400 1. Bowman — Physiography of the Central Andes. 



formed subsequent to the deposition of the alluvium now 

 found there. There has never been a lake in the region except 

 this glacial lake, for the deposits which the La Paz river is dis- 

 secting all about the city are distinctly not lake deposits. They 

 are the coarsest alluvium, the sort of material that mountain 

 torrents carry, only roughly sorted and bearing all the marks 

 of stream and not lake deposition. The clay beds found inter- 

 calated with these coarse deposits, and used for brick-making, are 

 glacial clays. They are pebbly, impure, local, and irregular in 

 occurrence, with nothing in their structure or position to war- 

 rant the hypothesis of deposition in the waters of a lake. Two 

 other hypotheses have been put forward in explanation of 

 the course of the La Paz river. The first is that the river is 

 antecedent in origin, having gained its course upon an initial 

 surface previous to the uplift of the Cordillera Real, and that 

 it has persisted in it during the slow upheaval of mountains in 

 its path. Two objections stand in the way of the acceptance 

 of this view. A stream as short as that part of the La Paz 

 west of the Cordillera Real (twenty or thirty miles), as Conway 

 has pointed out,* could scarcely withstand the tremendous 

 obstacles which these mountains afford. Nevertheless, if the 

 uplift were slow enough even this great task might be done by 

 a stream as small as the La Paz. In this event, however, we 

 should look for one of two results. A stream that has per- 

 sisted in a given course since the close of the Paleozoic must 

 have its relations to rock structure and to adjacent divides 

 well established. We cannot grant to such a stream to-day an 

 unstable headwater condition. Now it is the most striking 

 characteristic of the La Paz that its headwater section is under- 

 going the most vigorous dissection and has been cut back miles 

 within glacial and post-glacial time. If we restore that part of 

 its course recently cut away, we have a stream that is even 

 shorter than the already very short course indicated above ; 

 in other words, we have a mountain torrent, and it is precisely 

 this sort of a stream that we conclude that the La Paz must 

 have been in this region previous to the uplift of the pene- 

 plain so excellently preserved about the western base of the 

 Cordillera Real. The tremendous advantages of heavy preci- 

 pitation and excessive gradients of those streams that flow 

 eastward off the flanks of the Cordillera Real and the strong 

 warping in this direction of the old peneplain has given these 

 streams exceptional advantages over those tributary to the 

 interior basin. The consequence is not alone expressed in the 

 La Paz. On Steinmann's map the Sayacuira is shown crossing 

 this same mountain axis south of the Sierra Vera Cruz, 

 although here the height of the range is, to be sure, very much 

 * Climbing and Exploration in the Bolivian Andes, pp. 126-128, 1901. 



