This parish, so important in an economical point of view, on account
of its minal production, affords a vast fund of geological
information. Its northern part is composed of granite; its southern
part of various rocks belonging to the porphyritic group. Its granite
on the eastern side is like that of Alternun, and contains layers
which abound in porphyritic crystals of felspar. On the western side
it comprises several kinds of this rock; some characterised by the
proportions of shorl that enter into their composition; and others by
containing talc instead of mica, and by the felspar being prone to an
extensive decay, in which state it furnishes porcelain clay (or china
clay) for the potteries. A more particular description of these kinds
of granite will be given hereafter, in the accounts of St. Stephen’s,
and of the adjacent parishes. Carclaze tin mine must not, however, be
passed by, as it is one of the greatest curiosities in Cornwall. This
mine is worked “open to the day” (according to the miners’ term), that
is, like a quarry. It is of a considerable depth; and its superficies
exceeds several acres in extent. It is excavated entirely in a white
granite, somewhat similar to the disintegrating variety above alluded
to; and when the sun shines, the reflection of light is so exceedingly
dazzling as to be almost insupportable. The tin ore occurs here
intermixed with shorl and quartz, in the form of short irregular
veins, which traverse the granite in every direction, and so abundant,
that the whole rock requires to be pounded and washed to complete an
entire separation of the ore.
Hornblend rocks succeed the granite, and produce a red fertile soil.
These extend a little to the south of the town of St. Austell, and are
followed by a blue lamelar slate, in which the mines are situated.
This rock is much softer, and more argillaceous than the hornblend
slate, and decomposes into a light-coloured soil. The matrix of its
lodes abound in chlorite: it is probably a chlorite schist. This
formation is traversed by several beds of felspar, porphyry (elvan
courses), in the western side of the parish, which run north-east and
south-west, in a somewhat tortuous manner, and dip towards the
granite. One of these elvans, near Pentewan, has been extensively
quarried, and is much esteemed as a building material. This chlorite
slate also contains, in the cove at Duporth, a bed of compact
magnesian rock, abounding in asbestos; and passes on either hand into
the surrounding slate, by means of layers of talcous schist.
This parish has long been celebrated for its stream works, which are
diluvial beds containing tin ore. They are generally found in deep
vallies where rivulets flow, which are used in separating the tin ore,
by its inferior specific gravity, from common stones or pebbles; hence
the name of “stream-works.” The nature of these deposits varies
according to the positions which they occupy between the sea and the
granite; whence the stanniferous strata were derived.
Pentewan stream-work is one of the most interesting in the whole
county. Its lowest bed consists of pebbles, gravel, and tin ore, and
it rests on the solid rock. Immediately above this tin-ground is a
black stratum of vegetable remains, among which are stumps of trees,
standing erect, with their roots penetrating downwards into the bed of
gravel. This subterranean forest stands forty-eight feet below
high-water mark; showing that there must have been a change in the
relative sea level. On this vegetable bed reposes a thick stratum of
silt, intermixed with horns of deer, and with other relics of land
animals, and also with detached pieces of timber. This silt is of the
same description as that now forming in the Truro river, and in other
estuaries on the coast; and it contains layers of shells peculiar to
such situations.
This silt is covered by a deep deposit of siliceous sand, in which
occur various remains, principally of marine origin; and lastly, over
this lies another bed of silt like the preceding, which reaches to the
surface, where a thin marsh soil is now in a state of cultivation. The
upper bed of silt is nearly on a level with the sea, being separated
and protected from it by the interposition of a sandy beach.
Many theoretical observations and reflections would naturally present
themselves, after a statement of these facts; but such would be more
appropriate to a separate treatise, than to a series of notices on
individual parishes.
[11] The market continues to deserve this character; the
tolls, persuant to the charter of Queen Elizabeth, are
assigned to the relief and maintenance of the poor.