The eastern and south-western sides of this parish are situate on
granite, the greater part consisting of high and barren hills,
including Carnbrea and Carnkie. This rock is large-grained, and not
very prone to disintegrate; it is occasionally traversed by beds of
felspar porphyry.
On the boundaries of this granite, and in the adjacent slate, are
numerous tin and copper-mines, the most interesting of which are
Delcoath and Cock’s Kitchen; the latter extends into the parish of
Illogan, but is a continuation of the Camborne lodes. Delcoath has
been for many years the deepest mine in Cornwall. It stands at the
surface, about fifty fathoms (300 feet) above the level of the sea,
and the deepest shaft is about a hundred and eighty fathoms (1080
feet) below the sea level, making on the whole a depth from the
surface of nearly 1400 feet.
These mines exhibit the curious geological phenomena of alternating
granite and slate; that is, in sinking a perpendicular shaft, the
miner passes repeatedly out of one of these rocks into the other.
Various theoretical opinions have been entertained on this subject:
some geologists supposing that the layers are no more than irregular
protuberances from the main mass of the rock; whilst others consider
them as large veins dipping towards and communicating with the granite
at a great depth. But it is ascertained that these granite layers are
sometimes detached or insulated masses, whilst at other times they
form large veins or courses, which have regular bearings to a
considerable distance, and are then called elvans; to form, however, a
correct idea of the features of these phenomena, we must become
acquainted, not with the appearance only, but with the nature and
composition of these two rocks.
Granite and slate are usually considered, from their exterior
character, as very dissimilar, whereas in this situation their real
composition is nearly alike.
The granite immediately in contact with the slate, consists of compact
felspar, containing particles or crystals of felspar, quartz, and
mica, in variable proportions, but the whole generally increasing
towards the centre of the mass. So that the granite is changed into a
felspar rock or porphyry rock, scarcely ever resembling a well
characterised granite; while the slate in contact has received the
various names of greywacke, greenstone, clayslate, and killas; but it
appears to be a rock _sui generis_, consisting almost entirely of
compact felspar, coloured purple or blue by its intimate union with a
dark-coloured micaceous mineral, sometimes seen distinct on the
surface of the slate, and from which it appears to derive its lamellar
structure. The bases of these two rocks are therefore the same, and at
the point of contact it is often difficult to draw a line between
them; for the slate passes into white compact felspar by the gradual
disappearance of its colouring mineral; and this granite rock, by the
more and more additions of felspar, quartz, and mica, reassumes its
usual character.
On this view of the subject, it is easy to comprehend why the granite
and the slate alternate and mutually pass into each other; and an
explanation may also be given of the complicated phenomena of granite
veins in slate, when it is assumed that both rocks are not only of
contemporaneous origin, but likewise similar in their mineral
composition.
Between this mining district and a line drawn east and west across the
parish, a little north of the church town, the land is in most places
very good; but north of this line, at the extremity of the parish,
where it abuts on the sea, the ground is almost entirely uncultivated,
affording nothing more than a slight pasturage for sheep. At Godrevy
Point there is laid open an interesting section of diluvial deposits:
one of the beds, composed of shelly sand and pebbles, is consolidated
with sandstone and conglomerate.
[24] See Symons of Halt in Botus Fleming.
[25] Here the word “fire-side” is interlined; and at ‡ the
words “good now” in the same hand with the paragraph within
brackets.