164 THE GEOLOGY OF THE CANOBOLAS MOUNTAINS, 



typically white or light bluish-grey in colour, aphanitic in appear- 

 ance (although glassy examples occur), and may be more or less 

 porphyritic. They are extensively developed in the central area^ 

 particularly in and about the Devil's Hole valley, and about the 

 head- waters of the Cadianjulong Creek. The most characteristic 

 of these rocks is the Ortliophyrlc Arfvedsonite Coaiendite CK.bSS) 

 of which there are large exposures at the base of the Young Man 

 Canobolas, and in the Bald Hills. The former hill, with the 

 exception of the top, is built up almost entirely of this rock; so 

 also are the Bald Hills, with the exception of cappings of the more 

 basic trachytes upon two of them. The centre one of these three 

 hills has a rounded summit, exposing a bare outcrop of this rock, 

 devoid of soil and vegetation, which, with the white colour of the 

 rock, gives it a striking appearance. This rock polishes weil^ 

 taking on a beautiful honey-yellow colour mottled with dark 

 blue, and makes a handsome ornamental stone. A second type of 

 arfvedsonite-comendite (X.631) although not differing appreciably 

 in composition, differs somewhat in appearance, and has a char- 

 acteristic bluish-grey colour. This constitutes the bulk of the 

 Pinnacle, and is well exposed all along the east side of the 

 Devil's Hole. On Johnston's Pinnacle massive bold outcrops of 

 light-coloured ^egirine-trachyte occur, exhibiting columnar 

 structure. 



All of these leucocratic types of trachyte give bare rugged 

 outcrops, and the soil derived from their decomposition, as one 

 would expect from their composition, is very poor in character. 

 Columnar structure is not uncommon. The mode of occurrence 

 of these acid lavas suggests that they were erupted from a number 

 of different vents. Owing to their acid composition they solidified 

 rapidly, and consequently did not travel far, but built up steep 

 lava-cones. These are the oldest of the Canobolas lavas, and^ 

 wherever junctions occur, are seen to be overlain by the more 

 basic rocks (see figs.l and 2). 



(b) The Melanocratic Trachytes. — These differ from the leuco- 

 cratic types in containing little or no quartz, a relatively large 

 proportion of the ferromagnesian constituent, and are more 



