IV.—Historical Geology of Cornwall

Abstract

The best description of St. Michael's Mount, as it now exists, that I can find, is by Mr. Wm. Pengelly, F.R.S.,1 as follows: “The Mount is an isolated mass of granite measuring about five furlongs in perimeter at its base. At high-water it plunges abruptly into the sea, except on the northern or landward side, where the granite comes in contact with the slate, into which it sends veins and dykes, as may be well seen on each side of the harbour.Here there is a small plain occupied by a village, adjacent to which is the harbour, built in 1726−7, and as Mr. Johns, the harbour-master, has been good enough to write me, capable of receiving ships of 500 tons burthen” Its sityation is described as follows: “ The distance between the neatest point of Marazion Cliff and spring-tide high-water mark on the Mount is 1680 feet. A tidalb isthmus (Hogus) of highly inclined Devonian slate and associated rocks, in most cases covered with a thin layer of gravel or sand, is at spring-tide, in still weather, 12 feet below; and at low-water 6 feet above the sea-level. This ridge is dry in fine westher from four to five hours every tide, but occasionally during stoms and neap tides it is not passable for two or three days.”