73 



mind him that no definite base-line of the Paleeozoic roclcs can be 

 established by one transverse section only, which terminates in 

 the centre of a very complicated region. He must know, that 

 deposits which have no existence in a given territory set on and 

 expand in adjacent tracts. Although, then, the structure of the 

 Lake Country naturally gives him confidence in defining the base 

 of the Silurian system by the comparison of the Bala rocks with 

 those of Coniston, still it remains to be proved, whether the 

 north-western tracts of North Wales do not contain other fossili- 

 ferous bands inferior to those of Bala. As Mr. Sharpe has not 

 examined this part of the country, the question must be answered 

 by others ; and I rejoice to say that the reply is about to be made 

 by the geologist, who, above all others, is most conversant with that 

 region. 



You are aware, Gentlemen, that this is the very tract in which 

 Professor Sedgwick has so long worked, and from survey's of 

 which he gained that intimate knowledge of slaty structure, which 

 is now considered, thanks to his masterly memoir, an essential ele- 

 ment in practical geology. I dwell upon this point with peculiar 

 pleasure, because I well recollect the day when the truth of those 

 lessons, which I first learnt from my friend, were opposed by many 

 and accepted by few, though they now form part of the text-book 

 of the field surveyor. To a re-examination of this country, then, 

 Professor Sedgwick has devoted portions of the two last summers, 

 with the distinct object of ascertaining, first, whether he was correct 

 in his original opinion, on which I steadily relied, that great masses of 

 the slaty rocks of North Wales, sometimes containing fossils, dipped 

 under the Silurian rocks described by myself; and if so, secondly, 

 what zoological distinction could be established between such rocks 

 and those described as Lower Silurian. We were both awai'e, and 

 the point was fully commented upon in my own work, that the Bala 

 limestone fossils agreed with the Lower Silurian * ; but depending 

 upon his conviction that there were other and inferior masses also fos- 

 siliferous, we both clung to the hope that such strata, when thoroughly 

 explored, would off'er a sufficiency of new forms to characterize an 

 inferior system. The results of Professor Sedgwick's recent re- 

 searches would have been communicated to the Society before this 

 Anniversary, had not his other avocations prevented his visiting 

 London ; and as the memoir will shortly be read before you, I will 

 now so far allude to it only as to enable us to draw conclusions 

 respecting the base of the fossiliferous slaty rocks of North 

 Wales. 



Professor Sedgwick has reassured himself that there are fossili- 

 ferous slaty masses, of great vertical thickness, which rise out from 

 beneath the lowest Silurian rocks of North Wales hitherto described, 

 and occupying the region of Merionethshire and Snowdonia, ulti- 

 mately rest upon chloritic and micaceous schists (Menai Straits), 

 into which they do not pass. The lowest of these fossil bands, 



* See Silurian System, p. 308. 

 VOL. IV. PART I. G 



