THE DARTMOOR VOLCANO. 
165 
which rate the entire mass of North America would be reduced 
to the sea-level in four and a half millions of years. 
Our Dartmoor streams with their rapid fall do proportionately 
much higher work ; and that their volume and activity were much 
greater once than now, let the great gorges at Fingle, Holne, 
the Dewerstone, and Tavy Cleave attest. However the rivers 
may have been helped, to imagine that these ravines were worn 
by such streams as now occupy them is to assign to our latest 
geological epoch an antiquity perfectly unconceivable. 
Still there are times when even these modern rivers develope a 
remarkable amount of energy. We may see granite pebbles, two 
and three pounds in weight, ten or a dozen miles down their 
courses from the nearest point of origin ; and the old beds fre- 
quently contain large boulders far more remote. Granite pebbles 
were discovered on the bed rock of the Laira in sinking for the 
new Laira Bridge ; huge boulders beneath 75 feet of silt at the 
mouth of the Tavy; and Mr. T. M. Hall records them in the 
Taw, at Taddiport, twenty to twenty-five miles from a possible 
source. 
A good deal of this rock sculpture may indeed be due to 
ice action. 
"It is no more possible," recently wrote Professor Prestwich, 
"to judge of the rate of denudation during the Glacial period 
by the river and ice action of the present day than it is of the 
rate of flow of Greenland ice by modern Alpine experience. The 
enormous pressure and wear of ice 2000 to 6000 feet thick in 
contracted valley channels . . . the powerful disintegrating effects 
of extreme cold on rocks, the annual action of ground ice in 
rivers, and of the sweeping and devastating floods resulting from 
the melting of the winter's snow and surplus ice, combine to 
produce results of which it is impossible to judge by the moderate 
work of these temperate latitudes." 4 
The more severe, however, the degrading influences to which 
Dartmoor has been exposed, the more important must have been 
its mass, by comparison with that which remains. We are again 
and again forced back upon our hypothesis as an a priori necessity, 
for I do not see how any local elevation of the height required 
could have been other than volcanic. 
I do not know a better description of the operation of weathering 
4 Geology, ii. 553-4. 
