shales, limestones, &c.) known in descending order as the król, Infra-Król, Blaini, and Infra-Blaini beds. In the Król beds is a massive limestone (Król limestone) probably representing the limestone of the Pir Panjál range, which is most likely of Carboniferous age. The Blaini and Infra-Blaini beds are probably Silurian. The Lower Himálayan range ends at the Sutlej valley, west of which the continuation of the central range is followed immediately by the third of Sub-Himálayan range. This occurs almost always on the south of the Lower Himálayas ; it is composed of later Tertiary rocks (Siwáliks, &c.), which range parallel with the main chain. Generally the Sub-Himálayas consist of two ranges, separated by a broad flat valley ("dún" or "doon") ; the southern slope, overlooking the great Indo-Gangetic plain, is usually the steepest. Below Náini Tál and Dárjíling (Darjeeling), the sub-Himálayan range is wanting ; on the Bhután frontier the whole range is occasionally absent, and then the great plain slopes up to the base of the Lower Himálayan region. It is within the Sub-Himálayan range that the famous Siwálik beds occur, long since known for their vast stores of extinct mammalia. Of about the same age are the Machhar beds of Sind, which also contain a rich mammalian fauna. The Lower Manchhars probably correspond to the Náhan beds, the lowest of the Siwáliks ; they rest upon the Gaj beds, which are probably Upper Miocene. From this it would seem that the lowest Siwáliks are not older than Upper Miocene. The higher Siwálik 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 Siwáliks ; but they contian, at their base, a marine band with Pliocene shells. The Manchhar and Siwálik beds are chiefly of freshwater origin.

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 througout all the vast length of time represented by these formations there is here no direct evidence of any important break in succession, or unconformity. The lowest beds (salt marl, probably Silurian) and the highest (Siwáliks) 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 Indo-Gangetic Plain 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 924 feet above the sea. This point, by a time measured down the valley, but not following the winding of the rivers, 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 Rájmahál hills west of the delta of the Ganges, were continous with those of Sylhet, east of the delta. Marine Jurassic 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 lat4r Tertiary times. There are peat and forest beds, which must have grown quietly at the surface, alternating with deposits of gravels, 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 Himálayas may be reffered to the same period. Numerous minor disturbances occurred in the area which is now northern India during Palaeozoic and Secondary times, but the great dusturbance which has resulted in the formation of the existing chain of the Himálayas 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-Himálayan range were deposited upon uncontorted Palaeozoic rocks, but the whole has since been violently contorted and disturbed. The 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 probably that the elevation of the mountain ranges and the depression of the Indo-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 Himálayan region earthquakes are still common, whilst in Peninsular India they are rare.

Peninsular India.—The oldest rocks of this area consist of gneiss, which occurs in there districts:—a very large part of Madras, extending to Ceylon; 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 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 Vindhyan series generally sharply marked off from older rocks; but in the Godávari 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 more, 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 Bijápur—in each case a distance of over 700 miles. The upper Vindhyans 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 Gondwána 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 Gondwánas, 13,000 feet thick ; Upper Gondwánas, 11,000 feet thick. The series is mainly confined to the area of country between the Narbadá 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 Gondwánas are very constant in character, wherever they are found ; the upper numbers of the lower divisions show more variation, and this divergence of character in different districts becomes more marked in the Upper Gondwána series. Disturbances have occurred in the lower series before the formation of the upper.

The Gondwána beds contain fossils which are of very great interest. In large part these consist of plants which grow 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 Gondwána period. So vast was the time occupied by the deposition of the Gondwána beds that great changes in physical geography and in the vegetation repeatedly occurred. The plants of the Lower Gondwánas consists 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 Gondwánas; 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 Gondwána 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 Pánchet group) is more nearly related to that of the Upper Gondwánas than it is to the other Lower-Gondwána floras.

One of the most interesting facts in the history of the Gondwána series is the occurrence near the base (in the Tálcher 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. 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 or 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.

The Dámodar series, the middle division of the Lower Gundwánas, is the chief source of coal in Peninsular India, yielding more of that mineral than all other formations taken together. The Karharbári group is the only other coal-bearing formation of any value. The Dámodars are 8400 feet thick in Rániganj coal-field, and about 10,000 feet thick in the Sátpura basin. They consist of three divisions; coal occurs in the upper and lower, ironstone (without coal) in the middle division. The Ráníganj 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 Dámodar river, along which runt the road from Calcutta 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 H. Bauerman. The upper or Ráníganj 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 is from 12 to 18 feet, but occasionally a seam acquires a great thickness—20 to 80 feet. The lower or Barákhar 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 Sánktoria, near the Barákhar river, are fairly good for coke and gas. The best coal in India is in the small coal-field at Karharbári. The beds there are lower in the series than those of the Ráníganj field; they belong to the upper part o the Tálcher group, the lowest of the Gondwána series. The coal-bearing beds cover an area of only about 11 squre miles ; there are three seams, varying from 9 to 33 feet thick. The lowest seam 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.

The Dámodar 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 Dámodar river. Laterite (see below) is sometimes used as ore. It is very earthly and of a lower 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 of ghutin (concretionary carbonate of lime) 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 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 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 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. R. 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 east 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. 764-66). 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 .

[For the above section on Geology we are indebted to Mr W. Topley of the English Geological Survey.]

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