I. Bowman — Physiography of the Central Andes. 201 



Interior Features. 



The evidences along the west coast of South America of 

 recent tectonic adjustments are particularly clear because the 

 uplifts which express these readjustments have occurred on 

 the seashore, where a standard surface makes reasonably safe 

 comparisons of relative land levels possible. They are particu- 

 larly convincing by reason of the freshness of the uplifted 

 shore forms, a fact owing to the recency of their occurrence at 

 high levels and to the extreme aridity of the climate in which 

 these forms occur, with their consequent freedom from the 

 obliterating effects of rainwash, aided by chemical decay. 

 This juxtaposition of recently made shore forms and compe- 

 tent agent of formation enables physiographic determinations 

 to be made without hesitation. 



It is a matter of great significance, from both the geological 

 and the geographical viewpoint, to ascertain how generally this 

 crustal deformation affected the interior portions of the central 

 Ancles. None of the geological arguments so far advanced by 

 those who have examined the field has included any recogni- 

 tion of the clear evidences of crustal movements exhibited by 

 topographic and drainage lines, and allied phenomena. In 

 every case the arguments have rested upon purely paleontolog- 

 ical or structural evidence. Furthermore, in the majority of 

 cases, the analysis of drainage adjustments or directions is 

 made with sole reference to and in close conformity with the 

 presumptions raised by the strictly geological conclusions. 

 Thus, for example, it is often argued that formerly the great 

 interior sea of central Bolivia discharged into what is now the 

 Amazon basin by way of the La Paz river, whose gorge 

 through the Eastern iVndes forms one of the profoundest clefts 

 of that region. Now, whatever geological suggestions there 

 may be for such an assumption, it is certain that the physio- 

 graphic evidence (see Part II, The Eastern Andes) annuls the 

 supposition in the clearest possible manner. Nor is this the 

 only disadvantage which the present interpretations suffer. 

 As geological history, the record of fluctuations of land level 

 are of great importance even when such fluctuations are not 

 marked by sedimentary deposits. They may condition, for 

 example, the climate of the region and the character of the 

 sediments which may ultimately exist; or they may form, as 

 actually in the central Andes, the lost members in time of the 

 series of recent geological occurrences. Their omission 

 under the last-named circumstances means the omission of a 

 part of the geological record in as important a sense as if 

 glacial events and the forms which express them were omitted 

 from the record. The fluctuations of level in the interior of 



