630 BE. J. A. THOMSON ON THE [Dec. I913, 



is given to those which betray their origin most clearly. Many 

 of the rocks are best described as being of a nondescript 

 character. 



The comparisons made with similar rocks occurring elsewhere 

 in Western Australia are based mainly on the study of specimens 

 kindly presented by Mr. Gibb Maitland, many of which were also 

 analysed by the officers of the Survey. They show very clearly the 

 general similarity of the massive amphibolites of the different gold- 

 fields, and lead to the hope that each field will prove capable of 

 resolution into its original constituent rocks after careful peno- 

 logical study. 



The term greenstone is used throughout for rocks in which 

 chlorite and carbonates predominate to the exclusion of hornblende 

 and epidote or zoisite — that is, it is used in contradistinction to the 

 term amphibolite and in the sense of the German griinstein. 

 This is, no doubt, a restriction of the general usage; but it has 

 this justification, that it obviates the necessity for a new term,, 

 and meets a definite need. It is only in rare cases that it is not 

 possible to decide at once, on an inspection of the hand-speeimen, 

 whether a given rock is an amphibolite or a greenstone, and hence 

 the wider use of the latter term is unnecessary. The Kalgoorlie 

 greenstones are not mere surface-modifications of the amphibolites, 

 but retain their characteristics to the greatest depth to which mining- 

 has been carried : that is, over 2500 feet. The distinction between 

 amphibolites and greenstones (as here restricted) is of the utmost 

 economic import in "Western Australia. 



(a) The Sediments. 



There is some difficulty in obtaining specimens of the sediments 

 suitable for microscopic study. In the first place, the rocks them- 

 selves, with the exception of the conglomerate-bands, are seldom 

 exposed at the surface. As they have not proved auriferous, but 

 little exploratory work has been done on them, and there is an 

 absence of dumps from which fresh material may be obtained. 

 Moreover, in the immediate neighbourhood of Kalgoorlie, chert}-, 

 slaty, and graphitic rocks of undoubtedly igneous origin are so 

 abundant, that one hesitates to accept any rocks of similar appear- 

 ance as sedimentary. The processes of alteration have so far obscured 

 the peculiarities of the igneous rocks that no microscopic criteria 

 exist for distinguishing the two groups. 



Where undoubted sedimentary rocks crop out, as in the ridges 

 south-east of Kurrawang, they are of much coarser texture 

 than the slaty varieties above mentioned, and vary in character 

 from grits to coarse conglomerates. At the surface, they are 

 generally reddish or purple from the presence of oxide of iron. 

 Sections show that the grits consist chiefly of quartz and felspar 

 in subangular or elliptical grains, with their longer axes parallel.. 

 The felspars belong both to orthoclase and. to acid plagioclase, 

 and along with quartz often show strain-shadows. There are, in 

 addition, similarly-shaped areas of cataclastic quartz and felspar.. 



