The rocks of this parish are somewhat similar to those of St. Mervyn,
which is already stated to belong to the calcareous series. The rocks
are well exposed on the shores of Padstow harbour, and therefore they
merit a more particular description.
The southern part of this parish consists chiefly of a blue slate,
which in some strata is very fissile, and in other strata it separates
into thick lamellar pieces, and passes in the adjoining strata into
calcareous schist. This slate at Dinah’s Cave contains a bed of black
limestone, which may be a continuation of that on the opposite side of
the harbour at Rock ferry. At the latter place, the transitions
between the blue slate and the limestone are beautifully displayed.
The limestone is of the same nature as that in Veryan on the south
coast. Proceeding towards Slepper Point, at the entrance of the
harbour, after passing the town of Padstow, the cliffs consist of
alternate beds of compact and of schistose greenstone; these continue
to line the shore with broken rocks for more than a mile, and they are
succeeded by red and greenish slates, which are lamellar, of a fine
texture, and rather hard, resembling the mineral called novaculite or
hornestone; and connected with this slate occurs a compact crystalline
rock like that of Trevose Head in St. Mervyn, consisting of small
crystals of glossy felspar, and of a scaly pulverulent mineral of a
greenish colour, the nature of which is not very apparent. This
singular rock may be the equivalent of diallage rock, a member of the
serpentine series, as its position bears a great resemblance to that
of the Lizard magnesian rocks.
At Penniscen Bay, on the north side of the parish, the cliff exhibits
many alternations of limestone and slate; this limestone is more
crystalline than the black limestone of Rock Ferry, approaching nearer
to the Plymouth limestone in its appearance; but organic remains have
not yet been discovered in it.
The slate between the beds of limestone is a calcareous schist, and is
of a white colour, and earthy where it has been weathered; but when
perfect it is found, especially near the limestone, to be of a blue
colour with broad diagonal bands or stripes of brown, precisely like
the calcareous slate which occurs in Werrington, near Yealm Bridge,
where it is extensively quarried for chimney-pieces, and for other
domestic or ornamental purposes.