Thus, if to the wasting of a stone at the bottom, we add the accidents that may befal it in the wasting of its sides, we see at least the physical possibility of detached stones being put in motion, merely by their own weight. It is indeed remarkable, that some of the largest of these stones rest on very narrow bases. Those at the foot of Saleve touch the ground only in a few points: The Boulder stone of Borrowdale is supported on a narrow ridge like the keel of a ship, and is prevented from tumbling by a stone or two, that serve as a kind of shores to prop it up. Very unexpected accidents sometimes happen to disturb the rest of such fragments of rock as have once migrated from their own place. Saussure mentions a great mass of _lapis ollaris_[179] that lies detached on the side of a declivity in the valley of Urseren, in the canton of Uri. The people use this stone as a quarry, and are working it away on the upper side, in consequence of which it will probably be soon overset, and will roll to the bottom of the valley. [Footnote 179: Voyages aux Alpes, tom. iv. § 1851.] 357. In many instances it cannot be doubted, that stones of the kind here referred to are the remains of masses or veins of whinstone or granite, now worn away, and that they have travelled but a very short way, or perhaps not at all, from their original place. Many of the large blocks of whinstone which we find in this country, sometimes single, and sometimes scattered in considerable abundance over a particular spot, are certainly to be referred to this cause. But the most remarkable examples of this sort are the stones found at the Cape of Good Hope, on the hill called _Paarlberg_, which takes its name from a chain of large round stones, like the pearls of a necklace, that passes over the summit. Two of these, placed near the highest point, are called the Pearl and the Diamond, and were mentioned several years ago in the Philosophical Transactions.[180] From a more recent account, these stones appear to be a species of granite, though the hill on which they lie is composed of sandstone strata.[181] The Pearl is a naked rock, that rises to the height of 400 feet above the summit of the hill; the Diamond is higher, but its base is less, and it is more inaccessible. [Footnote 180: Vol. lxviii. p. 102.] [Footnote 181: Barrow's Travels into Southern Africa, p. 60.]