154 president's address — section c. 



carried down to a depth of 412ft., and passed through nothing else but 

 limestone — the Eucla limestone. The sequence of strata in the Western 

 Australian bores coincides in its essential particulars with that indicated 

 by the South Australian bores, and there can be very little doubt as- 

 to the identity of the two series of beds, whatever may be their age. 



No mention of the recent advances in Western Australian geolog}^ 

 would be complete without some reference to that extensive develop- 

 ment of residual deposits which have been found over the whole length 

 and breadth of the State. 



The term " laterite " has been officially adopted, though in a 

 somewhat more extended sense than its original application, for all 

 the deposits resulting from the decomposition and re-consolidation 

 of rocks in situ. 



The laterites of Western Australia consist largely of hydrated 

 oxide of iron and alumina, producing, on the one hand, deposits of 

 excellent iron ore, and, on the other, bauxite. In some parts of the 

 State the deposition of secondary silica in the lateritic deposits pro- 

 duces what are practically quartzites ; these, by an increase in the 

 ferruginous coloring matter, pass into a jasperoid form of laterite. 

 There are thus three forms of these laterites : an aluminous, a ferru- 

 ginous, and a siliceous, the composition being liable to vary con- 

 siderably over a small area, it being largely governed by the nature 

 of the underlying rocks. The structure is sometimes massive, and 

 almost homogeneous, but is more frequently pisolitic and nodular, in 

 which case the concretions are richer than the interstitial matter. 



The lateritic deposits naturally vary in their lithotogical characters. 

 They are often very porous, and weather into caverns and cavities of 

 all sizes. The surface of the rock is often covered with a glaze of 

 hydrated oxide of iron. When freshly broken the rock represents a. 

 mottled appearance, owing to the different shades of brown, yellow, and 

 red. The rock passes gradually into the underlying rocks without any 

 sharp line of demarcation. That ferruginous and siliceous laterites 

 are more commonly met with is due to the fact that deposits of this- 

 type are better able to resist disintegrating influences than the softer 

 varieties ; they thus not only remain themselves, but act as a protecting 

 cover for the rocks beneath. 



Mr. J. Beete Jukes, writing in 1850, in his almost classic " Sketch 

 of the Physical Structure of Australia," mentions the occurrence of 

 these lateritic deposits — as seen by him in the country between Perth 

 and York. He says : — " For a few feet below the surface the rock 

 was a singular concretionary ferruginous compound, which looked 

 like a clay or sandstone, that, being highly ferruginous, had formed 

 itself into a mass of small balls and irregular concretions of a black 

 oxide of iron or hematite. Below this ironstone (which is its nam& 

 in the colony), wherever the rock was exposed it appeared for many 

 miles to be granite, or some granitic compound." 



In another place he mentions, as occurring in one of the lateral 

 valleys of the Swan Eiver, " A thin capping of ironstone, forming a 

 line of small crags." 



