This parish is one of the great mining districts of Cornwall,
abounding in tin and copper ores, but more particularly in the former.
It differs, however, from all the other districts, in being remote
from the great central masses of granite. This peculiarity has often
attracted the notice of several observers, and has long been
considered as a strange anomaly by geologists.
The case is not, however, without example; for, although this tract is
distant from the granite both of Redruth and of St. Dennis, yet a
small mass of that rock does not exist at Cligga Point, on the
confines of the parish. This granite has, indeed, been called by some
an elvan, and by others a secondary formation of granite, as has been
also that of St. Michael’s Mount; but, although the rock is not in
this place of any great extent, it has all the mineralogical and
geological characters of the larger masses.
Large courses of granite elvan are common in the northern part of the
parish, containing short irregular veins and bunches of tin ore. These
courses are extensively exposed in the cliffs, and present a singular
appearance, somewhat resembling a bank of earth perforated by rabbits’
burrows, in consequence of the miners having taken the ore wherever it
has been exposed to view.
The Beacon, a high hill near the church town of St. Agnes, merits
particular attention. The lower part is formed of a schistose rock,
composed of granular felspar intermixed with particles of quartz and
minute scales of mica. Ascending towards the summit, the quartz
gradually increases in quantity, till at last it becomes the
prevailing ingredient of the rock, and preserves it against the
natural causes of decay; whilst lower down, where the felspar abounds,
the rock is extensively disintegrated. On the side of the hill, about
three or four hundred feet above the level of the sea, is a deep
deposit of diluvium, consisting of alternate layers of clay and sand.
To point out the origin of these layers, and to explain the reason of
their occurrence in such an elevated situation, would require long
details. For this, and for other interesting particulars respecting
the phenomena of this parish, the fourth Volume of Transactions,
published by the Geological Society of Cornwall, may be consulted.
St. Agnes’ Beacon was chosen as one of the principal western stations
in the great Trigonometrical Survey of England. The position of the
summit was then determined with extreme accuracy: Latitude 50° 18′
27″, Longitude 5° 11′ 55″.7. In time 20 m. 47″.7. Height above low
water 621 feet. See the Philosophical Transactions for 1800, pp. 636
and 714.
[5] It appears, however, by Mr. Tonkin’s notes, that St.
Agnes was deemed a distinct parish, and had a parochial
chapel in it, so early as the year 1396. A licence to build
a new chapel was dated Oct. 1, 1482. _Lysons._
[6] This gentleman was the lessee of the great mine before
described. Borlase says, “It is judged that the late Mr.
Donnithorne, who had the whole adventure, and worked it at
his own expense, in a few years last past got at least
40,000_l._ clear by this mine, and much more he might have
raised yearly if he pleased.”
[7] For the latter name Hitchins substitutes Thomas Warren,
Esq. and Mr. John Tregellas, of St. Agnes.