DIAMOND: OCCURRENCE IN INDIA 14.1 



tributaries of the Lower Ganges in Bengal, between the rivers Son and Khan, in latitude 

 25° N. Any other diamond localities outside the area just marked out (see map, Fig. 33) 

 are unimportant, and the reports concerning them are often uncertain. In general, many of 

 the reported localities for diamond are doubtful, there being no exact and reliable information 

 respecting them, or they are simply based on the existence of old mines. 



It is often supposed that all Indian diamond mines are of the greatest antiquity. In 

 many cases the date at which the workings were commenced is not known ; but the working 

 of the most important deposits known at the present day does not date back to very remote 

 periods, probably in all cases subsequent to the year 1000 a.d. and sometimes much later. 

 Of a few mines it is known exactly when work began, as will be mentioned below. 



Diamonds are found in India in compact sandstones and conglomerates, in the loose, 

 incoherent weathered products of these rocks at places where they lie on the surface, and in 

 the sands and gravels of those rivers and streams which have flowed over the diamantiferous 

 strata or their weathered debris and have washed out the stones from their former 

 situations. 



The diamantiferous sandstone of India is of very wide distribution. It belongs to the 

 oldest division of the sedimentary formations of the country which usually rest directly upon 

 the still older crystalline rocks, such as granite, gneiss, mica-schist, hornblende-schist, 

 chlorite-schist, talc-schist, and similar rocks. Fossils have not been found in these 

 sandstones, so that it is not possible to determine exactly to which of the European 

 formations they correspond in age ; they may however be safely stated to belong to the 

 Palaeozoic period, and possibly to the Silurian division of this period. 



The oldest bedded rocks known to Indian geologists are included in the Vindhyan 

 formation. Only the lower division of this formation is represented in the Madras 

 Presidency in southern India, and is there known as the Karnul series. In northern India, 

 for instance in Bundelkhand, these lower beds are overlain by the later beds of the Upper 

 Vindhyan formation. As far as is yet known, the diamantiferous sandstones of the whole 

 of India belong to this Vindhyan formation ; but while in the southern diamond districts, 

 and probably also in the districts of the Godavari and Mahanadi rivers, they belong to the 

 lower division, namely, the Karnul series, in northern India, for example in Bundelkhand, 

 they belong to the upper part of the Vindhyan formation. 



The Lower Vindhyan formation (namely the Karnul series) consists mainly of limestones 

 with interbedded clay-slates, sandstones, conglomerates, and quartzites. In southern India at 

 the base of this series of beds are beds of sandstone and conglomerates which are known as 

 the Banaganapalli group and here constitute the diamantiferous strata. The whole of the 

 Banaganapalli sandstones are on an average ten to twenty feet thick ; they are usually coarse 

 grained, sometimes argillaceous, at other times very compact and siliceous, and in places 

 felspathic and ferruginous ; they are dark in colour, being red, grey or brown. The pebbles 

 of the interbedded conglomerates have been derived from the denudation of older rocks, and 

 consist for the most part of quartzite, variously coloured hornstones and jasper, as well as 

 compact clay-slates. 



TTie diamonds are found in an earthy bed containing abundant pebbles ; this bed is 

 clearly and definitely marked off from other beds and is not repeated at any other horizon. 

 The diamonds, which may themselves be regarded as pebbles since they also show signs of 

 rounding, lie scattered singly among the other pebbles which are of the same materials as 

 those just mentioned. This earthy bed, in which alone the diamonds are found, is of little 

 thickness ; in exceptional cases its thickness is stated to be two and a half feet, but it often 

 measures less than a foot, and rarely exceeds this amount. 



