DEVONSHIRE 
33 
chap, xxix 
re plaeecl in a distance of less than forty miles by the peculiar “ Culm- 
measures ” of Devonshire — a series of black shales, grey sandstones and 
gnits, thin limestones and lenticular seams of impure coal (culm), which are 
u °t only singularly unlike in original characters to the ordinary Carbon- 
derous formations, but have been made still more unlike by the extensive 
and severe cleavage to which the Palaeozoic rocks of Devon and Cornwall 
have been subjected. That these Culm-measures are truly Carboniferous 
| s made abundantly clear by their fossil contents, though it has not yet 
' een possible to determine how far they include representatives of the great 
stratigraphical subdivisions in other parts of the country. 
If is to De la Beche that geology owes the first intimation of the 
occurrence. of interstratified igneous rocks in the Carboniferous series of 
Devonshire. As far back as the year 1834, in his singularly suggestive 
Deatise, Researches in Theoretical Geology, this eminent geologist ex- 
Pi'essed his opinion that not only were the “trappean” bands regularly 
intercalated in the sedimentary series and continuously traceable with the 
§ e neral stratification, but that they occurred at various localities in such 
ll manner as to raise the suspicion that these points may mark some of 
Dm centres of eruption. He particularly cited the example of Brent loi 
^ a remarkable volcanic-looking hill, composed in part of a conglomerate 
having every appearance of volcanic cinders.” 1 
In his subsequently published Report on the Geology of Cornwall, 
-Devonshire and West Somerset, De la Beche dwelt in more detail on the 
1 esults of his study of these rocks, which he had traced out on the ground 
ll ml expressed upon the maps of the Ordnance Geological Survey. Hardly 
lll l additions have since been made to our knowledge of the field-relations 
°I the rocks. It is to the maps and Report of De la Beche that we must 
DDl turn for nearly all the published information on the subject. I shall 
mefore give here a summary of what can be gathered from these 
Publications. 
In tracing the limits of the Culm-measures, De la Beche found that 
11,1 "udl-defi tied line could be drawn between these strata and the “ grau- 
) v ncke or Devonian formations underneath. The Carboniferous series lies 
j 11 H great trough, of which the axis runs nearly east and west, so that the 
owest members of the series rise along the northern and southern margins, 
nt De la Beche was struck with one remarkable contrast between the two 
PPosite sides of the trough — a contrast which marks the Devonian as well 
^ the Carboniferous formations of this region. On the south side an 
a undant and persistent group of intercalated bands of igneous, or as he 
'j aUecl them, “ trappean,” materials can be followed along the whole line of 
mndary, while no such group occurs on the north side. He found these 
juuds to be lenticular, traceable sometimes for a number of miles, then 
y' 111 4 out and reappearing on the same or other horizons. He mapped 
e ui the whole way from Boscastle on the west to near Exeter on the 
ast ’ an< I found that though the individual sheets might be short, the 
, 1 °P- at. p. 384. 
VOL. II 
2 Sheets 22, 23, 24, 25, 30, 31, 32 and 33. 
D 
