Vol. 69.] PETROLOGY OF THE KALGOOELIE GOLDE1ELD. 655 



While the porphyries range as dykes through the quartz- 

 dolerites, ' calc-schists,' and peridotites of the Kalgoorlie ridge, the 

 porphyrites are found chiefly in old dumps of abandoued mines 

 and water-shafts in the valley west of 'The Mile' on which 

 Boulder is built, and consequently it is not clear through what 

 rocks they are intrusive. The distribution of these dumps, and of 

 some outcrops on the rising ground, favours the view that there 

 is a broad band of porphyrite with a trend similar to that of the 

 other intrusives ; but, if so, it is a composite intrusion, for the 

 nature of the rocks varies in nearly every dump. Iu the hills 

 composed of epidiorites and quartz-dolerite-amphibolites west 

 of the Kalgoorlie-Boulder valley, there are various small dykes 

 of porphyrite ; and Dr. Maclareu has observed them as far to the 

 south-east as Mount Monger. 



The differences within the group lie partly in the varying pro- 

 portions of phenocrysts and groundmass, and partly in the relative 

 proportions of felspar, hornblende, and biotite. The rocks which 

 agree most closely with typical porphyrites show small phenocrysts 

 of black hornblende and white felspar in a white or yellow felsitic 

 groundmass. The more micaceous varieties are grey to black, and 

 devoid of apparent phenocrysts ; while the almost dioritic type 

 from the Power-House Reserve presents a distinctly plutonic aspect, 

 with only an occasional larger white felspar to bear witness to the 

 porphyritic structure. In some localities, the rocks are curiously 

 mottled by the presence of inclusions. These are mostly of a white 

 felsitic rock in a typical dark porphyrite, but to a limited extent 

 consist of still darker, more hornblendic varieties. The white 

 inclusions sometimes present great resemblance to the albite- 

 porphyries, but differ in the presence of occasional phenocrysts 

 of hornblende. It is, apparently, to these mottled rocks that 

 Mr. Campbell and Mr. Simpson have applied the term ' orbicular,' 

 although the analogies with napoleonite are of the slightest. 

 Another type of inclusion, occurring in the diorite-porphyrite of the 

 Power-House Reserve, consists of coarsely-crystalline, black, ' basic 

 segregations,' made up chiefly of hornblende and epidote, with 

 subordinate carbonates and magnetite. 



The rocks are never quite fresh, the felspars always containing 

 more or less sericite, and occasionally a little zoisite. Other 

 secondary minerals of common occurrence are uralite, epidote, 

 chlorite, albite, carbonates, rutile, magnetite, pyrite, and quartz. 

 The phenocrysts consist of apatite, felspars, hornblende, biotite, 

 and rarely quartz ; while the groundmass apparently consisted 

 chiefly of felspars with subsidiary quartz, hornblende, and biotite, 

 but is now much obscured by secondary minerals. 



Apatite occurs sparingly in small but stout hexagonal prisms, 

 and is generally turbid from alteration. In one rock it exhibits 

 schillerization and distinct pleochroism. Hornblende is found 

 in somewhat elongate pale-brown or green prisms, showing sharp 

 cross-sections with prism and B-pinacoid faces, or longitudinal 

 sections with the dome (Oil), and frequently twinned on A (100). 



