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Places established where they could be reached only by exceptional floods would be without water for years at a time unless wells were dug to reach it or tunnels driven to ground water into the piedmont, such as we have described at Pica (p. 20). A settlement formed near the head of a mountain torrent, where the stream flows in full volume and so vigor- ously as continually to rework the materials of its valley floor, would likewise have a precarious existence as at Algarrobal (p. 68). Between these two extremes, however, it is possible to establish fields and irrigating canals and to enjoy a reason- ably dependent supply of water. Precisely where a town will be located depends upon routes and trails to other towns and to the coast, so that there is some variation in the posi- tion of settlements along the western foot of the Andes. They are all alike, however, in that cach depends upon a mountain stream that has a steadily diminishing volume westward, toward the desert. Each failing stream—Aroma, Huaschijia, Tarapaca, Mamifia, Quisma, Chacarilla, Huatacondo, Mani, and others—is the locus of a village or a line of villages. Each stream is deeply incised below the level of a broad piedmont slope. This is not merely a local condition. It extends along the western border of the Andes for five hundred miles, from Copiapé in the south to Pisagua in the north.

Of all desert places in South America, the villages and settle- ments along the Andean foot in Tarapacd and southward to the end of the desert have their fortunes most intimately de- termined by the local seasons. So far as their daily life is con- cerned, the coast might as well be a thousand as a hundred miles away. There are exceptions, to be sure, as where a mine or a source of water supply affects a remote mountain settlement, but on the whole it is a singularly self-contained serics of communities. Before the devclopment of nitrate only naked desert confronted them westwards. It discouraged occupation and movement in that direction. They looked to the mountains for their trade relations and for a part of their subsistence, not to the sea. It is of far more importance to