THE METAMORPHIC ROCKS OF ADKLIE LAND. STILLWELL. 163 



and it is certain that the development of the garnet took place prior to the serpentini- 

 sation. It probably occurred before the development of the enstatite, because we have 

 previously noted garnet seams in deeply pleochroic hypersthene. 



Sometimes these altered crystals of hypersthene contain inclusions of biotite, or 

 of ilmenite surrounded by biotite sprays with associated quartz. But biotite crystals 

 are mostly confined to the margin of the serpentine hypersthene aggregate. These 

 biotites may have the normal garnet rim, produced, no doubt, in the same manner as 

 before by a biotite-plagioclase reaction. Such reaction may explain the garnet fringe 

 around the edge of the aggregate, but it will not reasonably explain the rims and seams 

 in the inner part of the aggregate. The garnet has also been derived in some other 

 manner. 



It seems necessary to account for the garnet without any reaction at all, and to 

 assume that the garnet is derived directly from the hypersthene. Holland* has reported 

 the decomposition of augite into garnet and felspar ; but there is no reason to suppose 

 that this instance cannot be explained as has been done in the garnet plagioclase 

 pyroxene gneiss (No. 953) of Stillwell Island, in which augite has reacted with labradorite 

 to produce garnet with andesine and quartz. In the same publication Holland refers 

 to the description by Brauns in 1888 of the formation of a lime iron garnet in a palaeopi- 

 crite by the alteration of augite in which the chemical analysis indicated a removal of 

 A1 2 S . L. Hezner mentions the record of a pseudomorph of garnet after augite by 

 Pelikan.f These instances, however, are probably not parallel with the present instance. 



Van Hise quotes the change of pyrope into enstatite, spinel, and quartz. J It is 

 not unlikely that this reaction is reversible, with suitable conditions, and pyrope may be 

 derived from enstatite, provided the suitable amounts of A1 2 3 are available. If this 

 is so then the type of reaction may be indicated thus 



SMgSiOg + A1 2 8 <- Mg 8 A1 2 Si, 12 



The hypersthene that enters into the reaction.very probably contains some alumina. 

 According to Dana, hypersthene may contain as much as 10 per cent, of A1 2 8 , and a 

 Victorian example in a titaniferous dacite was found by Richards to contain 4 per cent. 

 It can be conceived that the A1 2 8 content of the hypersthene may provide the alumina 

 in the above reaction, because the amount of serpentinised hypersthene is much greater 

 than the amount of garnet formed. The iron content of the hypersthene may separate 

 out as iron oxide as in the formation of enstatite or enter the garnet molecule. Any 

 content of lime in the hypersthene would also enter the garnet molecule. 



As a result of the examination of these four slides we think the most comprehensive 

 name is hypersthene biotite felspar gneiss. 



T. H. Holland. " Origin and Growth of Garnet*," Rec. G.S.I., XXIX., p. 20. 

 t Op. cit., L. Hezner, p. 67. 

 J " Treatise on Metamorphism," p. 304. 



" On the Separation and Analysis of Minerals in the Dacite of Mount Dandenong, Victoria," H. C. Richards, Proo 

 Roy. Soc. Vic., vol. XXI., n.s., p. 533. 



