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 One further observation upon water supply is necessary here to understand the nature of the climate and settlements of the Atacama region. Desert oases are of two general kinds: {1) those that lie on heights that reach into the zone of cloud and rain and (2) those that lie in hollows or valleys that have river water or ground water. Raton Mesa in New Mexico represents the first type; Copiapé and Vallenar the second—they lie deep down in the ground. So extremely dry is the Desert of Ata- cama that none of the heights in it, though they reach several thousand feet above the general level, catch a significant rain- fall. The nearest approach to the high-island-like oasis of Raton Mesa is to be found along the coast of Chile, as at Paposo, south of Antofagasta, where the heavy fog supports a little herbage—the counterpart of the grassy hills of wet weather seasons along the so-called lomas of coastal Peru.

The effect of a single shower on the southern margin of the Chilean desert (about Coquimbo) is noted by Darwin:

“…The farmers, who plant corn near the seacoast where the atmosphere is more humid, taking advantage of this shower, would break up the ground; after a second they would put the seed in; and if a third shower should fall, they would reap a good harvest in the spring. It was interesting to watch the effect of this trifling amount of moisture. Twelve hours afterwards the ground appeared as dry as ever; yet after an interval of ten days, all the hills were faintly tinged with green patches; the grass being sparingly scattered in hair-like fibres afullinchin length. Before this shower every part of the sur- face was bare as on a high road.

Going still farther northward toward the Huasco valley, Darwin took the coast road, ** which was considered rather less desert than the other."" The shower which he mentions above had reached (a fortnight before) about halfway to Huasco, and so far as it extended the ground was covered with a faint tinge