VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 127 



abound in some sections and their development in any one 

 grain is capricious, suggesting a possibility of primary 

 nature. 1 Interaction with the residual magma has been 

 mentioned. Between felspar and pyroxene there seems to 

 be chlorite. Alteration to chlorite is very frequent, show- 

 ing that the augite is aluminous. The possibility of the 

 derivation of large masses of iron oxides from the augite 

 has been noticed. 



The orthorhombic pyroxene is definitely hypersthene, 

 fairly rich in iron. It is pretty abundant and is strongly 

 trichroic with the following scheme: — X, pale reddish- 

 purple; Y, reddish-yellow; Z, greyish-green. 



Perhaps the most important feature of this mineral 

 generally is the development of the so-called schiller plates. 

 Although some crystals have an absence of schillerisation 

 still the great majority show this phenomenon to different 

 degrees. In particular in one crystal which is figured in 

 PlateVK, fig. 2, these schiller plates are seen to be developed 

 parallel to three planes. However, the dominant schiller 

 effect is that parallel to (100) with the minor effect in the 

 plane parallel to (010). 



In the augite the schillerisation is confined in most cases 

 to planes parallel to the orthopinacoid, with examples at 

 times of development in clinopinacoidal planes. 



The question of the origin of schiller structures is a 

 fascinating one. Prof. Judd 2 discussed it at some length, 

 and insisted on the secondary nature of the phenomenon, 

 attributing it to deep-seated conditions. He averred that 

 the degree to which this property is developed in rock 

 minerals is in general a function of the depth from the 

 surface. Essentially Judd's concept, therefore, was that 



1 Of. Harker, "Tertiary Igneous Rocks of Skye," p. 109. 

 1 Q.J.G.S ., 1886, p. 382. 



