229 



The mineral becomes clouded with limonite(?), beneath 

 which it passes into iiralite. This alteration appears to be 

 connected with the proximity of the felspars, for it is most 

 pronounced along their borders, and especially at their in- 

 tersections. It seems possible that the alteration of the fel- 

 spars made them more porous and thus better channels for 

 mineral solutions, the agents in producing the change. The 

 felspar of the rock is almost completely altered to mica, 

 partly fibrous and partly scaly of high birefringence, but 

 fairly low refractive index. There is some sign of mutiple 

 twinning still preserved, perhaps by the different orientation 

 of the mica flakes derived fi-om alternate twin lamellfe. Epi- 

 dote occurs in small colovirless grains in the felspar. Brightly- 

 coloured epidote (pistacite) is also present in a few scattered 

 and rather large grains. Pseiidoraorphs after olivine occur 

 composed of pale-green serpentine, bordered by magnetite. 

 Ilmenite is present in considerable amount. A chemical 

 analysis of this rock (A) is given on page 234. 



The rock composing "the dyke near the gorge, on the 

 old road five miles west of Blinman," is also an olivine-dia- 

 base. It is, however, rather different from the rock last de- 

 scribed. It is basaltic in appearance, fine-grained, but with 

 large grains of a dark-green ferromagnesian silicate, and 

 small pale-green felspar phenocrysts. Microscopically the 

 structure is porphyritic with a semi-ophitic base. The pheno- 

 crysts are augite, quite allotriomorphic, and 2-3 mm. in 

 length. These are sometimes not all of one grain, but are 

 aggregates of several grains, sometimes twinned and usually 

 enclosing ophitically a few felspar laths. The olivine occurs 

 in large idiomorphic crystals, and is entirely changed to dusty 

 magnetit-e, pale-green serpentine and talc(?), the last being 

 colourless and highly birefringent. The magnetite is present 

 in great amount, showing the highly ferriferous nature of 

 the original olivine. The plagioclase is largely altered to 

 mica, but some remains, still showing the multiple twinning, 

 which, by its extinction angle of 25° and refractive index 

 greater than that of the balsam, is probably labradorite. 

 Between the felspar laths, partly ophitic and partly granu- 

 litic, is titaniferous augite, quite fresh or in various stages 

 of alteration to a deep-green uralite, changing to yellow on 

 rotation, together with chlorite and a little epidote. Ilmenite 

 is present in some amount. Order of crystallization : — 



Olivine 



Felspar 



Ilmenite 



Augite 



