Palmer — Notes on the Andes of N. W. Argentina. 313 



gradients eastward to the Atlantic Ocean. It is largely covered 

 by a thick mantle of alluvium that conceals the structures of 

 the older rocks. The alluvium represents the combined 

 accumulation of materials decayed in situ and of materials 

 carried down from the mountains. The Bermejo, Salado, and 

 other rivers emptying into the Atlantic drain the Pampas. 

 Along the western edge of the Pampas there are a few small 

 mountains and ranges forming the previously mentioned pre- 

 Cordillera. Certain of these ranges were observed from the 

 railroad between Tucuman and Salta. These ranges are com- 

 posed chiefly of Paleozoic quartzites and schists, which for the 

 most part stand on edge. Farther east and beyond reach from 

 the railroad there are a number of outcrops of a Mesozoic red 

 sandstone, which is folded and overturned to the east, the 

 commonest dips being to the west. 



Mesozoic Formations. 



Leaving the Pampas and going westward into the foothills 

 region one ascends through brown and gray Paleozoic quartz- 

 ites, and then encounters a great succession of sediments, 

 which include red and gray sandstones, red shales, a small 

 amount of red conglomerate, and a little light-colored lime- 

 stone. The succession of these could not be worked out fully, 

 and no measurement of the thickness of the red sandstone, the 

 upper formation, could be made. This upper formation is 

 a brick-red sandstone, mostly of average coarseness, and well- 

 cemented by a mixture of calcite and iron oxide. The in- 

 dividual beds are from three inches to six feet thick and are 

 usually separated by thin beds or partings of shale. The sand- 

 stone beds are quite uniform in character, whereas the shales 

 are quite variable, some being fine, others gritty, and others 

 micaceous. There are a good many beds or lenses of conglom- 

 erate, containing pebbles of quartz, red sandstone, and shale, 

 as well as a few of quartzite, mica schist, and granite. These 

 pebbles are well-rounded and range in size from a quarter of 

 an inch up to four inches in diameter. Cross-bedding is very 

 common, especially in the conglomerate lenses, but can be 

 detected in almost every outcrop. The thickness of these 

 brick-red sandstones is at least 500 meters (1500 feet) and is 

 probably between two and five times as much as that. The 

 fossil evidence, which suggests that this formation is of Lower 

 Jurassic age, is discussed in Part II under " Ages of the 

 Formations." 



Beneath this red sandstone formation there are at least two 

 other formations which are believed to be Mesozoic on account 

 of their structural and geographic relations. The relations 



