BY W. N. BENSON. 155 



but which passes without a break into an obviously brecciated 

 rock, filled with fragments both of igneous rock and of limestone. 

 A microscopical examination of the apparently massive rock 

 shows that it also is brecciated. It consists of large shattered 

 crystals and angular fragments of quartz and felspar in a fine- 

 grained felsitic base. Even more remarkable is the agglomeratic 

 rock on the summit of the ridge between Silver Gully and Pipe- 

 clay Creek. This consists of a varied collection of rounded or 

 angular fragments of porphyritic or trachytic keratophyre and 

 magnetite-keratophyre in a matrix of trachytic keratophyre. 

 Though there are no lines of contact with the sedimentary rocks 

 exposed, it seems reasonable to consider this mass as the product 

 of the intrusion of a keratophyre-magma filled with cognate 

 xenoliths.* This is an extension of the conception of a brecci- 

 ated intrusion which is necessary to explain the structure of the 

 Hyde's Creek keratophyre. 



The main series of rocks to which the name quartz-keratophyre 

 is most directly applicable, run along the eastern side of Silver 

 Gully, across Pipeclay Gully to Cope's Creek. Their macroscopic 

 features have been already described. The composition of the 

 rock is fairly uniform throughout, but variations occur. The 

 predominant mineral is acid felspar. This generally, but not 

 always, forms phenocrysts lying in a pilotaxitic, trachytoid or 

 panidiomorphic granular base. With these are sometimes pheno- 

 crysts of augite with a large optic axial angle. The plagioclase 

 of the base is rarely easily determinable, being often rather 

 dusty .• it does not appear, however, to be more basic than oligo- 

 clase. Augite may occur in the base as small prisms, but is 

 generally changed to chlorite : magnetite, ilmenite or titano- 

 morphite may occur in small amount. The greatest diversity 

 arises in the mode of occurrence. A few of the rocks in this 

 mass are free from quartz, but the majority contain it in a 

 manner which raises suspicion as to its primary character. It 

 may be interstitial, or form in little irregular patches against 

 which the felspars are moulded, or it is present in intimate 



* Compare with this the " Eruptive pseudo-conglomerate" described by 

 Clemen ts(i>), p. 135. 



