/. Bowman — Physiography of the Central Andes. 209 



retained its initial altitude after uplift while the eastern por- 

 tion was depressed ; the basin thus formed receives a limited 

 rainfall, and even in the wetter climate once prevailing had no 

 outlet to the sea. 



The fact that this blocked quadrant was formerly baseleveled 

 appears from the relation between surface and structure that 

 obtains in the gorge of Rinconado, which enters the basin 

 immediately south of the twin peaks of Huasco. The gorge is 

 200 ft. in depth, with nearly vertical sides, and reveals a 

 clearly featured section. The granite-gneiss forming the body 

 of the block has many structural deformities, a fact which is 



Fig. 8. * 



variously expressed, in some places by faults impossible of 

 restoration on account of the homogeneity of the mass ; in 

 other places by anticlinal structures. The present surface of 

 the block is exceedingly regular and smooth except where 

 locally roughened by stream dissection ; and is remarkable for 

 the exact regularity with which it cuts across the irregularities 

 of structure. This truncated condition coupled with the 

 extraordinary evenness of the blocks in their general aspect 

 forms one of the most constant and striking features of the 

 region. The block now slopes northwest, occupying a tilted 

 position with respect to its plane of origin. 



The fault scarp of the western edge of the basin, the locus 

 of movement for the tabled block constituting the southeastern 

 quadrant of the basin, has been but slightly modified by stream 

 erosion since faulting. It possesses two important genetic 

 qualities which reveal its true nature. First of all, the material 

 composing the block thus scarped is essentially identical in 

 *Kefer to parallels for correction of scale. 



