IV.—On the Constitution, Origin and Dehydration of Laterite

Abstract

The objects of this note are three: (1) to call attention to the essential chemical similarity between laterite and bauxite; (2) to offer a theory for the formation of laterite in the moist tropics; (3) to suggest an explanation for its spontaneous dehydration.(1) Laterite has generally been referred to as a ferruginous clay; but if the term clay is restricted to substances having a basis of hydrous silicate of alumina, this definition is incorrect. The alumina in laterite exists, as it does in bauxite, in the form, of hydrous oxides. Kaolin must thus be removed finally from the list of weathering products; it is formed generally, perhaps exclusively, by the action of subterranean vapours on aluminous silicates. This conclusion necessitates a re-examination of many so-called argillaceous substances; many doubtless include the kaolin already existing in the kaolinized aluminous silicates before they are exposed to the weather. But it is probable that some of the red clays of past geological ages, formed under subaerial conditions, contain free hydrous oxides of alumina; and for those that are shown to contain hydrous silicate of alumina, it would be well to test the possibility of a secondary reunion of aluminic hydrate and free silicic acid.(2) To account for the fact that an aluminous silicate undergoes a more complete disintegration under tropical conditions than under the deep-seated and presumably high-temperature conditions of kaolinization, the writer suggests that laterite is due to the agency of lowly organisms, possibly akin to the so-called nitrifying bacteria.