Dr. Mac Culloch on the Granite Tors of Cornvjall, 75 



That tills would be the consequence of a gradual action merely me- 

 chanical Is undoubted, as the mass must ultimately acquire that 

 figure which, being the last result of the action of decomposition 

 is the one which will offer the greatest resistance to further 

 change. In a chemical view, the same must also to a certain extent 

 hold true; since any given particle, supposed cubical and placed at 

 the angle or edge, will be exposed to the action of the solvent on 

 two or more surfaces, while that on the side of the mass is exposed but 

 on one, hence the angular body must ultimately change its figure, and 

 approximate to a spheroidal form : it is easy however to see that the 

 influence of this cause will be retarded in a quickly increasing ratio, 

 and that it is insufficient to account for the extreme change of form 

 suffered by granitic masses. If it were sufficient in the case of granite, 

 it should equally produce in sandstones of prismatic fracture a determi- 

 nation to the spheroidal form. But in these we see that the process of 

 superior waste at the angles and edges, soon ceases to produce an effect 

 in modifying the figure of the mass, and that sandstone never assumes 

 the decidedly spheroidal forms which are exhibited by granite. 

 Mechanical causes of change are here out of the question. If we 

 now suppose the hardness of a mass of granite, or its resistance to 

 the disintegrating power of air and water, to vary in any given ratio 

 at certain distances from the centre, it is evident that the effect of 

 chemical action on the surface, will be to change the figure of that 

 mass, and that the ultimate effect will be to disclose the sphere 

 inscribed within that cube. Let us consider how far the facts bear 

 us out in this supposition. 



De Luc has observed in his Geology, that granite sometimes de- 

 composes into spheroidal forms, and he describes piles of this rock In 

 Silesia, resembling, as he says, Dutch cheeses. I need not quote more 

 authorities for a fact witnessed by innumerable observers. In our 



K 2 



