162 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
part at least plagioclase. One or two may be quartz ; one or two 
are a kind of viridite, probably replacing a pyroxenic mineral ; 
and one small grain resembles epidote. The rock fragments 
are all, so far as I can see, of igneous origin. Some are fairly 
clear, some a rich brown colour, some almost black with opacite ; 
some are homogeneous, except for a little opacite and some belonites 
or trichites of a dark grey colour, which often are grouped in 
more or less dendritic forms or bundles like rootlets. A few 
of these grains are still isotropic, but most of those which are 
transparent exhibit devitrification structure. Small spherulites, 
showing the black cross with the two nicols, are rather common ; 
one fragment seems part of a large spherulite. Other fragments 
show flow structure ; one is perlitic. Clearly several varieties of 
rock are present, but I think the majority may be referred to 
andesites, some of which may not be far removed from basalt ; 
others may have a tolerably high percentage of silica. I think 
the materials have undergone attrition, and have been deposited 
by water, but believe they have been obtained by the denudation 
of volcanic cones." 
It is very remarkable that this testimony to the existence of 
local acidic rocks of the volcanic series should have come to light 
while this lecture was in hand; but in all probability there is 
more behind. Still, the peak of Dartmoor disappeared so very 
long ago that the absence of all traces would have been less 
surprising than their preservation, even in this meagre though 
unmistakable form. 
An attempt to reconstruct the Dartmoor volcano must needs be 
largely hypothetical ; but we can reach sufficiently accurate results 
by applying conclusions derived from a study of other volcanic 
piles — ancient and modern. What we commonly find is a central 
cone, with the sides rising at angles of about thirty degrees, in the 
midst of a lower elevation formed by slopes of small gradients, 
six td ten degrees, extending a considerable distance. Hence 
there are two ways of estimating the size of a volcanic mountain. 
The igneous base from which the cone of Etna rises is forty miles 
in diameter \ but the mountain proper, as distinct from the lava- 
covered plains, is some ten miles only. Though truncated, the 
height of Etna is 10,834 feet. Professor Judd 3 assigned the 
Tertiary volcano of Mull a basal diameter of thirteen or fourteen 
miles; and on the analogy of Etna, gave it a height of 14,500 
feet. 
3 Qaar, Jour. Gcol. Soc. xxx. 259. 
