In many respects the extensive bauxites of the lower Amazon basin and Guianas coastal plain are similar. Being platform-type deposits they are confined to areas which have been tectonically stable over a prolonged period of time, despite periodic minor epeirogenic uplift and crustal warping. Detailed field and laboratory studies of the profile sections suggest that their origin was complicated and that they were in fact polygenetic in origin. It is suggested that an initial period of intense bauxitization in late Tertiary times was followed by a rise in base levels with deposition of Quaternary sediments. Conditions at this stage (notably in the Amazon basin) were more humid and ground-water podzols were formed, these now being represented by hard ferruginous "laterite" horizons. Subsequently these regions were subjected to renewed epeirogenic uplift and, as the existing drainage patterns became more incised, a period of limited secondary bauxitization ensued. However, some restricted areas continued to experience limited podzolization. Amazon bauxites in particular share many features in common with other Tertiary platform-type deposits notably in northern Australia. Their origins were broadly similar, although this factor is sometimes obscured by the effects of physical erosion.