190 National Geographic Magazine. 



Peschel examined our rivers chiefly by means of general maps 

 with little regard to the structure and complicated history of the 

 region. He concluded that the several transverse rivers which 

 break through the mountains, namely, the Delaware, Susquehanna 

 and Potomac, are guided by fractures, anteiior to the origin of 

 the rivers.* There does not seem to be suflicient evidence to 

 support this obsolescent view, for most of the water-gaps are 

 located independently of fractures ; nor can Peschel's method of 

 river study be trusted as leading to safe conclusions, 



Tietze regards our transverse valleys as antecedent ;f but this 

 was made only as a general suggestion, for his examination of 

 the structure and development of the region is too brief to estab- 

 lish this and exclude other views, f 



Lowl questions the conclusion reached by Tietze and ascribes 

 the transverse gaps to the backward or headwater erosion of ex- 

 ternal streams, a process which he has done much to bring into 

 its present important position, and which for him replaces the 

 persistence of antecedent streams of other authors.^ 



A brief article§ that I wrote in comment on Lowl's first essay 

 several years ago now seems to me insufiicient in its method. It 

 exaggerated the importance of antecedent streams ; it took no 

 sufficient account of the several cycles of erosion through which 

 the region has certainly passed ; and it neglected due considera- 

 tion of the readjustment of initial immature stream courses dur- 

 ing more advanced river-life. Since then, a few words in Lowl's 

 essay have come to have more and more significance to me ; he 

 says that in mountain systems of very great age, the original 

 arrangement of the longitudinal valleys often becomes entirely 

 confused by means of their conquest by transverse erosion gaps. 

 This suggestion has been so profitable to me that I have placed 

 the original sentence at the beginning of this paper. Its thesis 

 is the essential element of my present study. 



Phillipson refers to the above-mentioned authors and gives a 

 brief account of the arrangement of drainage areas within our 

 Appalachians, but briefly dismisses the subject. | His essay con- 

 tains a serviceable bibliography. 



If these several earlier essays have not reached any precise 



*Physische Erdkunde, 1880, ii, 442. 



f Jahrbuch Geol. Eeichsanstalt, xxviii, 1878, 600. 



iPet. Mitth., 1882, 405 ; Ueber Thalbildung, Prag, 1884. 



§ Origin of Cross-valleys. Science, i, 1883, 325. 



II Studien tiber Wasserscheiden. Leipsig, 1886, 149. 



