1851.] SEDGWICK SLATE ROCKS OF DEVON AND CORNWALL. 3 



was no longer any physical evidence opposed to the palseontological) 

 must be regarded as Carboniferous*. 



At the time this conclusion was published in 1836, we had only de- 

 termined the south base of the culm-trough at one point near Oak- 

 hampton. But soon afterwards the whole southern line was deter- 

 mined by myself with a near approximation to its present position on 

 our geological maps of Devonshire and Cornwall. 



But if the culm-trough of North Devon were of the Carboniferous 

 period, it seems to follow, almost as matter of course, that many of 

 the older rocks of North and South Devon must belong to the period 

 of the Old-red-sandstone. Their position in the natural sections, in 

 some places their mineral character, and all their fossils, so far as 

 then known in South Devon, tended to this conclusion. Yet for rea- 

 sons recorded in the former volumes of our ' Transactions,' this con- 

 clusion was not drawn by us until near the close of 1838. During 

 the early part of the summer of 1836, we determined correctly the 

 principal physical groups of the older Devonian rocks, and I will 

 briefly enumerate them in this place, and give them names, not for 

 the purpose of stating anything new or unknown, but for the purpose 

 of more easy reference to the physical structure of Cornwall, respecting 

 which I have never before stated my views, in any detail, to this 

 Society. 



The first and oldest of these groups may be conveniently called the 

 Plymouth Group, using these words in an extended sense, so as to 

 include all the limestones of South Devon and the red sandstones 

 superior to the Plymouth limestone. The equivalent to this group 

 in North Devon includes, I think, the Ilfracomb and Linton limestones 

 as well as the red sandstones of the north coast. 



The second group includes the slates expanded from Dartmouth to 

 the metamorphic group of Start Point and Bolt Head, and is, so far 

 as I know, without fossils : it may be called the Dartmouth Group ; 

 and its equivalent in North Devon is found in the slates of Mort 

 Bay, which end with beds of purple and greenish sand-rock and 

 coarse greywacke. It ranges nearly E. and W. across the county. 



The third group is not, I think, found in South Devon; but in North 

 Devon it is well-defined, commencing on a base-line of sandstone-beds 

 which range nearly east and west from Baggy Point (on the western 

 coast) to Marwood (which is a few miles north of Barnstaple), and 

 thence towards the eastern side of the county. This group is con- 

 tinued, in ascending order, to the slates on the north shore of Barn- 

 staple Bay ; but its very highest beds are seen on the south shore 

 of the bay, dipping under the base of the Culm-measures. 



The equivalent of this third and highest Devonian Group is found 

 to the south of the great culm-trough, in a group near the top of 

 which appear the limestone-bands and fossiliferous slates of Pether- 

 win. It may be called the Barnstaple or Petherwin Group, and I 

 shall generally use the latter name, because it enables us more directly 

 to connect this group with the older deposits in Cornwall. 



* See Report Brit. Assoc. 1836, Transact. Sections, p. 96; and Trans. Geol. Soc. 

 2 Ser. vol. v. p. 682. 



B 2 



