Page:Isaiah Bowman - Desert Trails of Atacama (1924).pdf/356

 and the bordering terraces that man is able to satisfy his material needs.

Central-western Bolivia marks the change between the northern portion of the Central Andes and that southern portion that embraces the great salars and interior basins that form the characteristic feature of the Puna landscape. The change is interestingly foreshadowed in the contrast between lakes Titicaca and Poopé. While the water of Lake Titicaca is slightly brackish, fish live in it and are caught for food; and its border is in many places fringed with recds that grow in shallow water. The great depression in which the lake lies is fed by rains and melting snows from the surrounding high- lands and mountains, principally the Cordillera Real; and so copious and regular is the supply that, instead of drying up, Lake Titicaca persists as a large lake and continuously over- flows its southern rim into the vast depression just south of it. The outlet of Lake Titicaca, the Desaguadero, pursues its course southeastward to Lake Poopé, more than 400 feet lower than Lake Titicaca. Though Lake Poopé spills a moder- ate amount of water westward into the great salars of Coipasa, there is so much water evaporated from the shallow basin of Poopé as to render the water too salt to be drinkable.

These changes in salinity of the water in the three successive basins arranged from north to south—Titicaca, Poopé, and Coipasa—are in almost exact sympathy with the rainfall, and as the rainfall diminishes we find man himself making cor- responding responses. Cultivation, or at least occupation for pasture, extends pretty much over the whole surface of the mountain and valley zone of Peru, as we have seen above; and the same is true of the Bolivian plateau south of Lake Titicaca. Midway between the two lakes a change takes place. The land adjacent to Titicaca is cultivated for cereals, the soil farther south becomes more sterile, and saline tracts more numerous. The effect is to divide the population into two ribbons (east and west of the Salar de Uyuni) instead of a continuous broad