THK MKTAMOKI'HIC ROCKS OF ADELIE LAND.-ST1LLWELL. 173 



finer grained type than No. 951, and, in the hand specimen, might be taken for a 

 slightly altered dolerite, because there seem to be fine-grained portions, representing 

 the unaltered dolerite, surrounded by more coarsely crystalline rock, representing the 

 altered part. No part, however, is found to be unaltered in section. 



In this section we find that the outlines of primary felspar and pyroxene of the 

 dolerite have quite disappeared. The former crystals are now replaced by a finely 

 diablastic aggregate of pyroxene and felspar. The individuals in the aggregates are 

 more granular in contrast to the vermicoidal appearance in the preceding. The 

 aggregates may contain small garnets and biotites with numerous small crystals of 

 ilmenite ; a little quartz has been detected and is probably associated with the formation 

 of garnet. We call these aggregates diablastic, because we consider them to be pro- 

 duced in the decrystallisation, or breaking down, of the primary pyroxene and labra- 

 dorite which results partly in the secondary pyroxene and a more sodic plagioclase. 

 This decrystallisation is followed by a recrystallisation, and we find here and there 

 granulitic aggregates of secondary pyroxene, including both orthorhombic and mono- 

 clinic forms, identical in kind with those produced in the Cape Gray rocks. 



The recrystallisation or the building of large crystals from smaller ones seems to 

 have taken place under conditions in this case which have favored the formation of green 

 hornblende and biotite. Hornblende and biotite possess an average grain size much 

 greater than the pyroxene, and are both much more abundant than in No. 951. The 

 large crystals of hornblende and biotite are frequently aggregated in clusters, just as if 

 each cluster were a metamorphic differentiation centre of hornblende or biotite. Horn- 

 blende and biotite are frequently intergrown, indicating that they have formed at the 

 same time. Often the hornblende clusters have a linear trend, and sometimes they are 

 circular, enclosing areas of the diablastic felspar and pyroxene (Plate VII., fig. 6). In 

 doing this, they provide the initial stages of the growth of the phenomena to be described 

 in No. 953. 



Hypersthene is again noted among the pyroxene, and apatite and odd grains of calcite 

 are present. 



The metamorphic character of this example dominates the igneous, and most of 

 the rock has suffered complete decrystallisation. It may be called a hornblende 

 plagioclase pyroxene gneiss. 



No. 952. In some instances the hornblendisation noted in the preceding has 

 proceeded to such an extent that a normal amphibolite has formed. No. 952 is an 

 example of this type, obtained from among the basic plagioclase augite rocks of Stillwell 

 Island. 



In the hand specimen this rock is similar to the fine grained, massive varieties at 

 Cape Denison. In section, it consists chiefly of hornblende and felspar (labradorite- 

 andesine), with small amounts of ilmenite, biotite, and garnet. Sphene, calcite, and 

 apatite have been detected. 



