184 AUSTKALASIAN ANTARCTIC EXPEDITION. 



There is a very strong family likeness in the chemical composition of the dyke 

 rocks from these four localities. Each analysis has the general characters of a basic 

 igneous rock and closely resembles that of the Cape Denison amphibolite (No. 629). 

 The minor differences can readily be explained as primary variations in the compositions 

 of the dykes at the separate localities. 



It may be recalled, however, that the mineral compositions of these four rocks 

 show great variation, and range from 45-3 per cent, pyroxene in the Cape Gray rock 

 to 2-3 per cent, pyroxene in the Garnet Point rock ; from 57-7 per cent, hornblende 

 in the Garnet Point rock to 3-4 per cent, hornblende in the Cape Gray rock ; from 19-7 

 per cent, garnet in the Stillwell Island rock to 1-2 per cent, garnet in the Garnet Point 

 rock, and from 6-7 per cent, ilmenite in the Stillwell Island rock to 1-2 per cent, in the 

 Garnet Point rock. These varying mineral combinations are independent of the 

 chemical composition and are interpreted as due to varying metamorphic conditions. 

 The similarity of the four analyses provides an argument for the general constancy of 

 chemical composition during metamorphism. 



The specific gravities of these rocks are all higher than the specific gravity (3-030) 

 of the Cape Denison amphibolite, No. 629, which is considered to be a product of more 

 superficial conditions. These higher specific gravities agree with the general deep 

 seated metamorphism of Cape Gray Promontory. The garnet amphibolite from Stillwell 

 Island has a value distinctly greater than the others, and this high value can be ascribed 

 to the same cause as the production of garnet. 



The general family likeness is reflected in the table of Osann group values and 

 projection values. These group values place each rock in the group of eclogites and 

 amphibolites. When the projection values are plotted they produce a cluster of dots 

 around the mean projection value of this group (fig. 14). 



The production of secondary pyroxene requires a high temperature factor, and the 

 production of garnet requires a high uniform pressure factor during the recrystallisation. 

 Rocks which contain these two minerals can confidently be classed as kata zone products. 

 The plagioclase pyroxene gneiss of Cape Gray is a kata zone rock, though only the 

 incipient forms of garnet are found. Hornblendisation of the pyroxene is looked upon 

 by Grubenmann as a meso zone characteristic, and, therefore, the hornblende plagioclase 

 pyroxene gneiss from the Cape Pigeon rocks represents a transition stage between the 

 kata zone type and the meso zone type. Other dyke rocks described from the Cape 

 Pigeon rocks are distinctly meso zone types. 



The garnet amphibolites from Garnet Point and Stillwell Island are members of 

 the garnet amphibolite family which Grubenmann places in the Meso division. The 

 development of both garnet and hornblende from the pyroxene and felspar has been 

 described from the same rock, but it cannot be considered to be proved that the garnet- 

 forming conditions are the same as the hornblende-forming conditions. An increase of 



