398 I. Bowman — Physiography of the Central Andes. 



and trend. This is especially well-marked in the region north 

 of Sucre, between Cochabamba and Oruro, where it was observed 

 by the writer, and again north of Oraro from Caracollo to Col- 

 quiri. Any explanation of the drainage must therefore begin 

 with the larger members of the stream systems directed in 

 somewhat the same way that they are arranged to-day. The 

 original drainage produced by the initial folding of the 

 mountains, a drainage sympathetic with respect to anticlines and 

 synclines of considerable regularity and great size, has been 

 completely modified. To-day, not only are the axes of the 

 major streams everywhere directed across these mountain axes. 



Fig. 28. 



6430 



ZZ* 



64*30' 



Fig. 28. Map of the Tojo region, southwest of Tarija, Bolivia. (From 

 Steinmann, as in fig. 21.) Disregard scale ratio. 



but even the tributaries that drain the lesser areas are not 

 infrequently in anticlinal valleys, though this arrangement is 

 not general because of the small extent of soft rock found in 

 the series uncovered by the erosion of the folds. 



Identically similar features are expressed in the region north- 

 east of the Cordillera Keal. (Fig. 15.) The streams draining 

 toward the northeast cut straight across the foot-hill region of 

 northwest trending ridges, and must likewise have gained 

 their courses before ridges, as we know them to-day, appeared. 

 Evans* assigns the stream courses of Caupolican Bolivia to a 

 remote period "before the evolution of the present features of 

 the country " and the present unsympathetic relation of streams 

 to structure as being brought about by subsequent u earth 



*Expedition to Caupolican Bolivia, 1901-1902 (Geog. Jour., vol. xxii, 1903). 



