Vol. 69.] PETROLOGY OP THE KALGOOELIE GOLDEIELD. 623 



or wall-like hillocks are due to the projection of reefs of handed 

 jasper, rising through the surface-deposits. Wherever the solid 

 rocks of the ridge crop out at the surface, or are exposed in shallow 

 trenches, they are nearly always oxidized, with the exception of a 

 few amphibolites. In these circumstances geological field-work is 

 rather difficult, and were it not for the material which may be 

 collected in the dumps of old mines and prospecting shafts, added to 

 the artificial sections displayed in the extensive mine-workings in 

 depth, the complexity of the field would never be suspected, and 

 geological mapping would be impossible. 1 



The ridge is composed of elongate masses of massive or sub- 

 schistose rocks, each trending in a north-north-westerly and south- 

 south-easterly direction, and the floors of the valleys on each side 

 appear to possess the same character. Where schistosity is deve- 

 loped, the planes of foliation have the same general trend as the 

 rocks themselves, with a steep dip averaging 60° west-south-west- 

 wards. The rock-junctions, where not faulted, appear to possess 

 the same direction of inclination ; but on this point few positive 

 observations can be made, for the majority of rock-junctions 

 traversed by mine-workings prove to be faults. The central mass 

 of the ridge is a large dyke or sill of altered, coarse, intrusive 

 quartz-dolerite, which traverses the higher hills of the ' North End' 

 in a band 200 yards to a quarter of a mile wide, and spreads out in 

 ' The Mile' to form a boss-like mass measuring about half a mile in 

 diameter. Three other dykes leave the boss, one running for about 

 a mile from the north-east side to lose itself under the surface- 

 deposits of the eastern valley, the others being practically the 

 southward prolongations of the northern two, and running for 

 some distance in the direction of Hannan's Lake. On most sides the 

 quartz-dolerites are surrounded by fine-gi'ained amphibolites and 

 greenstones of somewhat variable character, presumably the country- 

 rocks into which the dykes are intrusive. In the eastern part of 

 the ' North End ' a large intrusion of ultrabasic rocks approaches 

 closely the main quartz-dolerite dyke, but is separated by a thin 

 band of fine-grained amphibolite. On the somewhat scanty evidence 

 of widely-separated dumps on the flats west of the ridge, it appears 

 that another ultrabasic intrusion in the fine-grained amphibolites 

 runs parallel to the quartz-dolerite at some little distance from it. 

 Earther west in the Kalgoorlie-Boulder valley the few dumps that 

 are available reveal only porphyrites, but whether there is a broad 

 band of this rock or several separate dykes remains uncertain. The 

 quartz-dolerite of 'The Mile' is intersected by two thin dykes of 

 albite-porphyry, which have the common north-north-westerly and 



1 It is unfortunately impossible to reproduce with this paper a geological 

 map prepared by Dr. J. M. Maclaren and myself, since the information which 

 it contains as to the distribution of the rocks and lodes, and more particularly 

 as to the faulting that they have undergone, is the exclusive property of the 

 group of mining companies on whose behalf it was prepared. Nevertheless, 

 the reservations thus made concerning the exact boundaries of the rocks do 

 not materially affect the discussion of the phenomena described in this paper. 



