Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/85

From Wikisource
Jump to navigation Jump to search
This page has been validated.
Dr. MacCulloch on the Granite Tors of Cornwall.

That this would be the consequence of a gradual action merely mechanical 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 determination 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 decomposes 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