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



between these two formations could not be ascertained. One of 

 them is a line-grained, closely bedded, white or yellow-white 

 limestone, having a thickness of 60 to 100 meters (200 to 300 

 feet). There are a few oolitic bands in it from two to four 

 feet thick. The other formation is a soft and mealy drab 

 sandstone of fine texture. It has been but weakly cemented, 

 so that its more gritty portions form a friable sandstone, and 

 the remainder a gritty shale. The whole is so soft that out- 

 crops are rare. It seems to have a thickness of 100 to 125 

 meters (300 to 400 feet). 



Detailed study would undoubtedly enable the geologist to 

 work out the sequence of the limestone and the shale, and 

 possibly to subdivide them further on a lithological basis. 



This series of three (or more) Mesozoic sedimentary forma- 

 tions is folded into roughly parallel anticlines and synclines. 

 The folds average about north and south in trend, but the 

 strikes vary from K 45° W. to N. 40° E. Some of the 

 anticlines are so big as to reveal in the canyons the basal 

 quartzites and schists. Though most of the folds are open, some 

 are closed and overturned towards the east. In a few cases 

 there has been thrust faulting, with overthrust to the east, the 

 throws measuring hundreds of meters. One such overthrust 

 lies a few miles east of the Cuesta del Obispo, and has a throw 

 of at least a hundred meters (300 feet), and possibly (if there 

 were movements parallel to the plane of bedding) much more. 



Since these sediments are fairly coarse and show much cross- 

 bedding, it is probable that they represent piedmont and 

 flood-plain deposits formed by the erosion of a mountain system 

 composed of the Paleozoic metamorphic rocks and their intru- 

 sions of granite. They were at the time of their origin very 

 much like the superficial deposits now forming along the 

 eastern foothills of the Andes. The presence of abundant 

 calcite in the cement shows that the climate was and has been 

 as arid as it now is, with a precipitation of less than 50 centi- 

 meters and in many places less than 25. 



Paleozoic Metamorphic Pocks. 



Between the Cuesta del Obispo and the Calchaqui Valley 

 there are several alternations of Mesozoic sedimentary rocks 

 and Paleozoic metamorphic rocks. This alternation is due to 

 the rather deep erosion of the huge folds which once existed 

 here. The Paleozoic metamorphic rocks are chiefly schist and 

 quartzite, with a small amount of slate. It seems that the 

 quantity of quartzite decreases as one goes west towards the 

 Calchaqui Valley, where the Paleozoic consists almost entirely 

 of schist and slate. The schist is mostly gray or gray-brown. 



