This parish stretches north and south across the large patch of
granite, which is situated between Bodmin and St. Austell, and also
extends over the slate at its northern and southern extremities. The
extent of surface covered by the northern slate is more than a square
mile; but the southern slate forms only a small triangle north and
north-east of St. Blazey Bridge. The northern half of this parish
corresponds with the parish of St. Dennis in the nature of its rocks;
the southern half with St. Austell and St. Blazey. The most
interesting feature of this parish, in a geological point of view,
consists of its numerous and extensive stream-works. This part of
Cornwall, including the adjoining parishes, has long been celebrated
for the fine quality of its stream-tin, which is, as to the greater
part, either crystalline, or of the kind denominated wood-tin, on
account of its fibrous texture. The gravel containing this stream-tin
rests on the rock, or on its untransported debris; above this occurs a
regular stratum of decayed trees and plants; in the deepest
stream-works these are covered by another bed of gravel containing
tin, having also a superincumbent layer of decayed trees and plants:
but this second stratum of tin, always less in quantity than the
first, is seldom of sufficient value to pay for the labour of
separation. Over all occurs a layer of uncertain thickness, composed
of the debris of adjacent rocks, and sustaining on its surface a
coating of recent vegetable, and forming marshy, arable, or meadow
land, according to the accidental situation of the spot.