EOCKS m N.W. CAERNARVONSniRE. 275 



Prof. Hughes boldly writes down " Twt-Hill beds," and speaks of 

 " the most conspicuous divisional planes suggestive of bedding."' 

 On this I must point out that if a bedded rock could be so metamor- 

 phosed as to produce a quartz-porph3'ry, the first thing to disap- 

 pear, as such, would be the planes of bedding ; and if there are any 

 such planes to be found in a metamorphic rock, we must look for 

 them, not in the most conspicuous, but in the most obscure sub- 

 divisions of the rock-mass. Prof. Bonney brings the microscope to 

 bear upon the question, and declares he can distinguish between a 

 granite and a granitoid rock. The only points of difference that 

 can be tested are given in a note in Quart. Journ. Geol. Soc. 

 vol. XXXV. p. 306. One is that in a granitoid rock the quartzes 

 and felspars show more irregularity in their outlines, size, and 

 distribution ; and another that an indefinite felsitic matrix may be 

 seen. This latter point is surely more suggestive of a porphyry, 

 and there is little use in discussing the former, since it does not cover 

 half the structures found in rocks of the Twt-Hill type. In various 

 parts they put on various aspects, but it is impossible to trace the 

 changes : they come in sporadically and die out gradually, after the 

 manner of a granitic intrusive mass, varying its character according 

 to its circumstances of cooling, and so merging into a quartz- 

 porphyry. From the great similarity of the structures to those in 

 undoubtedly intrusive masses, I conclude that the Twt-Hill rock 

 must be of an intrusive character. But that it was intrusive into 

 the conglomerate is another matter altogether. True, the edge of 

 the conglomerate looTcs burnt in the Twt-Hill field-quarry, but 

 under the microscope this is shown to be due only to infiltration 

 of ferric oxide along the junction. Again, the lowest portion of the 

 conglomerate here is very remarkable. Its quartz-fragments are 

 quite angular, and even irregular ; they are sometimes complex, 

 as if they had come from such a rock as the neighbouring granite, 

 and then show signs of very severe pressure. There are fragments 

 also of quartz-schist* and other altered rocks, while the matrix is 

 composed of secondary quartz and sericite. The rock would cer- 

 tainly appear to be derived from others in the immediate neigh- 

 bourhood, one of which would have to be practically identical with 

 the granite here exposed. 



We must also consider what is the true age of the conglomerate. 

 It is always spoken of as the Cambrian conglomerate, though at 

 Twt-Hill field-quarry the succeeding rocks are scarcely exposed : 

 but the same conglomerate is, or was, seen, by the new church, and 

 not fifty yards away, down the sloping street, black shales are seen 

 (fig. 4), and these are continuous for the rest of the distance to the 

 point, quite close at hand, where Mr. Marr found Arenig fossils '^. 

 Any fault supposed to lie between Arenigs and Cambrians here is 

 entirely hypothetical, and when the beds are traced north is found 

 to be practically impossible. Hence it is that I conclude that this 

 conglomerate cannot be of Cambrian, but must be of Arenig age. 



* Callaway, Geol. Mag. 1881. 



t Quart. Journ. Geol. See. vol. xxxii. 1876. 



