Chap. XI.] DEVONIAN TYPES— NOETH AND SOUTH DEVON. 
273 
Upper Devonian rocks, from Lynton, through Ilfracombe, to Barnstaple, — 
the whole dipping under strata of Carboniferous age *, which, on the op- 
posite side of a wide trough, again rise to the surface, resting also upon 
Upper Devonian strata. 
The tract of North Devon has thus been selected as affording the best 
type of succession of the British rocks to which the name Devonian was 
applied, because it offers a clear ascending section, through several thousand 
feet of varied strata, until we reach other overlying rocks, which are un- 
deniably the bottom beds of the true Carboniferous system. Por, whether 
we advance from Barnstaple to the south or from Petherwin to the north 
(section, p. 272), we find ourselves in a wide f trough of overlying strata 
in which the slaty character is but little developed, and which are much 
softer, in places, than those of the flanking tracts. 
Now, although this overlying series is in mineral aspect as much unlike 
the Carboniferous strata of most other parts of Britain as the rocks of North 
Devon are unlike the ordinary Old Red Sandstone of England and Scot- 
land, we have proofs by fossils, besides the analogy with Pembrokeshire 
before spoken of, that the black limestones of Swimbridge and Venn &c. 
with Posidonomyse (or the calcareous band g of the section) do represent, 
on a miniature scale, a part of the Mountain or Carboniferous Limestone, 
that the next series of white grit and sandstone of Coddon Hill &c. stands 
in the place of the Millstone-grit, and that the overlying courses of culm 
with many remains of Plants are consequently the equivalents of some of 
the lower coal-bearing strata of other tracts to be described in the next 
Chapter %. In short, no one denies that in the Culm series of Devonshire 
we have the representatives of the Lower Carboniferous strata. 
The' objections, therefore (which have, however, been only very partially 
made), to the view taken by Professor Sedgwick and myself, that the 
Devonian rocks of the foregoing section are the true representatives in time 
of the great deposits of Old Red Sandstone of other parts, are quite un- 
tenable. In truth, the long period which was occupied in developing the 
enormously thick Old Red deposits of Wales and of large tracts of Scotland 
and Ireland must have produced equivalent accumulations elsewhere ; and 
the vast slaty series of North Devon immediately underlying the lowest 
Carboniferous beds occupies precisely the same position as the Old Red 
Sandstone of England, Scotland, and Ireland. Moreover, as every geologist 
knows that the crystalline feature of slaty cleavage was impressed upon 
* The coal-field, which is bituminous in Mon- 
mouth, Glamorgan, and Carmarthen, becomes an- 
thracitic in Pembroke, where the stone-coal series, 
much disturbed and broken, differs from that of 
Devon only in being much more productive. 
Possibly some of the culm-strata of Devon, devoid 
as they are of any workable coal, may yield bitu- 
minous products by the application of heat. 
t The thickness of these Lower Carboniferous 
strata must not be estimated by the breadth which 
they occupy on a geological map ; for, owing to 
countless convolutions, the very same beds are re- 
peated over and over in the same broad trough : 
the folds are well exposed near P»ude and at other 
places on the coast (see Trans. G-eol. Soc. loc. cit.). 
The bottom beds only of these undulations, or 
small portions of each side of the culm-trough (g), 
are represented in the section. 
I Some of the culm-beds of Devon maybe con- 
sidered subordinate to the Millstone-grit; but 
most of the culm overlies that rock, and is simply 
the equivalent of the culm of Pembrokeshire, of 
which hereafter. 
X 
