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



in the world. And yet it is safe to assume that to the casual reader 

 who has not visited the country the above publications present little 

 else than a mass of bewildering detail. 



" For reasons which can be readily understood, geological inquiry in Western 

 Australia has up to the present consisted chiefly of a series of unconnected 

 observations to the co-ordination of which we must look to the future." ^ 



One reason for this, as outlined below, is the difficulty of field 

 geologj'. A more important reason is the poor measure of comparative 

 petrological study hitherto attempted, and the failure to use those 

 principles of classification that have been worked out in areas of 

 crystalline schists in Europe and America. The object of this paper 

 is to give the main results obtained by the writer's petrological studies 

 and to put forward as a working hypothesis a classification of the 

 rocks which will serve as an introduction to a detailed discussion of 

 each group in subsequent papers. 



II. DiFPICULTIES OF FlELD GeOLOGT IN WESTERN AuSTEALIA. 



The greater part of the State possesses an arid, if not a desert, 

 climate, and shows, in consequence, surface features very different 

 from those familiar in more humid regions. The most striking of these 

 is the poor development of drainage systems in the whole region south 

 and east of the Murchison River. On most maps there are no rivers 

 shown within this area except in the immediate coastal districts, but 

 large and small lakes are depicted in great profusion.* These lakes on 

 inspection prove to be for the most part mere sandy depressions almost 

 devoid of water except for a few weeks after a downpour of rain, and 

 at other times containing only isolated pools of saline water. They 

 are not, however, disconnected basins, for they pass at each end 

 into dry vallej'S, and are clearly relicts of a former drainage system. 

 Topographical maps of the heai't of the country do not exist, so 

 that it is at present impossible to reconstruct the earlier system 

 and show how the lakes are connected. So flat are the present 

 valley bottoms that it is difficult at times to say which way the 

 present drainage runs. 



The general surface of the country is remarkably monotonous. 

 Low ridges and valleys follow one another like the waves of the 

 sea. The valleys are rarely V-shaped or gorge-like, and most often 

 show curves approaching those of the vertical section of a saucer. 

 Only rarely does a dome-shaped mountain rise above the general level 

 of the plateau. 



There is a large body of evidence to show that the present surface 

 has originated by the filling in of the valleys from one much more 

 deeply dissected. Not only is this suggested everywhere by the cross- 

 sections of the valleys and the alignment of the lake-basins, but it is 

 proved in many localities by the mining of deep leads to some 

 hundreds of feet below the surface of the present valley bottoms 



^ A. Gibb Maitland, Ann. Eep. Gaol. Surv. Western Australia for 1910, 

 Perth, 1911, p. 12. 



- Cf. H. P. Woodward, "The Dry Lakes of Western Austraha " : Geol. 

 Mag., Dec. IV, Vol. V, p. 363, 1897. 



