146 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
THE DAKTMOOR VOLCANO. 
BY R. N. WORTH, F.G.S. 
(Read October 18th, 1888.) 
Dartmoor is a broken tableland of Granite, of irregular outline. 
The greatest length, North to South, is 22 J miles; the greatest 
breadth, East to West, 18^ miles. The average breadth cannot, 
however, be taken as more than 1 5 miles ; nor the average length 
as more than 20. The area is therefore about 300 square miles. 
These are the measurements of the visible granite ; but the 
granite is surrounded by a belt of altered rocks — slates and grits 
— beneath which it dips, which have been changed in character 
by its proximity, and which indicate its wider presence at no 
great depth. Including this zone, the area affected by the physical 
phenomena of Dartmoor is not far short of 400 square miles. 
Other elevated granitic regions of similar character, which cannot 
be dissociated from Dartmoor in origin, occur at intervals to the 
westward. Chief are the Hingston, Brown Willy, Hensborough, 
Carnmenellis, Land's End, and Scilly bosses ; but more or less 
closely grouped with these are smaller patches, which, in several 
localities where mining operations are carried on, have proved 
to be connected with the main masses at no great depth, the 
surface interval being occupied by shallow basins of the slaty rocks 
commonly grouped as killas. There is every reason to assign 
all these granitic areas to one and the same geological age, and to 
one and the same series of causes ; and there is good ground also 
to suggest that the connection proved to exist between the minor 
masses exists, at a greater depth, between the larger; and that 
from Dartmoor to Scilly, therefore, we are dealing with one and 
the same granitic axis, most strongly marked in Dartmoor, and 
gradually decreasing in elevation and importance westward, where 
initial causes seem to have been least active and subsequent 
changes most prominent. Denudation seems to have increased 
in activity from the north-eastern flank of Dartmoor, where 
