304< Geological Society : Prof. Sedgwick on the 



tlie fossiliferous beds of Bala, includes all the higher portion of the 

 Berwyns, and all the slate rocks of South Wales which are below 

 the Silurian System. Its slate beds are less crystalline, and its 

 general structure is more mechanical, than the preceding group, and 

 it contains incomparably more fossils, which (though there are many 

 extensive portions of the group without fossils) are disseminated 

 through the more calcareous beds in great abundance. Many of the 

 fossils are identical in species with those of the lower division of the 

 Silurian System, nor have the true distinctive zoological characters 

 of the group been well ascertained. 



In many parts of South Wales it is separated from the Silurian 

 System by great faults and derangements of the strata, marked by a 

 broad band of rotten non-fossUiferous schist. At the north end of 

 the Berwyn chain it appears to pass by insensible gradations into the 

 lower division of the Upper System (the Caradoc Sandstone). 



(4.) The last natural group (the Silurian System). For all details 

 respecting this system the author refers to the abstracts of Mr. Mur- 

 chison's papers*, and to his forthcoming work. He then describes the 

 sections : 



(1.) East of the Berwyns, in which the Caradoc Sandstone is finely 

 developed ; containing the Llandeilo flagstone and other character- 

 istic calcareous and shelly bands. 



(2.) The sections north of the Berwyns, connecting Montgo- 

 meryshire with Denbighshire. The ascending series is described as 

 follows : — 



(1.) A series of beds several thousand feet in thickness, and ap- 

 parently forming a passage between the Upper Cambrian and 

 lowest portion of the Silurian System. 

 (2.) Bands of calcareous slate with numerous organic remains of 



the " Caradoc Sandstone." 

 (3.) Series of flagstones, more or less calcareous, with many or- 

 thoceratites and two species of cardiola, overlaid by, and as- 

 sociated with, irregular masses of roofing slate with a trans- 

 verse cleavage. 

 (4.) Flagstones and rotten slates, many parts in an imperfect 

 state of induration, and the whole surmounted by the Car- 

 boniferous Limestone. — Of the preceding section the lower 

 part of No. 3. is identical with the series of Long Mountain 

 in the Silurian sections of Mr, Murchison ; but No. 4. is 

 mineralogically unlike [any thing he has described, although 

 it has been found to contain some of the fossils of the Upper 

 Ludlow Rock. It appears from these details that the Silurian 

 System, although its subdivisions are obscure from the abs- 

 ence of the Wenlock and Ludlow limestones, is more fully 

 developed than in the group (No. 3.) of the great Cumbrian 

 section above described. 

 The author then briefly notices the slate rocks of Charnwood Fo- 

 rest, which he refers provisionally to the Upper Cambrian System ; 



[* See Lond. and Edinb. Phil. Mag., vol. iii. p. 224 ; vol. iv. p. 159, 228, 

 370, 450 J vol. v. p. 217 ; and vol. vi. p. 314, 37G.] 



