The alluvial deposits of the plain, as made known by the boring at Calcutta, prove a gradual depression of the area in recent 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 ft. was proved at the bore hole, but probably this represents only a 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 ft., and may be much more. At Lucknow a boring was driven through the Gangetic alluvium to a depth of 1336 ft. from the surface, or nearly 1000 ft. below sea-level. Even at this depth there was no indication of an approach to the base of the alluvial deposits. [Illustration: Map of India (Northern Part) Scale 1:7,500,000] [Illustration: Geological Map] The deposits of the Indo-Gangetic plain are of modern date and the formation of the depression which they fill is almost certainly connected with the elevation of the Himalayas. Both movements are probably still going on. 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.