THE METAMORPHIC ROCKS OF ADELIE LAND. STILLWELL. 175 



(2) There are seams and patches of granular green hornblende in cracks and fractures 

 of the relic pyroxene, and among the aggregates of secondary granular pyroxene. It 

 appears that the hornblende has formed directly from both the relic dusty augite and 

 from the secondary pyroxene. A large plate of relic augite may enclose parasitically 

 a granular aggregate of green hornblende crystals in which an ilmenite crystal may be 

 set as a nucleus, formed, as before, by the coalescence of the primary ilmenite inclusions. 

 Sometimes larger hornblende crystals have grown out of the aggregates of small granular 

 hornblende. 



(3) There is frequently a considerable amount of small secondary biotite associated 

 with the granular hornblende so intimately that there can be no doubt they have arisen 

 at the same time as the hornblende. Its formation depends on the chemical supply 

 of potash and water. 



(4) The large relic augites, which may be replaced by secondary pyroxene or 

 hornblende, are bordered by a zone of small garnets which may be partly idioblastic 

 (Plate VIII., figs. 4 and 5). Between the edge of the pyroxene crystal and the garnet 

 rim there is usually a thin zone of clear felspar (or quartz). The manner in which the 

 garnet rim follows the outline of the relic pyroxene can be easily seen when the section 

 is observed with a low power lens. The character of the felspar in the neighbourhood 

 of the garnet undergoes an obvious change when observed in polarised light (Plate 

 VIII., fig. 6). The formation of the garnet absorbs lime from the primary labradorite, 

 and we may find a labradorite crystal zoned with a rim of more sodic felspar. By analogy 

 with phenomena in metallic alloys, the manner of extraction of the anorthite from 

 the solid solution of plagioclase is strongly suggestive of solid diffusion. The reaction 

 that has taken place is one that has been quoted by Grubenmann,*and may be written 

 in this case 



Augite Labradorite Garnet Andesine 



CaMgSi 2 + 2CaAl 2 Si 2 8 , NaAlSi 8 8 ^ Ca 2 MgAl2Si,0 12 + CaAljSiA, NaAlSi 3 8 



Quartz 

 + Si0 2 



There is no evidence to lead us to ascribe these compositions to augite, felspar, and 

 garnet, but, by doing so, we can more readily understand how the garnet is formed 

 and the more sodic plagioclase produced. More augite may combine with another 

 anorthite molecule of the andesine, and a still more sodic plagioclase produced. The 

 separation of the quartz has been definitely no^ed in a second section No. 935 (2) from 

 the same specimen, and that it does appear with garnet is abundantly evident in 

 No. '953. 



Sometimes where a blastophitic structure can be recognised in No. 935 (2), and a 

 relic labradorite crystal crosses a plate of dusty augite, we may find no garnet border. 



Along the edge of the felspar there are numerous small rounded inclusions like incipient 



__ 



Die KruUllinen Schiefer, vol. I., p. 34." 



