316 Palmer — Notes on the Andes of JV. W. Argentina. 



this great graben is mentioned here as an element of the Cor- 

 dilleran structure. A block of the red Jurassic sandstone has 

 moved downward between two mountains of Paleozoic schists. 

 This downfaulted block, or graben, is about 8 kilometers wide 

 and at least 40 kilometers long (5 by 25 miles), and has dropped 

 some five thousand feet, relative to the bordering metamorphic 

 rocks. 



The Eastern Cordillera. 



West of the Calchaqui Valley is the Eastern Cordillera, 

 which in this region has no individual name. It consists of a 



Fig. 3. 



Fig. 3. Photograph taken on a lava-stream which flowed down the Que- 

 brada de la Paya. The lateral valley directly opposite the camera was 

 dammed, and a lake and characteristic lake deposits were formed. 



great mass of Paleozoic schists into which have been intruded 

 a number of granite and granite porphyry masses. One of 

 these at Piedras Blancas is interesting on account of the cubi- 

 cal shape of the feldspars. These weather out without losing 

 their peculiar form, and make an unusual sort of gravelly sand. 

 Many of the ravines on the east flank of the Eastern Cordillera 

 have had small streams of basic lava poured into them. The 

 lava dammed up the valleys, making temporary lakes which 

 are now drained, but whose recent existence is demonstrated 

 by almost undissected lake beds ; fig. 3 is from a photograph 



