136 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. lDcC. 16, 



entered the country single-banded, and while it was unknown in all 

 its peculiarities of physical structure. 



If, in the papers recently published in our Journal, there is a 

 difference between Mr. Sharpe and myself as to the interpretation 

 of certain phaenomena connected with igneous products (a subject 

 undoubtedly obscure and difficult), there is also a difference as to 

 facts, about the interpretation of which there can, I think, be no 

 final ambiguity. Thus, for example, Mr. Sharpe has placed an an- 

 ticlinal line in the Berwyn chain where I believe no such line exists, 

 and has thereby thrown into some confusion the relations of the 

 neighbouring country. This, however, involves an error of small 

 amount in comparison w'lih one introduced by an hypothetical fault 

 which he places along the strike of the great slate quarries of Pen- 

 rhyn and Llanberis. By the interpolation of this fault, he has been 

 enabled to refer the whole of the series, west of the Penrhyn and 

 Llanberis slates, partly to the upper division of the Lower Silurian 

 rocks, and partly also to the Upper Silurian rocks : I was staggered 

 at a statement so directly contrary to everything I seemed to know 

 of the structure of North Wales. But I resolved to give this new- 

 hypothesis a fair examination ; and I commenced my task by re- 

 visiting the great quarries of Penrhyn, Llanberis, &c. The result 

 I may state in a few words. My former views and sections were 

 correct. The Penrhyn slates underlie the system of Snowdonia : 

 they are among the older portions of the rocks of North Wales. Of 

 the truth of this there is not, I think, the shadow of a doubt. The 

 fact is proved by one of the plainest sections in North Wales. 

 Certain rocks on the shores of the Menai (called Upper Silurian in 

 the paper to which I am referring) are, I believe, older still. But 

 this is a point of indirect interpretation rather than direct proof, and 

 is made out by analogies which I must discuss in the course of this 

 paper. In short, the slates near Bangor and Carnarvon are among 

 the very oldest rocks of North Wales ; of course excepting the cry- 

 stalline and hypozoic groups of Anglesea and of the south-western 

 shore of Carnarvonshire, which I do not at present wish to notice. 



Sections from the Menai to the great Carnarvonshire Slate Quar- 

 ries, the top of Moel Hebog, ^c. 



Leaving then the consideration of the hypozoic groups, we find 

 along the shores of the Menai Straits, from Bangor to Carnarvon, in- 

 terrupted masses of dark-coloured earthy slates, which, were we to 

 judge only by mineral structure, might easily be confounded with 

 Upper Silurian rocks. On the shore near Bangor they alternate 

 with trappean conglomerates and trappean shales (schaalstein), and 

 in that respect cannot be distinguished from the old Cambrian slates. 

 The same alternations are seen on the road between Bangor and 

 Carnarvon. They are cut through by a great intrusive rib of sye- 

 nitic porphyry of a different epoch, which ranges very nearly with 

 the beds, and does not appear to disturb their general relations. 

 These rocks are now traversed by a tunnel connected with the new 

 railroad, and among the materials brought out I was not able to 



