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.
