I. Bowman — Physiography of the Central Andes. 379 



preparation of the eastern Andes, the more profoundly dis- 

 turbed of the two systems, for participation in the base-level- 

 ing that followed the formation of the western Andes, 

 undoubtedly accounts for the general development of the 

 peneplain upon both orographic systems. Our whole con- 

 ception of geologic time, and especially of the time required 

 for the completion of an erosion cycle, is so vague that neither 

 the termination of the first cycle nor the geologic age of the 

 mature slopes of the second can be fixed with any measure, 

 one might almost say with the least measure of certainty. If, 

 however, the existence of the peneplain and of the mature 

 slopes of a later cycle are once established and the age of the 

 former determined to be later than the Cretaceous, we are pre- 

 pared to accept the conclusion that however small a portion 

 of geologic time remains, that portion can not fail to have 

 been long enough to produce the results observed. In short, 

 that it is our conception of geologic time and particularly 

 that portion of it since the Mesozoic that requires adjustment. 



Current ^Explanation. 



Inasmuch as the interpretations here presented differ radi- 

 cally from previous ones, a brief review of current descriptions 

 and explanations seems advisable. The descriptions of the 

 central Andes in standard general references have this in 

 common, that the mountain forms of both the eastern and 

 western "ranges" (plateaus) are described solely with refer- 

 ence to the order of initial uplift and the degree of 

 dissection tacitly assumed to have been accomplished in a 

 single cycle of erosion. "The eastern ranges were folded 

 earlier than the western ranges, where the folds are most 

 marked."* "The western Cordillera is younger than the 

 eastern Cordillera and was covered by the sea during Paleo- 

 zoic and Mesozoic times, then uplifted to a great height and 

 still further modified by eruptive material even now, in many 

 places, in process of accumulation. " f These serve to indicate 

 the point concerning uplift, while the absence of any appre- 

 ciation of erosion cycles is recognized by the following 

 description, which may well serve as a type : " In this 'Bolivian 

 Switzerland' .... amid a chaos of precipitous heights, detached 

 crests, and masses, thrown together without any apparent 

 order, it seems difficult to detect any general plan.";); 



Another point may here be noted concerning the interpre- 

 tations so far published. While all ascribe an earlier uplift to 



* Herbertson, A. J. , The Continent of South America ( The International 

 Geography, p. 817, 1900 ). 



f A free translation of Sievers, Die West Cordillera (Slid- und Mittel-Amer- 

 ika, p. 390, 1903). 



{Keane, A. H., South America (Stanford's Compendium of Geography, 

 p. 240,1901). 



