VOLCANIC NECK AT THE JtfASIN, NEPEAN RIVER. 121 



brown material. The cementing material is distinctly 

 calcareous. The quartz sliows sharp extinction on the 

 whole, but some has been strained. The felspar is acid 

 oligoclase. The fragments of lava comprise basalt and 

 andesite, the latter sometimes glassy. The basalt is 

 hyalopilitic in fabric and much chloritised. The indeter- 

 minate brown individuals seem to suggest ostracod remains 

 but Mr. W.S. Dun expressed the opinion, on looking through 

 the slides, that the material was not of organic origin. 

 •Some examples of the rocks under description are extremely 

 fine grained and others very arenaceous in character. 

 Although there is a considerable divergence in some pro- 

 perties, they can be conveniently grouped together. The 

 question of the origin of these numerous inclusions cannot 

 be definitely answered, but there is a similarity between 

 some of the slides and sections of certain horizons in the 

 Narrabeen beds. Those inclusions at the Basin which con- 

 tain fragments of lava, as described above, suggest redis- 

 tributed tuffs. Other slides strongly resemble sections 

 kindly lent by Mr. W. L. Havard of an upper horizon of the 

 Wiauamatta stage which is associated with the calcareous 

 rocks of this age at Picton and elsewhere. It is possible 

 that the inclusions in question are derived from the Narra- 

 been stage or represent remnants of a roof of upper Wiana- 

 matta rocks, an obvious extension of those calcareous 

 horizons which covers considerable areas .in the County of 

 Cumberland. 1 If the latter is in part or wholly correct, 

 then the instance is recalled of the preservation in the 

 vent at Arran Island of blocks of Triassic and Cretaceous 

 strata, the sole surviving relics of a former series now 

 completely eroded away. 



(I)) Rhyolitic Rocks. — This type of inclusion is rare. One 

 specimen was found by the author and a slide of a similar 

 inclusion from the Basin was kindly lent by Mr. G. W. Card. 



1 The writer is indebted to Mr. Havard for information as to the extent 

 of these rocks. 



