210 I. Bowman — Physiography of the Central Andes. 



lithologic character with the downfaulted block forming the 

 southeastern quadrant just described. It possesses no textural 

 peculiarities which might account for the formation of the 

 great thousand-foot declivity of remarkable simplicity, straight- 

 ness, and detiniteness of trend. If further evidence were 

 needed that the succession of events were the formation of 

 structural irregularities, peneplanation, and block faulting, it is 

 to be found on the face of the scarp here considered, where 

 the flat upper surfaces of the block, a peneplain remnant of 

 unmistakable identity, is suddenly terminated by this great 

 rectilinear wall. Furthermore, not only the succession oi the 

 major events which those conditions signify, but also the fact 

 of faulting is established by the manner in which this recti- 

 linear scarp trends regardless of structure. While time did 

 not permit the detailed examination of the structural geology, 

 it was forcibly impressed by the evidence of several critical 

 localities that we have here that inharmonious relation of 

 mountain form and mountain front to structural axes so signifi- 

 cant of long intervals of topographic development between 

 periods of crustal deformation. 



It is the lack of correspondence between the trend of the 

 scarp and that of the structural axes that at once dismisses the 

 initial structure as the cause of the scarp and establishes the 

 fact that the scarp was formed in a second structural epoch. 

 That this second epoch is of much more recent date and quite 

 unassociated with the first in time, as the above consideration 

 shows it to be different in kind, is established by the length of 

 time required for the old flat surface to attain the perfection 

 here displayed and the well-nigh complete discordance it 

 exhibits with respect to the initial structure. The peneplain 

 thus bevels the first set of structures and is itself cut into large 

 well-defined remnants, or blocks, by the second and much 

 later set of structures — the fault planes which are the loci of 

 recent block displacements. 



At the time of major displacement the Huasco basin must 

 have been lowest in its northwestern extension, this being the 

 direction in which the southeastern block quadrant was tilted. 

 At present the salt lake of Huasco occupies a position south of 

 the center of the basin, having been displaced from its earlier 

 more northerly position by the filling of waste material in the 

 form of enormous alluvial fans heading in the valleys among 

 the lofty volcanoes which occur in this direction. These great 

 volcanoes (fig. 8) are a portion of the Sillilica range, the most 

 important members being Y. Sillilica, the twin peaks of 

 Huasco, Sacaba, and Mt. Divisadero. 



The volcanoes rest upon the broken and tilted fragments of 

 the peneplain which is clearly discernible about the borders of 



