Palmer — Notes on the Andes of N. W. Argentina. 329 



There are a few anticlines in the sandstone in the neighbor- 

 hood of Payogasta by which the metamorphics are brought 

 close to the level of the valley floor, so as to be exposed in 

 small gullies. On account of this relation we can not consider 

 the base of the sandstone as much below the present valley 

 floor. 



Considering the mountains on either side of the valley as 

 1200 to 2000 meters high, the throw of the faults would be 

 from one to two kilometers (three-quarters of a mile to a 

 mile and a quarter). 



The Relation of Folding to Faidting. 



A graben, or down-faulted crust-block, may attain its posi- 

 tion and structure as the result of any one of several processes. 

 It may have settled vertically under gravity as the result of 

 the pulling away of its sides by tensional forces in the earth's 

 crust, in which case it would find expression as a crust block 

 of downward-tapering cross section. Conversely, an upward- 

 tapering wedge-shaped crust-block may be depressed by com- 

 pressive forces acting horizontally on oppositely inclined 

 thrust fault planes. It may have remained lower while the 

 side blocks were pushed up, or it may have been a separate 

 great slice caught in a fault zone subjected to oscillatory move- 

 ment, the upward movements taking place dominantly on one 

 of the graben walls, the downward movements occurring chiefly 

 on the other. The last two causes might cooperate with either 

 of the first two in attaining the final result. 



The general relations of the Calchaqui graben, which bear 

 upon this problem, are as follows: 



The region is one of folded and overthrust rocks, which 

 have been cut by normal faults, as is true of all the eastern 

 border of the Central Andes. It is improbable that there 

 could have been a period of crustal tension, as all the struc- 

 tures which have been described are of types due to com- 

 pression. It is also improbable that the Calchaqui graben 

 block has remained at a constant level in the presence of such 

 tremendous and widespread forces as those which built the 

 Andes. 



It is clear that faulting has occurred on both sides of the 

 block and that the block has suffered compression. These two 

 facts, which are the dominant facts of its structure, are 

 explained by the hypothesis that in the elevation of the Andes 

 faulting occurred on this eastern margin and that due to the 

 oscillatory nature of the fault movements the block was rela- 

 tively depressed. There is no evidence at hand to show 

 whether the folding of the Jurassic red sandstone took place 



