The grand geological feature of this interesting parish is the large
mass of granular shorl rock, which towers above the adjacent country.
It has all the appearance and form (so far as regards the mechanical
structure of this mass) of a projecting tor of granite, such as
commonly cap the most elevated hills in Cornwall: but a nearer
examination shows that it is a compound of shorl and quartz. This rock
is entirely surrounded by a felspathic slate, being about a mile
distant from the granite. St. Mewan Beacon is an instance of another
large tor of shorl rock, a notice of which was omitted under the head
of St. Mewan; this rock differs however from that of Roach in having a
base of compact crystalline quartz, throughout which the shorl is
disseminated in various proportions. Its position is also different,
for that rises immediately out of the granite.
The southern part of Roach parish reposes on granite; the remainder on
rocks of the porphyritic series. It corresponds in its geological
structure with the parish of St. Dennis.