Prof. W. G. Woolnough — Laterite in W. Australia. 389 



material, are covered with crusts of salt and gypsum. The ridges 

 are, to a large extent, very rocky, and are frequentlj-, but not always, 

 capped by lateritic material. This laterite is again very different in 

 general aspect from that which mantles the western zone of the 

 Darling Plateau. It is not decidedly pisolitic, but is rather cellular 

 and cavernous in structui-e. Nevertheless, the effects of concretionary 

 action are abundantly apparent throughout, and it shows every 

 evidence of a mode of origin generally similar to that insisted on by 

 Simpson. The author is by no means so familiar with these eastern 

 areas, distant from Perth 200-400 miles, as he would like to be, but 

 quite numerous traverses of the area liave been made. In theAvestern 

 part of the Goldfields Belt, near Southern Cross (205 miles E.N.E.), 

 for instance, the laterite in many cases, if not always, lies directly 

 on the surface of the crystalline rocks and schists, without the 

 intervention of any extensive layer of thoroughly leached pipeclay. 

 The rock, however, is deeply weathered, and the lower parts of the 

 laterite crust represent an impregnation of the very much rotted 

 original rock. 



Further east, again, for instance, at Coolgardie (220 miles E.N.E.), 

 there is a partial return to the conditions of occurrence met with in 

 the Darling Plateau, The "Red Hill" in this town is a very typical 

 laterite-capped butte, strongly concretionary rock resting on a well- 

 leached substratum, but one which is much more ferruginous than 

 that wliich is characteristic in the extreme west. Throughout the 

 Eastern Goldfields, so far as I have been able to observe, there is very 

 little tendency to laterite formation on anything but the basic rock 

 types. Granites, quartzites, or siliceous schists are entirely free 

 from laterite coverings. In these eastern areas laterite cliffs often 

 constitute what are known as " Breakaways ", which form the shores 

 of the salt lakes. 



This brief and inadequate outline of the physiographic conditions 

 under which the laterite is distributed in the south-western portion 

 of Western Australia, indicates that the problem of its formation is 

 more complicated than Simpson has shown. The author is wholly 

 in accord with him with regard to the chemistry of the process of 

 laterite formation, namely, by leaching of the soluble constituents of 

 the subsoil, transportation of the materials in solution to the surface 

 by capillarity, and precipitation of certain of the dissolved matters 

 there, through alternate saturation and desiccation of the subsoil 

 consequent on seasonal alternation of extremely wet with intensely 

 dry seasons. Simpson, however, appears to believe that the laterite 

 may have formed 07i the surface of the Darling Peneplain, and it is on 

 this point that I disagree with him. The drainage of the area is far 

 too perfect at the present day to admit of the upward leaching of 

 solutions to any considerable extent. The rapid fall in the level of 

 water in wells sunk through the laterite capping on the plateau, 

 even at very considerable distances from the nearest deep valley, 

 indicates the effectiveness of lateral drainage under existing 

 conditions. In November and December, 1916, the author noted 

 a fall in the level of one such well (at 50 miles on the Perth-Albany 

 road) of upwards of 30 feet in six weeks. 



