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 for this habit of the river its water would be far more useful to the people of the valley, because cach stream must now be tapped far above a given settlement in order that water may be brought out at the terrace tops where arable land is found. Poma itself consists of a ranch, on which are a few huts surrounding the house of the owner, and directly up val- ley the village of two-score houses. From any vantage point in the valley one may sce right to the head of it fifteen to twenty miles away where snow lies on the high peaks of Acay during the winter. On cither side of the valley floor are the huts of the natives scattered at wide intervals, their flocks ranging over many acres of mountain side in search of pasture.

The site of Poma is the bed of a temporary lake, now partly dry land, partly swamp, where the hollows of the former lake floor have not yet been completely filled up or drained. The origin of the lake is found in geologically recent volcanic action. Four miles south of Poma are twin volcanoes (Fig. 66). When the flow first occurred the river was dammed up, and a lake several miles in extent was brought into existence. With the cutting down of the diverted stream into the rock at the edge of the lava flow at the western side of the valley the lake was drained. Thus it came about that an extensive arca of flat land in the midst of mountains watered by many streams from the adjacent high cordillera has given rise to a settlement far removed from any large center of population. In an air line from Poma to Rosario de Lerma it is 50 miles, and the distance is almost doubled by trail.

The valley behind the lava dam is topographically well adapted to the control of live-stock feeding. Steep mountains on either hand prevent the mules from straying too widely. Barley and wheat are raised up to 11,000 feet, as we observed on June 16 in traveling across the Cuesta del Obispo; but this is in a situation well protected by deep valleys roundabout from the cold-air drainage that threatens the cereal crops in sites nearer the cold cordillera. At Poma alfalfa is the chief