BY M. AUROUSSEAU. 307 



Walkom, 1911), and in those of the Bingara-Barraba district 

 (Benson, 1913, Pt. iii.). 



The effects of physical instability in the magma (Harker, 1909, 

 Ch. ix.) are widespread in the rocks described, but have not been 

 noted elsewhere in the series, except in the quartzes of Pokolbin. 

 The corrosion of the quartzes and felspars was probably due to 

 the sudden change from plutonic conditions on eruption, but the 

 resorbtion of the hornblendes and biotites, and the zoning of the 

 felspars were undoubtedly intratelluric. 



In four rocks, from three localities, the grouiidmass has the 

 exact composition of a felspar, being a devitrified felspar-glass. 

 A similar feature has been observed in a devitrified obsidian 

 from Mount Alford, Queensland, (Wearne and Woolnough, 1911, 

 p. 154) but I am unable to find record of the phenomenon else- 

 where. 



Combined with the low ferro-magnesian content of the rocks 

 under consideration, it indicates that they are trachytic in com- 

 position, though andesitic m habit. Trachytes are notably absent 

 from the collection studied, though present in the similar assem- 

 blages of Pokolbin (Browne and Walkom, 1911), and Tangorin 

 (Walkom, 1913). 



According to the criteria established by Iddings, the quartz 

 of all the acid rocks crystallised rapidly 



The accessories are interesting; the iron-ore is probably all 

 titaniferous (Walkom, 1913; Jaquet, 1901). Zircon is remark- 

 ably abundant (cf. Benson, 1913, Pt. iii.), while apatite is ex- 

 ceedingly rare, except in (390). 



The felspars are all comparatively fresh, and singularly free 

 from the ordinary types of alteration. On the whole, they are 

 more acid than those in the Pokolbin rocks. 



The pyroxene-amphibole-mica andesites of Martin's Creek, 

 Paterson, and Eelah, are so exactly alike, except in their degrees 

 of weathering and the consequences thereof, that I have no hesi- 

 tation in stating that they are derived from the same magma- 

 fraction. The similarity of the resorbtion-effects in both rocks 

 makes it highly probable that they were erupted at the same 

 time. 



