[ 631 ]

CHAPTER XXII.

GEOLOGY OF INDIA.

FOR geological purposes British India may be mapped out into the four geographical divisions of-the Himalayan region, the Indo-Gangetic plain, Peninsular India, and Burma.1

THE HIMALAYAN REGION.-The geology of this tract is more IIimácomplex and less fully known than that of the Peninsular layas. area. Until the ground has been carefully gone over by the Geological Survey, many points must remain doubtful; and large areas of the Himálayas (Nepál and Bhután) are still inaccessible to Europeans. The oldest rock of the Himalayas is a gneiss, differing in character from the gneiss of the Penin- Gneiss. sula, and from that of Assam and Burma. The Himalayan gneiss is usually white and grey, its felspar orthoclase and albite; it contains much mica and mica schist, and is 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 Ladákh) range.

axes.

The Central Himálayan region may be described as con- Central sisting of two gneissic axes, with a trough or synclinal valley gneissic 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 Ladákh axis comes 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

1 This section is based upon the official Manual of the Geology of India, by Messrs. H. B. Medlicott and W. T. Blanford, 2 vols., Government Press, Calcutta, 1879. Mr. W. Topley, of the English Geological Survey, conducted the preliminary condensation.

Lower Himá layas.

Krol

limestone.

SubHimálayas.

Siwálik beds.

axes occurs the basin-shaped 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 seem there to have followed each other without important breaks or unconformities; but after the deposition of the Cretaceous rocks of the Himálayan region, important changes appear to have taken place in its 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 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, etc.) known in descending order as the Krol, Infra-Krol, Blaini, and Infra-Blaini beds. In the Krol beds is a massive limestone (Krol 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 or sub-Himálayan range. This occurs almost always on the south of the Lower Himalayas, and is composed of later Tertiary rocks (Siwáliks, etc.), which stretch 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 Dárjiling (Darjeeling), the sub-Himálayan range is wanting; on the Bhután frontier the whole range is occasionally absent, and the great alluvial 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 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

SALT RANGE, GANGETIC PLAIN.

633

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 contain, at their base, a marine band with Pliocene shells. The Manchhar and Siwálik beds are chiefly of fresh-water origin.

The Salt Range in the north-west of the Punjab has, in Salt addition to its economic value, a special geological importance. Range. Representatives of most of the great European formations of Silurian and later epochs are found in it; and throughout 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 through the range. But the others cannot 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 Indosquare miles, and varies in width from 90 to nearly 300 Plain. Gangetic 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 sea-level. This point, by a line measured down the valley, but not following 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 Its slope sea, averages only about 1 foot per mile. It generally exceeds to the sea. this 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. Towards the sea, the slope becomes almost imperceptible.

There is no evidence that the Indo-Gangetic plain existed as Its geosuch in pre-Tertiary times. The alluvial deposits made known logical age. by the boring at Calcutta, have already been described in

Its alluvial sufficient detail. They prove a gradual depression of the area deposits. through the later Tertiary times. There are peat and forest

Its geological history.

Peninsular
India.

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 give only 5 to 10 fathoms, but they very rapidly deepen to over 300 fathoms. The sediment seems to be carried away from this hole by the set of the currents; so that it has remained free from silt 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 Paleozoic 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-Himálayan range were deposited upon uncontorted Palæozoic 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 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 greatly diminished intensity, have not wholly ceased. The alluvial deposits prove depressions to have occurred 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 here consist of gneiss, in three tracts :-throughout a very large part of Bengal and 1 Vide ante, chap. i. p. 26.

VINDHYAN AND GONDWANA BEDS.

635

Madras, extending to Ceylon; among the Aravalli ranges; and in 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 tracts. The Transition

rocks are of great but unknown age. The Vindhyan rocks which succeed them are of very old Palæozoic age, perhaps pre-Silurian. Yet long before the earliest Vindhyan rocks were laid down, the Transition rocks had been altered and contorted. In more recent times there have been local disturbances, and large faults have in places been found; but the greater part of the Peninsular rocks are only slightly disturbed, and the most recent of the great and widespread earth movements of this region date back to pre-Vindhyan times.

The Vindhyan series are generally sharply marked off from Vindhyan older rocks; although in the Godávari valley there is no rocks. well-defined line between these and the Transition rocks.

Vindh

yans.

The Vindhyan beds are divided into two groups. The Lower 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 (Soane) 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 Vindhyans Upper cover a much smaller area, but attain a thickness of about Vindh yans. 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 Paleozoic 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 fresh-water 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 interest- Gondwana ing and important of the Indian Peninsula. The beds are series. almost entirely of fresh-water origin. Many sub-divisions 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 (Soane) on the north, and the Kistna (Krishna) on