ese shoals
is equal to about one-third of the whole extent of England and Scotland.
The average height of the banks measures, according to Mr. Stevenson,
about seventy-eight feet; the upper portion of them consisting of fine
and coarse siliceous sand, mixed with comminuted corals and shells.[463]
It had been supposed that these vast submarine hills were made up bodily
of loose materials supplied from the waste of the English, Dutch, and
other coasts; but the survey of the North Sea, conducted by Captain
Hewett, affords ground for suspecting this opinion to be erroneous. If
such immense mounds of sand and mud had been accumulated under the
influence of currents, the same causes ought nearly to have reduced to
one level the entire bottom of the German Ocean; instead of which some
long narrow ravines are found to intersect the banks. One of these
varies from seventeen to forty-four fathoms in depth, and has very
precipitous sides; in one part, called the "Inner Silver Pits," it is
fifty-five fathoms deep. The shallowest parts of the Dogger Bank were
found to be forty-two feet under water, except in one place, where the
wreck of a ship had caused a shoal. Such uniformity in the minimum depth
of water seems to imply that the currents, which vary in their velocity
from a mile to two miles and a half per hour, have power to prevent the
accumulation of drift matter in places of less depth.

_Strata deposited by currents._--It appears extraordinary, that in some
tracts of the sea, adjoining the coast of England, where we know that
currents are not only sweeping along rocky masses, thrown down, from
time to time, from the high cliffs, but also occasionally scooping out
channels in the regular strata, there should exist fragile shells and
tender zoophytes in abundance, which live uninjured by these violent
movements. The ocean, however, is in this respect a counterpart of the
land; and as, on the continents, rivers may undermine their banks,
uproot trees, and roll along sand and gravel, while their waters are
inhabited by testacea and fish, and their alluvial plains are adorned
with rich vegetation and forests, so the sea may be traversed by rapid
currents, and its bed may here and there suffer great local derangement,
without any interruption of the general order and tranquillity. It has
been ascertained by soundings in all parts of the world, that where new
deposits are taking place in the sea, coarse sand and small pebbles
comm