INDIA main physical regions of India may be comprehended at a glance. On the north-west, north, and north-east lie the great mountainous borderlands, and separating them from the plateau of peninsular India is the wide Indo-Gangetic depression. From the Pamirs the Himalayas extend in the form of a scimitar as far as the great bend of the Brahmaputra, beyond which the mountain system of Burma runs in parallel ranges from north to south. To the south-west of the Pamirs, the borderland consists in the north of part of the Hindu Kush and its offshoots, and in the south of the parallel and concentric ridges of Baluchistan, which rise in the Sulaiman and Kirthar ranges to hills of considerable height. Be tween the Himalayas and the north-west borderland certain great physical differences ought to be noted. The former are higher and more continuous, and are formed of rocks of more ancient origin than are found in the latter except in the Hindu Kush. The passes across the Himalayas are few and difficult, while to the south-west of the Pamirs there are several routes by which India may be entered with comparative ease. In Burma the mountains appear to correspond in age to those of the north-west borderland.
The Indo-Gangetic plains may be considered as consisting of the basins of the Indus and the Ganges below the 1,000-foot contour line. The earth movements, which led to the upheaval of the lofty Himalayas in the north, led likewise to the formation of a great depression further south. This depression has gradually been filled up by the alluvium carried down by the rivers draining into it, a process of land building still being carried on in different parts of the region.
The general character of peninsular India, which is the oldest land mass of the country, is that of a plateau with a gradual slope from west to east. On the west it is buttressed by the lofty wall of the Western Ghats, between which and the sea lies a narrow coastal plain. The so-called Eastern Ghats merely form the low escarpment of the plateau, which stands back some distance from the Bay of Bengal, so that the coastal plain is much wider than on the west. The surface of the plateau is generally an area of open valleys and wide plains, broken up by a number of ridges running eastwards from the Western Ghats. Over the greater part of this region the rocks are of Archaean age, but the great volcanic outbursts, which took place at the end of Cretaceous and in early Tertiary times, have covered in the north-west an area over 200,000 square miles in extent with igneous rocks, generally known as Deccan trap. In various parts of the peninsula, but especially in the basins
of the Son and Godavari, there are patches of the Gondwana series formed during Carboniferous and later times, which are of especial importance as they contain the bulk of the coal supplies of India. Much of the east and part of the west coastal plain consist of post-Tertiary deposits.
The soils of these different regions vary greatly in different parts of the country. The alluvium of the Indo-Gangetic valley and of the coastal plains is, as a general rule, the most productive, but it varies in character from drift sands, on which nothing will grow, to clays so stiff that they cannot be drained. The Deccan trap furnishes in the upland regions a poor and infertile soil, but in the lowlands it affords a deep, black soil of a peculiar consistency, which makes it very retentive of moisture and thus renders it especially valuable in these districts where the rainfall is not great. The soils derived from the Archxan rocks also vary greatly in character and fertility, but, except in the valley regions, they may, as a general rule, be classed as poor.
CLIMATE.—India falls within the monsoon region of Asia, and its climate is mainly controlled by that fact. Two seasons may be recognised : the dry, or north-east monsoon lasts from the middle of December to the end of May, and the wet, or south-west monsoon, from the end of May to the middle of December. During the first part of the dry monsoon a belt of high pressure stretches across Asia but from the outflowing cold winds India is protected by the Hima layas. In January, its actual mean temperature ranges from over in the south to less than 55°F. in the north-west. As pressure is therefore highest in the north-west, winds flow down the Gangetic plains in a south-easterly direction, and, veering round by north over the Bay of Bengal, blow across southern India from the north-east. While crossing the ocean they pick up a certain amount of moisture which they deposit on the southern part of the east coast and in Ceylon. At the same time there is a slight amount of precipitation on the Himalayas and in northern India, as a result of storms in the upper air current moving from the equator towards the north.