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Traveling westward over the Puna de Atacama from Poma at the eastern foot of the mountains to San Pedro, west of the great cordillera, one leaves the last outpost of settled life at 11,000 feet, at the border of the mile-deep Calchaqui valley. Even by the time one has reached Poma the forest and scrub of the eastern foothills and front ranges have been passed, and the only extensive growth besides grass and shrubs is columnar cactus that extends mountainward far into the zone of frost to an elevation of 13,500 feet. The rim of the Puna de Atacama begins nearly a day’s journey west of Poma, and the trail climbs rapidly to a height of over 16,200 feet (4950 meters) at the Abra (Pass) de Penas Blancas. The last few hundred feet of the trail is over rough volcanic débris and above the upper limit of plant life.

On the western side of the pass the shepherds’ corrals and huts are at a higher elevation, ranging from 14,000 to 14,400 feet. The first substantial huts are at 13,500 feet, or more than 2500 feet below the pass. We passed the first dwelling sites on June 22, camping at 14,250 feet in a ravine on the western side of the pass; and the following day, June 23, we camped at the edge of the lake and borax flat at Pastos Grandes at 12,650 feet. The minimum temperature at our camp at 14,250 feet was 14° F. on the night of June 22.

The eastern mountain wall that forms the rim of the Puna is a divide between two quite unlike worlds. From the tops of the eastern spurs one Jooks far down into another climatic belt to what appears to be a world of plenty. The alfalfa meadows, the barley and wheat fields, the gardens and settlements of the Calchaqui valley are the chief elements in the border scenery. West of the divide there is a clinkery, lava-covered, or waste-strewn, arid surface. It has many minor divides, huge salars, or salt-covered basins; and its immense sheets