BY W. N. BENSON. 667 



plagioclase, augite, titaniferous magnetite, apatite, and a little 

 quartz and pyrites, but the proportions between the minerals vary 

 greatly, as also does the type of felspar present. The handsome 

 rock, forming the bold cliffs of Hanging Rock [N.T., 327] is one 

 of the most felspathic (See analysis). It is composed of large, 

 platy crystals of albite, very slightly decomposed. Between these, 

 and eating into the main crystals, is a matrix composed of small, 

 lath-like albite, sometimes with a roughly parallel structure, some- 

 times with an irregular, confused mat, like the felspar of some 

 trachytes. The pyroxene is in irregular grains, sometimes imbedded 

 in the large felspar crystals, frayed out at the ends, and rarely well 

 bounded; often also it forms small wispy patches lying in the 

 phenocryst or in the ^felspar mat (Plate xxv., fig. 3). The 

 pyroxene is almost entirely converted into actinolite. Ilmenite and 

 apatite, in small amount, make up the rest of the rock, together 

 with a few grains of epidote. The spongy nature of the felspar, in 

 this rock, seems good evidence of its secondary origin by recrystal- 

 lisation through the action of sodic solutions. In others [e.g., 

 M.B., 12; see analysis], the felspar is quite fresh in appearance, 

 not at all spongy, and is albite. The pyroxene, however, is entire- 

 ly changed to pale, fibrous amphibole. This rock is the "country" 

 of a small quartz-reef, south-east of Bingara, and is associated 

 with Tamworth rocks, though east of the serpentine-line. 



The majority of the rocks, however, have a plagioclase of about 

 the composition of andesine. Some slides [e.g., N.T., 197; also from 

 Hanging Rock], show well how albitisation is proceeding inwards, 

 with clarification of the dusty andesine. Its change of composition 

 is clearly not zonal, as it occurs chiefly on the exposed parts of the 

 crystals, i.e., where they project into the interstices. Concurrently, 

 the amphibolisation of the pyroxene is in process. This rock is 

 noteworthy for the abundance of the interstitial quartz, which is 

 clearly primary. In other rocks, [e.g., N.T., 172, from near the 

 Swamp Creek Falls (Nundle)], the pyroxene and felspar (ande 

 sine) are both comparatively fresh. This rock intrudes into a fine- 

 grained tuff, and contains small interstitial areas of pilotaxitic 

 rock like spilite. 



