chap. x. Sea-action at Eastern Foot of Andes. 289 



gentle, slope, from five to ten miles in width (as repre- 

 sented in the following diagram), entirely composed of 



No. 27. 

 Section of the Plain at the Eastern Foot of the Chilian Cordillera. 



Cordillera. Talus-plain. Level surface, Gravel 



2,700 feet above sea. terraces. 



perfectly rounded pebbles, often white-washed with an 

 aluminous substance like decomposed feldspar. This 

 sloping plain or talus blends into a perfectly flat space 

 a few miles in width, composed of reddish impure clay, 

 with small calcareous concretions as in the Pampean 

 deposit, — of fine white sand with small pebbles in 

 layers, — and of the above-mentioned white aluminous 

 earth, all interstratified together. This flat space runs 

 as far as Mendoza, thirty miles northward, and stands 

 probably at about the same height, namely, 2,700 feet 

 (Pentland and Miers) above the sea. To the east it 

 is bounded by an escarpment, eighty feet in height, 

 running for many miles north and south, and composed 

 of perfectly round pebbles, and loose, white-washed, or 

 embedded in the aluminous earth : behind this escarp- 

 ment there is a second and similar one of gravel. 

 Northward of Mendoza, these escarpments become 

 broken and quite obliterated ; and it does not appear that 

 they ever enclosed a lake-like area : I conclude, there- 

 fore, that they were formed by the sea, when it reached 

 the foot of the Cordillera, like the similar escarpments 

 occurring at so many points on the coasts of Chile and 

 Patagonia. 



The talus-like plain slopes up with a smooth 

 surface into the great dry valleys of the Cordillera. On 

 each hand of the Portillo valley, the mountains are 



