6S2 



III. South Devon section. — This section, in conformity with the 

 scheme given in the former paper, is as follows, in the ascending 

 order* : — 



(1.) A series of slate rocks subdivided into two groups, — the lower 

 containing a few calcareous bands, the upper group more calcareous 

 and ending with the Plymouth limestone. The two are considered 

 as one formation ; and the name, Ashburton bands, which had been 

 given to the calcareous beds of the lower division, is now withdrawn, 

 as the position of the Ashburton lime rock is considered ambiguous. 

 The name of Ugborough bands is not liable to the same objection. 



(2.) A great group of coarse red flagstone and slate, identical in its 

 structure with No. 3. of the North Devon section, and containing 

 some corals that do not appear in the mountain limestone, but are 

 found both in the Cambrian and Silurian systems. This group is 

 provisionally identified with No. 3. of the North Devon section. 



(3.) A great group of slate rocks without beds of limestone, and 

 very rarely with any traces of organic remains. By the suppression 

 of No. 4., this group is considered as the equivalent of No. 5. of 

 the North Devon section. 



(4.) Mica and chlorite slate, anomalous in structure and position, 

 and forming no part of the ascending series. 



The preceding identifications are only provisional, and many desi- 

 derata are enumerated ; but it is considered certain that the South 

 Devon section belongs, on the whole, to a lower series than the 

 North Devon. Neither of them are, however, supposed to descend 

 lower than the Upper Cambrian, or the higher part of the Lower 

 Cambrian, group. To place the South Devon section above the North 

 Devon, would be to violate all the analogies of structure derived from 

 other parts of England ; and would not, the author believes, be sup- 

 ported by any specific evidence derived from fossils. 



CLASSIFICATION OF THE ROCKS OF CORNWALL. 



The author states, that the Plymouth limestone, in its range west- 

 wards, gradually thins off, and comes to an edge about the middle 

 of Whitesand bay. The strike of the beds and the trending of the 

 coast prevent this limestone and all the upper groups of the South 

 Devon section from appearing again on the south-eastern side of 

 Cornwall. 



The inferior portion of the first group (No. 1 .) of the South Devon 

 section passes into Cornwall in a bi'oad zone, gradually acquires the 

 strike of the Cornish rocks, and so runs along the S.E. coast; and 

 finally passes from Falmouth bay to Mounts bay ; rising on its north 

 side towards the granite, and on its south side dipping under the 

 serpentine of the Lizard district. As in Devonshire, the group 

 contains beds more or less calcareous, and, rarely, thin beds of lime- 

 stone. 



In the same way, though not with the same clear evidence, the 

 calcareous slates rising from beneath the culm-measures near Laun- 

 ceston, double round the granitic promontory of Rough-Tor, and 



* See Proceedings, p. 562. 



