Palmer — Notes on the Andes of N~. W. Argentina. 317 



taken on such a lava flow and looking up a small side- valley in 

 which lake deposits can be seen. The freshness of topographic 

 form suggests that these flows are not older than late Pleisto- 

 cene. 



There are very few outcrops of the Paleozoic schists west of 

 the Eastern Cordillera. Most of the region is covered with 

 lava flows of Pleistocene age, by recent alluvial or piedmont 

 deposits, or by deposits of saline residues. The great amount 

 of volcanic material suggests that its eruption began long 

 before the Pleistocene, and that it occurred intermittently 

 through, possibly, the whole Tertiary. In the Quebrada de 

 Soncor, a canyon which runs down from the Martime or West- 

 ern Cordillera to the Salar de Atacama, there is exposed a 

 more recent flow of glassy mica andesite about 15 meters thick. 

 There is under this a less recent flow of hypersthene andesite 

 porphyry, which has been cut to a depth of at least 50 meters. 

 These flows, extending down from the Maritime Cordillera, 

 have their western margins at the edge of the Salar de Ata- 

 cama. Along the margin of the lava flows there are a score 

 or more of cones of dacite porphyry from twenty-five to seventy- 

 five meters high. 



Since the lavas are not very old, erosion has not progressed 

 far enough as yet to expose to any great extent the underlying 

 formations. Only three such exposures were observed. One 

 of these was on the northwestern margin of the Salar de Pin- 

 con, where there are flows of lava similar to those just described, 

 except that they are on the eastern side of the Maritime Cordil- 

 lera. These have been cut through so as to expose underlying 

 alluvial deposits. 



Again, in the valley of the Atacama Piver, north of San 

 Pedro de Atacama, the lava cap is less thick, and under it are 

 exposed at least one hundred and fifty meters of partially con- 

 solidated sands and gravels of a strong red color. These sedi- 

 ments bear large quantities of gypsum, perhaps 10% by volume, 

 indicative of arid conditions of deposition. The beds are 

 moderately folded and tilted. The gypsum stands out in the 

 form of horizontal shelves two or three inches wide, a structure 

 due to its resistance to the scouring of the persistent northwest 

 winds of the region. The lava here lies conformably on the 

 sediments or nearly so. 



On the southwestern flank of the Serranias de Purilacte 

 there are exposed quartzites, which were probably the source 

 of the sand and gypsum deposits just described. They seem 

 to be of great thickness, perhaps a thousand meters, and are 

 now tilted nearly vertical. The nature of the contact of the 

 red, gypsiferous sediments and this quartzite is completely 

 obscured by the broad alluvial deposits of the Llano de la 

 Paciencia. 



