622 DK. J. A. THOMSON ON THE [Dec. 1913, 



altered by contact, and consist of foliated hornblende and tremolite- 

 talc-scbists alternating with massive amphibolites which show 

 complete mineralogical reconstitution ; their auriferous deposits are 

 often of phenomenal richness, but are sporadically distributed, and 

 the effects of hydrothermal alteration on the walls of the veins are, 

 in general, slight. Norseman may be cited as an example of a field 

 lying partly within and partly without the aureole of a granite, 

 the apophyses of which, nevertheless, play a considerable role, even 

 in those parts that are unaffected by contact-alteration. Kanowna 

 is composed of rocks showing no trace of contact-alteration, but it, 

 too, is dominated by dykes of quartz-porphyry. At the other end 

 lies Kalgoorlie, an area in which neither granites nor quartz- 

 porphyries play any part. The rocks are mainly amphibolites and 

 serpentines showing little structural metamorphism, but a distinct 

 feature is the presence of albite-porphyry and porphyrite-dykes ; 

 the auriferous deposits are more regular in distribution than in the 

 other fields, and the metasomatic alterations of the country-rock 

 are very intense. 



In this paper the rocks of Kalgoorlie, which I have had the 

 opportunity of studying in the field, are described in detail, and, 

 in addition, comparisons are made with similar rocks from other 

 parts of the State, based on the study of specimens kindly supplied 

 by Mr. A. Gibb Maitland, Director of the Geological Survey of 

 Western Australia. Kalgoorlie appears to be more complex than 

 other fields of similar type, and it is possible that some of the 

 problems which it presents will become clearer if they can be 

 attacked in detail in areas of simpler constitution. 



The mines of Kalgoorlie are situated on a broad ridge about 

 4 miles long by a mile in width, trending in a north-north-westerly 

 and south-south-easterly direction. The southern end contains 

 the rich mines which have made the goldh'eld famous, and, on 

 this account, has been popularly termed ' The Golden Mile,' or 

 briefly ' The Mile.' The northern part, which is but moderately 

 auriferous, is locally referred to as the ' North End.' These local 

 names will be employed in this paper. The town of Kalgoorlie 

 occupies the floor and eastern slopes of a valley to the west of 

 the ' North End ' ; farther south, the suburb of Boulder lies in 

 the same valley to the west of ' The Mile,' and hence ' The Mile ' 

 is also spoken of as the ' Boulder Belt.' x This valley and a 

 similar one limiting the ridge on the eastern side are both deeply 

 filled with alluvium and wind-blown material ; while a mantle of 

 surface-deposits extends for a considerable distance up the sides of 

 the ridge itself, in some places completely envelopingit. Nearly all 

 the highest points are formed of hard, red, ferruginous laterite 

 resting on an earthy white material resembling kaolin, but the 

 laterite is not confined alone to the high points. Smaller irregular 



1 Boulder, the suburb of Kalgoorlie, must not be confused with Boulder 

 City (Colorado). The paper by Mr. T. A. Eickard, on ' The Veins of Boulder 

 & Ivalgoorlie' Trans. Am. Inst. M. E. vol. xsxiii (1903) pp. 567-77, refers to 

 the American telluride-field. 



