III.—The Residual Earths of British Guiana commonly termed ‘Laterite’

Extract

On pages 20–2 and 99–105 of the Geology of the Gold Fields of British Guiana I gave a condensed account of the residual earths derived from the gradual decomposition of igneous rocks in situ which characterize wide areas ia British Guiana as well as in the neighbouring countries of Venezuela, Dutch Guiana, French Guiana, and Brazilian Guiana. This deposit forms in many places a widespread very thick blanket-like coating to the igneous rocks from which it is derived, and owing in many places to its striking resemblance in general properties to the typical Indian formation described by Buchanan in 1807 it has been alluded to by many authors and by numerous mining engineers as ‘laterite’. I gave on p. 101 two analyses of lateritic deposits which I selected from many I had made as showing the general composition of the earths. Unfortunately I omitted to show in them separately, as I had done in the original analyses, the proportions of silica present as quartz and of that present in a combined state. If I had done this it would have been seen that the earths contained but little combined silica and a relatively high proportion of alumina presumably present in the state of hydrate. For instance, in the Tumatumari sample which I collected myself from a deep cutting in the laterite lying on the diabase of the Tumatumari cataracts a few feet only above the surface of the unaltered rock, out of 51·76 per cent. of silica 49·35 is in the form of quartz, leaving 2·41 per cent. in the combined state in the presence of 24·55 per cent. of alumina.