A small portion of the southern part of this parish, adjoining to St.
Dennis, rests on granite, the surface of which abounds in projecting
masses of shorl rock. Leaving this granite district about the Indian
Queens, on the Great Truro Road, a tin mine, called the Fat Work,
presents itself, and displays some interesting phenomena. The basis of
the rock is a compact felspar, sometimes almost colourless, at other
times dark blue and glossy; but more commonly both these kinds are
blended together in stripes or spots like agates. This rock next the
lode is much decomposed, the blue parts being changed into a light
pink; which shows that the colouring mineral is not hornblend. It is
probably shorl, a substance abounding in the quartz veins by which the
rock is traversed. The lode of this mine is very curious. It consists
of a large massive rock, fifteen fathoms in width, throughout which
shorl, and irregular veins of tin ore, are dispersed. The matrix
consists of a dark-coloured basis, interspersed by innumerable angular
and fragment-like portions of rock.
Near the Indian Queens a manganese mine was formerly worked, and the
adjacent moors abound in stream works.
A fine blue fissile slate occurs around the town of St. Colomb, and
continues for some distance northwards. Near Trewan it is found to
contain beds of compact rocks (not unlike those of the Mount Bay),
which inclose veins of actynolite. Still proceeding northward, quartz
so much prevails, as to occasion sterility in the land, displayed in
open downs like those of Roach, to which they adjoin.
Castle-An-Dinas is composed of a siliceous variety of the rock found
at Fat Work mine; and it appears to be one of the sources from whence
must have been derived the pebbles and striped shorl rock scattered
over Tregoss Moor.
[33] There is, or was, this inscription also:
D’s John Arundell, Mill. CCCC. verus Patronus hujus
Ecclesiæ, qui hanc Capellam fecit.
[34] Extracted from an “Account of certain Hill Castles,
near the Land’s End in Cornwall,” by William Cotton, Esq.
F.S.A. printed in the Archæologia, vol. XXII. where a plan
and section of Castle-An-Dinas will be found, taken with
greater care than that in Lysons’s Cornwall. In the
Gentleman’s Magazine, LXXII. p. 393, are engravings of two
stone weights found within the inner circle of this
fortress. The weight of one was seventeen pounds and a half;
and that of the other three pounds one ounce.