1852.] SEDGWICK ON THE LOWER PALAEOZOIC ROCKS. 153 



Berwyns to Bala, that my friend left me (and it was the last time we 

 ever met to work together in North Wales) with a most express de- 

 claration that the Bala groups could not be brought within the limits 

 of his system. At that time therefore (1834) he knew that the lower 

 beds of the Silurian series, and the upper beds of the Cambrian series, 

 could not be at once separated by their fossils. And this opinion 

 has been expressed by me, again and again, in my published papers, 

 as well as in the reiterated discussions before this Society. 



Such was my confidence in the decisions of my friend on any 

 question respecting Silurian rocks, that I accepted his determination 

 of the Meifod group, I might say, with implicit faith, and set it down 

 as Caradoc. But, in that case, the beds of Glyn Ceiriog, and many 

 other beds on their strike, and skirting the northern limits of my 

 Cambrian series, must also be Caradoc. I supposed, therefore, that 

 several masses of calcareous slate (such as those of Cader Dinmael, 

 Penmachno, &c.) might have been put in a false position in my field- 

 sections ; and that in truth they might be subordinate to the con- 

 glomerates, grits, and flagstones, &c., which range not far from the 

 Holyhead Road, at the base of the Denbigh flags ; in which case they 

 must come into a true Caradoc group. Such was the hypothetical 

 conclusion to which I was driven. 



I had then no opportunity of putting this hypothesis to the test ; 

 but the next time I visited North Wales (1842), in company with 

 Mr. Salter, I found at once that the calcareous slates above-mentioned 

 were not subordinate to the group I had called Caradoc sandstone. 

 On the contrary, they were all undoubtedly subordinate to the great 

 Bala group, and therefore a part of my Cambrian series. It followed, 

 therefore, that I had hypothetically tortured the upper groups of my 

 Cambrian series to make them fit to the lower groups of the Silurian 

 series. In this I had done wrong ; for the event has proved that my 

 Cambrian sections were right in principle, while the lower groups of 

 the Silurian sections were wrong. From this time (1842), I began to 

 lose my confidence in the stability of the base-line of the " Silurian 

 System." 



From 1834 to 1842 I had accepted Sir R. I. Murchison's conclu- 

 sion, and made the Meifod beds Caradoc or Silurian, and the Bala 

 beds Cambrian ; but the only hypothesis on which this conclusion 

 could be maintained was dissipated at the first so-called Caradoc 

 quarry which I examined in 1842 in company with Mr. Salter. I 

 need not allude to our joint labours in 1842 and 1843. I did not 

 during those two summers alter a single important Hne in my Cam- 

 brian sections ; but what did the subdivisions of the sections, mean ? 

 That was to be settled by the fossils, and I had a friend with me who 

 could give me, I thought, an oracular response. He concluded on 

 fossil evidence, and the conclusion was borne out by the sections, that 

 the Meifod and Glyn Ceiriog and Bala beds were nearly on one par- 

 allel. Hence, if the Meifod beds were Caradoc, the Bala beds must 

 also be Caradoc, or very nearly on its parallel. But if so, it followed 

 almost of necessity, that the great undulating masses of sandstone 

 between Mallwyd and Can Office must be Upper Silurian. And, by 



