STRATIGRAPHY '/ 6\) 



to the late diastrophism which gave rise to the erosion of the modern 

 topographic features." 



Machareti formation. — The strata above the fluvio-glacial grits and 

 conglomerates of the Mandiyuti formation and below the chert and lime- 

 stone which overlie the Bermejo series, as here defined, are named the 

 Machareti formation. The type locality is in the gorge of Vitiacua creek 

 a few miles west of the town of Machareti. The formation is at most 

 places about 1,000 feet in thickness, but varies from slightly less than 800 

 to a little more than 1,600 feet. 



These beds are mostly sandstones, but include also a considerable 

 amount of shale and clay. The sandstones are massive, irregularly bedded 

 and coarsely laminated. Their effect on the topography may be seen in 

 figure 13. They display various tints, from light yellowish brown or 

 cream to deep red or maroon. In places they are white and have a sugary 

 texture. Ordinarily the interbedded shales and clay are red; the shales 

 are non-fissile. 



VITIACUA LIMESTONE AND CHERT 



This formation has been observed in the north part of the Sierra de 

 Aguarague, and its probable equivalent has been mapped in two localities 

 in the north part of the Sierra de Charagua. Float derived from it was 

 seen in the Sierra de Mandiyuti and elsewhere, but it was not observed 

 in place in that range or in other localities where detailed work was done. 



The type locality is in the western part of the gorge through which Rio 

 Vitiacua traverses the Sierra de Aguarague, about a mile downstream 

 from the portals through the hogbacks which form the western slopes of 

 the range. At this exposure the Vitiacua formation consists of thin beds 

 of chert and cherty limestone separated by thin partings of clay. Some 

 of the chert is purplish pink and is massive or in extensive sheets averag- 

 ing 2 inches in thickness. The rest is brown or white and occurs in 

 irregular nodules or concretions scattered through the thin calcareous 

 strata. The entire formation at this locality is only a little over 15 feet 

 in thickness. 



In the north part of the Sierra de Charagua, 2 1 / 4i miles due west from 

 the village of Tacuru, several beds of coarsely crystalline, light gray lime- 

 stone are exposed in the gorge of Tacuru Creek. The limestone is 6 feet 

 thick and is more resistant than the underlying sandstones or overlying 

 shales. It contains no chert at this place, but scattered among the calcite 

 crystals are numerous grains of white quartz, well rounded and washed 

 entirely clean of clay particles. In a similar stratigraphic position near 

 the western end of the Saipuru gorge, a few miles farther south, in the 



