465 
being in the 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 Mr. 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 pheenomena 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 the phenomena 
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 fissuring, or that they were not equally 
_ affected by the same movement. } 
In conclusion, Mr. Lyell alludes to Mr. Murchison’s description + 
of the transverse valleys or fissures which divide the Aymestry and 
Ludlow beds into knolls in the 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- 
verpool, in 1837, Mr. Lyell inferred that the fossiliferous strata in- 
vaded and altered by granite in the neighbourhood of Christiania 
belong to the Silurian period, in consequence of their containing 
Graptolites and Catenipora{; and in this communication he states 
* See Mr. Murchison’s Silurian System, chap. xvii. 
+ Silurian System, p. 236 et seq. 
{ See Seventh Report of the British Association, Notices and Abstracts, 
p. 67; and Athenzum for 1857, No. 516, p. 683. 
