VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 115 



on a large scale, of the result of the intratelluric differenti- 

 ation, which is evidenced at Dundas and elsewhere. 



The present paper is based on observations made in 1919, 

 and is intended mainly as a contribution to our petro- 

 graphical knowledge of these interesting occurrences in the 

 Sydney district. 



It is very probable that a more complete suite of inclus- 

 ions exists than is described below, as the outcrop of the 

 dyke containing the xenoliths is very small and decomposed. 



General Geology. 

 A geological sketch map of the neck and the surrounding 

 country is given in fig. 1. The neck presents an elongated 

 shape in plan, being about one and three-quarter miles long, 

 and about twelve chains in mean width. The maximum 

 width does not exceed thirty chains. The general trend 

 of the vent is E. 18° N. 



At its surface outcrop it is intrusive almost entirely into 

 Hawkesbury sandstone. The definite relations of the 

 igneous rock to the surrounding sediments at the eastern 

 end of the neck are somewhat obscured by the recent 

 alluvium of the Nepean and also by the fact that the soils 

 derived from the breccia, the Wianamatta shales and the 

 river alluvium are similar in appearance, but the neck 

 probably breaks through the Wianamatta shales. The 

 filling of the neck is composed almost wholly of a dark 

 greenish breccia, and within this there are blocks which, 

 though termed " inclusions," are identical with certain 

 fragments in the fine-grained matrix, and thus have distinc- 

 tion in size only. On the other hand both foreign and 

 cognate fragments occur within the breccia. 



Examples of the former comprise blocks of quartzite 

 ranging in size up to several feet in diameter. Judged by 

 their general felspathic residuum they are probably meta- 



