158 Sir Roderick I. Miirchison on the 



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 cha- 

 racterised respectively by Upper and Lower Silurian fossils. There 

 are, indeed, districts in which one portion of the Upper Silurian 

 group is unconformable to another ; and, again, lines of dislocation 

 producing unconformity occasionally affect the subordinate members 

 of the lowest group itself, which Professor Sedgwick recognises as 

 one natural whole. But we already know from 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 believ- 

 ing, 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 I'eality 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 inseparable.! When I published the ' Silurian 

 System,' I then knew 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 

 shewn, that out of 74 species of shells and Crustacea in the Upper 

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

 rian forms), 9 at least range into the Lower Silurian rocks of Britain, 

 whilst 14 is the number, if the Lower Silurian type of Northern 



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



t The researches of the Government 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 system only. 



