Scotland does not yield the mollusks or zoophytes of Devonshire, nor is there recorded in the latter district more than the faintest trace of the ichthyolitic wealth of the North. Though this fact may still have difficulties connected with it, they have ceased to be chronological, for Sir R. I. Murchison tells us "that the same fossil fishes, of species well known in the middle and upper portions of the Old Red of Scotland, and which in large tracts of Russia lie alone in sandstone, are in many other places found intermixed, in the same bed, with those shells that characterize the group in its slaty and calcareous form in Devonshire, the Rhenish country, and the Boulonnais. This phenomenon, first brought to light in the work on Russia, by myself and colleagues, demonstrates more than any other the identity of deposits of this age, so different in lithological aspect, in Devonshire on the one hand, and central England and Scotland on the other. The fact of this intermixture completely puts an end to all dispute respecting the identification of the central and upper masses at least of the Old Red of Scotland with the calcareous deposits of Devonshire and the Eifel."[1]
In a paper "On the Slate Rocks of Devon and Cornwall," read before the Geological Society of London in 1851, Professor Sedgwick stated his views respecting the division of these rocks into three groups, as follows:—
"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 limestones. The equivalent to this group in North Devon includes, I think, the Ilfracombe 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 Morte Bay, which end with beds of purple and greenish sand-rock and coarse greywacke. It ranges nearly east and west 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 continued in ascending order to the slates on the north shore of Barnstaple 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 Petherwin. It may be called the Barnstaple or Petherwin group."[2]
