BY THE REV. J. MILNE CURRAN. 223 



Another reaction that renders the olivine of the ground-mass 

 distinct enough from the augite is effected by treating the slice 

 with warm HCL, until on gently drying the olivines gelatinize 

 slightly, when they can be stained by fuchsin. The olivines will 

 then stand out in marked contrast to the augites. 



In many of the slides patches of an isotropic glass can easily be 

 detected, particularly with the help of the quartz plate. The glass 

 often seems of a light wine-red colour by transmitted light. I notice 

 that prolonged treatment with acid has no appreciable effect on 

 this substance. The glass is, therefore, not of a tachylytic nature, 

 but more acid in character. 



From what has already been said it will be gathered that there 

 are two generations of olivine, augite and felspars in tbe Bathurst 

 basalts. This is quite in keeping with the observations that have 

 been made on similar rocks in the Old World. In the peridotites 

 it is common to find olivine in the ground-mass and the same 

 mineral as porphyritic crystals. In many dolerites labradorite and 

 augite form the principal ingredients of a ground-mass in which 

 the same minerals occur porphyritically* I have met with no 

 explanation altogether satisfactory of this common condition of 

 igneous rocks. In the paper just referred to Dr. Bonney remarks 

 that although an explanation of these anomalies does not seem 

 hopeful, we may bear in mind that the temperature of consolida- 

 tion for a mineral out of a magma is not necessarily identical with 

 that of the isolated mineral, as one substance acts as a flux on 

 another. 



As throwing some light upon this interesting question of the 

 separation of minerals from a molten magma, the following extract 

 from a paper by Professor Judd applies to our own rocks. 



" In some instances the mechanically injured condition of the 

 crystals and other appearances strongly suggest their actual trans- 

 port from below in the midst of the materials of the surrounding 

 ground-mass. But in others the porphyritic crystals exhibit zoned 

 structures and other characters not found, perhaps, in the deeper- 



* See Professor T. G. Bonney, Q.J.G.S., Vol. xn., p. 79. 

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