354. Besides glaciers and torrents, which have no doubt been the principal instruments in producing these changes, other causes may have occasionally operated. Large stones, when once detached, and resting on an inclined plane, from the effects of waste and decomposition, may advance horizontally, at the same time that they descend perpendicularly, and this will happen though they be not urged by any torrent, or any thing but their own weight; for the surface of the ground, as it wastes, remains higher under the stone, and for a little way round it, than at a greater distance, on account of the protection which it receives from the stone, as in the instances at Saleve, just mentioned. The stone itself also becomes rounded at the bottom; and thus the surface in contact with the ground is diminished in extent, and the two surfaces rendered convex towards one another. It must therefore happen, that the support, continually weakening, will at length give way, and the stone incline or roll toward the lower side, and may even roll considerably, if its centre of gravity has been high above its point of support, and if its surface has had much convexity: Thus the horizontal may very far exceed the perpendicular motion; and, in the course of ages, the stone may travel to a great distance. A stone, however, which travels in this manner, must diminish as it proceeds, and must have been much greater in the beginning than it is at present. 355. This kind of motion may be aided by particular circumstances. When a stone rests on an inclined plane, so as to be in a state not very remote from equilibrium, if a part be taken away from the upper side, the equilibrium will be lost, and the stone will thereby be put in motion. That stones which lie on other stones, may, by wearing, be brought very near an equilibrium, is proved by what are called _rocking-stones_, or in Cornwall _Logan stones_, which have sometimes been mistaken for works of art; but are certainly nothing else than stones, which have been subjected to the universal law of wasting and decay, in such peculiar circumstances, as nearly to bring about an equilibrium of that stable kind, which, when slightly disturbed, re-establishes itself.[178] The Logan stone at the Land's End, is a mass of granite, weighing more than sixty tons, resting on a rock of granite, of considerable height, and close on the sea shore. The two stones touch but in a small spot, their surfaces being considerably convex towards one another. The uppermost is so nearly in an equilibrium, that it can be made to vibrate by the strength of a man, though to overset it entirely would require a vast force. This arises from the centre of gravity of the stone being somewhat lower than the centre of curvature of that part of it on which it has a tendency to roll; the consequence of which is, that any motion impressed on the stone, forces its centre of gravity to rise, (though