KOCKS IN N.W. CAERNARVONSHIRE. 281 



thus bounding the felsite on the west, and running through the 

 corner of the road where the great conglomerate changes its strike. 

 A small fault does come out on the shore between the Eaths and the 

 Ferry. Thus the general strike of the beds on the east side of this 

 fault would be to the east of north, instead of to the west of north, 

 as on the other side of Bangor valley. But if my view be correct, 

 all this is a matter of little consequence. The puzzle is to account 

 for the very large pebbles of felsite and other volcanic products when 

 the felsite near at hand must have been covered up by earlier de- 

 posits, and there is none other near to choose from. 



Thus the Pre-Arenig rocks near Bangor form one indivisible 

 series, but what is their age ? Are they also Pre-Cambrian ? The 

 supposed proof of this has rested on the assumj^tion of a basal Cam- 

 brian conglomerate, and the assumption is not justified. With 

 Prof. Eamsay the only question was between what we now call 

 Upper or Lower Cambrian, i. e. between " a " and " 6," and he 

 concluded, from a comparison of these rocks with others in Glyn- 

 lifou Park, that they were Cambrian rather than Lingula-flags. 

 Another reason that led him to this conclusion was the similarity 

 of the conglomerates near Bangor to those near Llanberis, which are 

 undoubtedly Cambrian. Any difference between these was ac- 

 counted for by metamorphism. We can, however, dispense with 

 much metamorphism, and yet compare these rocks with Cambrians. 

 There is little to choose, microscopically or otherwise, between some 

 of the rocks near Gorsbach and some near Llyn Padarn, nor between 

 some varieties near Bangor and the rocks in the heart of the Cam- 

 brian near Llandwrog, nor between the grits north of Perfeddgoed 

 and some west of Dinas Dinorwic. The included fragments may 

 be somewhat different, but they are exactly the same type of rock. 

 The great peculiarities of these Bangor beds are their great total 

 thickness without reaching the great workable slates, and the 

 presence of numerous and various conglomerates. They are called 

 volcanic, but they are arranged in beds after the manner of sub- 

 aqueous deposits, and can only be said to be volcanic as having been 

 derived from the denudation of volcanic products. They are, of 

 course, of later date than the volcanic eruption whose remains are 

 seen in the porphyries. Thus their general features attach them 

 to the Cambrian, and their peculiarities are what we might expect 

 at the base of such a series. Is not the cause of any minor dif- 

 ferences from other Cambrian rocks simply that we never see the 

 base of the Cambrian elsewhere in North Wales ? 



This opens up another question, and forces us to the study of the 

 rocks by Llyn Padarn, since these are supposed to represent the basal 

 Cambrian, and to lie upon a Pre-Cambrian axis. 



Llyn Padarn and Mcel Tryfaen. 



The great conglomerates which lie on the east of the masses of 

 porphyry, on either side of Llyn Padarn, are considered by every one 

 to be the base-bed of the Cambrian. According to Prof. Eamsay, as 



Q. J. G. S. 1^0. 174. u 



