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



before, during, or after the faulting. It is slightly more pro- 

 bable, perhaps, that faulting and folding occurred simultane- 

 ously, for it is probable that in the release of the vertical 

 forces in the crust, horizontal stresses and strains would have 

 been developed. 



That the habit of the Jurassic red sandstone and its related 

 red sandstones in northwestern Argentine is one of block-fault- 

 ing seems certain. Pierre Denis* has described the Serrania 

 de la Lumbrera, one of the elements of the pre-Cordillera, as 

 an up-faulted block with gentle folds. Bailey Willis, in a 

 personal communication to Professor Bowman, said that he 

 depended to quite an extent on the dominant block-faulting of 

 the region in locating wells for the water-supply for the 

 Argentine Government's new railroad. 



Resume. 



In summing up the geologic history of the region traversed 

 by the Expedition we find evidence of the following periods 

 and events : 



(1) The deposition of Paleozoic (and possibly Archean) 

 sediments. 



(2) The metamorphism of these to schists, quartzites and 

 slates by regional processes in mountain-making movements at 

 the end of the Paleozoic. This was either accompanied by, 

 or followed by, the intrusion of granites and granite porphyries. 



(3) The erosion of these to maturity. 



(4) The deposition on the eastern side of the Cordilleras 

 of Jurassic sediments, with a marine basal portion, but chiefly 

 terrestrial. 



(5) A period of vertical movements, probably oscillatory 

 and differential in character, forming the Calchaqui graben ; 

 these were accomjmnied by horizontal compression normal to 

 the axis of the Cordilleras, which folded and faulted the 

 Jurassic rocks. 



(6) A period of great volcanic activity, yielding rhyolites 

 and andesites, beginning in the Tertiary and running into the 

 Pleistocene and recent. 



(7) Some glaciation in the Eastern Cordillera. 



(8) The present period of erosion, with very slight volcanic 

 activity. 



* Annales de Geographic, July 15, 1913, Paris. 



