The church is situated on the borders of the granite, which extends to
the extreme northern part of the parish, and is for the most part of
that common kind which is shipped at Penryn for the London market.
Proceeding southward from the church town this granite is bounded by a
belt of rocks belonging to the porphyritic series, such as the felspar
and hornblend rocks; but in the vicinity of the Helford river these
are succeeded by the calcareous series.