Owing partly to their indurated state, and partly to
their great antiquity, they are usually found in the heart of
mountainous districts, which have undergone great disturbance,
and have been subjected to an enormous amount of denudation. In
some cases, as in the Longmynd Hills in Shropshire, they form
low rounded elevations, largely covered by pasture, and with few
or no elements of sublimity. In other cases, however, they rise
into bold and rugged mountains, girded by precipitous cliffs.
Industrially, the Cambrian Rocks are of interest, if only for
the reason that the celebrated Welsh slates of Llanberis are
derived from highly-cleaved beds of this age. Taken as a whole,
the Cambrian formation is essentially composed of arenaceous
and muddy sediments, the latter being sometimes red, but more
commonly nearly black in colour. It has often been supposed that
the Cambrians are a deep-sea deposit, and that we may thus account
for the few fossils contained in them; but the paucity of fossils
is to a large extent imaginary, and some of the Lower Cambrian
beds of the Longmynd Hills would appear to have been laid down
in shallow water; as they exhibit rain-prints, sun-cracks, and
ripple-marks--incontrovertible evidence of their having been a
shore-deposit. The occurrence, of innumerable worm-tracks and
burrows in many Cambrian strata is also a proof of shallow-water
conditions; and the general absence of limestones, coupled with
the coarse mechanical nature of many of the sediments of the
Lower Cambrian, maybe taken as pointing in the same direction.
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