THE DARTMOOR VOLCANO. 
153 
The basic in like manner passed from the plutonic gabbro, 
through augitic-gabbro and dolerite, into basalt-lava, and finally 
into the basalt-glass known as tachylyte. 
These are but main varieties, and the unvarying result of 
petrographic research since then has been to prove full inter- 
mediate gradations from the plutonic to the volcanic types, so 
that one and the same original fluid mass of molten matter 
may put on all these varieties of lithological form. Granite, 
if it reaches the surface, becomes lava. 
In the paper from which I have quoted, Professor Judd traced 
the history of the great Tertiary volcanoes of Mull, Ardnamurchan, 
Eum, Skye, St. Kilda, and of the Grampian chain — in each and 
all of which volcanic rocks have been associated with great central 
granitic masses ; and in his concluding passages observed : 
"From the active volcanoes of Etna, Vesuvius, and Skaptar 
Jokul, the step to the ruined piles of the Mont Dore and the 
Cantal is an easy one, and with these last we have little difficulty 
in perceiving the parallelism of the phenomena displayed by the 
rocks in the central mountain group of Mull. From the condition 
of the volcanic rocks in Mull to that in Skye the transition is 
obvious, and from the latter to Beinn Nevis, and thence to the 
granitic bosses of Cairngorm, the Moor of Eannoch and the Ross of 
Mull, can easily be made ; nor will any difficulty be experienced 
in passing from these latter to the wide-spreading tracts of granite, 
such as that of Leinster. . . . 
"Whilst, on the one hand, we are led to conclude that great 
tracts of granite like that of Leinster may once have been 
surmounted by vast volcanic piles, we cannot, on the other 
hand, doubt that the subaerial volcanic phenomena of Iceland, 
Sicily, and the Andes are accompanied by innumerable igneous 
injections in subjacent strata, and the formation of masses of 
granite, syenite, diorite, and gabbro at great distances beneath 
them." 
For Leinster read Dartmoor, and the parallel is complete. 
We find then that the granite of Dartmoor is an igneous 
intrusive rock, formed at a great depth beneath the surface, 
and therefore exposed by the removal of the rocks beneath which 
it consolidated. We learn also that granite is but a phase ; and 
that the same matter now found as granite would, had it issued 
