GEOLOGIC STRUCTURE 759 



height southward toward Rio Vitiacua. Where crossed by that river, 

 these hogbacks reach more than half way to the crest of the range; the 

 fault displacement appears to have decreased as the folding of the strata 

 increased. 



AGUARAGUE FAULT AND ANTICLINE 



The beds forming the Sierra de Aguarague are crumpled and broken 

 into a long, narrow, sharply compressed, anticlinal fold, sheared through- 

 out four-fifths of its length by a thrust-fault. In places the fault dis- 

 placement is the more important structural feature and gives rise to the 

 prominent fault-scarp Avhich forms the east front of the range at its 

 northern end near Boyuibi and throughout the greater part of its length 

 south Of Yacuiva. Elsewhere, however, the fold is the dominant struc- 

 ture ; the fault is of minor importance, and in places is almost, if not 

 entirely, wanting. Thus, between Machareti and Yacuiva the eastern 

 slopes of the range, as well as the western, are dip slopes, and the fault- 

 scarp, if present, is near the middle of the sierra instead of along its 

 eastern front. 



Xear the northern end of the range the geologic structure is well dis- 

 played in the gorge of Rio Vitiacua. There the thrust-fault is nearly as 

 far from the eastern series of hogbacks along the outer slopes of the range 

 as it is from the western ones. The western limb of the faulted anticline 

 dips westward at angles which increase from about 20 degrees on the 

 western margin of the range to more than 60 degrees midway the length 

 of the canyon. The eastern flank of the anticline inclines toward the east 

 at angles which vary between 7 degrees and 45 degrees. The thrust fault- 

 plane is inclined toward the west at an unknown but probably rather 

 steep angle. On either side of it there is a zone of shattered and con- 

 torted strata. The easily flexed beds of Los Monos shale in this shattered 

 zone are intricately folded and crumpled, so that their dip varies much 

 in degree and direction. The western side of the fault-plane is the up- 

 throw side and forms the hanging wall. Displacement along the plane 

 must have amounted to several hundred feet. 



From Rio Vitiacua southward to a point a few miles southwest of 

 Tarairi the thrust-fault retains about the same relative position with 

 respect to the east front of the Sierra de Aguarague as that indicated in 

 the Vitiacua Canyon. West and northwest from Tarairi the east front 

 of the mountains is a remarkably regular and unbroken slope, inclined 

 at an angle of about 25 degrees toward the east. As viewed from the 

 trail east of the mountains, it appears likely that the fault is a major 

 structural feature immediately west of this dip slope along the middle 



