320 



schists, and mica schists. The whole region has suffered consid- 

 erable regional metamorphism, in common with the metamor- 

 phism shown by the eastern beds of the Mount Lofty Ranges 

 on the mainland. The differentiation of contact and regional 

 metamorphism for the area under consideration would 

 demand much careful field and petrographic study. 



III. Characters of the Rock Types. 



(a) THE MAIN GRANITE. 



Throughout the mass, the granite maintains a very con- 

 stant mineralogical and textural character. In hand specimens 

 the rock is more or less even-grained, with the occasional 

 development of phenocrysts of unstriated felspar. The most 

 striking feature of the rock is the presence of subidiomorphic 

 crystals of quartz, showing a remarkable blue opalescence. 

 This quartz is also developed in the minor intrusions associ- 

 ated with the granite. The felspar shows well-developed 

 cleavages, and often contains inclusions of biotite. Occasion- 

 ally a felspar phenocryst may show, with the aid of a lens, 

 an intergrowth with quartz — suggestive of a graphic inter- 

 growth. According to the freshness of the rock the felspars 

 are seen as greyish, pink, or tending to greenish in colour. 

 The dark mineral is biotite. In addition small quantities of 

 iron pyrites can be detected in some specimens. Under the 

 microscope the minerals developed are seen to be quartz, 

 microcline, plagioclase, biotite, and, as accessories, muscovite, 

 apatite, ilmenite, pyrites, and zircon. As secondary minerals 

 there are present epidote, leucoxene, sericite, kaolin, and 

 chlorite. 



Quartz occurs, firstly, as subidiomorphic grains with well- 

 developed cracks, and showing undulose extinction. These 

 represent the grains seen in hand specimens. Minute 

 inclusions are very numerous, and in many cases appear to be 

 laid out in strings. Many of these undoubtedly are fluid or 

 gaseous inclusions, whilst others appear to be solid, and 

 probably represent rutile needles. Whilst in reflected light 

 this quartz is characterized by a bluish opalescence, in trans- 

 mitted light it has a distinct yellowish to reddish yellow 

 appearance, according to the thickness viewed. 



Again, some of the quartzes when carefully examined 

 show zones of alternate clear and opalescent layers in reflected 

 light, and, in transmitted light, these show up as colourless 

 and yellowish areas respectively. These zones appear to 

 follow the outlines of the growing crystal. D 2 2o=2'650. It 

 is not proposed to consider this question in the present paper, 

 as the subject is reserved for a subsequent communication. 



