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Page:Popular Science Monthly Volume 57.djvu/174

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exceptional instances land forms initiated by deformation, so recently as to have suffered as yet only insignificant sculpture, may exhibit much irregularity. The most striking example of this kind, an example of the very highest value in the systematic study of land forms, is that afforded by the diversely tilted lava blocks of Southern Oregon, as described by Russell.[1]

Turning now to the second line of advance, it is noteworthy that so keen an observer as Lesley insisted, as late as 1856, that the peculiar topographical features of Pennsylvania, which he knew and described so well, could have been produced only by a great flood. But the principles of the uniformitarians were constantly gaining ground against these older ideas; and after the appearance in England of Scrope's studies in Central France and of Greenwood's polemic little work on 'Rain and Rivers' (1857), victory may be said to have been declared for the principles long before announced by Hutton and Playfair, which, since then, have obtained general acceptance and application.

Yet even the most ardent uniformitarians would, in the middle of the century, go no further than to admit that rain and rivers could roughen a region by carving valleys in it; no consideration was then given to the possibility that, with longer and longer time, the hills must be more and more consumed, the valleys must grow wider and wider open, until, however high and uneven the initial surface may have been, it must at last be reduced to a lowland of small relief. The surface of such a lowland would truncate the underground structures indifferently; but when such truncating surfaces were noticed (usually now at considerable altitudes above sea level, as if elevated after having been planed, and therefore more or less consumed by the erosion of a new system of valleys), they were called plains of marine denudation by Ramsay (1847), or plains of marine abrasion by Richthofen (1882). To-day it is recognized that both subaërial erosion and marine abrasion are theoretically competent to produce lowlands of denudation; the real question here at issue concerns the criteria by which the work of either agency can be recognized in particular instances. In the middle of the century, not only every plain of denudation, but every line of escarpments was held by the marinists to be the work of sea waves; and it was not till after a sharp debate that the bluffs of the chalk downs which enclose the Weald of southeastern England were acepted as the product of ordinary atmospheric weathering, instead of as the work of the sea. Whitaker's admirable essay on 'Subaërial Denudation,' which may be regarded as having given the victory in this discussion to the subaerialists, was considered so heterodox that it was not acceptable

  1. 4th Ann. Rep. U. S. Geol. Survey, 1883.