THE DARTMOOR VOLCANO. 
155 
previously exist. 8 On the other hand, as pebbles and fragments 
of Dartmoor rocks are found in the local breccias and con- 
glomerates which represent the margins of the ancient Triassic 
sea, it is clear not only that Dartmoor must have been erupted, 
but that some of its underlying rocks must have been liable to 
denuding influences — i.e.> that some of its granites must have been 
exposed at the surface — before these derived beds were formed. 
Thus we are led to conclude that before the elevation of the 
Dartmoor region began the area was occupied by Carboniferous 
and Devonian rocks, the former resting upon Devonian, and the 
latter possibly in part upon Silurian, as traces of that system 
seem to exist in Devon as well as in Cornwall. Both the 
Devonian and the Carboniferous rocks of the Dartmoor area had 
been disturbed by igneous action before the upheaval of the 
granite. Not only have we lava- and ash -beds contemporaneous 
with the Devonian rocks, especially in the south-west of the 
county, but around the Moor there are bands and bosses of 
intrusive plutonic rocks partially following the outline of the 
granite in great curves, as at the north-western corner, and 
specially prominent near Mary Tavy and Tavistock, where they 
form the eminences of White Tor and Cocks Tor — the highest 
non-granitic points of the county. These gabbros, as they have 
frequently been called, though chiefly falling among the modern 
epidiorites, are the vestiges of a widespread pre-Dartmoor igneous 
activity in the intrusion of basic igneous rocks, and their exceptional 
development in the Mary Tavy district has been thought to indicate 
that we have there exposed some of the lowest stratified rocks 
brought up by the granite in this county. It may be that 
they represent the earlier stages of the eruptive epoch which 
eventually gave us Dartmoor, for their relations to the granite, 
here and also in Cornwall, are too persistent to be accidental. It 
is not unusual for one eruptive centre at one time to produce 
basic and at another acidic rocks. 
Thus we have to account for the presence of the granite, the 
disappearance of the rocks which once covered it, and the 
relations between the granite and its stratified surroundings. 
8 Certain of the veins on the north of Dartmoor contain fragments of 
Carboniferous rocks, some not quite detached, some with perfect angles. 
(G. W. Ormerod, Trans. Devon. Assoc. ii. 128.) 
VOL. X. M 
