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 are built not in the mountains but along their border. While the mountains offer serious transit difficultics they are diffi- culties not so much of distance as of elevation and snowfall, and in any case they are difficulties that are rather quickly passed. In the absence of mineral wealth, railways are built toward the mountains with the intention of crossing them, not for the purpose of exploiting the subsoil. They follow the shortest and the most practicable routes. Each one of the transitable passes is known for the relative difficulty of the road, the probability of snowfall, and the access to markets on either side. They were among the first topographic features to have a determining influence upon cities and even upon the course of history, as in the founding of Mendoza and in the development of the cattle trade and the goods trade from the older Spanish colony in Chile to the colonists who went out from there to settle along the eastern border of the Andes, As we go northward the simple character of the mountains is lost. Beginning in latitude 30° the mountains have long extensions southeastward toward the plains of central Argen- tina. These offshoots enclose or nearly enclose embayments of the plain that extend like arms of the sea up the intervening valleys. The characteristic relation of basin and plain con- tinues along the entire eastern border past the northern boundary of Argentina and through Bolivia into southeastern and eastern Peru. It is a Central-Andean type of mountain border—as characteristic and persistent a feature as the drain- age of the nitrate desert or the longitudinal valley of Chile. The general trend of the eastern wall of the Andes in its course across northwestern Argentina is not from south to north as in Patagonia but from southwest to northeast. As the mountain zone broadens, so does the mountain border become more complex. Every physiographic complexity is reflected in altered human relationships: the location of the trails, the size and situation of the cities, the whole scheme and structure of the economic life. Within the mountain belt of the Puna region in the broader Andean zone we have onc of the most desolate regions in the world. Population is all but absent except in a few miscrable villages crouching on the