GEOLOGY, BY DR. BOASE.
There are few spots that have excited greater geological interest than the serpentine tract of the Lizard, and no part of it will be found more instructive than this parish.
By far the greater part of St. Kevern rests on magnesian rocks; but north of a line drawn from Porthalla, nearly due west to Goonhilly Downs, the rocks belong to the calcareous series. The latter rocks may be seen on the coast from Porthalla to the Nare Point, and will be found to resemble the series between Gorran and the Dodman Point. On the left side of Porthalla Cove the blue slate abounds in veins and in irregular nodules of calcspar; and at low-water-mark a more compact variety is exposed, which evidently forms the passage into the black limestone, loose fragments of which are sometimes found on the shore. In a small creek within the Nare near Bostowda, is a large patch of conglomerate, the pebbles and fragments of which have been derived from the rocks which line the banks of the river Hellas as high up as Gweek; but which bear no resemblance to the rock of the immediate vicinity. This is the most decided instance of a fragmentary rock in Cornwall.
The hollow occupied by the little stream which discharges itself at Porthalla divides the calcareous shale from a rock of totally different nature, videlicet serpentine,
several varieties of which form the neck of land stretching thence to Dranna Point.
At Porthoustock a glossy lamellar rock, already noticed as joining the serpentine at Cadgwith, forms each side of the cove; but here, on proceeding to the Manacles Point it may be seen passing into diallage rock: the latter extends so far as Coverack, and also inland to the foot of Goonhilly Downs. At Coverack the diallage rock appears to pass into serpentine; but here again, as at Porthalla, the junction is a concrete. The varieties of serpentine near Coverack Pier are numerous, and several of them may be seen passing into each other, which in other parts of the Lizard district form large and apparently independent masses. From Coverack to Kennick Cove the cliffs are very bold, and display different kinds of serpentine and diallage rocks, and at Blockhead a large stratum of indurated steatite, beautifully marked with brown arborescent figures on a yellow ground. At Kennick Cove, red and olive green serpentine, abounding in scales of diallage, and traversed by numerous veins of asbestos, talc, and calcareous spar, are exposed to view on a grand scale; and at Gwinter, a little north of the cove, diallage rock is accompanied by layers of beautiful violet-coloured jade, or compact felspar, containing large plates of diallage as metalloide as at Coverack.
It may be noticed here, that all the uncultivated land extending over serpentine formation, is clothed with the most beautiful of European heaths; the Erica Vagans of Linnæus, so named on account of its being found in various parts of the world on particular spots. Hudson named it “Multiflora” from its splendid inflorescense; and Dr. Withering, with some others, didyma, with reference to double antheræ on each flower. This heath bounds itself almost within a yard to the limits of the magnesian earths.