channel, which he at last effected with great
difficulty.[461]

In the estuary of the Thames at London, and in the Gironde, the tide
rises only for five hours and ebbs seven, and in all estuaries the water
requires a longer time to run down than up; so that the preponderating
force is always in the direction which tends to keep open a deep and
broad passage. But for reasons already explained, there is naturally a
tendency in all estuaries to silt up partially, since eddies, and
backwaters, and points where opposing streams meet, are very numerous,
and constantly change their position.

Many writers have declared that the gain on our eastern coast, since the
earliest periods of history, has more than counterbalanced the loss; but
they have been at no pains to calculate the amount of loss, and have
often forgotten that, while the new acquisitions are manifest, there are
rarely any natural monuments to attest the former existence of the land
that has been carried away. They have also taken into their account
those tracts artificially recovered, which are often of great
agricultural importance, and may remain secure, perhaps, for thousands
of years, but which are only a few feet above the mean level of the sea,
and are therefore exposed to be overflowed again by a small proportion
of the force required to move cliffs of considerable height on our
shores. If it were true that the area of land annually abandoned by the
sea in estuaries were equal to that invaded by it, there would still be
no compensation _in kind_.

The tidal current which flows out from the northwest, and bears against
the eastern coast of England, transports, as we have seen, materials of
various kinds. Aided by the waves, it undermines and sweeps away the
granite, gneiss, trap-rocks, and sandstone of Shetland, and removes the
gravel and loam of the cliffs of Holderness, Norfolk, and Suffolk, which
are between twenty and three hundred feet in height, and which waste at
various rates of from one foot to six yards annually. It also bears
away, in co-operation with the Thames and the tides, the strata of
London clay on the coast of Essex and Sheppey. The sea at the same time
consumes the chalk with its flints for many miles continuously on the
shores of Kent and Sussex--commits annual ravages on the freshwater
beds, capped by a thick covering of chalk-flint gravel, in Hampshire,
and continually saps the foundations of the Portland limestone. It
receives, b