234 ANNALS NEW YORK ACADEMY OF SCIENCES 



Range, in fact most of them have cut veritable canyons and, in many- 

 parts, are still continuing the process of incision. The inter-stream 

 spaces are, as yet, barely scratched by tributaries to the master lines of 

 drainage. The pampas thus far named are but high-lying plains, with 

 their initial surfaces well preserved. They are veritable deserts. 



To the southeast of the Vitor Valley we find the pampas of La Joya 

 and Chachendo, the two being partly separated by outliers of the West 

 Range, while nearer the coast and on the northwest side of the valley 

 of the Tambo we find the pampas of Islay and Tambo. 



Crossing the Tambo Valley, we meet the lower plains adjacent to the 

 shore line and usually known as the La Punta Plains. Crossing the 

 coastal ridge, however, we come upon a higher-lying plain of enormous 

 dimensions and usually known under the name of Clemesi. It extends 

 with little interruption to the valley of the Moquequa. It decreases very 

 materially in elevation as we pass from the Tambo to the Moquequa 

 Valley. On the Moquequa, the plains have a width of from twenty to 

 twenty-five miles. 



Of all the pampas so far mentioned in the southern coastal plain, the 

 lava flows already mentioned form no inconsiderable part of the deposits 

 making the Siguas- Vitor La Joya sections. Excellent sections may be 

 seen on the Siguas and Vitor valleys. Here can be seen two distinct 

 flows, as indicated by the inter-stratification with sediments. The max- 

 imum development of the flows seems to have taken place along a line 

 or section from Arequipa to Quilca, the former town being located well 

 within the west slope of the West Range of the Andes, and on the con- 

 tact, of the flow with older formations. 



Just what may be the correct correlation of the sediments of these 

 sections with those of the central and northern division of the coastal 

 plain has not been determined with any degree of certaint}^; it is mis- 

 trusted, however, that they represent rather late phases of Tertiary de- 

 position. It is important to note, as well, what an enormous amount of 

 elevation has taken place, since their deposition, since we now find some 

 of these at least 1,000 feet above sea level. There is no proof that the 

 lava plains as a whole ever stood at sea level. The fact, however, that 

 salt deposits are known to occur well up to the higher plains indicates 

 a depression of such extent that a part at least of the mud-flow must have 

 been near sea level. 



Returning to our description of the areal extent of the southern divi- 

 sion, we find that the Pampa de Clemisi has suffered but slight erosion. 

 Near the inner edge of the Cadena de la Costa are some very fine salt 

 deposits. The best sections are to be found in the slopes of the Moque- 



