STRATIGRAPHY 743 



with occasional streaks or splotches of greenish gray. This is overlain by 

 massive light-colored sandstones in beds of varying thickness, many of 

 which are between 2 and 10 feet thick. For the most part, these strata 

 are comparatively soft and crumbling. 



Between 600 and 900 feet above the base of the formation, there is a 

 sandstone member of great resistance to erosion. Its beds display varying 

 tints of red alternating with white or light gray; some of them are sepa- 

 rated by occasional partings of red clay. These strata are all very irreg- 

 ularly bedded and for the most part are composed of very small grains of 

 quartz, which give a rather fine texture to the rock. Still higher in the 

 stratigraphic sequence the sands become coarser and the beds more 

 massive, so that in the middle of the formation, 1,800 to 2,200 feet above 

 its base, the sandstones are of great thickness and show much cross-bed- 

 ding and many irregular laminations, as though deposited by shifting 

 deltaic currents. This portion of the formation is grayish white. Above 

 these beds the strata are comparatively soft and not well exposed, but at 

 the top of the formation there are 200 to 300 feet of very resistant white 

 and gray quartzitic sandstones. These are composed of well rounded 

 quartz grains firmly bound with siliceous cement. There is little or no 

 mica nor feldspar associated with the quartz grains, which for the most 

 part are very clean. Cross-bedding is noticeable, but not nearly so promi- 

 nent as in the underlying beds. These resistant top strata of the Tacuru 

 formation give rise to the conspicuous portals through which the streams 

 leave the mountain gorge and debouch on the undulating lowland to the 

 east. 



This quartzitic upper member of the Tacuru formation forms the outer- 

 most hogbacks on either side of the Charagua range. Its influence on the 

 topography is illustrated in figure 3, in which this resistant member of 

 the Tacuru formation may be seen rising from the margin of the Chaco 

 on the east side of the Sierra de Charagua, a short distance south of 

 Saipuru. In figure 14, which shows the south side of the valley of Rio 

 Charagua at the point where this river enters the Sierra de Charagua 

 from the west, the top member of the Tacuru rises steeply to form the 

 hogback partially visible at the left of the photograph. The slightly cross- 

 laminated sands in this portion of the Tacuru formation are also shown 

 in figure 15, a bluff rising from the banks of Rio Charagua at the eastern 

 entrance to the Charagua gorge. 



In the Cuestas de Oquita the Tacuru formation is about 3,500 to 4,000 

 feet in thickness. The exact contact between it and the underlying strata 

 was not observed, nor was the Vitiacua limestone noted in those hills 

 through which a cross-section was run along the channel of Rio Parapiti. 



