Silurian Bocks of North fFalee, ^c. 161 



All that I knew of the fossils of the co-called Cambrian rocks, at 

 the period of the publication of my work was, that the chief forms 

 which occurred in their Eastern or upper division, as at Bala, were 

 specifically the same as my Lower Silurian types, — a strong proof 

 that there was a gradual transition from the one to the other, and that 

 ths boundary-lino drawn upon my map between the tracts named 

 Siluria and Cambria was merely provisional, and had been laid down 

 on no fixed geological principle. Thus, in speaking of some of these 

 Bala fossils, my words were,—" As these shells abound also in the 

 Lower Silurian rocks, it would seem that as yet no defined line of 

 zoological division occurs between the Lower Silurian and Upper 

 Cambrian group ; and that as our knowledge extends^ we may pro- 

 bably fix the lowest limit of the Silurian beneath the line of demarca- 

 tion which has for the present been assumed "* Surely woi'ds could 

 not more explicitly shew, both that the boundary-line marked in the 

 map was not a natural limit, but simply a geographical (not a true 

 geological) line which ran between my working-ground and that of 

 Professor Sedgwick; whilst they clearly indicated my belief, that the 

 Silurian system, as distinguished by its fossils, would probably be car- 

 ried further downwards when the region should have been thoroughly 

 explored. 



In fact, the problem to be then solved was — whether the enormous 

 masses of rock which seemed to rise out from beneath the Bala beds 

 would bo found to contain a distinct group of organic life \ If not, 

 I for one should never have thought of considering them as consti- 

 tuting a system equivalent to the Silurian. 



In the mean time, other geologists began to occupy themselves in 

 North and South Wales. Mr Bowman made an excellent addition 

 to our acquaintance with the former, by shewing the exact equiva- 

 lents of some of my Upper Silurian strata in Denbighshire; and whilst 

 Professor Sedgwick was renewing his labours, Mr Sharpe also entered 

 the Welsh arena, and contributed, as the Society knows, many new 

 and valuable data. But whoever was the inquirer, no fossils were 

 reported or heard of but " Silurian." In South Wales, Sir H. de la 

 Beche and the Government Surveyors shewed, that the rocks of 

 certain quartzose and slaty tracts which had been loosely mapped 

 as Cambrian by myself, were, in truth, physically, as well as zoolo- 

 gically, the same Lower Silurian strata I had described on their 

 eastern and southern frontier, and that thus, with the exception of 

 certain limited districts occupied by very ancient unconformable grey- 

 wacke void of fossils, as at St David's, &c. (like that of the Longmynd 

 in Shropshire), the lowest fossiliferous strata in South Wales were all 

 conformable and connected masses. Even in a geographical sense, 

 therefore, the question whether the so-called Cambrian rocks could 

 be characterised by peculiar fossils, seemed to be thus narrowed 



* Sil. Syst., vol. i., p. 308. 



