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



of colourless ellipsoids of tremolite within the brown hornblende 

 corresponds rather to the former proportion of olivine in the rock 

 than to that of augite ; and this leads to the belief that such 

 ellipsoids in lustre-mottled amphibolites may be, at least in part, 

 the result of the replacement of olivine. Unless, however, there 

 are remnants of olivine still retained in a rock, it is unsafe to 

 postulate the former presence of the mineral, as microscopic 

 criteria for distinguishing tremolite formed from pyroxene and 

 from olivine respectively appear to be non-existent. 



A priori, three types at least of lustre-mottled amphibolites 

 may be distinguished : — 



(1) Those formed from hornblende-peridotites ; (2) those formed from 

 hornblende-olivine-dolerites or gabbros ; and (3) those formed from horn- 

 blende-dolerites or gabbros without olivine. 



Of these types only the last-named has as yet been found in 

 Western Australia. A fourth possible type might be formed from 

 a pyroxenite containing much original hornblende, or an augite- 

 hornblendite, if the augite occurred in poecilitic crystals. The 

 pyroxenite-amphibolite from Ravensthorpe described above approxi- 

 mates closely to this category, but the lustre-mottling is very local 

 in character. 



Lustre-mottling is not confined to igneous rocks containing 

 poecilitic hornblende, but is also found in some that contain 

 poecilitic augite. Amphibolites derived from such rocks would 

 scarcely be expected to retain this character so clearly as those 

 derived from hornblendic rocks, for the process of uralitization does 

 not develop compact crjstals, such as would give rise to smooth 

 cleavage-surfaces like those found in residual brown hornblende. 



(h) The Epidiorites. 



These rocks are uralitized, saussuritized, and leucoxenized gabbros 

 or ophitic dolerites, and are of a type so common that detailed 

 descriptions are unnecessary. They have a very small develop- 

 ment in Kalgoorlie, where they occur only as a local facies of 

 the quartz-dolerite amphibolite, but are more abundant in the 

 ridges to the west of the town ; while an incompletely-uralitized 

 dyke is found 3^ miles south of Bulong on the Mount Monger 

 Road. No analysis of this type from Kalgoorlie has been made. 



Epidiorites are of very common occurrence among the amphi- 

 bolites of Western Australia, and their dei'ivation from ' diabase ' 

 has in some cases been recognized by Dr. Vogelsang, Mr. Simpson, 

 and Mr. Gibson. In 1909 I described several in detail from the 

 Northern Goldfields, including pegmatitic varieties (G.S.M. 6401, 

 6410, 6414, 6434, 6437, 6442). 1 The following analyses (p. 643) 

 refer to rocks of this class. 



1 Bull. Geol. Surv. W. Austral. No. 33 (1909) pp. 135-36, 138, 140, 146, 

 147, & 150 ; also figs. 45 & 46. 



