St. Anthony is situated in the calcareous group of the slate
formation, and is composed of a glossy blue and fine grained slate,
which alternates with a coarse blue rock abounding in scales of mica,
and is more or less lamellar according to its proportion of this
mineral: this compact rock readily disintegrates, assuming various
yellowish tints, and an arenaceous appearance; indeed, it is seldom
seen but in that condition, and it has therefore been sometimes
mistaken by geologists for a variety of sand-stone.
At Porth there is a narrow neck of land, on the upper part of which
lies a sandbank abounding in perfect shells, which are arranged in
layers, and appear similar to those of the adjacent beach. This bank
is at least thirty feet above high water-mark, and it is covered with
a stratum of earth in cultivation. The sand is silicious, and becomes
in the lower part intermixed with pebbles, resembling in this respect
the banks on the shores of Mount’s Bay. This affords an example of an
ancient beach elevated above the one now in existence. The whole coast
of Cornwall furnishes numerous instances of this occurrence, and the
former beach is uniformly at the same elevation above that actually in
existence, indicating that the sea must, at some former period, have
joined the land at a line now higher than the present beach by that
difference.