THE DARTMOOR VOLCANO. 
159 
Ben Nevis and at Glencoe, and the district generally is as free 
from them as Dartmoor. But if we have no beds of felstone, we 
have what amounts to much the same thing — veins of felsitic 
rock — in the el vans ; and I can speak from personal observation 
of the similarity between some of our felsites, or quartz-porphyries, 
and those of Glencoe. 
The fact that our el vans 9 are of later date than the consoli- 
dation of the granite which, in common with the associated rocks, 
they traverse, has long been recognized; but sufficient emphasis 
has not hitherto been laid upon the different physical conditions 
which the formation of these two classes of rocks, from what is 
practically the same magma, or mother-liquor, indicate. The 
elvans point incontestably to the long continuance of violent 
eruptive action under another stage of environments than those 
attending the formation of the granite. Some of them run for 
miles as dykes of unknown depth, and it is idle to assume that 
the forces producing their intrusion expended themselves under- 
ground, and that all upward movement stopped short of the 
surface in the more prominent examples. They are essentially 
illustrations of volcanic as distinct from plutonic activity, and 
while not in themselves true lava, are the next thing to it, and 
readily graduate into it. 1 
These elvans demand closer study than they have had, and 
some of them illustrate the wide range of rock varieties produced 
from one original magma in a very remarkable manner. The 
Grenofen elvan, which at the Grenofen quarry is rather a 
syenitic-granite with a little interstitial felsitic matter than an 
elvan, is cut in the tunnel near Shillamill on the new line from 
Tavistock, and might there be regarded as an ordinary loose- 
textured granite — a fact easily accounted for by the greater depth 
of consolidation. Still further west, on Morwell Down, at a 
greater elevation, it has a more distinctive felsitic type, and 
presents a semi-vitreous compact ground -mass, in which quartz, 
felspar, and mica are porphyritically developed. The Eoborough 
9 The indefiniteness of this local term, while disqualifying it for strict 
scientific reference, gives it a special value here, where strict definition is 
not required, and is indeed at this stage undesirable. 
1 Since this lecture was delivered, a paper by the writer on the " Elvans 
and Volcanic Rocks of Dartmoor" has been read before the Geological 
Society. 
