158 AUSTRALASIAN ANTARCTIC LXPEDiTlON. 



possible, and at the same time an explanation why the quartz of the related type (No. 

 979) is replaced by orthoclase in this example (No. 947)*. 



(KH) 2 (MgFe) 2 A1 2 (Si0 4 ) s + CaAl, Si 2 8 + 4Si 2 

 Biotite Anorthite Quartz 



^ 2(MgFe)0,CaO, A1 2 3 , 3Si0 2 + 2KA1 Si s 8 

 Garnet Orthoclase 



The related rock (No. 979), in which there is very little garnet but much quartz 

 and plagioclase, can then be explained as a rock type in which the biotite side of this 

 equation is expressed. No. 947, in which there is considerable garnet and orthoclase 

 and practically no quartz, may be looked upon as a rock type in which the garnet side 

 of this equation is expressed. 



We have interpreted the reaction of the hypersthene with felspar in these 

 hypersthenic rocks to biotite and ilmenite as associated with a change of kata zone 

 conditions to meso zone conditions. If, after these changes, the biotite reacts with 

 quartz and felspar to produce garnet and orthoclase, we should, on the same reasoning, 

 interpret the cause as a reversal to the kata zone conditions. For this we cannot imagine 

 any variation in the depths of the earth's crust, because there is no similar evidence 

 in any other variety of gneiss on Stillwell Island. We can only imagine that the 

 temperature and pressure have been increased locally, possibly by neighbouring chemical 

 reactions which have liberated heat and caused expansion of volume. If this were so 

 the area subjected to the reverse conditions would be highly localised. 



The specimen No. 947 is an irregularly shaped piece about Sin. long and, roughly, 

 l^in. square in section. A second section was cut from the opposite end of the specimen, 

 distant Sin. from the first section. In this section no garnet is found, but large pieces 

 of quartz. Hypersthene is present, again showing some alteration to ilmenite and 

 biotite, but none of the biotite is rimmed with garnet. This result was surprising, 

 and a third section was cut from the middle of the specimen, half-way between the two 

 previous sections. In this middle section some garnet is found, but less than in the 

 first section. It again borders biotite in the same remarkable manner. There is some 

 quartz in the section. We have also noticed in this section a large crystal of pale-green 

 mica with included ilmenite. This green mica is evidently an intermediate stage between 

 delessite and biotite, but it is not possible to say in which way the reaction is going. 

 Hence the supposition of the highly localised distribution of the garnet rims seemed 

 to be confirmed. 



The rock may be described as a hypersthene alkali felspar gneiss, in which the 

 hypersthene has first partly changed to ilmenite and biotite. This change has been 

 followed in localised portions of the rock by a partial reaction of biotite with quartz 

 forming garnet and orthoclase. 



Similar conclusions may be formed about the primary igneous origin of No. 947, 

 as in the case of No. 979. 



* The (KH) molecule is reckoned as K 2 for simplicity. In the analyses of some biotites the K 2 is in great excessover 

 the H ; O, and this is assumed to be the case here. 



