34 NaUonal Oeograjphic Magazine. 



whose genesis is attributed to retrogressive (rilckwarts fort- 

 schreitende " or " rtickschreitende " ) erosion, is vaguely separa- 

 ted into several ill-defined classes and sub-classes determined by 

 structure, climate, and various other conditions. The second of 

 Lowl's categories is also recognized by Phillipson. Still more 

 recently, Richthofen, neglecting antecedent drainage, designated 

 the superimposed class of Powell epigenetic, and formulated a 

 classification of the remaining types of continental depressions 

 (Die. Hohlformen des Festlandes) as (a) orographic depressions 

 (Landsenken) ; {b) tectonic valleys, and (c) sculptured valleys; and 

 the last two categories are separated into classes and sub-classes, 

 corresponding fairly with those of Lowl, determined by their 

 relations to structure and by various genetic conditions. 



These several classifications have much in common ; their 

 differences are largely due to the diversity of the regions in which 

 the investigations of their respective authors have been prosecuted; 

 but combined they probably comprehend all the topographic 

 types which it is necessary to discriminate. 



The American classification and nomenclature, particularly, is 

 unobjectionable as applied to montanic hydrography; but it does 

 not apply to the perhaps equally extensive drainage systems and 

 the resulting topographic configuration developed on emergent 

 terranes either {a) without localized displacement or [h) with local- 

 ized displacement of less value in determining hydrography than 

 the concomitant erosion, terracing and reef building ; neither 

 does it apply to the minor hydrography in those regions in which 

 the main hydrography is either antecedent or consequent ; nor 

 does it apply even to the original condition of the superimposed 

 or antecedent drainage of montainous regions. 



Upon terranes emerging without displacement and upon equal 

 surfaces not yet invaded by valleys, the streams depend for their 

 origin on the convergence of the waters falling upon the uneroded 

 surface and affected by its minor inequalities, and for their direc- 

 tion upon the inclination of that surface. They are developed 

 proximally (or seaward) by simple extension of their courses by 

 continued elevation, and distally by the recession of the old and 

 the birth of new ravines ; and since in the simple case it follows 

 from the law of probabilities that the receding ravine will retain 

 approximately the old direction and that the new ravines will de- 

 part therefrom at high angles, the drainage systems thus inde- 

 pendently developed become intricately but systematically rami- 

 fied and more or less dendritic in form. Lowl, Phillipson, Richt- 



