I. Bowman — Physiography of the Central Andes. 205 



sea level, constitutes it one of the most interesting and import- 

 ant physiographic units to be found anywhere upon the earth, 

 particularly as large portions of it, by virtue of the pronounced 

 aridity of the climate even at these high elevations, have 

 remained in a relatively undissected and therefore safely deter- 

 minable condition up to the present time. 



Every step of the traveler's approach to tins great uplift 

 increases his astonishment at the perfection of the but slightly 

 modified peneplain remnant there disclosed. The view repre- 

 sented in tig. 5 gives one but an inadequate notion of the 

 perfection it displays. 



Fig 5. 



Fig. 5. The "crest" and western slope of tlie Maritime Andes (plateau) 

 as seen from east of Pica. Chile, 



On the left (north) are the unexplored Altos de Sitilca, pre- 

 sumably volcanic, and on the right (40 miles farther south) are 

 the gentle outlines of the Chacarilla mountains. Between 

 these two volcanic piles is the unbroken level of the plateau. 

 The winding precipitous gorges (quebradas) nick its edge 

 deeply, but at a distance even this mark of dissection is 

 unobservable and only the impressively level sky-line stands 

 out sharp and distinct. A straight-edge projected under the 

 line does not enable the eye to discover a single important 

 departure from horizontality in the whole forty-mile section. 



The same regularity marks the descent of the western edge 

 of the plateau beneath piedmont deposits. The average of 

 many observations gives 7 C as the mean value of this descent 

 while the mean descent of the overlapping piedmont surface 

 lies somewhere between 3° and 5°. This relation is expressed 

 in fig. 6. 



One of the best localities for the study of the peneplain sur- 

 face whose warped slope descends beneath the piedmont is 

 found a long half-day's journey east of Pica, or about TO miles 

 east of Iquique and a short distance north of the trail to Lake 

 Huasco. The altitude is 5,800 ft* (A. T.) There, in the 

 bottom of the Quisma gorge is a pronounced unconformity 

 showing sandstones, conglomerates, and more recent alluvium, 



* Aneroid determinations are to be understood in even 7 case unless it is 

 specifically indicated otherwise. 



