392 Prof. W. 0. Woolnough — Laferite in W. Australia. 



of laterite on their alluvial terraces in the neighbourhood of the 

 highlands. This is markedly the case, for instance, with the Preston 

 River at Donnybrook (210 miles S.). It is quite possible that 

 laterite may be actually forming at this and similar points, and the 

 formation may be explained as follows : — 



Most of the streams in question are fed by the subsoil drainage 

 which flows beneath the laterite capping of the plateau. Their 

 Avaters are, even in the wet season, hard and somewhat mineralized. 

 As the di-y season advances they all, with few exceptions, become 

 unpleasant for drinking purposes. Many of them deposit iron 

 abundantly, and give rise to iridescent films of iron oxide on the 

 surfaces of pools. These films have been mistaken repeatedly for 

 indications of petroleum. "When the active flow of the streams 

 ceases early in the summer, capillary action through the porous 

 alluvium of the terraces may induce the upward concentration of the 

 dissolved salts and give rise to normal laterite. It is to be noted 

 that some of these river-terrace laterites are much more like the 

 ordinary plateau laterite than are the detrital laterites. For this 

 reason it may be advantageous to recognize a third river-terrace type 

 of laterite in addition to the two varieties (solid high-level type and 

 secondary detrital type) defined by Simpson. This mode of occurrence, 

 if it is correctly iindei'stood by the author, may possibly explain the 

 very common occurrence of laterite in the breakaways on the shores 

 of salt lakes in the Eastern Goldfields. 



The author doubts whether the occasional torrential rains of the 

 arid areas are competent to produce laterite as supposed by Simpson. 

 The latter authority (ioc. cit., p. 401) mentions falls of 3-30 inches of 

 rain at Mulline in one day and of nearh' 4 inches at Coolgardie in 

 two days. It is the universal experience that such torrential down- 

 pours cause comparatively little saturation of the soil. Even in very 

 porous sandy areas the proportion run off to soakage is very high, and 

 it is extremely doubtful whether the cycle of events necessary for 

 laterite formation could follow such sudden downpours as those 

 mentioned. Matters are quite otlierwise when the rainfall is seasonal 

 in character, as is very typically the case in the areas nearer the coast. 



To explain the heavier laterization of the goldfields than of the 

 wheat belt I would suggest that the question must be referred back 

 to the previous geographic cycle. It has been shown that there has 

 been a net uplift of the Darling Peneplain of nearly 1,000 feet on the 

 west, and probably of considerably more on its eastern side. If the 

 theory of laterite formation under low altitude conditions is correct 

 it follows that the goldfields laterite must have been formed when 

 the land stood much lower than it does now. 



The presence of marine fossils at peneplain level at Norseman 

 (350 miles E.S.E.) indicates that, at no very distant epoch, the sea 

 extended much further inland than the present head of the Great 

 Australian Bight. It is well known that in comparatively recent 

 geological time the climate of Central Australia was much moister 

 than it is at present, and it is reasonable to suppose that a much 

 increased humidity was experienced in the Goldfields area. Under 

 these circumstances it is verv easv to account for the extensive 



