On the eastern side adjoining to the Downs, the rocks are quartzose,
but the greater part of this parish is composed of slates belonging to
the calcareous series, which extend to the sea shore. At Tremper
Bridge, and in the islands lying at the entrances of St. Columb Porth,
beds of black limestone occur, associated with talc, schist, and blue
slates.
The most interesting feature, however, of this parish is the
testaceous sandstone at New Quay, and on the shores of Fistral Bay.
The land adjacent to the sea at those places is covered with sand,
which above high-water mark is lapidified, as is also a bed of pebbles
on which the sand sometimes rests. For the particulars of this
phenomenon the first and the fourth volumes of the Cornwall Geological
Transactions may be consulted.