8 • 



In addition to the porphyritic granite the rocks of Port 

 Elliot include a more acid, even-grained granite, a relatively 

 coarse-grained mass of red aplite, another mass of greisenised 

 soda granophyre, and a dyke of soda aplite. The granites are 

 intersected by pipes and narrow dy^^es of potash aplite and 

 veinlets of coarse pegmatite, with frequent patches of quartz 

 and tourmaline. 



Forphyritic Granite. — This constitutes the greater part of 

 the igneous rock at Port Elliot. It extends from about Green 

 Bay right around past Commodore Point, with one wide gap 

 which is occupied by a sandy bay. 



In general appearance it bears a certain resemblance to 

 the Granite Island rock, but on the more southerly parts of 

 the outcrop the phenocrysts are smaller and the opalescent 

 quartz crystals are perhaps more abundant and of a deeper 

 blue. 



Sharply-bounded inclusions of schist are found, some of 

 large dimensions, and in one place a rounded enclosure of 

 porphyritic diorite, similar to that of Granite Island, was 

 seen. 



The weathering of the granite is distinctive. Whereas 

 the Granite Island rock has a rough and breccia-like 

 surface, that of Port Elliot is smooth, the felspar phenocrysts 

 being by no means prominent. This is due, in part at least, 

 to the fact that much of the rock is being constantly wetted 

 by waves or spray, which protect it from the destructive 

 influences, both mechanical and chemical, of the atmosphere; 

 at Granite Island, in the region of calm water, this protection 

 is not available. 



Even-grained 6^r«m#e.— Megascopically this bears some 

 slight resemblance to the porphyritic granite, but differs from 

 it in the absence of large crystals of felspar and quartz, in 

 the smaller proportion of ferro-magnesian constituents, and in 

 the finer grain. This type of rock is first met as one 

 approaches Port Elliot along the shore from the Victor Har- 

 bour side. It weathers to a reddish-pink colour on the surface 

 and for a little distance down, but at depth shows the blue- 

 grey tint due mainly to the presence of opalescent quartz. As 

 one goes east this even-grained granite is seen to enclose masses 

 of the porphyritic granite and to invade the latter in tortuous 

 tongues. There has been a certain amount of contamination 

 by the older rock, for at the contact the even-grained granite 

 acquires sparingly phenocrysts of felspar and large blue quartz 

 crystals, a feature which dies out with increasing distance 

 from the contact. 



The junction between the two granites is not sharp, but 

 of a "swirling" nature, somewhat like that between the granite 



