PETROLOGICAL NOTES ON FURTHER ROCK SPECIMENS. 317 



Except for an inevitable difference in size the plagioclase of the finer parts of the 

 rock is much the same as that found in the coarser. 



(6) The Acid Hypersthene Gneisses. 



This division recorded by Stillwell is unrepresented among the present series 

 of specimens. 



(c) The Metamorphosed Dyke Series. 



Sir Douglas Mawson recorded in his diary (p. 171 of present series), " irregular 

 bands of black rock exist as at Cape Denison : some are not much altered, and others 

 are full of fine garnet." 



Dr. Stillwell then continues to note that some of the rocks exhibit " remarkable 

 stages of incipient alteration." An alternative hypothesis seems to be suggested by 

 the more recent work of Harker and Wiseman in connection with the so-called epidiorites 

 of the Scottish Highlands : that these rocks are high grade metamorphic types, where 

 hornblende is giving place to two pyroxenes, garnet is developing and granulitic texture 

 is characteristic. 



No. 972. Hornblende-Plagioclase-Pyroxene Gneiss. 



This is a dense, fine-grained rock. Its apparent uniformity is resolved on close 

 inspection into dark masses of relatively coarser material and dark spots whose 

 grain-size is considerably smaller. An ill-defined band where biotite and quartz are 

 megascopically visible also tends to relieve the uniform nature of the rock. 



The biotite and hornblende are not without signs of alteration. The grains of 

 these minerals are arranged in the typical decussate manner, but particularly in the case 

 of the hornblende, instead of being compact they are definitely poikiloblastic due to a 

 development of quartz. Associated with the biotite and hornblende, but more 

 particularly with the biotite, are diablastic and mynnekitic intergrowths of quartz and 

 orthoclase. The biotite prisms are still oriented in the manner referred to above, but 

 owing to the nature of their development unchanged pyroxene granules remain as 

 inclusions. The relationship between these intergrowths and the biotite (and hornblende) 

 is revealed by the bays and bights in the ends of the prisms and frequently by their 

 presence in the more crystalloblastically powerful sides of the prisms. 



The granulitic areas are composed of small rounded grains of pyroxene set in a 

 matrix of plagioclase, probably labradorite, which has been formed by recrystallisation. 



An imperfectly formed vein of quartz traversing the rock suggests that there 

 has been a possible acquisition from without of silica which may have been effective 

 in supplying this material to the ampbiboles which are usually below the meta-silicate 

 stage in rocks of this kind. 



The existence of considerable hydrostatic pressure is evidenced by the presence 

 of garnet crystals, and, this pressure, no doubt, has played a considerable part in the 

 recrystallisation of the pyroxene. 



