184'?.] MURCniSON ON THE SILURIAN ROCKS OF N. WALES, ETC. 175 



monstrated to be zoologically synonymous with Lower Silurian, Pro- 

 fessor Sedgwick will not abandon the name he formerly applied to his 

 great physical group, though such name was used before its fossil 

 contents were known. 



The question then is simply : Will geologists find it possible to use 

 two terms to designate the very same succession of animal life upon 

 the surface of the globe ? — such terms being relatively made to de- 

 pend upon the greater or less thickness of the strata and their di- 

 versity of lithological structure ? 



If then greater thickness of the masses be abandoned as a reason 

 for a separate name, some geologists might contend that a physical 

 separation of the upper and lower groups, or the unconformability 

 of the one to the other, would afford grounds for such a distinction. 

 But even this feature is wanting, in reference to the two groups 

 characterized respectively by Upper and Lower Silurian fossils. 

 There are, indeed, districts in which one portion of the Upper Silu- 

 rian group is unconformable to another; and again, lines of dislo- 

 cation producing unconformity occasionally affect the subordinate 

 members of the lowest group itself, which Professor Sedgwick recog- 

 nises as one natural w^hole. Butwealreadyknowfrom the survey of the 

 Government geologists, that all the Silurian strata roll over in con- 

 formable folds throughout South Wales, and that there is no general 

 break between the masses occupied by Upper and Lower Silurian 

 fossils in Wales, any more than within the limits of the Silurian region 

 first chosen as a pattern. Indeed, I have strong grounds for belie- 

 ving, that the very rocks of Bala, about which so much discussion has 

 taken place, will prove to be the physical equivalents of the schists, 

 flags, limestones and sandstones which in South Wales have been 

 described by me as Llandeilo flags. 



There being, then, no unconformity between my Silurian and 

 the Cambrian of Professor Sedgwick, the only remaining ground for 

 changing the name, is the opinion which he seems to entertain, that 

 the Silurian system, as originally described, is in reality made up of 

 two natural-history groups, and ought therefore to have two names. 

 On this point also it is scarcely necessary that I should go beyond 

 the clear evidences recently afforded by the Professor himself, of the 

 great interchange of fossils between the Upper and Lower Silurian 

 groups*, to convince every one that they are so knit together in Britain 

 as to be geologically inseparablef . When I published the ' Silu- 

 rian System,' I then knev^^ that a limited number of species only 

 passed from the upper to the lower group, but in succeeding years I 

 learnt, that many more were common to the two groups both in 

 Great Britain and in other countries. My last memoir on Gothland 

 has shown that out of 74 species of shells and Crustacea in the 

 Upper Silurian rocks of that island (46 of which are British Upper 



* See also Quarterly Journal Geol. Soc, ante, vol. ii. 



t The researches of the Governraeiit Geological Surveyors will presently bring 

 to light facts which will place beyond all doubt, that even in North Wales or 

 Cambria itself, many Upper Silurian forms are intermixed with those of Lower 

 Silurian age ; in short, Professor Edward Forbes has assured me, that in North 

 Wales there is but one natural-history svstem onlv. 



