180 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



are precisely represented, after numberless undulations, by the oldest 

 fossil-bearing sandstones and schists of North Wales. 



That several of my sections, where they traversed the boundary-line 

 between the Siluria and Cambria of my original map, are erroneous, 

 as my friend states, is necessarily true. Yet even here, the main 

 physical lines are for the most part faithful to nature. Thus, in 

 Montgomeryshire on the east slope of the Berwyns, there are schists 

 which conformably underlie true *'Llandeilo flags" as represented 

 by me * ; but these, instead of being any longer called Cambrian, 

 have been shown by the Government Surveyors to be part and parcel 

 of the Llandeilo formation. The same authorities have further shown, 

 that the overlying and undulating masses of sandstone, which from 

 many of their fossils I had termed Caradoc sandstone (Meifod), are 

 really the upper members of the now more accurately defined Llan- 

 deilo group. I may here observe, that the unconformity between 

 the Llandeilo and the Caradoc, which Ramsay and Avelynf were 

 the first to trace through certain counties, is after all a local phse- 

 nomenon ; for the same authors have ascertained, that, in the vicinity 

 of Bala itself, the equivalent of the Caradoc sandstone overlies con- 

 formably the limestone which is identical with that of Llandeilo J. 

 And thus I am led to adhere more than ever to my old and simple 

 classification of *' Upper and Lower Silurian" rocks, and not to divide 

 the latter into two groups, particularly when it is now ascertained 

 that so very many characteristic species are common to all the lower 

 parts of the series, whether sandstones, limestones, schists, or slates. 

 Caradoc and Llandeilo may be conveniently separated in certain tracts, 

 but, as respects general views, they constitute one natural group. 



Again, my old sections in Pembrokeshire, such as that near 

 Llandewi Felfrey §, which also indicate lower schists conformable to 

 Llandeilo limestone, are correct, if the name of Cambrian be omitted. 

 In other sections |1 along the original frontier there are even strata co- 

 loured Cambrian which overlie the Lower Silurian, proving that at all 

 events I endeavoured to draw fairly what I saw, though I did not 

 then attempt to solve such enigmas. 



With these physical obscurities along the frontier-line I never 

 grappled, because they led me away from a region where all was 

 comparatively clear into a highly complicated tract, the survey of 

 which Prof. Sedgwick had undertaken : and now that the Government 

 Surveyors have shown that our mutual boundary-line was a mere hy- 

 pothesis, and that the so-called " Cambrian" is absolutely composed 

 of undulations of my Silurian rocks, there is no question at issue. 



I trust that on reflection my friend Professor Sedgwick will see, 



* ' Silurian System/ pi. 29. fig. 9. 



t Quart. Journ. Geol. Soc. vol. iv. p. 294 et seg. 



X Such local unconformity can indeed never be admitted as a ground for 

 breaking up any natural group. In parts of South Wales, for example, one 

 portion of the coal is unconformable to another ; and in Brittany, the great break, 

 differing essentially from that in the Silurian rocks of Wales, which is above 

 alluded to, occurs beneath the Llandeilo or lowest Silurian strata, and between 

 these and the unfossiliferous slates or Cambrian. 



§ * Silurian System,' pi. 35. fig. 5. || Ibid. pi. 34. fig, 3. 



