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



basalt, which would distinguish it at once from the leucite basalts 

 that are known to occur at Harden, Byrock, and Cobar. Com- 

 pared with the basalts immediately around, those of Orange and 

 Carcoar for instance, the Bathurst rock is distinctive enough. 

 This is most easily detected in preparing thin slices for the micro- 

 scope. Long before the slice is sufficiently thin, the Orange basalt 

 is seen, by transmitted light, to consist of a felted mass of plagio- 

 clase, with augites and olivines for the most part wedged between. 

 The Bathurst slice on the contrary will show micro-porphyritic 

 minerals in a holo-crystalline base with an abundance of magnetite 

 and drop-like grains of augite. Basalts of this type are not 

 uncommon in Europe and America. The resemblance extends 

 even to such minute details as the serpen tinization of the olivines, 

 and the sharp well-marked features of the iron oxides. Zirkel's 

 remark, relative to the American basalt, applies well to this 

 Australian example. " It is worth while," he says, " to pause and 

 remark that in these widely remote quarters of the globe the 

 product of the solidification of a molten mass, although exposed 

 to many casualties, has nevertheless maintained a surprisingly 

 close identity of microscopical composition."* 



Basalt in the Field. — A glance at the map accompanying this 

 paper will show the exteut of the basalt. It marks the course of 

 an old river valley. At the outside it is not more than 150 to 200 

 feet in thickness where it lies deepest. It can be studied well at 

 the quarries on the Bald Hills, where stone is obtained for road 

 purposes. Perth railway station is very convenient to the hill 

 marked F. Here the basalt forms one of those table-topped hills 

 which, in the western district, are invariably recognised, even from 

 a distance, as ba sal tic. The road from Perth to Evans' Plains 

 crosses a saddle in the hills. On this road sections of decomposed 

 granite are exposed, where the weathering of the rock can be 

 noted. About half way up the hill water-worn pebbles will be 

 found, increasing as we ascend. These have weathered out from 

 the drift that lies between the granite and basalt. As soon as no 



* Zirkel, I.e. p. 233. 



