I. Bowman — Physiography of the Central Andes. 375 



fragments of the clay remained. These were then covered by 

 a second thick layer of sand and thus preserved. 



The frequent occurrence of irregular and locally variable 

 conglomeratic layers throughout the sandstone series, the inter- 

 stratified clay lenses, the great thickness of the conglomerates 

 and coarse sandstones, establish the conclusion that the sand- 

 stone series represents a piedmont river deposit. As an index 

 of the close proximity of highland and aggrading flood-plain 

 upon which such coarse deposits were formed, might be cited 

 our observations upon the degree of coarseness of the bar 

 material in the Chapare downstream from San Antonio at 

 the base of the Andes, tig. 13. At and above San Antonio 

 the river is full of bowlders, often of huge size. At Santa 

 Rosa, less than twenty miles away, the bars are composed of 

 sand and pebbles of 8, 10, and 12 inches in diameter. Six 

 miles below Santa Rosa one can discern pebbles only in 

 patches upon the upstream sides of the bars, and fifteen miles 

 farther downstream, or forty miles by river and perhaps twenty 

 or twenty -five miles in a direct line from the edge of the 

 existing upland, it is impossible to find any pebbles whatever, — 

 the bars are wholly of sand. Such a distribution of coarse 

 material along the valley of the Chapare supplies data for the 

 conclusion that the eastern front of the Andes was not far 

 from the conglomerate formation of a particular locality and 

 that the progressive eastward uplift of the Andes accounts for 

 the wide distribution of the conglomerates throughout the 

 sandstone series that now constitutes the eastern border of the 

 Andine Cordillera. This conclusion seems the more warrant- 

 able because the conditions of lofty mountains and adjacent 

 flat plains, with a strong and sudden break between, are now 

 most favorable for the widest development of conglomerates ; 

 and as we have just seen, these are now formed in an 

 extremely narrow belt (less than thirty miles) along the moun- 

 tain front. 



This brief outline of the geologic structure of the region 

 may now well be followed by a word concerning the geologic 

 history of the central Andes as a whole. With these three 

 groups of facts before us — the structure, the geologic history, 

 and the general physiographic aspect of the region — we shall 

 be prepared to discuss the physiographic conclusions and the 

 detailed stratigraphic and physiographic evidence upon which 

 they rest. 



The western or Maritime Andes consist chiefly of Mesozoic 

 strata interstratified with and intruded by igneous rocks, and 

 are of later age than the eastern Andes. The whole rock series 

 is surmounted, in addition, by volcanic piles which give what- 

 ever variety and mountain alignment these mountains possess. 



