402 I. Bowman — Physiography of the Central Andes. 



Cordillera Real. They are therefore superposed courses 

 which were developed upon the old peneplain when what is 

 now ridge and valley stood at a common level. The uplift 

 whereby the second cycle of erosion was inaugurated was a 

 broad uplift, the streams incised their valleys slowly within the 

 heart of the plateau although on its borders dissection pro- 

 gressed with extreme rapidity because of the marked break 

 within short distances between low plain and strongly uplifted 

 plateau. Only the weaker tributaries have had their courses 

 modified. These have developed along the belts of weaker 

 rock and are arranged in consequence in strikingly linear 

 courses at right angles to those of the master streams. The 

 " total-eindruck " is strikingly like that derived from a study 

 of our own Appalachian drainage system where the trellised 

 pattern of the drainage bears evidence of the two-cycle origin 

 of the members of each drainage system. 



A striking feature, and one that gives a high degree of con- 

 clusiveness to the interpretations here offered, is the occurrence 

 of such large blocks of undissected remnants of the peneplain 

 at high levels. The reconstruction of an old surface in many 

 instances depends solely upon the plane of the hill top levels 

 and its discordance with respect to structure ; in the present 

 case the surface itself, practically undissected over wide areas, 

 affords a convincing quality to the interpretation. The 

 relatively slight degree of dissection that the peneplain 

 remnants of the western Andes display in so many places is 

 owing in small part to their favorable situation with respect 

 to the runoff of the mountains (or the lack of lofty moun- 

 tains) behind them, and in larger part to the extreme aridity 

 of the climate. The existence of the peneplain, and the slight 

 extent of residual mountains upon it, led to a much more 

 even distribution of rainfall than is at present the case. At 

 no time in their history were the Andes so high as they are 

 to-day ; and at no time were the regional climatic contrasts 

 so sharply marked and extreme as they are to-day. These cli- 

 matic contrasts were offered at the close of the first deformative 

 period in which the peneplain of the great denudation cycle was 

 elevated and have been strengthened by the even greater uplift 

 which closed the second cycle of erosion. Contrasts in degree 

 of dissection in the cycles since the great denudation have 

 therefore been gaining in strength, and in the cycle of vigor- 

 ous dissection recently inaugurated these are at a maximum. 

 In the western Andes are the relatively undissected plateau 

 remnants ; the eastern Andes show upon their margin some of 

 the profoundest dissection that can be found upon the earth 

 to-day. 



Geological Department, 

 Yale University. 



