MICEOSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 215 



properties of black mica. Besides the orthoclase, striated felspars 

 can be picked out in reflected light. In this respect, the granite 

 differs from most of the Australian rocks that I have observed. 

 Under the microscope Slice 60 is fairly characteristic of the rock, 

 except that the hornblende constituent is not well represented.. 

 There are half-a-dozen characteristic flakes of biotite in this slice. 

 These are all found to be strongly dichroic. When rotated above 

 the lower nicol, they are a light yellow when the vertical axis of 

 the crystal is parallel to the plane of vibration of the light, and 

 dark indeed almost black, when the cleavage corresponding to 

 the lateral axis is parallel to this plane. There is rather more 

 triclinic felspar in this slice than is usual in slices of true granites,, 

 so that, were it not that I had studied the rock in the field, I 

 should be inclined to assign it a place among the quartz-mica- 

 diorites. The quantity of triclinic felspar present is most easily 

 discovered by examining the rock under crossed nicols. The 

 quartz is of a character, usual with granitic rocks, showing broad 

 plates of colour with the nicols crossed. Under high powers, the 

 quartz is seen to contain a large number of fluid cavities. Some 

 of these appear quite full, and many show that they contain 

 bubbles — probably of carbonic acid gas. Some of the most minute 

 of these cavities have bubbles exhibiting spontaneous movements. 

 I have examined a great number of these and noticed that the 

 smallest bubbles are the most active. Some vibrate too fast for 

 the eye to follow their motions while others move more slowly 

 round the containing cavities. Slices 62 and 63 show rather 

 more of the fluid inclusions in the quartzes than is usually seen. 

 I never noticed a gas cavity in the felspars of this rock. 



A rather distinctive character of quartz in this rock is the 

 presence of large amounts of hair-like bodies that run for long 

 distances through the silica. Occasionally they branch at the 

 ends, I have also noticed them bend away at a sharp angle, and 

 also to radiate from a single point. Similar bodies termed trichites 

 have been observed in other granites. They occur in a granite at 



