152 J. A. Thomson — Bocks of Western Australia. 



can hardly be doubted, however, from the numerous references to 

 extensive tracts of gneiss and gneissic granites outside the actual 

 mining fields of the State that there is a fundamental system of 

 Arcliaean gneisses similar to that of most other parts of tlie world, 

 and probably of earlier age than the auriferous series. Seeing that 

 so many of tlie granites are known to be intrusive into the auriferous 

 series, it is remarkable that the distinction between newer granites 

 and older gneisses has not been more clearly held in view by the 

 officers of the Geological Survey of the State,' particularly as the 

 former are intimately connected with tlie gold occurrences, and 

 the latter, except where intruded by later granites, are free of 

 all economic minerals. 



The Auriferous Series. — The rocks of this series are wonderfully 

 uniform both in original petrological nature and in their present state 

 of alteration over the whole State, and it is a perfectly justifiable 

 assumption to regard them as of similar age. A large series 

 of reliable chemical analyses has been made of them, but the 

 accompanying petrological study has been very meagre, and vitiated 

 by a lack of comparative studies. In most of the Bulletins of the 

 Geological Survey it is briefly stated that the rocks are of the genei-al 

 type common to the goldfields of the State, and they are divided into 

 massive and foliated varieties, and again into coarse-grained and 

 fine-grained. Excellent petrological studies were carried out by 

 Vogelsang^ in 1897, and he showed that some at least of the 

 amphibolitos had originated from 'diabases'. Yet in spite of this 

 and of the numerous analyses, the opinion is still occasionally put 

 forward that the series may be of sedimentary origin. Vogelsang 

 has, however, generally been followed by Simpson and Gibson, who 

 have described more massive varieties as epidiorite, diorite, and 

 amphibolites after ' diabase ' and pyroxenite, and chloritic and 

 sideritic rocks derived from amphibolite. No attempt has been made, 

 except in Kalgoorlie, to map the various members of the series 

 separately. It is greatly to be hoped, now that a petrologist has been 

 added to the staff of the survey, that this will be attempted in all 

 the principal goldfields, for otherwise the geological maps produced 

 are practically valueless to the mining community. 



The banded jaspers or ' ferruginous quartzites ' which penetrate 

 the amphibolites and hornblende schists of the auriferous series are 

 generally admitted to be of the nature of lode formations or bands of 

 highly replaced rocks. That they are anterior to the main deposition 

 of the gold is also generally recognized. 



The Granites and accompanying Dykes. — The granites generally form 

 the boundaries of the auriferous series, and have seldom, therefore, 

 been completely mapped. The smaller intrusions appear to be boss- 

 shaped, while the larger ones are aligned in ]N'.-S. bands. At the 

 junction the auriferous series is almost invariably represented by well- 

 foliated hornblende schists, whose foliation is parallel to the line 



^ Witness the practice of depicting granites known to be intrusive as ' Gn.' on 

 the geological maps. 



* K. Schneisser and K. Vogelsang, Die Gold/elder Australasiens. English 

 translation. The Goldfields of Australasia, H. Louis, London, 1898. 



