THE DARTMOOR VOLCANO. 
163 
If we are to treat Dartmoor as a single mountain, we cannot 
reckon an average basal diameter of less than sixteen miles ; with a 
circumference of fifty, against the forty of Mull and the thirty of 
Etna • and from this we might fairly infer a total height of 18,000 
feet, or three and a half miles. 
If we are to regard the two elevated sections of the Moor as 
representing the degradation of a twinned peak we should not 
reach so high ; but the conclusion that these higher regions of the 
present day indicate the original contour involves the assumption 
that denudation had proceeded on tolerably average rates over the 
whole tract; whereas we know that denudation has been more 
active on the south, since not only is the elevation less, but 
Devonian rocks are exposed, in which the work of alteration is 
greater, indicating greater heat and depth, while the more irregular 
boundaries suggest a nearer approximation to the base of the 
eruption. Existing denuding agencies are most powerful where 
the Moor is highest. Still, the transverse trends by Fur Tor and 
Eox Tor may give some indication of the basal outlines of a 
crater. 
But we can enquire in other directions. We have seen that 
there is a fairly gradual rise from the outskirts of the granite. 
This varies in different parts of the Moor, but in the main 
approximates the six to ten degrees already cited as that of the 
exterior slopes of volcanoes, until we reach the 1500 feet range. 
If we take that area contour as representing the base of the 
central pile or ridge, which would describe an irregular oval some 
seven and a half miles east to west by fifteen north to south, 
a slope of thirty degrees on the shorter axis would give a total 
height of nearly three miles. 
To treat the dip of the granite under the marginal rocks as the 
general slope would imply a regularity of elevation contrary to 
experience, and involve a theoretical height of at least four and a 
half miles. 
Upon the whole, therefore, I think we may fairly put the height 
of the central portion of the Dartmoor volcano at three and a half 
miles, or 18,000 feet. This would involve the disappearance at 
the peak of more than three miles, or 16,000 feet. 
No doubt these are large figures, but they are not by any means 
exceptional ; and the removal of enormous masses of superincum- 
bent rock has to be faced in any event, in order that the granite 
