MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 217 



than to a granite. This undoubtedly lends some weight to my 

 own opinion that these inclusions represent fragments of clastic 

 rocks, caught up in the granitic magma. It is hard to see how 

 they can be due to any selective influences in the cooling mass. 



Slice 70. — Granite from Moruya, New South Wales. This is 

 a rock well known in New South Wales and Victoria. It is 

 extensively used for building and ornamental purposes. The 

 columns in front of the General Post Office, Sydney, are of this 

 material. In colour and general structure it resembles the Bathurst 

 rock. Under the microscope the following minerals are recognized 

 1. quartz, 2. orthoclase, 3. triclinic felspar, 4. biotite, and 5. 

 hornblende. The orthoclase is nearly always cloudy and seldom 

 idiomorphic. In one instance on this slide, cross-banding or cross- 

 twinning shows beautifully under crossed nicols. The triclinic 

 felspars (oligoclase) are, as usual, well brought out in polarized 

 light. Both felspars occur without any crystalline outline. The 

 quartz is moulded around the felspars and hornblende. The biotite 

 shows a wavy structure as if subjected to a bending or twisting. 

 Large masses of quartz, although seemingly continuous in ordinary 

 light, are shown in polarized light to consist of many distinct patches 

 each one of which shows a different interference colour surrounded 

 by a prismatic girdle. From a microscopic examination, one 

 would conclude that the rock is of an enduring character, the 

 felspars though cloudy not being kaolinized to any considerable 

 extent and there is no notable excess of fluid cavities. I notice 

 too the absence of the slender hair-like bodies that are present in 

 many other Australian granites. 



Slice 72. — Hornblende Biotite-Granite from Burdekin River, 

 Queensland. There is a large development of this rock above 

 Bowen on the Burdekin River, and on the Queensland coast. 

 Macroscopically it would be described as a grey granite, hornblende 

 being plentiful in every specimen. Under the microscope the 

 quartz is water-clear and contains an abundance of fluid cavities. 

 Many of these cavities resemble the forms figured by Mr. Sorby 



