220 ON THE GEOLOGY AND PETROGRAPHY OF BATHURST, N.S.W., 



Hexagons (sum = 720°) : 



(i.) a 117° (ii.) a 113° 



b 132 b 133 



c 118 c 111 



d 120 d 111 



e 123 e 123 



f 107 f 124 



717° 715° 



In hand specimens, the Bathurst basalt bears a strong resem- 

 blance to the Rowley Regis basalt of Staffordshire. It is not 

 unlike, in its texture, a basalt in my own collection from Madeira. 

 It differs, however, from the basalt flows of the same age about 

 Orange and Dubbo. It is commonly known as " blue metal," 

 and I think that the Bathurst rocks have a decidedly bluer 

 shade than the generality of western basalts. This peculiar 

 blue-black is noticeable only on fractured surfaces, polished' 

 surfaces being very dark or almost black. 



Microscopic Structure of the Basalt. — I have cut thirty slices of 

 this basalt, collected at various points between Perth and Mt. 

 Pleasant. Five slices were cut from rocks from the high hill 

 overlooking Perth. Ten slices were made from the columnar 

 basalt in the quarries already referred to, and were taken from an 

 average depth of fifteen feet from the surface. A few slices were 

 collected from the hill known as the Pinnacle, and the remaining 

 number from Mt. Pleasant. Under the microscope there is no 

 essential difference between any of the slides. In fact there is 

 not even a structural difference between the slices from the most 

 widely separated localities. 



In grinding down the sections, the first mineral to show is 

 olivine. While the section is still comparatively thick, the micro- 

 porphyritic crystals of olivine are seen as clear spots in the, as yet, 

 opaque slice. The next mineral, recognisable is invariably the 

 augite, and as the section thins down the plexus or network of the 

 tiny felspars becomes visible. As the section grows thinner, the 



