Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/82

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Dr. MacCulloch on the Granite Tors of Cornwall.

duced by chemical action of air and water, without the necessity of any mechanical violence. However difficult it may be to give a very satisfactory account of this peculiarity, the fact is undoubted.

There is less difficulty in accounting for their separation from each other at their surfaces of contact, after the fissure has been formed, if we consider that they are liable to lodge water where the surface is horizontal, or to detain moisture where it is vertical.

That the wearing of these granites on the surface arises from the action of water, will be evident on examining the stones themselves, and the result of their disintegration. Wherever a stone is disintegrated by the most usual process, the oxidation of the iron which it contains, a change may always be observed to have taken place from the surface downwards to a more or less considerable depth in the stone. Sometimes even the whole mass of rock will appear to have undergone this gangrenous process at once, and to have become a bed of clay and gravel. But in the case of the granite now under view, it is evident that the change is merely superficial, and that no process of oxidation has taken place. Indeed, many of the varieties of which the mica and felspar are nearly white, contain so little iron that they are hardly subject to decomposition from this cause, however much they may, in such particular cases as that of the St. Stephen's granite, resolve entirely into gravel and porcelain clay. The most satisfactory proof however that the mere agency of water is sufficient to disintegrate this granite, is presented by those objects which perhaps in consequence of the Druidical speculations of Dr. Borlase are best known by the name of rock basons.

On the flat surfaces of these stones are frequently to be observed excavations, assuming some curved figure with rounded bottoms. Occasionally they are circular in their boundary, and as regularly spheroidal internally as if they had been shaped by a turning lathe.