34 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
trappean zone was continuous as far as the southern margin of the 
Carboniferous series could be seen, except where it had been broken 
through by the great granitic mass of Dartmoor. He ascertained that 
t le intercalated trappean rocks are not confined to the Culm-measures, 
but occur also in the contiguous portions of the “ grauwacke ” or Devonian 
system. 
But further, lie clearly recognized that the bands of igneous material 
which he mapped included both “greenstones,” together with other varieties 
ot massive eruptive rocks, and also volcanic ash or tuff, though he did not 
attempt to separate these out upon the maps, but contented himself with 
representing them all under the same colour. He admitted that some 
doubt might be entertained as to the age of the greenstones, for some of 
them might be intrusive and therefore later than the sedimentary deposits 
between which they lie. But he contended that there could be no un- 
certainty with regard to the trappean ash or tuff, which being regularly 
interstratified in the Carboniferous series, must be contemporaneous with it. 
He pointed out that many of the greenstones, as well as fragments in the 
conglomerates or ashes, were highly vesicular and must originally have been 
in the condition of pumice. 
As an illustration of the centres of eruption from which these materials 
were ejected, De la Beche drew special attention once more to the con- 
spicuous eminence of Brent Tor and the rocks in its neighbourhood. His 
remarks on this subject are well worthy of being quoted — “The idea that 
m the vicinity of Brent Tor a volcano has been in action, producing effects 
similar to those produced by active volcanoes, forcibly presents itself. 
That this volcano projected ashes, which, falling into adjacent water, 
became interstratified with the mud, silt and sand there depositing, seems 
probable. That greenstones and other solid trappean rocks constituted the 
lavas ol that period and locality, here and there intermingled with the ash, 
appears also a reasonable hypothesis. Upon the whole there seems as 
good evidence as could be expected that to the north and north-west of 
Tavistock, ash, cinders and liquid melted rocks were ejected and became 
intermingled with mud, silt and sand during this ancient geological epoch 
corresponding with the phenomena exhibited in connection with volcanoes 
ol the present day, more particularly when they adjoin or are situated in 
the sea, or other waters where ejected ashes, cinders and lava can be inter- 
mingled with ordinary mud, silt and sand.” 1 
It remains for some future observer to fill up the outlines thus sketched 
by De la Beche, by tracing the respective areas of lavas and tuffs, distin- 
guishing the various petrographical types, separating the intrusive from the 
interstratified sheets, identifying the necks and bosses that may mark 
centres ot eruption, and expressing these various details upon maps on a 
sufficiently large scale. 
A serious difficulty in this research arises from the effect of the profound 
alteration which has been produced on the igneous rocks by the cleavage of 
1 Op. cit. p. 122. 
