SUBMARINE DENUDATION. 
105 
Submarine Denudation. —When we attempt to estimate the 
amount of submarine denudation, we become sensible of the 
disadvantage under which we labor from our habitual inca¬ 
pacity of ofeerving the action of marine currents on the bed 
of the sea. We know that the agitation of the waves, even 
during storms, diminishes at a rapid rate, so as to become 
very insignificant at the depth of a few fathoms, and is 
quite imperceptible at the depth of about sixteen fathoms; 
but when large bodies of water are transferred by a current 
from one part of the ocean to another, they are known to 
maintain at great depths such a velocity as must enable 
them to remove the finer, and sometimes even the coarser, 
materials of the rocks over which they flow. As the Missis¬ 
sippi when more than 150 feet deep can keep open its chan¬ 
nel and even carry down gravel and sand to its delta, the 
surface velocity being not more than two or three miles an 
hour, so a gigantic current, like the Gulf Stream, equal in 
volume to many hundred Mississippis, and having in parts 
a surface velocity of more than three miles, may act as a 
propelling and abrading power at still greater depths. But 
the efficacy of the sea as a denuding agent, geologically con¬ 
sidered, is not dependent on the power of currents to pre¬ 
serve at great depths a velocity sufiicient to remove sand 
and mud, because, even where the deposition or removal of 
sediment is not in progress, the depth of water does not re¬ 
main constant throughout geological time. Every page of 
the geological record proves to us that the relative levels of 
land and sea, and the position of the ocean and of continents 
and islands, has been always varying, and we may feel sure 
that some portions of the submarine area are now rising and 
others sinking. The force of tidal and other currents and 
of the waves during storms is sufiicient to prevent the 
emergence of many lands, even though they may be under¬ 
going continual upheaval. It is not an uncommon error to 
imagine that the waste of sea-clifis affords the measure of 
the amount of marine denudation of which it probably con¬ 
stitutes an insignificant portion. 
Dogger-bank. —That great shoal called the Dogger-bank, 
about sixty miles east of the coast of Northumberland, and 
occupying an area about as large as Wales, has nowhere a 
depth of more than ninety feet, and in its shallower parts is 
less than forty feet under water. It might contribute to- 
Tvards the safety of the navigation of our seas to form an 
artificial island, and to erect a light-house on this bank; 
but no engineer would be rash enough to attempt it, as he 
would feel sure that the ocean in the first heavy scale would 
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