The granite being of unequal hardness, its constituent crystals become separated by the action of the weather into an incoherent gravel, which in Cornwall is called growan. The process may be seen in full activity on any tor. Sometimes water lodges on a slab, and finding a soft spot begins to decompose it; then, when this is the case, the wind swirls the water about, and with it the grit is spun round and round, and this continues the work of disintegration, and finally a rock basin is produced.
Of these rock basins some fine samples exist: that on Caistor Rock has had to be railed round, to prevent sheep from falling in and being drowned. Mis Tor has another, the Devil's Frying-pan. There are plenty of them to be seen in all conditions, from the rude beginning to the complete bowl.
At one time it was supposed that they were Druidical vessels employed for lustration, and archaeologists talked long and learnedly concerning them. But what is quite certain is that they were produced by Nature unassisted.
When a hard bed of granite lies on one that is very soft, the latter becomes disintegrated and eaten completely away. The hard bed is left either balanced on one point or more, or else has its centre of gravity so placed as to precipitate it from its position. Plenty of rocks may be seen in all these conditions. If it should chance that a rock remains poised on one point, then possibly a little pressure at one end of the slab will set it in motion. This, then, is known as a logan, or rocking stone, which antiquaries of old pronounced to have been employed
