Page:Isaiah Bowman - Desert Trails of Atacama (1924).pdf/75

 foreign enterprise until recent years, when Chilean capital has been invested in the business in increasing amounts. The nitrate deposits are found along the line of a great de- pression, the continuation northward of the well-known “longitudinal valley” of Chile. The Coast Range rises ab- ruptly several thousand feet above the sea, and between its moderate eastern slopes and the great western wall of the Andes is a depression with no outlet to the sea, The extreme smallness and infrequency of the rainfall have already been discussed. As we have said, more than a decade may pass without a single shower. Up in the mountains, however, the snow falls every winter, sometimes in local storms, sometimes over a vast area and reaching down to eleven and twelve thou- sand feet on the western mountain flanks. There are also rather frequent summer showers above eight and ten thousand feet. From these two sources the mountain streams derive their water supply and come down through steep-walled can- yons to the great, broad-spreading alluvial plain at the western foot of the mountains. The streams have lost volume in their descent over the waste-strewn floors of the canyons and by evaporation, so that they reach the border of the piedmont as trickling brooks rather than powerful mountain torrents. Whatever of land waste they have carried along with them to the mountain border is here deposited, so that there is a steady building-up on the outer or western fringe of the piedmont from year to year.

When there are unusually heavy mountain snows and rains the streams reach the border of the desert in greater volume and spread their mantle of waste over many square miles of the desert, and in rare years of extraordinary rain and snow the streams may come down in such volume as to flow out over the nitrate pampa, as they did in 1906 when they flooded the pampa as far as the railroad line near the western border. Were these rare floods more frequent in occurrence, lakes would be formed and there would be outlets to the sea and the nitrate would be dissolved and washed away. It is the very great dryness of the climate and the infrequency of the floods that make it possible for the nitrate to remain.