ROCK SPECIMENS FROM CENTRAL AND WESTERN AUSTRALIA. 301 



The rock is certainly an augite-hornblende lampropbyre, 

 probably a camptonite. It is the first rock of this class so 

 far found in Western Australia. 1 



A rock of very peculiar character may be described here, 

 as it has some faint resemblance to the camptonite just 

 described, (Oavanagh Range, 31/7/91). It is probably the 

 rock referred to by Streich as tachylite, of which Stelzner 

 remarks: — "This rock is of such an extremely fine grain 

 that I cannot determine it, even with the aid of the micro- 

 scope on rock sections." The hand specimen is a dark 

 aphanitic rock with some superficial resemblance to a 

 tachylite, but contains a few clear patches of quartz and 

 small geodes containing pyrites. The section (Fig. 4, 

 Plate XIV) shows a number of small elliptical and larger 

 irregularly shaped areas formed of small rods of almost 

 opaque material grouped together like bundles of faggots; 

 between these bundles and acting as a cement are 

 clearer areas consisting of irregular biotite flakes and an 

 indeterminate green mineral in a fine grained quartz base. 

 The green mineral possesses a higher birefringence and 

 lower refringence thau the biotite, but a similar absorption 

 and a pleochroism from opaque to dark green or yellow. It 

 appears to be uniaxial or feebly biaxial, is optically nega- 

 tive with positive elongation. The dispersion is very strong, 

 comparable to that of chloritoid, from which, however, it 

 differs in its direction of maximum absorption and its lack 

 of polysynthetic twinning. Most often it occurs in shapeless 

 plates, but occasionally gives lozenge-shaped sections. 

 These differ from hornblende only in the absence of cleavage 

 planes. The mineral thus appears to be intermediate 

 between biotite and chloritoid in its characters, and may 

 possibly be pseudomorphous after hornblende. 



1 The rocks described as camptonites by Simpson and Glauert from the 

 Philips River Goldfield appear to the writer to be really contact-altered 

 amphibolites, Bull. 35, Geol. Surv. W.A., 1909, pp. 42-3. 



