oo 



84 DAVIS — PLAINS OF MARINE AND SUBAERIAL DENUDATION. 



pendently of Ramsay's writings, in his great work on China, attention 

 heing led to the problem by the occurrence of unconformable marine 

 strata lying on smooth foundations, as observed in liis eastern travels. 

 He concludes that the " oldland " platform cannot have been produced 

 by atmospheric wasting or l)y running water ; tliese agencies produce 

 valleys separated by ridges. Truly the valleys multiply and widen and 

 the ridges weaken, but reduction to a lowland can be reached only locally 

 and in small dimensions. Moreover, change in the altitude of land works 

 against complete denudation ; yet, although such a result is unattain- 

 able by subaerial agencies, it may be accomplished by the waves of the 

 sea beating on the coast. Three cases are considered : a still-stand of the 

 land for an indefinite period, a slow elevation and a slow depression. 

 The still-standing land would be cut inward to a limited distance, after 

 which the waves would be exhausted on the platform of their own carv- 

 ing. During elevation slight effect could result, for the work would 

 always l)e beginning anew. Slow depression alone can produce regional 

 abrasion, for then the power of the waves is maintained by the continued 

 sinking of the bottom, while detritus accumulates on it. In contrast to 

 structural plateaus (Schichtungsplateaus), plateaus of denudation have 

 no relation to the structures across which they are cut or to the valleys 

 which are sunk beneath their level after general elevation. As examples, 

 the Ardennes and the uplands of the middle Rhine are first mentioned, 

 these bein;; explained as producible only b^^ sea waves ; never b}^ flowing 

 water or other subaerial agents. Another example given is the western 

 slope of the Sierra Nevada of California, now uplifted and dissected.* 



The substance of the above is repeated in Richthofen's " Fiihrer fur 

 Forschungsreisende," t emphasis being given to the association of plains 

 of denudation witli unconformably overlying sediments, to which the 

 English school directs insufhcient attention. Subaerial agents are de- 

 scribed as excavating valleys in uplifted plains of denudation, but not 

 in producing the plains (images 171-173, 670, 671). The prevalence of 

 superposed streams in certain dissected uplands of abrasion is noted 

 (pages 671, 672), but no contrast drawn between these examples and 

 others in which the streams are systematically adjusted to the structures. 



Cornet and Briart have made special study of the greatly deformed 

 Paleozoic rocks of Belgium, which they believe once rose in loft}' moun- 

 tains. Although they regard subaerial agencies competent to produce 

 the " complete ablation " of a land surface, they conclude that it was 

 probably the waves of an encroaching sea that contributed largely to 

 destroy what remained of their ancient mountains in Cretaceous time. J 



* China, 1882, vol. ii, chap, xiv, sec. 3. t Berlin, 1880, pp. 353-301. 



X Le relief du sol en Belgiqne. Ann. Soc. Geol., Helg., iv. 1877, pp. 72-113. 



