MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 19$ 



and Carcoar, Forest Reefs, and Cudal on the other. These specimens- 

 were taken from the Orange-Cargo Road on the highest shoulder 

 of the mountain which the road traverses. In hand specimens 

 the rock is of a dark colour showing grey to brown on weathered 

 surfaces. It is thickly set with glistening crystals of a felspar, 

 many of which show repeated twining in reflected light. Under 

 the microscope, in plane light, the large crystals are seen spread 

 through an exceedingly fine-grained intersertally composed base. 

 The lath-shaped felspars in this base measure on an average the 

 one-thousandth part of an inch up to the one-fivehundredth in 

 length. The character of this base is very uniform in all the slides. 

 Under high powers some isotropic or partly devitrified granules 

 are seen, together with some irregular grains of magnetite. The 

 porphyritic felspars all clearly of allogenic origin. Some of these 

 felspars are broken and others greatly corroded by the liquid 

 magma. On this slide one remarkable large twin is seen with 

 three of its sides sharp and idiomorphic, while the fourth shows 

 every sign of being corroded and eaten into by the surrounding- 

 magma to a considerable extent. On the edge of the slice two 

 good examples of broken porphyritic oligoclase can be observed. 

 The oligoclase is, in some instances, beautifully zoned. A line of 

 inclusion marks certain zones within the crystals showing a former 

 outline. The building up of the crystal was stayed in its growth 

 at this point, and a subsequent growth produced the outer part. 

 When these crystals are examined with crossed nicols, it will be 

 noticed that just as the point of extinction, for the central core, 

 has been passed, a dark outer zone of extinction makes its appear- 

 ance, gradually extinguishing as the centre lightens. A very fine 

 example will be found on the edge of this slice next to the num- 

 bered label. There is one other remarkable plagioclase on this 

 slide. In plane light, the outlines are noticed to be idiomorphic 

 and the crystal seems filled, towards the centre, with portions of 

 the base. Besides the base matter there is also included a large 

 olivine and a larger augite. In the centre, the main plagioclase 

 is represented by plates of its own material, distributed like a 



