130 



G. D. OSBORNE. 



the presence in the magma of pyroxene (here augite), which 

 has one ion in common with olivine, has " accelerated" the- 

 freezing of the latter mineral. This suggested explanation 

 of an apparent anomaly on the theory that rock magmas 

 behave as chemical solutions, has been used by Harker in the 

 interpretation of certain features of the ultrabasic rocks 

 of Rum. 1 The augite crystallised soon after the olivine and 

 before the felspar, but the augite-pleonaste system was a 

 late product of solidification, being subsequent to the 

 adjacent felspar. 2 



The felspar is subidiomorphic with stoutish habit, exhibit- 

 ing albite and pericline twinning. Strain is expressed by 

 undulose extinction and peripheral shattering. The alter- 

 ation in cracks and irregular patches has given rise to 

 sericite, chlorite and calcite. The extinction angle in 

 sections parallel to the brachypinacoid give, in different 

 slides, values ranging to bytownite of about the compo- 

 sition Ab29An7i. These determinations are in general 

 substantiated by the results of the analysis. 



Olivine occurs in rounded grains, and in places is com- 

 pletely pseudomorphed by iddingsite and bowlingite. 

 Secondary carbonates are also present. The augite is fairly 

 fresh and of a pale greyish-green colour. It is generally 

 concentrated locally and shows incipient alteration to 

 chlorite. 



Granophyric intergrowth of pyroxene and pleonaste is 

 present, showing the same general features as the similar 

 occurrence at Dundas. An example is figured in Plate VII, 

 fig. 6. Dr. Benson 3 accounted in one way for the inter- 

 growth, by postulating the solution of felspar of a partially 

 solidified gabbroic magma by a peridotitic magma on 



1 Harker, "Natural History of Igneous Rocks," pp. 170, 171, 205. 



2 Benson, Journ. Roy. Soc. N.S.W., 1910, p. 519, et seq. 



3 Eenson, loc. cit., p. 521. 



