PHYSICAL GEOGRAPHY 727 



its eastern neighbor. This regular increase in summit altitude from 

 range to range acids much strength to the hypothesis that summit eleva- 

 tions within each sierra are remnants of a former lowland plain. At 

 a very few localities there are upland surfaces of the plateau or mesa 

 type which may be considered as larger remnants of that same quondam 

 lowland. One of these is especially worthy of mention. It is situated 

 about half way between the canyons of Rio Saipuru and Eio Charagua, 

 high on the eastern slope of the Sierra de Charagua. Its surface has an 

 area of about a square mile and bevels the upturned sandstone strata. 

 It is evidently an erosion feature. The highest summits of the range 

 rise as jagged peaks 300 or 400 feet above it. Elsewhere in the same 

 mountain group there are occasional upland surfaces with gentle slopes 

 which bevel the upturned sediments and are ordinarily surmounted by 

 slightly higher rounded knobs. These upland flats are evidently erosion 

 forms developed by streams with low gradient traversing an old-age 

 topography. 



Again, in the Sierra de Aguarague there are certain topographical 

 features suggesting former peneplanation. Between Camatindi and 

 Tarairi the summit of this range for a dozen miles is an eastward- 

 dipping hogback. The remarkably smooth dip slope of this hogback 

 rises to a crest which for long distances is as regular as a rooftree. Above 

 this roof like ridge there are somewhat higher peaks at either end, such 

 as that shown in figure 11; these were doubtless monadnocks rising 

 above the peneplain which beveled the inclined strata. 



Although the data are not as complete as might well be desired, what 

 few facts are known all seem to point toward a two-cycle origin of the 

 present topographic features. At a comparatively recent date in geologic 

 history, subsequent to the crumpling and faulting of this region, an old- 

 age topography seems to have been developed quite generally throughout 

 the territory now occupied by the mountain ranges. Rivers meandered 

 across the lowlands toward the east in much the same courses as those 

 now followed by the master streams. That cycle of erosion was closed 

 by regional uplift, which warped the surface slightly and elevated its 

 western margin somewhat higher than its eastern. The rejuvenated 

 streams quickly incised their valleys and reduced their channels to the 

 new baselevel. In the areas of weaker rocks, wide valleys were quickly 

 carved and with the aid of the tributary streams the modern inter- 

 montane lowlands were developed. Wherever more resistant strata had 

 been brought above drainage by folds and faults, the larger streams 

 have thus far been confined in the most youthful of canyons, and the 

 hogback hills and serrate ranges have towered above their surroundings 



