644 dr. j. a. thccjison on the [Dec. 1 9 13, 



A. Epidiorites with Micropegmatite (Quartz- 

 Dolerite-Aniphibolites). 



The amphibolitic variety forms a fairly-persistent band of 

 varying width on the west side of the main dyke, and in parts of 

 the ' North End ' extends right across the outcrop. The rocks 

 are, in general, not so deeply oxidized as the chloritic varieties, and 

 may be collected at the surface in many places, notably near the 

 Warden's House and in the eastern railway-cutting through the 

 Hannan's Proprietary Lease (4222 E). South of ' The Mile ' 

 there is also a good exposure in a quarry in the old Leviathan 

 Boulder Lease (1072 E). 



In hand-specimens the rocks are mostly coarse-grained and of 

 a greenish-grey colour. The bright cleavage-surfaces of dark-green 

 hornblende are conspicuous, and appear to be scattered about in a 

 dull-white saussuritic material. In general, it is impossible to 

 recognize the presence of quartz without the help of a lens, nor is 

 leucoxene so conspicuous as in the greenstones. The rocks at the 

 southern end of the field have a darker green coloration, due to the 

 prevalence of epidote, and are traversed by venules of quartz and 

 •epidote. 



Despite the alterations that have taken place, the structural 

 relations between the minerals of the original rock are very com- 

 pletely preserved. Saussuritic pseudomorphs of earlier columnar 

 plagioelase are embraced ophitically by leucoxene and by uralitic 

 pseudomorphs of pyroxene. In the interstices of the framework 

 thus constituted a variable amount of micropegmatite or quartz 

 occurs. The earlier felspars have rarely escaped complete saussuri- 

 tization, and are generally replaced by almost opaque aggregates of 

 zoisite, or more rarely of brownish-green epidote. The previously- 

 existing pyroxenes are completely altered in the Kalgoorlie area to 

 an almost colourless, markedly-fibrous uralite, which projects in a 

 ragged fringe into the quartz-felspar intergrowths. Apparently, 

 also, original hornblende was present: for there is a small 

 amount of bright-green, pleochroic, compact hornblende on the 

 outer borders of the uralite, and in crystallographic continuity with 

 it. The marked contrast between the two varieties of amphibole 

 must arise from some difference in mode of origin, and the simplest 

 explanation is that the one is original and the other secondary. 

 The iron-ores are occasionally preserved without alteration, but 

 more often are represented by leucoxene and occasionally by 

 granular sphene. The interstitial quartz and felspar often form 

 micrographic intergrowths, but these are seldom so well-defined as 

 those seen in the greenstones. The later felspar is obscured by 

 inclusions of zoisite and sericite, but in places is fresh enough to 

 allow the species to be recognized as an intermediate plagioelase. 

 Both the micropegmatite and the interstitial quartz are rich in 

 inclusions of apatite. 



The analysis of a Kalgoorlie rock of this type (tabulated on p. 645) 

 is very similar to those of fresh quartz-dolerites from Great Britain. 



