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 band (Desaguadero Valley). In just the proportion that they become divided docs the character of the settlements change. Where it is a continuous belt there is more frequent com- munication anc larger towns. Farther south the settlements in the central part of the basin become small and widely scattered.

The salars themselves have no towns upon them because they are quite uninhabitable owing to the absence of fresh water and pasture and the very strong contrasts of temperature from night to day, the heat of midday being unbearable for practically the whole of the year and the sunlight extremely trying. In addition, Lake Poopé is almost surrounded by a saline marsh. South of Coipasa is the great Salar de Uyum. It fills almost the entire width of the great table-land between the interior borders of the eastern and western cordilleras. South of it the salars are broken up into smaller units by local volcanic eruptions.

Thence southward the salars continue over a broad stretch of country (see the map, Fig. 87) occupying a portion of east- ern Chile southeast of Calama, southwestern Bolivia, and northwestern Argentina.

With diminishing rainfall southward the size of the salars diminishes, for we can only have the greatest salars where we have relatively flat topography, broad basin floors upon which the waters may be spread out in a thin sheet, and a substantial amount of rain. If the rainfall becomes very light the salars will be broken up into small units in the hollows of limited basin floors, and this is the situation southward in the Puna de Atacama.

The contrast between the Puna de Atacama and the plateaus of Bolivia and Peru is still stronger if we study them with respect to their eastern approaches and the coming and going