Studies in Tropical Soils II. Some Characteristic Igneous Rock Soil Profiles in British Guiana, South America

Abstract

1. Five typical British Guiana soil profiles, derived under humid tropical conditions from igneous rocks of different silica contents, are described, and the results of their detailed petrographical, chemical and ecological examination are presented and discussed.2. Each kind of rock has produced a sandy, acidic, nutrient-deficient surface soil, but the katamorphic changes that have occurred during rock alteration, weathering and leaching have been fundamentally different.3. The basic and intermediate igneous rocks have given rise within the zone of alteration to “primary gibbsitic laterite”, covering the parent rock in a thin crusty layer. Within the zone of secondary changes, the laterite has been resilicated, presumably through the agency of ground water containing soluble silica or alkali silicates (partially in the more basic rock profiles, and completely in the intermediate types), into kaolinitic earths, coloured yellow and red by hydrous iron oxides. Residuary gibbsite and iron oxides have accumulated in the more basic profiles as concretionary gravelly masses.