MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 207 



broken. Sections, nearly parallel zo the clinopinacoid are recog- 

 nizable by the cleavage and the angles of extinction. There is a 

 great deal of green chloritic or serpentinous matter in the slice. 

 This can be detected by the unaided eye on holding the slide up 

 to the light. It might be difficult to account for this mineral, 

 were it not that it is sometimes found completely filling the out- 

 lines of what must, at one time, have been olivine. This secondary 

 product is easily removed by dilute hydrochloric acid. In polarized 

 light, the slice is decidedly doleritic in structure. There is a con- 

 siderable amount of isotropic glass, in which hair-like bodies often 

 showing a radial structure are developed. The exact nature of 

 these I have not been able to determine. 



Slice 23. — Basalt from Cemetery Hill, Carcoar, New South 

 Wales. In hand specimens this seems to be denser than the 

 average western basalts of New South Wales. Its crystalline 

 structure is just apparent to the unaided eye. Under the micros- 

 cope well developed laths of plagioclase is the most abundant 

 mineral. Augite is present in large irregular aggregations pierced 

 by the laths of felspar. The glass, both isotropic and devitrified, 

 is rendered almost opaque by magnetite dust. In a few places on 

 this slide, the magnetite is aggregated into rods that cross each 

 other at right angles, forming a latticed structure reminding one 

 of the well known structures figured by Zirkel. 



Slice 27. — A micro-porphyritic Basalt from Mt. Pleasant, 

 Bathurst, New South Wales. This slice was taken from a hill 

 that forms part of a basaltic flow near Bathurst. This rock is a 

 fine type of the micro-porphyritic structure of some basalts. The 

 constituents present are 1. plagioclase, 2. augite, 3. magnetite, 

 4. olivine, 5. glass, and 6. partly devitrified base. A streaming of 

 the felspars is splendidly developed in some of these slices (Nos. 27 

 to 35). The augite is in a perfect state of preservation, and indeed 

 the more unstable olivine is altered hardly more than along the 

 cracks. The augite occurs in two ways — first, as allotriomorphic 

 granules in the ground-mass, and secondly in large idiomorphic 

 crystals which occasionally give very fine optical sections. 



