184 ON THE GEOLOGY AND PETROGRAPHY OP BATHURST, N.S.W., 



Fracture : even but brittle. Heated in the closed tube, gave off a 

 little water. Dissolves completely in hydrochloric acid. Contains 

 silica, alumina and lime ; proportions not determined. Before the 

 blow-pipe intumesces to a porous mass. This mineral I take to 

 be prehnite. 



8. Felspar. — Orthoclase occurs as a leading constituent in the 

 Bathurst granite. Near White Rock, and other places, it occurs 

 in a porphyritic granite as crystals from half an inch to two inches 

 long. Under the microscope it is more cloudy than plagioclase, 

 which sometimes accompanies it. In most old rocks, when 

 examined in thin slices, orthoclase usually appears more or less 

 impure, on account of foreign substances and cleavage planes that 

 exist in it. In this respect Bathurst orthoclase follows the general 

 rule. No analysis of this mineral has been made, so far as I am 

 aware ; but from the intense colours, afforded by Szabo's methods, 

 I am inclined to think the percentage of potash is high. Typical 

 orthoclase contains silica 64*6, alumina 18*5, potash 16*9. 



Plagioclase. — It is rare to find a thin section of Bathurst 

 granite entirely free from plagioclase, but there is no predominance 

 of this mineral anywhere in the district over the monoclinic felspar, 

 by which the granites might pass locally into quartz diorites. 

 About four years ago I sent some slices to Mr. A. W. Howitt, 

 then of Sale, and he determined that the triclinic felspar of the 

 Bathurst granite was, in all probability, oligoclase. Triclinic 

 felspars, as one should certainly expect, are abundantly developed 

 as microscopic lath-shaped bodies in the basaltic rocks. Any 

 slice of the Bathurst basalt will show this clearly. See PI. xiv. 



Albite. — Mr. Howitt detected this felspar in some micro-slices I 

 submitted to him in 1886. It occurred as minute veins in ortho- 

 clase, placed approximately in the direction of the ortho axis. 



9. Augite. — This monoclinic pyroxene is known only as a micro- 

 porphyritic constituent of the basalts. Sections, approximately 

 parallel to the clinopinacoid, are readily obtainable. It also 

 occurs as minute grains in the ground mass of the basalts. Its 



