surfaces may be buried under lava flows. Neither the Coast Range of Chile nor the so-called) Pre-Cordillera along the eastern front of the Andes of northwestern Argentina is noted for its volcanic material but rather for its sedimentary and intrusive material modeled on smooth lines.
Were these generalizations limited to a small area they might have correspondingly small significance. On the con- trary, they are characteristic of the whole Central Andes. More than that, the studies that Willis has made in northern Patagonia and others have made in Peru, Ecuador, and Co- lombia reveal in effect a similarity of topographic features throughout the whole Andean realm.
Central and Southern Andes Compared
Northward from the southern tip of South America the Andes Mountains are composed within a rather narrow zone, and in a single airplane view their entire extent from Chile to Argentina could be seen on a clear day. The several chains would be seen to lie so closely parallel and with such narrow longitudinal valleys between them as to have in general a rather simple appearance. One would also see the streams that drain the Argentine plain north of Patagonia extending their headwaters effectively throughout the mountain country, for the most part to the very summits and crests of the main divide. In the same way the streams running westward to the Pacific interfinger with companion streams on the other side of the Cordillera. Every mountain hollow is reached by tiny headwater tributaries. It is effective through-flowing drainage of the normal type. The desertic mountain-and-plateau country of western Bolivia and northwestern Argentina would lie out of the picture.
The relation of man to the mountains in the Andes south of the Puna is as simple as the general physiography. The valleys among the mountains are desirable for the pasture they afford in the summer season. The rivers and lakes of the mountains or along the mountain border, as in Patagonia, are sources of water for irrigation and in part even for navigation. Towns
