164 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
should be exposed at all. The active presence of a volcano really 
lessens the amount of work left to be done by the ordinary 
process of denudation. 
Mull is a Tertiary volcano — quite an infant of days as compared 
with Dartmoor — and its highest point is but 3172 feet. 
Diagrammatic Reconstruction of Dartmoor. 
It must be understood that this sketch is diagrammatic and has no 
pretensions to scale, and that it is mainly suggestive and purposely made 
as simple in its conditions as possible. All below the horizontal line, which 
represents datum, is purely hypothetical ; and all above the curved line, 
which indicates generally the present surface contour of the Moor and 
bordering rocks from north to south. This superstructure has been removed. 
A represents Carboniferous rocks ; B, Devonian ; C, Granite ; D, Felsite ; 
E, Volcanic Material and Ejectamenta. The wedge-like intrusion of the 
granite has tilted and broken through the upper or Carboniferous rocks, 
and has thrust as well as heaved the lower or Devonian. 
A few moments' consideration will show that if Dartmoor had 
not some such magnitude as here assigned, it could never have 
sustained the enormous wear and tear to which it has been 
exposed. Not a shower that falls or wind that blows upon it 
but wastes it somewhat; and when we reflect that such degradation 
has been going on not merely for thousands, but for millions of 
years, and in earlier days at a much more rapid rate than now, 
what is left can only be a wreck of what must have been. 
The Mississippi is estimated to carry into the sea every year 
a quantity of sediment equivalent to the lowering of its Avhole 
drainage basin by the six -thousandth part of a foot of rock, at 
