| 
| 
| 
| 
& 
Subdivisions of the Paleozoic Strata of Great Britain. 347 
sandstone. ‘These masses are the degenerate representatives of 
the Old Red Sandstone. Among the imbedded pebbles we find 
both Cambrian and Silurian fossils; and on the upper surface of 
the older rocks (whether Cambrian or Silurian) on which the con- 
glomerates rest, are remarkable examples of mechanical abrasion. 
Il. Middle Paleozoic Division. 
Devonian Series or Old Red Sandstone. 
(1.) In Herefordshire and South Wales. 
1. Cornstone group, - - - Cephalaspis, ce. 
2. Red sandstone and conglomerate, Holoptychius, &e. 
The aggregate thickness eight or nine thousand feet. 
(2.) In Devonshire and Cornwall. 
1, Liskeard or Ashburton group. 
a, Great Devon limestone. 
2, Plymouth group. 6. Calcareous slates. 
ce. Coarse red sandstone and flagstone. 
Coars $ 
3. Dartmouth slate group.2 Devon, with beds of red, green, and variegated sand- 
sto . 
g : O 
4, : a. Marwood sandstones. 
Exel Barnsta- 6. Petherwin slate and CZymenia limestone; calcareous 
slates of Barnstaple. ‘ 
The agregate thickness of this series is very great, but is not computed. 
von and Cornwall: for the Herefordshire cornstone has charac- 
teristic fishes without characteristic shells and corals; while the 
Devonshire and Cornish series has characteristic shells and corals, — 
but is without fishes. ; 
In a former scheme the Liskeard and Plymouth groups were 
United; but they may, I think, be conveniently separated. The 
Herefordshire fish-beds (cornstone) I had formerly placed ove 
the Plymouth group, on the understanding that certain supposed 
fish-beds of Cornwall were probably of the age of the Dart- 
Mouth group. But the Cornish fish-beds having now disap- 
peared from the sections, I think it much the safest plan to place 
the Cornstone group below all the groups of Devonshire; espe- 
Cially as it seems in some places to pass downwards into the tile- 
Stone of the Ludlow group, and therefore appears io give us 
(what we do not find in Devon and Cornwall) a base to the De- 
Vonian series. On this hypothesis we might arrange the Devo- 
Mian series of Herefordshire and the sub-groups of the Silurian 
Seties in a regularly ascending and unbroken numerical order ; 
ut the numerical series would be defective in its last term. The 
har in Scotland seem however to be adverse to this hy- 
thesis, 
