212 ON THE GEOLOGY AND PETROGRAPHY OF BATHURST, N.S.W., 



Muscovite. — Muscovite is, comparatively speaking, rare. Mr. 

 A. W. Howitt first pointed out its presence to me. It is common 

 enough in the aplite and kindred rocks on the borders of the 

 granite country. But in the main body of the granite it has been, 

 up to the present, detected only under the microscope. 



Magnetite is readily recognised in every slice by its remaining 

 opaque in the thinnest sections, and by its peculiar lustre in 

 reflected light. Sphene is another rare constituent. It appears 

 in clear brownish-red granules. Sometimes wedge-shaped crystals 

 can be seen with dark or almost opaque edges. 



I have selected four fairly typical slides from my rock slices, 

 and I will give a short description of their microscopic characters. 



1. (Slice 48). The general appearance of this slice under the 

 miscroscope is that of a holo-crystalline rock. Some of the micas 

 and hornblende show ci'ystalline faces, but the quartz and felspars 

 are, for the most part, allotriomorphic. With crossed Nicols, a 

 considerable quantity of plagioclase becomes visible, but it is 

 altogether subordinate in amount to the orthoclase. The quartz 

 occurs in broad plates, filling up the interspaces between the other 

 minerals, and showing in polarized light the customary brilliant 

 colours. Glass cavities and fluid cavities are very abundant in the 

 quartz. Fluid cavities, with bubbles of gas, can be readily found 

 with a magnifying power of about seventy -five diameters. By 

 using i immersion lens, cavities containing spontaneously moving 

 bubbles can be detected. The Bathurst granite affords abundant 

 material for studying this wonderful phenomenon. The slide I 

 am describing contains many good examples of spontaneously 

 moving bubbles. Some of these bubbles move round the cavities 

 slowly, reminding one of the movements of a rotifer in search of 

 food. Others are stationary until the slice is slightly heated, when 

 the bubbles are seized with a sort of trembling motion and suddenly 

 start off travelling round the cavity. I have noticed many in 

 which the movements are so rapid that it is difficult for the eye to 

 follow them in their course. In this slice brown dichroic mica is 

 abundant. It becomes almost dark in some positions as the 



