Trof. T. Sterry Runt — On Cambrian and Silurian. 507 



mably." (Siluiia, 4th ed, p. 46.) In each of these cases, on the 

 contrary, according to Eamsay, there is observed ''a break very 

 nearly complete both in genera and species, and probable ixncon- 

 formity ;" the evidence of the palajontological break being furnished 

 by the careful studies of Salter ; while that of the stratigraphical 

 break, as we have seen, leaves no reason for doubt. (Ajem. Geol. 

 Surv., vol. iii. part 2, pp. 2, 161, 234.) The student of Siluria soon 

 learns that in all cases where Murchison's pretensions were con- 

 cerned, the book is only calculated to mislead. 



The reader of this history will now be able to understand why, 

 notwithstanding the support given by Barrande, by the Geological 

 Survey of Great Britain, and by most American geologists, to the 

 Silurian nomenclature of Murchison, it is rejected, so far as the 

 Lingula-flags and the Tremadoc slates are concerned, by Lyell, 

 Phillips, Davidson, Harkness and Hicks in England, and by Lin- 

 narsson in Sweden. These authorities have, however, admitted the 

 name of Lower Silurian for the Bala group or Upper Cambrian of 

 Sedgwick — a concession which can hardly be defended, but which 

 apparently found its way into use at a time when the yet unravelled 

 perplexities of the Welsh rocks led Sedgwick himself to propose, for 

 a time, the name of Cambro-Silurian for the Bala group. This want 

 of agreement among geologists as to the nomenclature of the Lower 

 Paleeozoic rocks, causes no little confusion to the learner. We have 

 seen that Henry Darwin Eogers followed Sedgwick in giving the 

 name of Cambrian to the whole Palaeozoic series up to the ba'se of the 

 May Hill Sandstone ; and the same view is adopted by Woodward in 

 his Manual of the Mollusca. The student of this excellent book will 

 ■find that in the tables giving the geological range of the mollusca, 

 on pages 124, 125 and 127, the name of Cambrian is used in Sedg- 

 wick's sense, as including all the fossiliferous strata beneath the May 

 Hill Sandstone. On page 123 it is however explained that Lower 

 Silurian is a synonym for Cambrian, and it is so used in the body of 

 the work. 



The distribution of the Lower and Middle Cambrian rocks in 

 Great Britain may now be noticed. The former, or Bangor group, to 

 which Murchison and the Geological Survey restrict the name of 

 Cambrian, and which they sometimes call the Longmynd, bottom or 

 basement rocks, occupy two adjacent areas in Caernarvon and 

 Merionethshire; the one near Bangor, including Llanberris, to the 

 north-east, and the other, including Harlech and Barmouth, to the 

 south-east of Snowdon; this mountain lying in a synclinal between 

 them, and rising 3571 feet above the sea. The great mass of grits 

 or sandstones appears to be at the summit of the group, but in the 

 lower part the blue roofing-slates of Llanberris are interstratified in 

 a series of green and purple slates, grits and conglomerates. (Some 

 of the Welsh roofing-slates are however supposed to belong to the 

 Llandeilo.) (Mem. Geol. Surv., vol. iii. part 2, pp. 54, 258.) The 

 Harlech rocks in this north-western region are conformably overlaid 

 by the Menevian, followed by the true Lingula-flags, or Olemis beds, 

 of the Middle Cambrian. Upon these repose the Tremadoc slates, 



