E. 8. Simpson — Laterite in Western Australia. 403 



hardened. Such conditions seem to me to favour, if not indeed to be 

 necessary to, the accumulation of laterite. 



It would appear that the accumulation of latei'ite is a very slow 

 process, and cannot take place where erosion is moderately rapid. It 

 tends, therefore, to grow on well-defined peneplains, such as the 

 1,000 feet Darling Ilange peneplain, and on the upper very gentle 

 slopes of valleys, where denudation is slow. Bare rock, clay, sand, 

 and secondary laterite carrjang sand and clay are found at lower 

 levels. In Western Australia primary laterite is found chiefly 

 between. 500 and 1,500 feet above sea-level. Higher altitudes than 

 1,500 feet are rare within the extra-tropical parts of the State. 

 They are usually devoid of laterite. 



The climatic conditions favouring its formation and preservation 

 -having been dealt with, the details of its growth need elucidating. 

 It has already been stated that laterite is an efflorescence resulting 

 from normal weathering, the first stage in the process being the 

 conversion of felspars into kaolin and the partial saturation of the 

 sub-surface water with carbonates of iron, manganese, lime, and 

 magnesia, with hydrous silica, titania, and alumina, and finally with 

 alkali salts. With the advent of dry weather this solution begins to 

 evaporate at the surface, and as it evaporates further supplies are 

 brought to the surface by capillary action. A slow but appreciable 

 movement of water from below to the surface goes on till the 

 weathered rock is desiccated. Mere contact of this water with the 

 atmosphere is sufficient to precipitate all the iron and manganese as 

 hydrated peroxides, two important laterite constituents. Evaporation 

 increases the speed of this precipitation, and at the same time leads to 

 the precipitation of all the other substances dissolved in the water. 

 Alumina and silica in the proportions Alo Og to 2 Si Og are co-precipitated 

 as colloidal halloysite (lithomarge of Fermor), A.\ Og . 2 Si Og . 2 H3 O. 

 An excess of silica over this proportion seldom if ever occurs in the 

 water, an excess of alumina is precipitated as hydrate, probably 

 primarily as gibbsite, AI3 Og . 3 Hg 0. Titanium is precipitated as 

 metatitanic acid, Ti Oo . x Hg 0. During the growth of the laterite 

 small amounts of sand and clay become mechanically included. 



All the important constituents of laterite are thus accounted for. 

 But in addition to these the evaporating water must have deposited 

 at the same time alkali salts and carbonates and sulphates of lime and 

 magnesia, compounds occurring only in traces in normal primary 

 laterite. The history of the rock is evidently not yet complete. It 

 must be continued over the dry season, during which the typical 

 laterite materials are rendered practically insoluble by consolidation 

 and desiccation, into the succeeding wet season. The soluble alkali, 

 lime, and magnesia salts are then soon taken up by the rain-water 

 and carried off in the streams. The laterite has then reached its 

 final composition. 



In structure West Australian laterites are almost always pisolitic, 

 the typical form of an amorphous substance slowly growing by 

 precipitation from solution. The nodules vary in size from that of 

 a pin's head up to about 1 inch in diameter, the average being one- 

 eighth to one-quarter of an inch. The spaces between the nodules 



