ROCKING-STONES, or LOGGANS, are large masses of rock so finely poised as to move backward and forward with the slightest impulse. They occur in nearly every coun try. Some of them appear to be natural, others artificial; the latter seem to have been formed by cutting away a mass of rock round the center-point of its base. The former are chiefly granitic rocks, in which feldspar and porphyry are abundantly present; and these ingredients becoming rapidly decomposed, and the dust and sand .washed away by rains, what was formerly a solid rock, soon assumes the appearance of a group of irregularly-shaped pillars, having a rhomboidal horizontal section, and separated into portions by horizontal and vertical fissures. As decay proceeds, the edges of the blocks forming the pillar are first attacked and disappear, as is also the ease with greenstotte and basalt, and the pillar now becomes a pile of two or more spheroidal rocks, resting one upon the other. Should a mass of rock be so situated as to preserve its equilibrium in spite of the gradual diminution of its base or point of support, a rocking-stone or loggan is the result. For an exposition of the principle regulating the stability of equilibrium of rocking-stones, see S'r:»31LITY. Various explanations have been given of the uses of
these singular objects. They are supposed to have been used in very early times for purposes of divination, the number of vibrations determining the oracle; hence it came to be believed that sanctity was acquired by walking round them.
Some rocking-stones occur near to remains of ancient fortifications, which seems to bear out a statement in one of the poems of Ossian. that the bards walked round the -stone singing, and made it move as an oracle of the fate of battle. In Greece, rocking stones occur as funeral monuments, and are generally found on conspicuous places near the sea. Rocking-stones are numerous in Yorkshire, Derbyshire, Cornwall, and Wales. One near Land's End, in has been computed to weigh no less than 90 tons. Near Warton Crag, Lancashire, are no less than seven of these stones. In Scotland, they occur in the parishes of Kirkmichael, Dron, and Abernethy, Perthshire, and in the parish of Kells, Kirkcudbrightshire. In Ireland they are found in many places; one situated at a place called Islandmagee, on Brown's bay, is popularly believed to acquire a rocking tremulous motion at the approach of sinners and malefactors.