324 



of occasional phenocrysts of the characteristic opalescent 

 quartz, bunches of biotite, and very sparingly an occasional 

 phenocryst of felspar. A few scattered grains of pyrites are 

 also present. 



The specific gravity of this rock is 2'625. D 1 *° = 2'623. 



Under the microscope the quartz phenocrysts have the 

 same characteristics as possessed in the normal granite. 



Biotite is spread sporadically through the rock with 

 its accompanying chlorite and epidote. Pyrites is also present. 



Quartz and microcline are developed, showing allotri- 

 morphic boundaries. The microcline shows Carlsbad twin- 

 ning, but the grating structure may be absent or represented 

 only by submicroscopic lines of light and shade. 



Some plagioclase is present, and has the properties of 

 oligoclase albite. 



Secondary mica and kaolin are developed as alterations 

 of the felspar. In a phenocryst of microcline the micasation 

 may be well developed. 



A primary micrographic intergrowth of quartz and 

 microcline is often present round the borders of a large 

 quartz grain. A determination of alkalies in this rock gave 

 K 2 = 5'58%, Na 2 = 2'63%. 



The rock is a Biotite Microcline Aplite. In parts it 

 has a distinct granite-porphyry facies, but its relationship 

 to the associated intrusions is better indicated in the name 

 given. 



(b) The Pink Aplite. — This aplite occurs as a distinctly 

 intrusive mass along the coast immediately south of Cannon 

 Hill. Its junction with the granite is in most places 

 markedly sharp. The mass shows a somewhat variable 

 texture throughout its extent ; the greater part is of very 

 fine grain, but this grades into a coarser variety, in which 

 are developed phenocrysts of blue quartz and a ferromagnesian 

 mineral, biotite, also makes its appearance. It is an aplite 

 with development in part of a distinct granite-porphyry facies. 



It is in this rock that occur the quartz tourmaline 

 nodules already described in detail in a previous paper. < 3 ) 



A number of quartz veins occupy fissures in the aplite, 

 and associated with these veins occurs a zone of altered aplite 

 which appears to be of the nature of a greisen. 



A further alteration of the aplite is the production along 

 fissures of a white kaolinized product. A number of quartz 

 geodes are also developed. Microscopic description of this 

 aplite is dealt with on page 325. 



(c) The WhMe Pegmatite (Aplite). — There are a number 

 of occurrences of this type all of which are not noted on the 



(3)Tilley: loc. cit. 



