Dr. Mac Culloch on the Granite Tors of Cornwall. 71 



sures in different directions, but most commonly tending to the per- 

 pendicular and horizontal. By those it is divided into masses of- a 

 cubical and prismatic shape. Of the exceptions to this rule there is 

 one among many other instances, in Shaugh rick near Plymouth. If 

 we examine a rock of this kind near the surface of the soil, we shall 

 find that the fissure is a mere mathematical plane, separating the two 

 parts, and that the angles are sharp and perfect. If we turn our 

 attention to granites which from their greater elevation above the 

 present soil appear to have been longer exposed to air and weather, 

 we shall find, as the first step to change, a gentle rounding of the 

 angles, such as is exhibited in the drawing last cited, the Vixen 

 Tor. By degrees the surfaces which were in contact become sepa- 

 rated to a certain distance, which goes on to augment indefinitely. 

 As the wearing continues to proceed more rapidly near the parts 

 which are most external, and therefore most exposed, the masses 

 which were originally prismatic acquire an irregular curvilinear boun- 

 dary, and the stone assumes an appearance resembling the pieces 

 which constitute the Cheese- wring. If the centre of gravity of the 

 mass chances to be high and far removed from the perpendicular of 

 its fulcrum, the stone falls from its elevation, and becomes constantly 

 rounder by the continuance of decomposition, till it assumes one of 

 the various spheroidal figures which the granite bowlders so ofteii 

 exhibit. A different disposition of that centre will cause it to pre- 

 serve its position for a greater length of time, or in favourable cir- 

 cumstances may produce a logging stone. 



It is not necessarv to call in the aid of long- continued friction or 

 distant transportation to account for the rounded form of these 

 granite bowlders. The changes which they undergo in their places 

 of rest, by their more rapid disintegration at the angles than at the 

 sides, are sufficient to prove that this spheroidal shape may be pro- 



