Prof. W. G. Woolnougli — Laterite in W. Australia. 393 



laterite formations of the region. The wheat belt was probably 

 sufficiently distant both from the known coastline on the west and 

 from the problematical coastline on the east to experience so light 

 a rainfall as to preclude extensive laterite formation of the normal type. 



Before leaving the subject of origin of laterite it maybe of interest 

 to point out that somewhat similar formations abound in Australia 

 under conditions pointing very conclusively to conditions of formation 

 identical with those laid down by Simpson for the Western Australian 

 laterite. 



In South Australia, where eruptive rocks are comparatively rare, 

 and where marine sediments and schists predominate, the place of 

 laterite is taken by almost ubiquitous " travertine", an impure lime- 

 stone with highly perfect concretionary and pisolitic structures 

 encountered under conditions quite similar to those of the western 

 laterite. 



Throughout the area occupied by the highly siliceous Upper 

 Cretaceous Desert Sandstone formation in Central Australia, very 

 widespread concentration of silica has followed a course identical 

 with the concentration of lateritic materials. The result has beeii 

 extensive opalization of the sandstone and the formation of 

 porcellanites and quartzites. 



Particularly in the wheat belt and goldfields of Western Australia 

 it is usual to find granite outcrops " case-hardened " to a depth varying 

 from a few inches to several feet. This phenomenon is evidently due 

 to a superficial concentration of materials derived from the somewhat 

 soft and crumbly internal portion of the rock, and it is to be explained 

 in the same way as the production of laterite. This "case-hardening" 

 of granite outcrops is an important factor in the production of the 

 " gnamma-holes " or natural tanks in which so much of the scanty 

 water supply of the arid interior is conserved. 



SUMMAET. 



The author is of opinion that — 



1. Laterite in Western Australia is formed by the leaching of 

 subsoil salts during seasons of heavy rainfall, and capillary attraction 

 of the solution to the earth's surface during intervening dry periods; 

 the dissolved matter being deposited in a concretionary fashion in the 

 surface layers of the soil. 



2. Laterization can occur only in areas where drainage is almost 

 at a standstill. This usually involves the existence of a peneplain 

 almost at sea-level. 



3. High-level laterite is a criterion of elevation of the land. 



4. Difference in laterite level suggests faulting, which can often 

 be proved by collateral evidence. 



5. Outstanding differences of opinion with regard to broad features 

 in the physiography of Western Australia may be reconciled by 

 recognition of the essentiall}' low-level nature of laterite. 



