72 Dr. Mac Culloch 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 disin- 

 tegrated 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 super- 

 ficial, 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 Druldical spe- 

 culations 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. 



