IV.—On the Formation of a Laterite from a practically Quartz-free Diabase

Abstract

After I had forwarded the paper on “The Residual Earths of British Guiana commonly termed ‘Laterite’”, published on pp. 439–52, 488–95, and 553–62 of the GeologicalMagazine for October, November, and December, 1910, I visited the neighbourhood of Christianburgh and Akyma on the Demerara River in British Guiana and examined a small long-deserted quarry in a very fine-grained diabase intrusive in hornblende - granitite - gneiss near the latter pluce. I obtained from the lateritic earth lying on the mass of unaltered diabase a boulder of that rook in which the central parts were ideally fresh, whilst the outer parts were decomposed into laterite. I separated these parts with great care into an outer red one, apparently an ordinary ferruginous laterite, and an inner one, buff-coloured with whitish spots in it, which was in actual contact with the unaltered rock. When I obtained the boulder these crusts were quite soft, but after a few days' exposure to the air they hardened considerably.