136 president's address — section c. 



The geological structure of Kalgoorlie is not of that extreme 

 simplicity which at one time had been anticipated. The staple forma- 

 tion consists, as at Norseman, largely of certain schistose rocks, some 

 of which are distinctly of sedimentary origin. The sedimentary beds 

 are represented by rocks, which range from soft shales to jasperoid 

 slates, grits to flinty quartzite, fine conglomerates or breccias to a fairly 

 coarse boulder conglomerate. 



In intimate association with the sedimentary beds are a series 

 of hornblendic rocks ; but whether these occur in the form of lava- 

 flows, or are intrusive, is as yet by no means clear. There are, however, 

 in addition ■^o these, certain undoubtedly intrusive igneous rocks, 

 sodafelsite, and porphyrite. 



Most of the rocks of Kalgoorlie appear to have been highly altered 

 by dynamical causes, with an accompanying recrystallisation of their 

 constituents, many of them have been carefully analysed and micro- 

 scopically investigated in the laboratory of the Geological Survey and 

 the results made available. Time will not admit of detailed reference 

 being made to the deductions to be drawn from these investigations, 

 beyond the fact that the " lode formations," for which the field is famed, 

 consist of a series of almost vertical banded schists of lenticular habit, 

 and apparently owe their origin to the dynamo-metamorphism of 

 a plagioclase-augite rock. Many of these ore-lenses are of great length, 

 and in some cases of considerable breadth ; at times, however, the 

 lateral continuity of the lenses is interrupted by overthrust and normal 

 faults of very variable downthrow. 



For a systematic study of the metasomatic history of the auriferous 

 deposits, leading as such must do to the discovery of facts which will 

 materially guide scientific mining, Kalgoorlie offers few, if any, rivals 

 in Australia. 



So far as observations have, however, at present been carried, 

 there do not appear to be any scientific reasons for believing that the 

 mines of Kalgoorlie have by any means reached the limits of ore depo- 

 sition, or that the lodes will not prove, as a whole, productive in 

 depth. 



The Pilbarra district (lat. 21°) affords better and more continuous 

 sections than are generally to be met with in any of the other districts 

 which have been examined. It thus reveals geological structures 

 which are not to be found in the more southerly districts, and on this 

 account serves to throw light upon obscure points in connection with 

 the geology of other fields. These sections furnish very important 

 evidence regarding the terrestrial movements to which these older 

 rocks have been subjected. The Pilbarra district contains very large 

 areas of granite, granodiorite, and gneiss, which, however, are not the 

 oldest rocks. In every case where their relation to the schists can be 

 observed along the margin it is seen that these granites are intrusive, 

 having gradually eaten its way into and partially absorbed them, and 

 several sections may be noticed in which the granite sends out tongues 

 and veins into them. One of the best localities in which the relation 

 of the granite to the metamorphic rocks may be seen is at the Wodgina 



