/. Bowman — Physiography of the Central Andes. 373 



Art. XXXVII. — The Physiography of the Central Andes : 

 II. The Eastern Andes ; by Isaiah Bowman. 



During the field examination of the eastern Andes of 

 Bolivia, not only were the local geology and physiography 

 noted, but many widely separated commanding points were 

 also gained from which was viewed the general aspect of the 

 eastern plateau. The results were everywhere strikingly 

 similar. The conclusions based upon an examination of these 

 wide expanses of the plateau surface and their discordance 

 with respect to structure harmonize with those resting upon a 

 study of drainage features and lead inevitably to the conclu- 

 sion that peneplanation is the dominating fact in the physi- 

 ography of the region. Lest it seem that this view is held 

 without sufficient consideration of the geologic structure and 

 of the relation of the plane of baseleveling to it, the following 

 geologic descriptions are introduced. They but serve to 

 emphasize the conclusions already stated by the striking 

 structural variations they indicate, variations practically unex- 

 pressed in the plateau surface, save where residual masses have 

 survived the baseleveling process. 



Geologic Features. 



The rocks of the eastern Andes may be roughly classified 

 into two great groups, the eastern sandstone series and the 

 western schistose series. The sandstone series consists of 

 shales, conglomerates and sandstones ; the schistose series 

 consists of slates, quartzites and quartzite schists. Both are 

 structurally disturbed, but the disturbances in the schists are 

 of a more profound order and have resulted in rnetamorphic 

 effects whereby the schistose structure was imposed upon the 

 entire western series. Every gradation may be observed in the 

 scale of these disturbances from those of microscopic to those 

 of mountainous proportions. In all sections there are notable 

 intrusions of igneous material. In the slates of Santa Yera 

 Cruz it is quartz porphyry and granite, the lead, tin and zinc 

 of commercial interest being found in fissures of the quartz 

 porphyry. In the western series of rocks almost every variety 

 of geologic structure may be found in a day's ride from east 

 to west across the grain of the rock ; in the eastern sandstone 

 series the structures vary from folds to block-faulted mono- 

 clines, so that within limited areas the latter structures show 

 dip and strike of more or less constant value. 



Specific structural values for definite localities are almost 

 without physiographic interest or importance, so generally do 

 they conform to the generalizations that have just been noted. 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 166.— October, 1909. 

 25 



