THE METAMORPH1U ROCKS OF ADBLIE LAND. STILLWELL. 171 



Apart from the schistose character the important difference from No. 773 is the 

 large percentage of hornblende. The grain size of the hornblende tends to be a little 

 larger than that of the secondary pyroxene. The green hornblende is found developing 

 directly from the relic platy augite and from the secondary pyroxene. Hornblende 

 grains are found indiscriminately associated with the granular pyroxene, and they 

 border plates of primary augite and arise parasitically within them. Further, horn- 

 blende with small crystals of ilmenite, completely replaces the pyroxene in some of the 

 crystalline layers of the rock. Biotite is much less important in this rock and shows a 

 very strong tendency to be confined to the pyroxene areas. 



The percentage mineral composition of this section is 



Felspar 33-6 



Pyroxene 24-4 



Hornblende 39-5 



Ilmenite 2-3 



Biotite 0-2 



Apatite Present 



This composition cannot be directly compared with No. 773, because it is a schistose 

 rock, and because the section has been cut at about 30 to the plane of schistosity instead 

 of 90. The difference in the ratio of the felspar to the ferromagnesian cannot be con- 

 sidered to demonstrate a change in chemical composition. The expression is, however, 

 useful to demonstrate the degree of hornblendisation. There is far too much pyroxene 

 for the rock to be considered an amphibolite. It is a transition type between the plagio- 

 clase pyroxene gneiss (No. 773) and an amphibolite, and should, therefore, be called a 

 hornblende plagioclase pyroxene gneiss. 



Stittwett Island. 



The basic rocks observed by Sir Douglas Mawson on Stillwell Island were considered 

 by him in the field to be altered dykes. " Irregular bands of black rock," he says in 

 his diary, " exist as at Cape Denison : some of these are not much altered, others are 

 full of fine garnet. The black bands usually extend long distances, and all have the 

 appearance of original dykes. A vertical section of one, exposed in a cliff face, showed 

 that it dipped regularly at 45 to the west." The island contains some of the most 

 interesting members of this dyke series, and remarkable stages of incipient alteration 

 are found. 



No. 951. This example has a coarser grain than most members of the dyke series. 

 The gram size is sufficient to suggest a primary gabbro, because there is little alteration. 

 On the other hand the rock may be a completely recrystallised example, and the " little 

 alteration " may be the incipient development of a second metamorphic phase. The 

 latter interpretation is rather supported by the granulitic texture and the recrystallised 

 character of the surrounding rocks. In section the rock consists of felspar, augite. 



ite, ilmenite, and apatite. 



