Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/77

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Dr. MacCulloch on the Granite Tors of Cornwall.

Transactions of the Geological Society, 1st series, vol. 2 plate page 0581.jpg

The Logging rock (Pl. III.) is situated on a peninsula of granite, in the parish of St. Levin, which stretches out about 200 yards into the sea, its isthmus still exhibiting remains of the ancient fortification of Castle Trereen. The mass of granite which forms this peninsula is split both perpendicularly and horizontally by numerous fissures, and is thus divided into a number of cubical and prismatic masses. A similar disposition in all the rocks of this shore has caused them to assume those singular forms which are so conspicuous at the Land’s End. The appearance of the perpendicular fissures on approaching the Logging rock from the isthmus is so remarkable, that we might for a moment fancy it the effect of stratification, as geologists have in other instances been tempted to suppose. Crystals of tourmaline are found in this granite, which has supplied the cabinets of collectors with so many specimens as to be too well known to need description.

The general height of the mass of rock on which the logging stone is placed varies from 50 or 80 to 100 feet, and it exhibits almost all round a perpendicular face to the sea. It is divided into four summits, on one of which, near the centre of the promontory, the stone in question lies. If the whole peninsula be viewed laterally, the conformity of the rocking stone to the mass on which it stands and to the other small stones which crown the summits, is such that the eye cannot detect it, so perfectly it seems in its place. It is in the front view only that it appears detached, as if occupying an accidental and not its natural and original place. Its general figure is irregularly prismatic, and four sided, having at its lower part that protuberance on which it is poised. So inclined is the plane on which it rests, that it appears at first sight as if a slight alteration of its position would cause it to slide along the plane into the sea; standing as it does within two or three feet of the edge of