MODERN GEOLOGY 



trine of the extremely slow denudation of land surfaces. 

 The enormous amount of land erosion will be patent to 

 any one who uses his eyes intelligently in a mountain 

 district. It will be evident in any region where the 

 strata are tilted as, for example, the Alleghanies 

 that great folds of strata which must once have risen 

 miles in height have in many cases been worn entirely 

 away, so that now a valley marks the location of the 

 former eminence. Where the strata are level, as in 

 the case of the mountains of Sicily, the Scotch High- 

 lands, and the familiar Catskills, the evidence of de- 

 nudation is, if possible, even more marked ; for here it 

 is clear that elevation and valley have been carved by 

 the elements out of land that rose from the sea as level 

 plateaus. 



But that this herculean labor of land-sculpturing 

 could have been accomplished by the slow action of 

 wind and frost and shower was an idea few men could 

 grasp within the first half-century after Hutton pro- 

 pounded it; nor did it begin to gain general currency 

 until Ly ell's crusade against catastrophism, begun 

 about 1830, had for a quarter of a century accustomed 

 geologists to the thought of slow, continuous changes 

 producing final results of colossal proportions. And 

 even long after that it was combated by such men as 

 Murchison, Director - General of the Geological Sur- 

 vey of Great Britain, then accounted the foremost 

 field-geologist of his time, who continued to believe 

 that the existing valleys owe their main features to 

 subterranean forces of upheaval. Even Murchison, 

 however, made some recession from the belief of the 

 Continental authorities, Elie de Beaumont and Leo- 



