Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/84

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Dr. MacCulloch 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 composition 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 produce 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."

Whatever disputes and doubts may have existed relating to the stratification of granite in general, I 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, just 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.