214 TIic (!colu(iy of tlic Gold Fh^hlx of British (rniana. 



The valley deposits are not as thick, as a rule, as are the hill-side 

 formations. They have an overburden of sandy clay up to four feet in 

 thickness, which lies on i^e-arranged gravel seldom more than four feet 

 in thickness. The bedrock is in places a reddish laterite, in others a 

 greyish kaolin. 



The great bulk of l)oth the hill and the stream gravels consists of 

 (juartz pel)bles, with some of more or less silicified felsite and a few of 

 concretionary ironstone. The hea^■y minerals, in addition to diamcjnds, 

 which are found in small pieces in the gravels, are ilmenite, schorl, 

 pleonaste, colourless to faintly coloured corundum and spinel, and 

 occasionally topaz. Minute crystals of zircon are present in small 

 ((uantities in the sands and in the greyish kaolin of the bedrock. With 

 the exceptions of the schorl and the veiy sparsely occurring topaz 

 these minerals are found in the basic rocks of the district, and may 

 have been derived from the erosion of the laterites resulting from their 

 decomposition. These, as well as the fragments of schorl and of topaz, 

 are all waterworn and rounded. 



The mode of occurrence of diamonds in this district offers no 

 indications as to their original source, and it is perhaps in our present 

 state of knowledge safest to regard them as having been deriAed from 

 the disiiitegration of the conglomerate lieds which formerly co\-ered 

 the country. 



At the same time there is nothing in their mode of occurrence 

 which renders unlikely their derivation, with certain of their accom- 

 |)anying minerals, from laterites resulting from the decomposition of 

 l)asic rocks. 



The diamonds found some years ago at Omai were in a bluish-grey 

 clay, a pnjduct of the decomposition of gabbro or diabase in situ. 

 The majority of these diamonds were of -sery small size, running from 

 fifty to si.xty to the carat ; they were generally very perfect octahedi-al 

 crystals ; and whilst most of them were colourless some were red, some 

 pink, others green, and a good many yellow or orange. I could not 

 obtaiTi at Omai any e\idence to connect their occurrences witli the 

 sandstone and conglomerate formation. 



Those found near the Kuribrong were associated %\ith sericitic and 

 chloritic rocks, which may be metamorphosed sediments ; but as the 

 fliamonds are there found not far from the escarpment of the sandstone 

 and conglomerate formation, they may have had a similar origin to that 

 suggested for those found in the Putareng District. Similarly those 

 obtained from the ])lacers in the Potaro Goldlield may have come from 

 the breaking up of the sandstone and conglomerate formation, gi-eat 

 boulders of which are found near the workings of the Inflexible 

 Syndicate. 



