Page:A Treatise on Geology, volume 2.djvu/221

 CHAP. IX. attention to Von Buch's hypothesis, and has recorded the result (Edinb. New Phil. Journal, July, 1836). He noticed the radiation of valleys from the Cantal, their narrowing from the centre of the elevation outwards, and their wanting lateral valleys. These radiating valleys, so numerous, from a single mountain, appear to have originated in fissures of disruption. The alternation of "stratified" tufa, with trachyte, under a capping of basalt, in the slopes of the mountain, is an argument of weight with professor Forbes, and leading to the same conclusion. "Upon the whole," says this careful observer, "it seems to me that the evidence of earthquakes subsequent to the deposition (in whatever way) of the Cantal and Mont Dor, is a fact so indisputable as to render the argument about craters of elevation to a great extent merely verbal."—"There seems, therefore, so much of probability, and so little of extravagance, in Von Buch's theory, that we wonder how it could possibly have given rise to such animated opposition."

Let us turn from volcanic districts to others in which stratified rocks have been subjected to vertical displacement, in order to see in what forms the dislocated rocks are combined. Are there in such rocks "hollows of elevation" such as may be compared with the erhebungs cratere of Von Buch? It appears that there are such elevations, unless, with regard to the lake of Laach, we reject the obvious inference from its general figure, and are prepared to doubt the exactitude of the description of the "valley of elevation" of Woolhope. Such cases are, however, rare; they seldom occupy an exactly, or even approximately, circular area: the Woolhope valley is elongated, the Laach crater imperfect; the valley of elevation of Kingsclere is very little allied to a conical mountain; Greenhow Hill, in Yorkshire, though a double or transversely divided elliptical elevation, is, perhaps, as good a case in point as any that can be mentioned in England, to show the analogy which really obtains between the elevation of