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 peaks on the west. They are real volcanic cones, many of them so recent as to be little dissected and so high as to be covered with snow most of the year. It isa great lava-covered volcano-studded wall.

Upon the eastern mountain wall of the Puna de Atacama and upon the peaks that crown it there is wrung out of the air almost the last vestige of its moisture. It results that the country to the west of that wall is exceedingly dry. The precipitation is so light that through-flowing drainage is absent over a wide area. It is a dryness that is broken in a significant way but once in a period of years, and precipitation then usually takes the form of a blanket of snow which may accumulate to a depth of several feet and remain for two weeks or so. The pass at the head of the ravine of Pefias Blancas is at an elevation of 4950 meters (16,236 feet). When we crossed it in July, 1913, there were snowbanks under the steeper lava cornices near the pass, and some snow lay in small patches on the eastern side of the pass. The early morning temperature at our last camp just east of the Western Cordillera on the San Pedro trail was below zero Fahrenheit. The stream and the pool near camp were frozen solid so that we led our pack train across it as across a floor of rock. Yet there was no snow on any of the slopes round about or on the cones in the distant view. It was only when we reached the pass in the main chain that a few tiny patches and strips of snow appeared high up in the sharp but small ravines cut in the volcanic cones that stand on either side of the gateway. ‘The stream water derived from rains and melting snows is gathered in local basins whose margins are rimmed by belts of relatively steep alluvium and whose floors contain either lakes or marshes or salt deposits or all three in varying sizes and combinations. In one basin salt may prevail as a floor deposit; in another it may be borax. The salty deposits at the bottoms of the basins are residues from evaporation and include chlorides of sodium, potassium, and magnesium, some sulphates and carbonates, and borax. The composition varies from place to place according to the sources of the material from hot springs which occur rather frequently throughout the Puna and from rain water that