West of this range the country exhibits a great variety of surface structure, and is humid and densely wooded; east of the range it consists of a broken tableland, arid or semiarid, with a general elevation of 5000 ft.
The elevated plateaus between these ranges are semiarid and inhospitable, and are covered with extensive saline basins, which become lagoons in the wet season and morasses or dry saltpans in the dry season.
It is true that a great variety of evidence is afforded by the composition of the rocks, that glaciers have left their traces in glacial scratchings and transported boulders, also that proofs of arid or semiarid conditions are found in the reddish colour of rocks in certain portions of the Palaeozoic, Trias and Eocene; but fossils afford the most precise and conclusive evidence as to the past history of climate, because of the fact that adaptations to temperature have remained constant for millions of years.
In some parts of the semiarid districts in the interior of the province irrigation is being successfully practised for the purpose of bringing land under profitable cultivation for fruit.
They are able to go without food and drink for long periods, and inhabit the arid and semiarid plateaus of the Andes and the steppes of Patagonia.