J. A. Thomson — Rocks of Western Australia. 149 



(e.g. Kanowna). The material filling the valleys is not, however, all 

 or mostly of an alluvial nature. The sands which cover the lake-beds 

 are largely wind-blown, and the same is true for much of the coarser 

 material in the upper parts of the valleys. The whole area is swept 

 by strong cyclonic winds, which convey the material broken by the 

 usual agencies of desert erosion into the nearest hollows, while the 

 finer sand is driven on to the lowest depressions, the lake-beds. 



The arid climate has influenced the surface features in yet another 

 way. The ground water level is deeper than is usual in more humid 

 regions, and consequently oxidation of the rocks has proceeded to 

 greater depths. This is especially the case in heavily mineralized 

 belts, where the production of sulphuric acid from the sulphides 

 has given the descending waters stronger powers of attack. The 

 result is the rotting away of large bodies of rock and an increased 

 eflSicacy of wind erosion. Side by side with the destruction of the 

 solid rock has gone on a reconstruction of surface deposits of various 

 kinds — laterites, surface quartzites, 'cement,' and 'cement gravel'. 

 The laterites are predominatingly aluminous where resting on granite 

 or gneiss, and ferruginous where resting on basic rocks. Owing 

 to their hard nature, they now form the caps of many of the smaller 

 hills and ridges. The 'cement' consists of oxidized rock fragments 

 bound together by a calcareous matrix. The ' cement gravel ' 

 consists of rounded or ellipsoidal balls of reddish - white material, 

 which when broken shows a concentric structure. It appears to 

 consist mainly of calcite, but whether the balls have formed by 

 accretion or by replacement is not quite certain. The ' cement 

 gravel ' is equally abundant on the sides of the ridges and in the 

 valleys. 



In consequence of the large areas occupied by the clastic materials 

 filling the valleys, the deep-going oxidation of many of the rocks, 

 and the widespread covering of surface deposits, the opportunities of 

 examining the solid rocks are greatly curtailed. The rock, if it may 

 be called rock that is at the same time hardest and least subject to 

 oxidation, is jasper, and this forms the summits of many of the ridges. 

 Other ridges are formed of practically unoxidized amphibolites or 

 serpentines. Geological mapping is only possible by the acquisition 

 of a local knowledge of the nature of the oxidized products of known 

 rocks, and by inference from the surface deposits and the plant 

 cecology. The study of rock contacts is seldom possible. Under all 

 the circumstances it is no matter of surprise to find that our 

 knowledge is practically confined to the immediate vicinity of the 

 goldfields, where the underlying rocks are exposed by mining and 

 prospecting operations. 



III. General Statement of the Problem:, and Discussion of 



PeEVIOTJS LlTERATtTEE. 



With few exceptions the gold-bearing deposits are enclosed within 

 basic schistose rocks, which are foliated in a general north-south 

 direction and are highly inclined. The direction of foliation varies 

 from N.E.-S.W. to N.W.-S.E. The schistose rocks alternate with, 

 or enclose, lenticles of more massive rocks of similar composition. 



