I. Bowman— Physiography of the Central Andes. 211 



their flows but winch becomes indistinguishable beneath the 

 mass of volcanic detritus in the heart of the range. An impor- 

 tant feature of the outlying volcanoes such as the volcano 

 Pelaya, on the southern border of the Borateras de Isma, is 

 the apparently complete adjustment of their lava flows to the 

 present attitude of the tilted blocks which they surmount. It 

 is especially noteworthy here by virtue of the strong tilt 

 imposed upon the block that is the impediment ; and indicates, 

 for this case at least, the fact that some of the volcanic flows 

 were later episodes than the block faulting. The volcano in 

 point is on the western side of the borax plain called the Salar 

 de Empesa (see general map of Bolivia). One of the flows 

 from its crater forms the steep wall on the south side of the 

 bay, the Boratero de Isma, that juts toward the west from the 

 main depression. The principal basin itself is but the down- 

 faulted block whose nature is sufficiently w r ell indicated by the 

 800-foot fault scarp that forms its southeastern margin and 

 visible from the west side of the lake as a steep and nearly 

 straight wall, cutting across a thick series of earlier and now 

 deformed mass of igneous rocks. 



These descriptions of the present condition of the once 

 lower and flatter peneplain, and the proof of its existence, 

 enables the brief presentation of a few facts which seem to 

 indicate two periods of uplift separated by alternate intervals 

 of quiet. The first is the occurrence of terraces just within 

 the edge of the plateau. These are well-defined and are 

 clearly not of structural origin, the rock in this locality, Que- 

 brada Quisma, being massive crystalline. The descent of the 

 upper valley slope from the topmost level of the plateau is rela- 

 tively gentle and the top of the terrace descends with still 

 gentler grade to the exceedingly steep descent of the gorge- 

 like inner valley. The terrace is conspicuous virtually to the 

 head of the valley — ten miles eastward — and continues down 

 stream to within a half mile of the edge of the plateau. Here 

 it disappears, the flat upper slope being displaced by a continu- 

 ous and steep descent to the valley bottom. Such a terrace 

 originates the conception of two uplifts. The first was fol- 

 lowed by valley development to the point of well-graded val- 

 ley slopes, although the dissection of the flat plateau surface 

 had only been begun on account of the aridity which charac- 

 terizes the region. The second uplift is marked by that deep 

 dissection which the now incised stream has accomplished. 

 The inner valley is a narrow gorge with persistently steep and 

 in places vertical sides, a contrasting condition with respect to 

 the outer valley, which suggests that the time that has elapsed 

 since the last uplift is short compared with the interval between 

 the two uplifts. 



