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 arise in more closely compacted settlements. But the fixed climate of desert and mountains, the open character of the country, the thinness of settlement, and the limited popula- tion which the region can support tend to keep the trails in fixed locations, and we may read their history from the earlicst colonial times, 1f not earlier, down to the present.

The Puna de Atacama, for example, can be crossed in almost any direction, but actually it is crossed in certain definite directions determined by the trails that follow the valleys on the border of the Puna. The passes of the border are reached by trails that climb by moderate grades to clevations only one or two thousand feet higher than the Puna basins and valleys: but on the outer side of the Puna there are very steep and rocky descents, and the valleys that offer naturally graded roads determine the location of trails. In the Pre-Cordillera— the lesser ranges that lie east and south of the great wall that constitutes the border of the Puna—the trails follow the valleys through the belt of woodland and cross by passes that have been in constant use since the time of the conyuistadores. The Calchaqui valley was the seat of a dense native Indian population in the earliest colonial times and has continued to be a center of agriculture down to the present. A north-south trail from Bolivia to northwestern Argentina passes through it, and its importance, though variable, has continued down to the present.

Farther south, in the region of Tucuman, is the pass of Pucaré, a crossing place between Andalgala and Tucumén, where the trail climbs up over the southward continuation of the Sierra de Aconquija. Troops of cargo mules pass con- tinually over this trail, carrying the wine, hides, and dried fruits of Andalgalé to the town of Tucuman and returning with sugar, tobacco, and rice. At first a mere trace, the trail has grown in importance with the development of the bordering ranches, haciendas, and towns. As the region became thickly settled in the border tracts where water may be had, the trail became more frequented and was in almost constant use up to the time that a railroad was built to Andalgala.