146 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Feb. 25, 



changed, that the successive beds are seen to plunge, with a northern 

 dip, under the rocks forming the base of the great deposits of Den- 

 bigh flagstone which compose the true Silurian series of North Wales. 

 In this view, the physical separation of the Cambrian and Silurian 

 series is not hypothetical, but perfectly natural ; and the zoological 

 separation, taken on the whole, is, perhaps, as complete as the phy- 

 sical. Sections, illustrative of these points, I have exhibited many 

 times before this Society, and I must not now attempt to reproduce 

 them. 



The first question which arises, when we attempt to separate the 

 great Cambrian series into subordinate groups, is this : — What is the 

 base of the whole series ? Properly speaking, there is no true mine- 

 ralogical base in North Wales, unless we take the metamorphic rocks 

 as a kind of hypothetical base ; but the lowest groups of the whole 

 series may, if I mistake not, be seen on a part of what I have called 

 " the great Merioneth anticlinal," and also among the red-coloured 

 slates which rise from below the great quarries of Nant Francon and 

 Llanberris. 



If this conclusion be true, there are two base-lines, on either of 

 which we might construct an ascending section through the Cambrian 

 rocks ; and, knowing the importance of putting this view to the test, 

 I employed my friend John Uuthven, in 1 846, to seek for fossils in the 

 dark slates of the Menai, and also among the beds which form the 

 great ascending section east of the Bangor slate-quarries. But he 

 failed in finding any fossils in the dark slates, and we both failed in 

 finding the Lingula-beds where I expected them ; although it was ob- 

 vious, from analogy, that they ought to be found a little above the 

 coarse grits (Harlech grits) which overlie the Bangor slates. 



During the following year I wrote to my friend Mr. Jukes, inform- 

 ing him of my assumed base-line, and of the position I had, from the 

 first, given to the Harlech grits, where they were represented in the 

 Caernarvon chain ; but I added that my scheme was defective in fossil 

 evidence, since I had failed in discovering the Lingula-flags above the 

 beds, which seemed very well to represent the Harlech grits. In his 

 reply he informed me that Professor Ramsay and his fellow-labourers 

 hadfound, and found in their right place, the beds for which I had 

 more than once sought in vain. I have stated these facts, in a few 

 sentences, to show that I have no wish to appropriate to myself dis- 

 coveries which are due to others, and that I have never put forward 

 any views respecting the grouping of the Cambrian series in a rash 

 and hypothetical spirit. I now consider it beyond all doubt that 

 there are, as stated above, two base-lines (on the same geological ho- 

 rizon), on which we might proceed to construct the successive groups 

 of an ascending natural section through the Cambrian and Silurian 

 series. 



The Bangor group (No. 1). — In the accompanying tabular view 

 under the term "Llanberris slates" are included not merely the 

 slates of the great quarries of Llanberris and Nant Francon, but 

 a series of slates and hard grits, with a few bands of porphyry, which 

 undulate towards the west, and are partly cut off by a great mass of 



