The extreme western part, adjoining Mabe, consists of granite of a
very crystalline variety, and excellent for building; it is exported
in great quantities from Penryn. Nearly the whole western half of the
parish is coarse; but the eastern part is well cultivated. The soil
rests on a species of hornblend slate, which furnishes deep soil by
its decomposition.
On the sea shore at Swanpool a most interesting phenomenon occurs in a
bed of felspar porphyry, (elvan course,) which runs north-east and
south-west for several hundred feet: near low water it exhibits that
appearance called by the miners a heave. The course terminates
abruptly, and begins again about twenty feet further to the south,
from whence it goes on as before the heave.
This porphyry is decomposed at the surface and to some depth, into a
fine white clay, from which bricks of a good quality are made on the
spot.