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It happens that the broadening of the mountain belt of the Andes of northwestern Argentina takes place in just the zone in which regular easterly winds begin to appear. ‘The farther north we go, the more regular become these winds; and in central-castern Bolivia, where the mountain belt is broadest, and thence northward through eastern Peru the easterly winds are the regular trades. It is in this belt that forest almost completely displaces grassland. Northwestern Argentina is the region in which the change takes place from desert plain to grassland and woodland in contrast to the truce forest that clothes the eastern slope of the Andes farther north. On a simple mountain border against which blow prevailing winds we always find a belt of maximum precipitation, and there, too, the heaviest forests grow. Such a belt varies in height above sea level according to the general geographical situation and the height of the surrounding country. In the Himalayas it is from 4000 to 5000 feet, in the western slopes of the Sierra Nevada Mountains in California it is at a comparable altitude, anc in the case of the Appalachian Mountains of eastern North America there appears to be increasing rainfall to the summits (4000 to 6000 feet); but the belt of densest forest in the Appalachians is lower than the belt of maximum rain because of the effects of winter snow, cold, and wind, which result in there being a cold timber line rather than a dry timber line at the top of the forest.

We found the zone of maximum precipitation on the moun- tains west of Salta to be marked by a belt of temperate-zone woodland between 4500 and 6000 feet. Above the woodland, scattered groves grow in favorable places, and belts of timber extend up the shadier and moister valley floors. The higher country bears a thin cover of herbaceous vegetation which gradually changes to the scattered clumps of ichu grass at the highest elevations. Up to 10,000 feet barley is grown; above that elevation potatoes are the chief vegetable product. The grasslands are the seat of pastoral population groups. In the forest, agriculture and grazing are combined. Below the forest,