E. S. Simpson — Laterite in Western Australia. 401 



contemporaneous and inseparable from one another. Dr. Termor 

 writes thus : — 



" When a rock breaks down into a day hydrated aluminium silicate 

 is to be regarded as the pure end product, all oxides being removed 

 in solution. When a rock is converted into laterite, on the other 

 hand, the reverse holds ; aluminium and other silicates are decomposed, 

 and the silica is removed in solution, presumably in the colloidal 

 form, whilst the oxides of iron, aluminium, titanium, and manganese, 

 which were relatively soluble under the clay-forming conditions, are 

 relatively insoluble under laterite-forming conditions. The oxides of 

 calcium, magnesium, sodium, and potassium are apparently soluble 

 under both sets of conditions. I do not propose to advance here any 

 reasons to account for these two diverse modes of alteration of 

 rocks. . . ." 



Such an apparently paradoxical postulate seems to require detailed 

 explanation on the part of the author. Surely Dr. Fermor is putting 

 the cart before the horse Avhen he speaks of the concentrated 

 accumulation of lateritic material as due to the leaching away of the 

 non-lateritic products of Aveathering. The formation of an ordinary 

 efflorescence of salt on the surface of salty ground, or of an efflorescence 

 of gypsum or potassium vanadate on the surface of a brick are 

 phenomena identical with the formation of laterite, but one does not 

 explain them by saying that in the one case the insoluble sand and 

 clay of the soil is dissolved away, leaving an accumulation of insoluble 

 salt, or that in the other case the baked clay of the brick is dissolved 

 away, leaving an accumulation of gypsum or vanadate of potash. 



The same remarks, indicate the absurdity of referring to a laterite 

 as a true residual, or as a replacement of a rock, i.e. a deposit 

 accumulating in the actual space originally occupied by the solid rock 

 which has yielded the materials which compose the laterite. The 

 primarj^ laterites of Western Australia are deposited, like other 

 efflorescences, outside the surface of the parent rock. Two typical 

 sections are shown in the accompanying figure (p. 402). 



Reverting to the question of the distribution of rainfall favourable 

 to the formation of laterite, the extra-tropical portion of Western 

 Australia may be divided into two provinces. One of these lies 

 within 50 to 100 miles of the western coast, where the average 

 annual rainfall is between 20 and 40 inches, distributed between 

 well-defined wet and dry seasons. Typical laterite localities in this 

 area are Mundaring and Greenbushes, the rainfall of which are — 



Mundaring. Greenbushes. 



Inches. Inches. 



Average annual rainfall ... 41 37^ 



Average November to March . 2J 3 



Average April to October . . . 382 ^^i 



The other province includes the Eastern Goldfields, and has no 

 defined annual wet and dry seasons. The average rainfall is between 

 -7 and 1 2 inches, one - quarter of which frequently falls in 

 a single day, the usual intervening climatic conditions being intensely 

 arid. As typical examples, Mulline, with an average of 11 '2 inches, 

 had a fall of 3-3 inches on one day in 1907 ; Coolgardie, with an 



DECADE V. — VOL. IX. — NO. IX. 26 



