394 I. Bowman — Physiography of the Central Andes. 



The assignment of a common origin to the now deformed 

 peneplain of the eastern and western Andes, as expressed in 

 the discordance between complex structures and flat-topped 

 plateaus common to both, demands attention to certain parts 

 of the great interior basin of western Bolivia. This flat-floored 

 basin was explained (Part I) as primarily the product of a 

 depressed block of the uplifted peneplain, and its borders as 

 fault scarps. The time of origin of the basin is established 

 by reference to the slopes already described. The uplifted 

 peneplain remnants or the plateau tops terminate abruptly all 

 about the rim of the basin. Several of the views already 

 noted (Huynuni, the pass of Apacheta, etc.) are either within 

 or almost within sight of the western border of the eastern 

 plateau. This abrupt termination of a well-developed base- 

 leveled surface indicates that its further extension has been 

 faulted down, even if more direct evidence of faulting were 

 not available. We are therefore assured that the basin was 

 not in existence as an enclosed tract with high bordering scarps 



Fig. 26. 



Fig. 26. Sketch showing three-cycle features prevalent throughout most 

 of the Central Andes. 



at the time that the development of the peneplain was com- 

 pleted, although it may well have existed as a valley or as an 

 extremely shallow basin. The Mesozoic invasions of the sea 

 were carried inland to this region, but no marine Tertiary is 

 found here. 



In contrast to the topographic discordance presented at the 

 rim of the great interior basin is the correspondence of develop- 

 ment between scarps and bordering basin in the second cycle 

 of development, which advanced to the point of maturity. Fig. 

 12, in the Maritime Andes, shows a region which is organized 

 with respect to the interior basin, and everywhere along the 

 coach-road from Oruro to La Paz we remarked the thoroughly 

 subdued slopes of the hills and plateau scarps. Oftentimes the 

 structure beneath the thin waste cover shows through in 

 slight corrugations, but in no place has such structure any 

 important expression in the topography. The slope formed 

 upon the dip of the schists and slates are scarcely more gentle 

 or regular than those formed across the outcropping edges of 

 the strata. No sudden break either in slope arrangement or 

 stream gradients marks the debouchure of the main tributary 



