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The Puna de Atacama is the greatest barrier within the Cen- tral Andes, the most extreme case of lofty, desolate plateau and winter cold. Farther north the high mountain valleys of the Andes are moister and warmer, farther south the moun- tainous zone is narrower. If the plateau of Bolivia is the widest part of the Andean system it is not the driest nor is it the high- est. If the Andes south of the Puna on the border between Chile and Argentina are colder because of a more southerly latitude the zone of cold is narrow, it takes but little time to cross it, it offers little or no foothold for life today, and it of- fered no scope for civilization in the past. Naturally the effects of the environment upon such inhabitants as there are in the Puna or about its borders and upon the ways of communica- tion are not only marked today but may be traced back by historical and archeological evidence into the racial life and culture of the Indian population.

We often think of mountains as tending to divide peoples. We picture their height, their rigorous climate, their intricate system of roads, the steep ascents and descents, as barriers to free intercourse. There abound specific and accurate illustra- tions of this principle, but we must also carry in mind an opposite effect, namely, that mountains in some cases tend to hold people together. In fact, one must go further and recog- nize that part of the population of a given region may exhibit one of these two responses while part of the population in the same region at the same time may exhibit the opposite response.

Let us take minerals by way of illustration. In the Central Andes there are a number of famous mineral localities, such as Potosi, Corocoro, and, farther south, in the Puna de Atacama, the less famous but rather widely known San Antonio de los Cobres. There are many mineral localities in the Desert of