Geological Society. 337 



being In tlie latter case an upper oolite or coral rag, reposing on Ox- 

 ford clay, which is succeeded by an inferior oolite resting upon lias, 

 in the same manner as the Aymestry limestone reposes on the lower 

 Ludlow mudstone strata, and the Wenlock limestone on the Wen- 

 lock shale ; and there being in both countries two escarpments of cal- 

 careous rocks, each having at its base a soft, argillaceous formation. 



The fact which j\lr. Lewis pointed out to the author is, that in 

 the Wenlock Edge, the lower escarpment, consisting of Wenlock 

 limestone, forms an uninterrupted ridge ; while the upper escarpment, 

 composed of Aymestry limestone and associated Ludlow rocks, is di- 

 vided into many knolls by transverse breaks ; but that after crossing- 

 the Onny, we find that, in the district between Shelderton and Ay- 

 mestry, the phenomena are reversed, the upper or Aymestry limestone 

 escarpment being undivided, and the lower or Wenlock limestone 

 ridge being formed of knolls. The cause of this difference, Mr. 

 Lyell is of opinion, exists in the variations in the thickness of the 

 limestones, and their consequent amount of resistance to denuding- 

 agents. In the Wenlock Edge, the calcareous strata which form 

 the summit are from 50 to 80 feet thick *, and there are many solid 

 beds in the underlying shale ; on the contrary, in the upper escarp- 

 ment, the capping of Aymestry limestone is inconsiderable : and in 

 the district between Shelderton and Aymestry, where thepheenomena 

 are reversed, the Aymestry limestone, with the accompanying solid 

 beds of the upper Ludlow, is from 80 to 90 feet thick, but the Wen- 

 lock is of inconsiderable dimensions. In each instance, moreover, 

 the two escarpments are so near to each other, that it is highly im- 

 probable that there could have been any great difference in the 

 amount of fracture and Assuring, or that they were not equally 

 affected by the same movement. 



In conclusion, Mr. Lyell alludes to Mr. Murchison's description f 

 of the transverse valleys or fissures which divide the Aymestry and 

 Ludlow beds into knolls in tlie ridge which ranges parallel to Wen- 

 lock Edge ; and he calls upon those geologists who may have the op- 

 portunity, to examine carefully the escarpment of the Edge itself, for 

 the purpose of ascertaining if there be any traces of the prolongation 

 of these fissures. Should they be found to exist, Mr. Lyell says, 

 the comparative integrity of the escarpments may be attributed with 

 still greater confidence to the resistance of the limestone beds which 

 constitute its upper part. 



7. " Notes on the Silurian Strata in the neighbourhood of Christ- 

 iania, in Norway," by Charles Lyell, Esq., V.P.G.S. 



In a paper read at the Meeting of the British Association at Li- 

 ver])ool, in 1837, Mr. Lyell inferred that the fossilifcrous strata in- 

 vaded and altered by granite in the neighbourhood of Christiania 

 belong to the Silurian period, in consequence of their containing 

 GraiJtolitcs and Cateniporat ; and in this communication he states 

 tliat, by the assistance of Mr. Lonsdale, he has been enabled to as- 



• See Mr. Murcliison's Silm-ian System, chap. xvii. 

 + Silurian System, j). 230 ct ,?(?</. 



X See Seventh Report of the Hritisli Association, Notices and Abstracts 

 p. 'J7; and Athena;um for 1837, No. 51G, p. G83. 



