ON THE CORDILLERA 127 



the Malargiie depression, below 35 S. lat., still presents 

 the typical scenery of the central Andes. The dejection- 

 cone of the Atuel resembles that of the Mendoza. The 

 fringe of torrential deposits, distributed in cones over 

 which the waters spread, is due to the rapidity of the 

 disintegration of the rocks in a desert climate. Keidel 

 has pointed out the part played by the summer rains 

 in transporting mobile elements, which the water 

 drops as soon as the slope diminishes ; the amount 

 of precipitation being too slight to permit the forma- 

 tion and spread over the plain of a regular network 

 of streams. From the Rio Grande onward the dejec- 

 tion cones disappear. The streams tend to become 

 permanent, and sink into narrow valleys. The summer 

 rains cease, and the water produced by the melting of 

 the snows has only a feeble capacity for transporting 

 stuff. The soil of the Cordillera is protected by a denser 

 vegetation. The first thickets of molle appear in the 

 valleys, the first scattered cypresses on the slopes, 

 at the Rio Agrio, a tributary of the Neuquen. Then 

 the forest invades the mountain : at first, from 38 S. 

 lat. to 39 30' S. lat., a resinous forest of araucarias. 

 At length, at Lake Nahuel Huapi, the forest assumes 

 the general appearance which it has as far as the 

 Magellan region. It is chiefly made up of different 

 kinds of beeches. The coihue (Notofagus dombeyi) 

 is the most conspicuous for about three quarters of 

 a mile, rising above an impenetrable undergrowth of 

 bamboo. Higher up the domain of the lenga (Notofagus 



diorites, which have not been exposed further north except at the 

 base of the western slope, opens out in the Patagonian Andes, of which 

 it is the main body between Lake Lacar and the Gulf of Ultima 

 Esperanza. In fine, the Patagonian Andes are characterised by 

 volcanic formations. They are seen on the eastern slope about 36 S. 

 lat., in the lava-flows and ashes of Payen and Tromen. Further south 

 volcanoes with acid lava and characteristic cones are restricted to 

 the centra] zone (Lanin, etc.) and the Chilean flank, but flows of fluid 

 basic lava cover enormous stretches at the eastern fringe of the Andes, 

 and they have spread over a good deal of the Patagonian tableland 

 outside the Andean region. 



