Vol. 69.] PETROLOGY OF THE KALGOORLIE GOLDFIELD. 631 



Next in abundance is biotite, partly in stout flakes irregularly dis- 

 tributed, partly in thin plates with a well-marked parallel structure. 

 It appears to be of authigenous character. A fair amount of 

 chlorite seems to result from the alteration of the biotite. A little 

 epidote, carbonates, and iron-ore complete the list of minerals. 

 The rocks have evidently been subjected to a considerable amount 

 of shearing. 



The matrix of the conglomerates is of similar composition, and 

 is distinctly schistose. The enclosed pebbles consist chiefly of 

 haematite-jaspers, liver-coloured quartzites, and quartz- and albite- 

 porphyries. The last-named are indistinguishable from the albite- 

 porphyries found at Kalgoorlie. 



A conglomerate of somewhat similar character, traced by Dr. 

 Maclaren in a ridge 19^ miles north-north-east of Kanowna, on the 

 Kurnalpi Eoad, differs from the Kurrawang Conglomerates in 

 containing pebbles of amphibolites, mostly of basic character. 



(b) The Fine-Grained Amphibolites. 



These rocks are found in various dumps, chiefly in the ' North 

 Eud' and on the western side of the main dyke of quartz-dolerite, 

 but also less commonly on its eastern side. The known occurrences 

 are too isolated for us to judge by these alone of the geological form. 

 The rocks pass gradually into fine-grained greenstones, which have 

 a wider distribution, occupying most of the Kalgoorlie ridge 

 outside the main quartz-dolerite dyke. 



Unlike the coarser amphibolites described below, the fine- 

 grained amphibolites retain no recognizable relict-structures. 

 Their geological history cannot, therefore, be ascertained by 

 microscopic study in the Kalgoorlie field. Mineralogically, they 

 have the composition of amphibolites derived from basalts or more 

 basic rocks. 



If they were of very limited distribution and confined to the 

 immediate neighbourhood of the coarser intrusive rocks, it would 

 be possible to consider them as the result of alteration of the 

 chilled margins of these intrusives ; but, along with the fine- 

 grained greenstones derived from them, they occupy an area con- 

 siderably broader than the widest dyke known in the field, so that 

 this explanation becomes impossible, although perhaps some of the 

 rocks here described are such chilled margins. Similar fine- 

 grained amphibolites and greenstones, however, attain a consider- 

 able development in other districts where coarse intrusives are 

 unknown : as, for example, the ' Six Mile,' Kanowna. Since they 

 apparently result from the alteration of basic igneous rocks, the 

 most probable explanation of their origin is that they represent an 

 older series of lavas and tuff's into which the coarser amphibolites 

 are intrusive. A distinction may thus be made between the older 

 and the younger amphibolites and greenstones. 



Mineralogically, the amphibolites consist chiefly of hornblende 

 and zoisite, with smaller amounts of felspar, sphene, chlorite, 

 carbonates, and quartz. The hornblende is a pale variety, except 



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