A line drawn N. W. and S. E. about a mile due N. from the church,
would divide this parish into nearly two equal parts, the northern
parts consisting of granite, and the southern of schistose rocks,
belonging to the porphyritic series.
The granite stretches completely across the large central mass of this
district, and contains many varieties. Next to the slate, the rock
generally contains a considerable portion of shorl, which sometimes
takes the place of the mica altogether; both of these kinds abound in
large veins and courses of quartz and shorl rock. But the most
abundant variety is that extensively decomposed into a white friable
mass; of this substance the more compact and perfect parts are
quarried, and exported under the name of China stone, whilst the
earthy parts are washed to separate the fine argillaceous particles
from the grains of quartz, and from other hard minerals with which
they are mixed in their natural state. The water, rendered white like
milk by this operation, is received in large pits, where the earthy
part subsides, and then after being dried is sent to the potteries
under the name of China clay. The manipulations which this substance
undergoes during its preparation are simple, but very tedious; and
they afford employment to several hundred persons. The extent of this
branch of commerce is so great, that about seven thousand tons of
clay, and five thousand tons of China stone are exported from Cornwall
every year, of which by far the greater part is raised in this parish.
The rocks of the slate series are similar to those of St. Mewan and
St. Austell, containing also metalliferous veins.