1 64 POPULAR SCIENCE MONTHLY. 



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.* 



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 ap- 

 plication. 



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 con- 

 siderable 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 subaerial 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 'Subaerial 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 



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



