400 E. S. SinijiSOH — Laterite in Weniern Australia. 



subject from the point of view of one having an intimate knowledge 

 of Indian laterite, but not a first-hand acquaintance with those of 

 other parts of the world. The present writer, during the last fifteen 

 years, has devoted much time to the study of the laterite of extra- 

 tropical Western Australia, both in the field and in the laboratory, 

 and is therefore in a position to supplement some of the work of 

 Dr. Fermor, whilst inclined to diifer from him in some of his 

 deductions. 



Broadly speaking, the laterite of Western Australia maj- be divided 

 into two classes — 



(1) Primary Laterite (true laterite, high-level laterite), formed in 

 situ out of soluble material derived from the weathering rock 

 immediately underlying it. 



(2) Secondary Laterite (lateritite, low-level laterite), composed 

 largely of the mechanically transported fragments of primary laterite. 



The study of the latter being dependent on and of secondary 

 importance to the study of the former, it is the primary laterites 

 which have been most closely examined, and vrill be dealt with 

 almost exclusively in this article. 



Primary laterite in AYestern Australia, and probably elsewhere, is 

 a product of normal processes of weathering, accompanied by 

 abnormal conditions of rainfall and denudation. It is not necessarily 

 confined to tropical areas : immense deposits in this State are extra- 

 tropical. It is simply confined to areas underLaid by suitable rocks, 

 and subject to all three of the favourable conditions just mentioned. 



By " suitable rocks " is meant rocks composed largely or wholly of 

 metallic silicates, especially in this State, granite and greenstones of 

 common types, as well as amphibolites, epidiorites, chlorite-schists, 

 and other similar rocks. 



By "normal processes of weathering" is meant the attack of the 

 outcrops of such rocks with rain-water carrying atmospheric 

 carbonic acid, the result being the formation of kaolin (crystalline 

 AlsOj. 2Si02. 2 H:2 0),halloysite (amorphous A LOg. 2Si02 .'2H2O), 

 and quartz or hydrous silica, together with solutions of aluminium 

 hydrate, titanium hydrate, ferrous carbonate, calcium carbonate, 

 magnesium carbonate, manganous carbonate, etc. 



The abnormal conditions of rainfall tending to produce laterite are 

 such that the weathering rocks and any products are alternately 

 saturated with water and then again completely desiccated. 



A slow rate of denudation is essential. 



Primary laterite is a true efloresce7ice, that is, a deposition on the 

 surface of the ground of substances dissolved by underground water, 

 brouglit to the surface by capillarity, and there deposited as solid 

 matter owing to aeration and evaporation of the water.^ 



Primary laterite in Western Australia, wherever sunk through, is 

 found to overlie an almost pure pipeclay, and this in turn a crystidline 

 rock. Dr. Fermor holds kaolinization to be a process incompatible 

 with laterite formation.'^ In Western Australia the processes are 



^ Vide E. S. Simpson, Bull. 6 of the Geol. Surv. of Western AustraUa, 

 pp. 39, 78, 79. Published 1902. 

 ' Geol. Mag., 1911, p. 4-59. 



