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



have been unable to Imitate the process in our laboratories. It is also 

 not improbable, that the quantity of potash which enters into the com- 

 position of felspar, may confer on it a similar property, and that even 

 in a greater degree, although direct experiments are wanting to prove 

 this fact. Whichever of these bodies is acted on in the case of this 

 disintegration, the quantity of matter actually dissolved is probably 

 very little ; we can even conceive it possible that the mere alternation 

 of the states of moisture and dryness, combined with frequent 

 changes of temperature at the surface, may be sufficient to pro- 

 duce this effect without any actual solution of the substance of the 

 rock. It is a matter of more difficulty to assign the cause of the 

 change of figure which the masses undergo, by what process Nature 

 *' mutat quadrata rotundis." 



Yv'^hatever disputes and doubts may have existed relating to the 

 stratification of granite in general, 1 believe there is no one now who 

 conceives the granite of Cornwall more than that of Arran or Mont 

 Blanc to be stratified. The favourers of different hypotheses, must 

 each be allowed for the present to adopt the opinions which to them 

 seem the best founded, and it must depend on the conclusions which 

 shall ultimately be adopted, relative to the aqueous or igneous origin 

 of granite, whether these fissures are to be considered as the effects of 

 contraction produced in the mass by the evaporation of water, or by 

 the abstraction of heat. The cause of their peculiar form remains for 

 the present involved in the same difficulties which attend on the 

 more regular prismatic figures found in the trap rocks. But the 

 fissures themselves having been formed in whatever way we 

 chuse to suppose, we have still a difficulty unsolved, and that is the 

 tendency which they exhibit to wear more rapidly on the angles and 

 edges than on the sides, and thus to assume the spheroidal forms 

 which facilitate the ultimate ruin and migration of the summits. 



