MICROSCOPIC STRUCTURE OF SOME AUSTRALIAN ROCKS. 223 



Slice 125. — Diabase porphyrite from near Cowra, New South 

 Wales. In hand-specimens this rock resembles one variety of 

 verd antique. It takes a fairly good polish. Under the micro- 

 scope it is seen to consist of porphyritic felspars enclosed in a 

 micro-crystalline paste. This paste or ground-mass carries a 

 great number of lath-shaped felspars, which show a streaming 

 structure in a perfect manner, as they now round larger crystals. 

 Grains of magnetite are abundant in this base. This is a Pre- 

 Tertiary rock and before secondary alteration had produced the 

 chloritic material, to which the rock owes its green colour, it 

 must have in its structure and composition, resembled a por- 

 phyritic basalt. 



The slices mounted on the Slides, numbered 119 to 125, are all 

 specimens of this rock. The ground-mass is of a light greenish- 

 yellow colour by transmitted light, and the large porphyritic 

 felspars of a milk-white shade. The latter have all become 

 clouded, no trace of the original glassy appearance remaining. 



Slices 126 to 129 inclusive. Diabase porphy rites, Red Hill, 

 Wellington, New South Wales. These rocks are not at all 

 unlike the specimens, just referred to, from Cowra. The rock is 

 best seen in a railway cutting, near the Springs (now Dripstone) 

 Railway Station, Western Line, New South Wales. It is in- 

 trusive in Silurian slates, which it has altered and contorted. In 

 hand-specimens large crystals of felspar, from one-eighth to one- 

 half an inch in length, can be recognized with the unaided eye, 

 embedded in a dark olive-green paste. When examined under 

 the microscope the only minerals recognizable are felspar, augite, 

 magnetite and calcite. The felspars and augites occur both por- 

 phyritically and as constituents of the base. The calcite is a 

 secondary infiltration, filling the joints and fine cracks in the mass. 

 This rock presents a very handsome appearance, and could be 

 utilised for industrial purposes. 



Slice 133. — Frost's Station, near Cobar, New South Wales. 

 The specimen from which this slice was cut occurs adjoining Slice 

 81. The exact nature of the rock it is not easy to determine, but 



