THE ARGENTINE UPLANDS. 357 



Ijarullcl ranges, the eastern lying entirely within Argentine territory, and on an 

 average about 5,000 feet lower than the western. 



The extinct Overo volcano (15,550 feet), which connects this eastern ridge with 

 the loftier range, not far from Mount Maipo (17,G70), is encircled by one of those 

 glaciers, which afford the best opportunity for studying the formation of the 

 nieve pénitente, "j)enitent snow," "so called from the eccentric resemblance to cowled 

 ' friars penitent,' affected by the frozen masses under the action of sun and wind. 

 The crystalline parts, which resist evaporation and the melting process, ramify 

 in the tti-angest fashion, in many places leaving the black ground exposed 

 between the fantastic blocks of ice, which sometimes stand five or six feet high."* 



Malakgue anp the Southern Cordilleras. 



South of Overo the Argentine chain is interrupted by the vallej^ of the Eio del 

 Diamante, beyond which rise other crests disposed in lines parallel with the main 

 range. A breach in which rises the Rio Atucl leads to the Planchon pass 

 (9,920 feet), one of the most frequented in the Argentino-Chilian Andes. 



Farther south the orographic system broadens out. Here the Argentine chain 

 of the Malargue (Malalhué) volcanoes, which contrasts with the Jurassic forma- 

 tions of the main range, is developed to the east of the deep longitudinal valley 

 of the Rio Grande or Upper Colorado. 



Some CO miles farther east, beyond a closed lacustrine basin remnant of a 

 former inland sea, rises the lofty Nevado de San Rafael (16,190 feet), an almost 

 isolated fragment of mountain masses which appear to have formerly been far 

 more extensive than at present. Farther south the Cerro Payen, undoubtedly of 

 igneous origin, dominates the A'alley where the Rio Grande and the Rio de las 

 Barrancas unite to form the Colorado. Near the Buta-co pass, which crosses the 

 Malargue chain at a height of 4,9S0 feet, is seen the Cura Cokalio, or " divine 

 stone " of the Araucanians, a huge sandstone mass, which has fallen across the 

 track from a neigrlibourino: cliff. 



According to Host, the Chos malal or Bum mahuida, in the eastern pre- 

 Cordillera, has an altitude of no less than 16,400 feet. It is an extinct volcano 

 surrounded on all sides by ashes, scoriœ, and lava streams. The whole of the 

 eastern Cordillera in this region is believed to consist of these igneous rocks, which 

 were ejected at two different epochs, the first represented by black trachytes, the 

 second by basalts. A sill 7,610 feet high separates the volcano from the western 

 Cordillera, and forms a divide between the waters flowing in one direction towards 

 the Colorado, in another to the Rio Negro through its Neuquen affluent. The 

 Andes have few more romantic sites than this pass in the " Argentine Switzer- 

 land," which commands a wide prospect of pastures and woodLmds, bounded north- 

 east by the gigantic Cerro Payen, north-west by the Campanario cone, with its 

 crown of picturesque rocks affecting the form of ruined towers. Southwards the 



* Vol. XVIII., p. 416. 

 57 



