R. D. Roberts — On the Twt Hill Conglomerate, Carnarvon. 195 



beds having led him to refer the conglomerate to the base of the 

 Cambrian. Prof. Hughes's wide experience and acknowledged 

 ability as a field geologist give great weight to his opinion on 

 matters of stratigraphy, but certain difficulties have presented 

 themselves, which cannot be lightly passed over. Apart from the 

 alleged passage of the conglomerate into granitoidite, described by 

 Prof. Bonney, Mr. Houghton and Dr. Callaway, there is the fact 

 that the Cambrian conglomerate of the Bangor district and other 

 parts of Carnarvonshire is largely made up of pebbles of felsite, 

 while the Twt Hill conglomerate is composed chiefly of quartz 

 pebbles and contains no felsite. These objections I believe I am 

 in a position to meet. 



A recent visit to certain sections in Anglesea, and a re- examina- 

 tion of the beds at Twt Hill, under specially favourable conditions, 

 enables me to offer additional evidence strongly confirmatory of 

 Prof. Hughes's view that the bed in question is the Cambrian con- 

 glomerate. 



The following is the substance of notes taken during several visits 

 to the Twt Hill quarry, and confirmed on the 6th of January, when 

 I last visited the district. The quarry was on that occasion free 

 from brambles and undergrowth, which in the summer somewhat 

 obscured the section, and the relations of the beds were clearly 

 shown. On the N.N.W. side of the quarry — western corner — the 

 granitoid rock is succeeded by the conglomerate, which in some 

 parts is fine, in others coarse. The pebbles are mostly quartz ; very 

 rarely one of dark quartzite or schist may be found. They vary 



Fig. 2. Section in Twt Hill Quarry from atoc (Ground Plan). 



from the size of a grain of wheat to that of a pigeon's egg. The 

 quartz pebbles frequently exhibit a glazed appearance, and the 

 matrix contains crystals of iron pyrites disseminated through it. 

 The conglomerate stretches E.N.E. and dips S.S.E. at an angle of 

 about 50°. Succeeding; it and bounding: the southern rim of the 

 quarry is a bed of sandstone, which may also be detected a few feet 

 beyond the quarry. That this rock is not similar to the bottom rock 

 (granitoidite) is clear from the character of the bed, but to remove 

 all shadow of doubt, I have procured a section of the rock for the 

 microscope, and the result completely bears out the stratigraphical 

 inference. Standing in the quarry, and facing towards the N.N.W. , 

 the conglomerate is seen dipping down the rock-face towards the 

 spectator, clinging as it were in patches to the underlying granitoidite 

 so as to give an appearance of beds dipping to the S.W. It is 

 much decomposed by the weather, and the pebbles may with ease 

 be picked out. The rocks are much traversed by fissures, which 

 frequently show slickensided surfaces indicating movement, and in 



