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 Geological Magazine 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.