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 rather closely confined to slopes with a southern exposure or to valley floors with an abundance of ground water.

Above the forest the slopes are covered with an extraordi- nary growth of cactus. It has in places the suggestion of a for- est aspect (Fig. 96). Between the upper edge of the forest and

. 99—Composition of cloud zone and woodland and grassland belts on eastern border of the Puna de Atacama under characteristic topographic conditions. In A the border ranges are low, the prevailing east wind of summer carries cloud, the belt of maximum precipitation is located on or near the Puna border, and both it and the low border ranges have corresponding belts of scrub and grass. Both border and outer valleys are irrigated, and the outer one may be very dry. The inner one has a large and important main stream with an abundance of tributaries. The high Puna is above the belt of heavy rains at all seasons and has only a meager vegetation of clump grass and moss. In B the high border ranges cut off the rain-bearing winds so effectively that there is a belt of pronounced rainfall (seasonal) and a growth of woodland (1) with grassy belts above and below it, The deep border valley {4} and the low outer valley (3) are both irrigated. The pasture belt at 2 in B is less pronounced than at 1 in A, being sustained by occasional showers only as the cloud belt is driven westward at rare intervals or the deep border valley fills with cloud. The valley floor of B 4 is semiarid in parts, and the stream flow is uncertain, although the high pastures of the bordering upper slopes are dependable in summer, and springs are fairly numerous.

the settlement of San Fernando in the Escoipe valley (1800 meters) are the densest cactus stands that I have seen any- where in South America. They are distinctly better developed upon the hillsides; the floor of the valley is occupied by grasses and shrubsrather than by cactus. Farther up the valley (Fig. 62) we come to the eastern slopes of the Cuesta del Obispo, and here the trail zigzags upward to a height of 3300 meters at the crest.