developed, and is of comparatively recent geological age. It 

 transgresses over the granite at Port Elliot, and patches of it 

 are to be seen on the east and south of Granite Island as 

 well as at Rosetta Head. 



The igneous rocks form a complex of distinct but closely- 

 related intrusions comprising granite of two different kinds, 

 albite syenite, quartz mica diorite, soda ^ranophyre and 

 aplite, and potash aplite. A number of doleritic intrusions 

 are, as will be shown, possibly related to the more acid rocks. 



As indicated above, the igneous rocks outcrop only at the 

 three prominent points — Port Elliot, Granite Island, and 

 Rosetta Head — in addition to the small islets, which were 

 not visited. 



(1) GRANITE ISLAND. 



Fo7"phyritic Granite. — The island is composed for the 

 most part of porphyritic granite, which is the normal or pre- 

 ponderating type, and is found also at Port Elliot and Rosetta 

 Head, as well as on West Island, and apparently on most of 

 the other islets too. It is a coarse-grained biotite granite, 

 very conspicuously porphyritic in felspar of a greyish-pink 

 colour, and studded with bluish opalescent quartz crystals 

 and grains up to nearly half an inch in length, which impart 

 to the rock a very striking and characteristic appearance. 

 The granite weathers in such a way that the felspar pheno- 

 crysts, some of them upwards of 2 in. in length by about 1 in. 

 in breadth, stand out from the rest of the rock, causing it 

 to have, from a little distance, the appearance of a conglomer- 

 ate or breccia (pi. i., fig. 2). 



Jointing is very conspicuous, and is very noticeable near 

 the breakwater, where the direction of the joints is a little 

 east of north. Here the joint-faces exposed by quarrying show 

 slickensides, as well as a skin of sericitic mica, resulting doubt- 

 less from differential movement during the uplifts and 

 subsidences to which the area has been subjected since the 

 consolidation of the granite. 



Inclusions, both small and great, of the invaded schists are 

 fairly numerous : one of these, on the west side of the island, 

 is about 12 ft. long. As a rule the boundaries of these 

 inclusions are quite sharp, no apparent assimilation having 

 taken place. In a number of cases, however, small inclusions, 

 about 3 or 4 in. long, appear to have been altered and 

 partially assimilated. 



Quartz Mica Diorite. — The granite is traversed in a 

 roughly meridional direction, from side to side of the island, 

 by two broad dyke-like masses of dense bluish rock, in parts 

 fairly thickly studded with large felspars and smaller quartz 

 crystals similar to those of the porphyritic granite, but in 



