Palmer — Notes on the Andes of N. W. Argentina. 327 



main points suggestive of the continuation of the fault along the 

 base of the Cumbres del Obispo as a line of inherent weakness 

 are as follows : 



(1) The abnormal profiles of the ravines show that the 

 gradients along the lower slopes of the otherwise exceptionally 

 smooth mountain flank have been steepened in recent times. 

 (See fig. 8.) 



(2) The mountain flank has a very straight outline from 

 north to south. In other words, contours drawn on this flank 

 would approximate to straight lines, a condition not to be 

 expected as a result of erosion, pure and simple, on schists of 

 variable resisting power. 



(3) There are several small volcanoes, now extinct, located 

 on or very close to the line of contact. For example, "El 

 Volcan " near La Poma. 



(4) The Argentine Weather Bureau reports from its station 

 at La Poma occasional slight earthquakes, which are of local 

 origin and have purely local effects, as if crustal adjustment 

 in the immediate vicinity were still in progress. 



From these lines of evidence it is concluded that the Cien- 

 aga fault continues northward along the base of the Cumbres 

 del Obispo, forming the contact between the Jurassic red sand- 

 stone and the Paleozoic metamorphic rocks. 



The argument for the southward extension of the western 

 fault from Pircas is by analogy to the eastern fault. The con- 

 tact between the metamorphics and the sandstone is strikingly 

 brought out by the great contrast in color between these two 

 rocks. From any vantage point in the central portion of the 

 valley both the eastern and western contacts can be traced 

 with ease for ten or a dozen kilometers to the north and south. 

 The contact on the west side is strongly deflected by every 

 spur and gully which it crosses, the deflection being vertical, 

 not horizontal, which indicates that the contact-plane is very 

 steep at all points. In view of this uniformity it is argued 

 that the western boundary is a fault extending southward along 

 the valley from Pircas. 



The eastern fault is not a line of uniform displacement, but 

 of somewhat differential movement. The fault is of the rotary 

 type. This is seen by comparing figures 8 and 9. Both were 

 taken from about the middle of the valley looking eastward to 

 the Cumbres del Obispo. Fig. 9, which was taken six or eight 

 kilometers north of La Poma, shows slopes much more 

 maturely dissected than figure 8, which was taken about 

 opposite La Poma. Since the slopes are less mature to the 

 south, there must either have been more recent movement or 

 have been greater movement. 



