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 tinued erosion of the desert hills. It is noteworthy that upon this map and the similar one of the Lquique region (Fig. 94) uot a single stream appears outside the mountain belt. Running wa- ter is entirely absent except immediately after rains, which fall at rarest intervals many years apart. The salts of the basin floors are still exposed where they crystallized out as the lakes they represent dried up, though in some cases they are covered with a veneer of dust and alluvial silt or sand.

It is an interesting discovery that the effects of aridity are sclf-stimulating and cumulative. In the West we have the expression “Too dry to rain,” and this is literally true, for once the air becomes sufficiently dry it takes an immense amount of moisture-laden invading air to displace it or to furnish moisture enough to offset evaporation in the lower air even when there are local showers in the upper air. The rain in such cases sometimes actually fails to reach the ground.

The self-stimulating effects of dryness are seen equally well or better (because more constantly in evidence) in drainage features. Between Fiambalé and Uyuni is a broad stretch of country in which this is best seen. Only a few basins are able to collect sufficient waters to overflow the lowest point on their rim and thus to join their fluvial system with that of a basin at a lower level. Given a greater amount of rain we shall have, let us say, substantial streams like the Lipez, that flows into the Salar de Uyuni from the south, or the Mauri, that comes out of the Western Cordillera in central-western Bolivia to join the Desaguadero. The effect is to cause a junction of a number of large streams upon a basin floor in the form of a large lake or large salar or both. Upon the southern borders of the Puna de Atacama, particularly in the basin of Fiambald, for example, we have the water supply gathered together in sufficient volume to cause channel ways to exist over the whole of the basin floor, though the streams are of the intermittent variety; and at the southern end of the basin these waters are gathered together from the subsoil in sufficient quantity to enable the stream to maintain its course through the narrow southern exit of the basin to Tinogasta and beyond.