VIEWS OF AMERICAN WRITERS. 391 



A characteristic example of this manner of treatment is found in the 

 valuable works by Lepsius on the mountains of the upper and middle 

 Rhine,* in which the Schiefergebirge and other ancient mountains are 

 fully treated as to structure, although little is said of their form and still 

 less of the origin of their form. 



The following citations are from works in which land form and sculpture 

 are more fully considered. 



The increasing importance attributed by Sir A. Geikie to subaerial 

 agencies in his later writings has already been noted. Professor James 

 Geikie goes further in this direction and says : 



"Valleys continue to be deepened and widened, while the intervening mountains, 

 eaten into by the rivers and their countless feeders and shattered and pulverized 

 by springs and frosts, are gradually narrowed, interrupted and reduced until 

 eventually what was formerly a great mountain chain becomes converted into a 

 low-lying undulating plain," t 



Gosselet, in his comprehensive monograph on the Ardennes, says that 

 the tilted, folded and faulted strata of their uplands haye been, as it were, 

 planed down by the combined action of atmospheric disintegration and 

 pluvial wearing. Both the Jurassic and Cretaceous formations are de- 

 scribed as lying on oldland soils, where they overlap the Paleozoic strata. J 



The elaborate treatise on " Les formes du terrain " (1888), by de la 

 Noe and de Margerie, clearly maintains that pluvial denudation may not 

 only produce valleys, but it may wear down the divides between the val- 

 leys (page 106). The escarpments or cross-valleys of the Weald in south- 

 ern England may be explained without calling on marine erosion, as most 

 of the English geologists have done (pages 135, 136). Plateaus of abra- 

 sion, without a cover of unconformable strata, maj^be " simply the result 

 of prolonged subaerial erosion." If unconformabl}^ covered, it still re- 

 mains to be seen how far the abraded surface is — 



" The modification by wave action of a hardly different surface, produced by the 

 prolonged work of streams which had long before attained faintly graded slopes, 

 and which had by the aid of atmospheric agents almost completely destroyed pre- 

 existing inequalities of form " (page 188). 



Penck concludes that the final aim of subaerial denuding agents is to 

 reduce a land almost completely to a plain,§ but his account of the 

 Schiefergebirge of the middle Rhine does not explicitly state whether the 

 " abrasionsplateau " of their uplands is of marine or subaerial origin. || 



*Die Oberrheinisclie Tiefebene und ihre Raudgebivge, Forsehungen zur deut. Landeskunde, i, 

 1885, 35-91 ; Geologievon Deutschland, 1887. 

 tMoLintains, their origin, growth and decaj'': Scot. Geog. Mag., vol. ii, 1886, p. IGO. 



I L'Ardenne. M6m. Carte geol., France, 1888, pp. 802, 8(i8, 837. 



§ Das Endziel der Erosion und Denudation, Verb, viii deut. Geographentag, 18P9, pp. 91-100. 



II Landerkunde des Erdtheils Europa, i, 1887, p. 316. 



