ON THE POSITION OF CULM DEPOSITS IN DEVONSHIRE. 539 
Professor Sedgwick and Mr. Mnchison. 
— The authors began by observing that 
this was a mere outline of a more 
detailed memoir on the physical struc- 
ture of Devonshire, which they were 
about to lay before the Geological So- 
ciety of London. In the published 
geological maps of that country, the 
whole system of the older slate rocks 
was represented under one colour, with- 
out any attempt at subdivision ; and 
one colour also represented different 
limestones, without any discrimination. 
The object of the authors was, to re- 
medy these defects, — to ascertain and 
represent the true position of the suc- 
cessive deposits and their natural sub- 
divisions, so as to compare them with 
corresponding deposits in other places. 
They also wished to determine the true 
place of the remarkable carbonaceous 
deposits of central Devon, which had 
been previously regarded as belonging to 
the lowest portion of the grauwacke 
formation. A section was exhibited of 
part of that country, from the north 
coast to one of the granite peaks of 
Dartmoor immediately south-west of 
Oakhampton. 
\The following is a description of the 
Diagram which will he found to art 
fi9- 7-] 
ascei^ding series of devonian rocks. 
Cambrian Hocks, {a) Slaty schists, with some 
calcareous courses and 
oruauic remains. 
(6) Purple, red, and grey- 
saiidstoiies, with beds of 
iron ore in upper mem- 
bers— -peculiar fossils 
near tbeii junction iviiU 
lire over l> ing limesloiies- 
Veins of lead and copper. 
Upper Cambrian (c) Calcareous aroupof Combe 
Martin 8c llfraeombe — 
fossils very abmidani — 
slaiey cleavaue. 
or Devonian Rocks, (d) Slates with quaitzose veins 
and beds-^-incolierent 
schists, &c. Mauganeset 
mines. 
(g) Slaty sandstones and 
scbisis - cleavage pass 
ing thro’ the beds of 
organic remains. 
if) Ditto, yviili cuiicielionary 
limestones, and many 
well kuoyvn Silut ian fos- 
sils, chiefly of the lower 
pan of the system. 
(g) Culmiferous or black lime* 
Slone, with poitionsof 
stone coal, and fossils 
distinct from any 'oimd 
in the inferior groups- 
Wavelliie occurs in the 
beds below this lime- 
stone. 
Ch) Culm beds with underly- 
ing and overlying sue. 
cessions of sandstone 
and shale often highly 
pyritous, with many no- 
dules of iron ore, fre- 
quently coiiiaininu coal 
tiiants.and never iiffect- 
ed like the older rocks 
by slaty cleavage. 
(i) New ted sandstone resting 
uiiconfoi mably on the 
« at bonaceous detiosits. 
(k) Giatiiteof Daitmoor and 
Ehan Dyke, Itoiii eiupt- 
ed tlirouijh the culm de- 
posits. 
In the ascending order this section 
exhibits — 
1. A system of slaty rocks, containing 
a vast abundance of organic remains, 
generally in the form of casts. These 
rocks sometimes pass into a fine glossy 
clay slate, with a true transverse cleav- 
age ; sometimes into a hard, quart- 
zose flagstone, not unusually of a 
reddish tinge ; sometimes into a reddish 
sandstone, subordinate to which are beds 
of incoherent shale. In North Devon 
they are very rarely so calcareous as to 
be burnt for lime, but in South Devon, 
rocks of the same age appear to be 
much more calcareous. 
2. A Series of rocks characterized by 
masses of hard thick-bedded red sand- 
stone, and red mecaceous flagstone, 
subordinate to which are bands of red, 
purple, and variegated shades. The red 
colour occasionally disappears, and the 
formation puts on the ordinary appear.^ 
ance of a coarse, silicious grauwacke, 
subordinate to which are some bands 
of imperfect roofing slate. In this series 
are very few organic remains. It is 
several feet in thickness, occupying the 
whole coast from the west end of the 
Valley of Rocks to Combe Martin. 
3. The calcareous slates of Combe 
Martin and Ilfracombe, of very great 
aggregate thickness, abounding in 
organic remains, and containing in a 
part of their range at least nine distinct 
ribs of limestone burnt for use. This 
limestone is prolonged into Somerset 
shiie, and appears to be the equivalent 
of that on the flanks of Quantock Hills. 
4. A formation of greenish and lead- 
coloured roofing slate of great thickness, 
and occupying a well-defined zone in 
North Devon, its upper bed alternating 
with and gradually passing into a great 
deposit of sandstones of various colours 
and mycaceous flagstones. These sili- 
cious masses alternate with incoherent 
slates, and are in some places surmount- 
ed by great masses of red unctuous 
Siluiiaii Rocks. 
Culm Deposits 
=C«al-field of 
Pembroke. 
