I. Bowman — Physiography of the Central Andes. 389 



between Saeaba and Inca Corral, about thirty miles northeast 

 of Cochabamba. The photograph represents the headwater 

 section of a small tributary of the Juntas, a river which com- 

 bines with the San Antonio to form the Chapare at the east- 

 ern base of the Andes. Standing upon the farthest ridge in 

 the left background and looking still further in the direction 

 of the view (E), one would see deep dissection and partial 

 obliteration of the mature slopes there developed, because of 

 the greater rainfall there and greater proximity to the plains 

 whose low altitude constitute them virtually a baselevel of 

 erosion. The mature slopes of tig. 22 are preserved because of 

 greater distance from the plains. The streams beyond the 

 ridge indicated have a direct course of about fifty miles to the 

 plains, and the rock in which their courses are cut is soft sand- 

 stone and shale ; the stream draining the valley of fig. 22 has a 

 roundabout course fully twice as long, and is held up through- 

 out the first fifty miles by hard slates and quartzite schists. 

 An analysis of the slopes of the figure leads one to appreciate 

 how long an interval of time they represent between the forma- 

 tion of the peneplain above them and the deep dissection now 

 elsewhere in progress and soon to be expressed in this valley 

 also. Geologic structure is here unexpressed to a degree not 

 less great than in the case of the baseleveled surface above. 

 The more resistant quartzites and the less resistant slates, and 

 the most variable dips, are all alike brought to a uniform slope 

 expression. A heavy sheet of loose waste cloaks the rock 

 beneath and outcrops are, for the most part, concealed. The 

 smooth catenary curves of opposite slopes from hilltop to 

 adjacent hilltop scarcely need description, so well organized do 

 they appear in the photograph. In fact, organization is the 

 keynote of the landscape hereabouts. The orderly arrange- 

 ment of slopes, the continuity of the waste cover, the regular 

 gradient of streams and valley floors, the complete subjugation 

 of rocks of varying hardness, all alike attest the perfection of 

 mature topographic development. Every element thus far 

 described is contrary to the supposition of but one cycle of 

 erosion inaugurated by one uplift. These slopes are not pro- 

 duced by the present drainage ; they are being destroyed by it, 

 or, as here, are about to be destroyed by it. Only ten miles 

 down the valley, stream incision is already accomplished and 

 hastened waste removal is disturbing the delicate organization 

 of slopes, renewing the rock outcrops and causing the uneven 

 expression of hard and soft rock. It requires long-continued 

 erosion at a much less elevation than that at which the region 

 stands to-day to produce by weathering and stream erosion 

 such a smooth perfection of maturity. A second profound 

 uplift inaugurated a third cycle of erosion, the one just begun, 

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



