I. Bowman — Physiograj/hy of the Central Andes. 399 



movements and erosive action ". We have here the only hint 

 in the whole literature of a feature as general as it is import- 

 ant and one which harmonizes so well with the conclusions 

 reached along other lines that it lends to it a very high 

 degree of credibility. 



The baseleveling of this whole region supplies the condition 

 which is required to explain the streams arrangements. The 

 existence of the peneplain is well established and its existence 

 meant a high degree of discordance, not only between surface 

 and structure, but also between drainage lines and mountain 

 axes. Upon such a baseleveled surface streams flow with a 

 minimum correspondence between their ultimate courses and 

 their consequent courses as determined by the initial outlines of 

 the folds. The warping of a baseleveled surface effects fur- 

 ther changes, which, as the warping may be entirely inharmo- 

 nious with respect to the orogenic movements, may still further 

 disarrange the drainage systems and cause at last an entirely 

 inharmonious relation between streams and structures. The 

 warping has the further effect of renewing dissection and by 

 such renewal exposing the once buried rock to the effects of 

 differential erosion. The rejuvenated streams begin anew to 

 carve out mountain range and valley as the dissection of the 

 softer rock by tributary streams follows upon the transverse 

 incision of the master streams. There thus comes about pre- 

 cisely that arrangement of streams that is exhibited in the 

 eastern plateau to-day. Master streams flow regardless of the 

 mountain ranges, so called. Some of the smaller tributaries were 

 developed along belts of weaker rock subsequent to the uplift 

 of the region ; others were developed in harmony with the 

 original mountain structures. 



Perhaps the most interesting drainage feature of the whole 

 region is the course of the La Paz river, concerning which there 

 has been a great deal of speculation and an equal amount of 

 erroneous explanation. Its striking transection of the greatest 

 mountain chain of Bolivia (fig. 14), its invasion of the great 

 interior basin of Bolivia and the soft material in which it is 

 cutting in the headwater region to-day, have drawn the atten- 

 tion of every student of Bolivian geography. A widely cur- 

 rent explanation is that at one time Lake Titicaca discharged 

 eastward through the gorge of the La Paz river, but that this 

 gorge was blocked by debris from the surrounding mountains, 

 thus giving an enclosed quality to the Titicaca system. The 

 explanation would be called absurd, were it not for its advance- 

 ment by well-known geographers. It is, therefore, necessary 

 to say that the highest strand line of the old lake that once 

 existed here is over a thousand feet below the level of the edge 

 of the basin drained by the La Paz river, and that the lake was 



