~ Parr II. Szcr. ii. § 7.] MARINE DENUDATION. 447 
it is certainly too small to be generally appreciable. In the same way, 
if we are told that the constant wear and tear which is going on before 
our eyes in valleys and watercourses, does not effect more than the 
removal of one line of rock in eight and a half years, we may naturally 
enough regard such a statement as probably an underestimate. But 
if we only permit the multiplying power of time to come into play, 
the full force of these seemingly insignificant quantities is soon made 
apparent. For we find by a simple piece of arithmetic, that at the 
rate of denudation which has been just postulated as probably a fair 
average, a valley 1000 feet deep may be excavated in 1,200,000, a 
period which, in the eyes of most geologists, will seem short indeed. 
Objection may be taken to the ratios from which this average rate 
of denudation is computed. Withcut attempting to decide what this 
average rate actually is—a question which must be determined for 
each region upon much fuller data than are at present available—the 
geologist will find advantage in considering, from the point of view 
now indicated, what, according to the most probable estimates, is 
actually in progress around him. Let him assume any other appor- 
tioning of the total amount of denudation, he does not thereby lessen 
the measurement of that amount, which can be and has been ascer- 
tained in the annual discharge of rivers. A certain determined | 
quantity of rock is annually worn off the surface of the land. If, as 
already remarked, we represent too large a proportion to be derived 
from the valleys and watercourses we diminish the loss from the 
open country ; or, if we make the contingent derived from the latter 
too great we lessen that from the former. Under any ascertained or 
assumed proportion the facts remain, that the land loses a certain 
ascertainable fraction of a foot from its general surface per annum, 
and that the loss from the valleys and watercourses is larger than 
that fraction, while the loss from the level grounds is less. 

3. Marine denudation—its comparative rate. 
From the destructive effects of occasional storms an exaggerated 
estimate has been formed of the relative potency of marine erosion. 
That the amount of waste by the sea must be inconceivably less than 
that effected by the subaerial agents will be evident if we consider 
how small is the extent of surface exposed to the power of the waves 
when contrasted with that which is under the influence of atmospheric 
waste. In the general degradation of the land this is an advantage 
in favour of the subaerial agents, which would not be counterbalanced 
unless the rate of waste by the sea were many thousands or millions 
of times greater than that of rains, frosts, and streams. But in 
reality no such compensation exists. In order to see this, it is only 
necessary to place side by side measurements of the amount of work 
actually performed by the two classes of agents. Let us suppose, 
for instance, that the sea eats away a continent at the rate of ten feet 
in a century—an estimate which probably attributes to the waves a 
