660 DR. J. A. THOMSON ON THE [Dec. I913, 



(f) The Jaspers and Graphitic Schists. 



An account of the petrology of Kalgoorlie would uot be com- 

 plete without some notice of the jasper-lodes and the bands of 

 graphitic schist, which traverse rocks of almost every nature. 

 Most of them run parallel to the foliation-planes of the enclosing 

 rocks, but some are oblique, notably a strong jasper-lode east of 

 Hatman's Eeward Mine in the ' North End,' which cuts the 

 foliation-planes at an angle of 50°. 



The jaspers are light or dark siliceous rocks, generally with a 

 fine banding which may be extravagantly contorted. The banding- 

 is due mainly to variations in the size of the grain of the quartz- 

 mosaic of which the rocks are composed : it is not due to 

 bands containing different minerals, as in the case of the haematite- 

 jaspers of other goldfields. Sections reveal little beyond a very 

 tine quartz-mosaic containing small sporadic rhombohedra of 

 carbonates or limonite pseudomorphs of the same. 



The jaspers are generally accompanied on each side by slaty 

 rocks, which bear a close resemblance to ordinary sedimentary 

 slates and phyllites, and similar rocks are also found replacing the 

 jaspers along the strike. Few opportunities of tracing the down- 

 Avard continuation of the jaspers are available ; but they appear 

 in some cases to be replaced by graphitic schists (for instance, 

 in the North End Development G.M.L. 1731 E, and in the Union 

 Jack G.M.L. 535 E). 



The band that has been most closely studied runs from the 

 Boulder Main Reefs G.M. through the Great Boulder and Boulder 

 No. 1 to the Golden Pyke G.M. and beyond. The outcrop can be 

 traced in places by white slaty rocks, while jaspers and slaty 

 rocks are seen in the railway-cutting east of the main sbaft of 

 Boulder No. 1 G.M. At the 100-foot level from Philip's Shaft in 

 the Great Boulder G.M., the band consists of white quartz-sericite 

 schists. A section of one of these shows a very fine quartz-mosaic, 

 with much fine schistose sericite, a feAV minute prisms of rutile, 

 and small pseudomorphs of limonite after pyrite or siderite. At 

 greater depths this band is represented by graphitic schists, as 

 may be seen at many places in the Great Boulder Mine. In 

 section, the graphitic schists are almost opaque, from the presence 

 of fine graphite dust and small crystals of pyrite ; but the presence 

 of quartz and sericite can generally be verified. A specimen from the 

 eastern boring (from the main shaft at the 2350-foot level) contains 

 quartz, carbonates, sericite, rutile, and clinochlore, in addition to 

 graphite and pyrite. Wavy bands of sericite, deeply stained by 

 fine graphite dust, wrap round clearer areas occupied by carbonates 

 or by large pyrite-crystals enveloped by quartz and clinochlore. 



At several places within this band of graphitic schists the 

 presence of albite-porphyry has been verified, but it does not appear 

 to form a continuous dyke. The same association of porphyry and 

 graphitic schist occurs at the 400-foot level of the Union-Jack 

 Mine. The graphitic schists are sharply marked off from the 

 quartz-carbonate-sericite schists in which they occur, and also 

 from the albite-porphyry. It seems most probable that they have 



