154 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
to the light, have formed a trachytic-lava. 6 Just as we find 
water in the solid form as ice, the fluid as water, the vaporous as 
steam, the gaseous as oxygen and hydrogen — the change is less 
in the thing than in its conditions. 
The next question is whether we have any evidence that the 
intrusive mass, of which the granite of Dartmoor formed a 
basal part, ever reached the surface as a volcano. We have seen 
that this granite must have consolidated at a great depth beneath 
vast masses of rock now removed. Can we gather what these 
rocks were 1 
We have had the figures of Mr. Sorby and Mr. Ward touching 
the pressure under which various granites have consolidated, the 
mean for the Cornish and Devon granites being the equivalent of 
55,000 feet of rock. But it is not to be understood that our 
present Dartmoor granite was ever buried beneath ten miles of 
strata. Mr. Sorby remarks that "in some cases the pressure 
was probably much greater than that of the superincumbent 
rocks, for otherwise they could never have been fractured and 
elevated ; whereas in other cases it may have been much less, 
if the internal pressure had been in any way relieved." 7 In other 
words, if the weight of the overlying rocks were equal to the 
pressure of consolidation there would be practically no surface 
change. If it were less, the extra force indicated would be 
expended by the eruptive granite in upheaval and distortion, 
to which relief might or might not be given by a volcanic 
vent. 
Now as there is abundant evidence of a great elevating force 
in and around Dartmoor, in the tilting of the slates, &c, it is 
clear that the superincumbent mass cannot have been anything 
like the ten miles whose equivalent pressure has been calculated. 
The Dartmoor granite rises through Devonian strata on the 
south, and partially on the west and south-east ; through Carboni- 
ferous strata on the north, partially on the west, and to a larger 
extent on the east. And as it sends veins not only into the 
Devonian, but into the Carboniferous rocks, it is plainly younger 
than they are, since it could not transverse that which did not 
6 Mr. J. A. Phillips, in 1875, illustrated the practical identity between 
granite, el van, and rhyolite. — Quar. Jour, Geol. Soc. xxxi. 338. 
7 Quar. Jour. Geol. Soc. xiv. 491. 
