MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 221 



seen that the material of the felspars has been removed or pro- 

 foundly altered, as when the slice is placed between crossed 

 nicols, the felspars remain dark while the surrounding rock 

 lightens. 



Slice 87. — Basalt, Nerriga, New South Wales. The following 

 minerals are present in this rock : — 1. plagioclase, in felted 

 aggregations ; 2. augite, in microporphyritic crystals ; 3. olivine ; 

 and 4. de vitrified glass. It will come under the heading of 

 basalts. 



Slice 93. — Augite-porphyrite, from Blayney, New South Wales. 

 This rock is quarried for road purposes from an open cutting near 

 the Roman Catholic Church, Blayney. In hand-specimens it 

 resembles a diorite ; but, when sliced for the microscope, it is. 

 found to be a very characteristic diabase. When examined 

 under the microscope we find it is a rock as interesting as it is 

 rare, so far as our present knowledge of Australian rocks goes. 

 In general structure it appears as a fine-grained porphyritic rock, 

 with a slightly greenish shade. The porphyritic ingredients are- 

 mainly a monoclinic pyroxene, augite and magnetite. These 

 crystals are set in an exceedingly fine-grained ground-mass of the 

 same materials. The pyroxenes are oftentimes beautifully zoned,, 

 and generally of a greenish colour by transmitted light. The 

 porphyritic crystals are idiomorphic and have their usually 

 sharp angles rounded, and generally show corroded outlines. 

 The black patches of magnetite are seen, with a low power, to be- 

 rather aggregations of magnetite grains than individual crystals. 



Slice 94 is another example of the same rock. A rock closely 

 resembling this diabase occurs at Buchan, on the Snowy River,. 

 Victoria. It has been described by Mr. A. W. Howitt as a 

 diabase porphyrite. 



Slice 95 is a slide of the Buchan rock kindly sent me by Mr. 

 Howitt.* 



Slice 96. — Diabase, Carcoar, New South Wales. On the upper 

 reaches of the Combing Creek, in a line with Mount Macquarie,. 



* Proc. Eoy. Soc. Victoria, Vol. xviii., p. 7. 



