Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/760

736 736 INDIA [PHYSICAL 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. Its age. 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 Rajmahal 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. Alluvial The alluvial deposits of the plain, as made known by deposits, 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 &quot; swatch of no ground &quot; ; 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 Paleozoic 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 move ments, 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 India, The oldest rocks of this area con sist of gneiss, which occurs in three districts : a very Pen ,. large part of Bengal and Madras, extending to Ceylon ; !arl j. 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 Via. are of very old Palaeozoic age, perhaps Pre-Silurian. But r ci 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 Vind hyan 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 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 Bijapur in each case a distance of over 700 miles. The upper Vind- hyans 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 neigh bouring land would be found. The Gondwana series is in many respects the most Gond interesting and important series of the Indian Peninsula. w4na The beds are almost entirely of freshwater origin. Many seriei subdivisions have been made, but here we need only note the main division into two great groups : Lower Gondwanas, 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 (Equisetacece 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