le of Portland._--The peninsulas of Purbeck and Portland are
continually wasting away. In the latter, the soft argillaceous
substratum (Kimmeridge clay) hastens the dilapidation of the
superincumbent mass of limestone.

In 1655 the cliffs adjoining the principal quarries in Portland gave way
to the extent of one hundred yards, and fell into the sea; and in
December, 1734, a slide to the extent of 150 yards occurred on the east
side of the isle, by which several skeletons buried between slabs of
stone, were discovered. But a much more memorable occurrence of this
nature, in 1792, occasioned probably by the undermining of the cliffs,
is thus described in Hutchin's History of Dorsetshire:--"Early in the
morning the road was observed to crack: this continued increasing, and
before two o'clock the ground had sunk several feet, and was in one
continued motion, but attended with no other noise than what was
occasioned by the separation of the roots and brambles, and now and then
a falling rock. At night it seemed to stop a little, but soon moved
again; and, before morning, the ground from the top of the cliff to the
water-side had sunk in some places fifty feet perpendicular. The extent
of ground that moved was about _a mile and a quarter_ from north to
south, and 600 yards from east to west."

_Formation of the Chesil Bank._--Portland is connected with the mainland
by the Chesil Bank, a ridge of shingle about seventeen miles in length,
and, in most places, nearly a quarter of a mile in breadth. The pebbles
forming this immense barrier are chiefly siliceous, all loosely thrown
together, and rising to the height of from twenty to thirty feet above
the ordinary high-water mark; and at the southeastern end, which is
nearest the Isle of Portland, where the pebbles are largest, forty feet.
The fundamental rocks whereon the shingle rests are found at the depth
of a few yards only below the level of the sea. The formation of that
part of the bar which attaches Portland to the mainland may have been
due to an original shoal or reef, or to the set of the tides in the
narrow channel, by which the course of the pebbles, which are always
coming from the west, has been arrested. It is a singular fact that,
throughout the Chesil Bank, the pebbles increase gradually in size as we
proceed southeastward, or as we go farther from the quarter which
supplied them. Had the case been reversed, we should naturally have
attributed the circumstance t