282 PEOF. J. P. BLAKE OX THE CAMBRIAN 



I understand him, the conglomerates first lay on. the hidden land 

 from whose denudation they are derived. Then came an eruption 

 of iDorphyry, which crept between the two and absorbed into its 

 substance the lower part of the conglomerate. According to Prof. 

 Eonney and Dr. Hicks, the porphyry was poured out as a lava, and 

 the conglomerate was derived from its denudation. The proofs of 

 this latter view are numerous. Pirst, there are the abundant 

 pebbles of felsite found in the conglomerate, which are certainly 

 like the porphyry below ; and it seems unnecessarily going out of 

 the way when Prof. Kamsaj" says they are like some Bala felsites. 

 Another argument may be derived from the way in which these 

 conglomerates cling to the porphyries all along their range. They 

 are seen in contact on Clegyr and on the south of Llyn Padarn, and 

 though Dr. Hicks supposes there are some schists between them at 

 ]\Ioel Tryfaen, his dark porphyritic-looking rock, which comes in 

 the adit next the felsite, has been recognized by Prof. Bonney as the 

 conglomerate itself; and in a small cutting of the slate railway on 

 the west-south-west of the summit of the hill, the conglomerate is 

 actually seen side by side with the felsite, neither passing into it, as 

 supposed by Prof. Eamsay, nor unconformable to it, since there is 

 no bedding in the felsite. 



Eurthermore Prof. Bonney has pointed out the flow-structure 

 in the porphyry. I may add that, as observed by Prof. Hughes, to 

 the west of Clegyr, and in immediate contact with the conglomerate, 

 there are great masses of agglomerate, and these are repeated in 

 connexion with another flow at a lower level, having a nose of 

 breccia at its termination near Llys Dinorwic. We can scarcely, 

 therefore, hesitate to admit, in the light of such observations, that 

 these are really, practically at least, subaerial flows, and that the 

 conglomerates have been derived in part from their denudation. 



But the next question is, what is the age of the porphyry ? in 

 other words, what did it flow upon ? Prof. Bonney and Dr. Hicks 

 do not seem to have inquired into this. They take it to be Pre- 

 Cambrian, and are apparently hopeless of finding any base or floor 

 for it. If such a floor is to be found, it must be on the western side, 

 since the conglomerate lies to the east : but here there is a great 

 mass of Cambrian strata, and it is the relation of the felsite to these 

 which we must examine. In some parts the junction is marked as 

 a fault, but in the rest it is not. On examining the ground we soon 

 find that the western boundary is for the greater part hypothetical, 

 as it is confessed to be in the explanatory memoir*. The ground 

 is covered by drift, and no junction can generally be seen. The only 

 exception to this is on the north-east side of the river Eothell, where 

 the drift is absent. This, then, is the crucial spot. If there is a 

 fault here, our question must remain unanswered ; but if there is 

 not, and the felsite is older than this western Cambrian, as well as 

 the eastern conglomerate, then the beds on the western margin must 

 turn up and be found lying upon, or dipping away from it, and so 



* Mem. Geol. Surv. vol. iii. 1866. 



