These commercial currents are of very ancient, probably pre-Columbian origin. Boman has discovered ears of maize in the prehistoric tombs of the Puna de Atacama.[18] The Puna, at a height of 11,000 to 12,000 feet, is permanently inhabited, unlike the high valleys of the Cordillera de San Juan, which are occupied only during the summer season by Chilean shepherds. It is primarily a pastoral and mining region, but it has some tilled land, at more than 6,700 feet above the level of the valleys. The higher limit of annual cultivation in the cold districts, which is fixed by the summer temperature, does not fall in the same way as that of arboriculture in warm districts, because trees suffer from the winter frosts. The Indians of Cochinoca and Susques sow lucerne and barley for fodder, and the _quinoa_ and potato for food. Transport between the Puna and the _valles_ is carried on by the inhabitants of the Puna, and is not shared by the _vallistas_. They are especially active in the north, in the province of Jujuy. Belmar shows how important the sales of the Puna woollen goods were by the middle of the nineteenth century.[19] These fabrics were used by the mill-owners of the Rio Grande de Jujuy to pay for the work of the Indians of the Chaco, whom they employed in the sugar-cane harvest. The competition of the manufactured products of Europe now menaces the domestic weaving of the Puna, just as the competition of the flour of the Pampa menaces the cultivation of cereals in the _valles_. [18] Eric Boman, _Antiquites de la region andine de la Republique Argentine et de la Puna de Atacama: Mission scient. G. de Crequi-Montfort et E. Senechal de la Grange_ (Paris, vols. i. and ii. 1908). [19] Belmar, _Les provinces de la Federation argentine_ (Paris, 1856). Besides this traffic of local interest the _valles_ serve for a traffic of a higher, almost a continental character. It seems certain that during the pre-Spanish period the road from the Peruvian tablelands to Chile avoided the inhospitable desert of the Puna de Atacama, entered the region of the _valles_ to the east, and crossed the Cordillera in the latitude of Tinogasta, or even a little further south. That was the route of the armies of the Incas, which in the fourteenth century came as far as Maule. The pre-Columbian roads, of which Boman has found traces between the Valle de Lerma and the Valle Calchaqui, seem to correspond with this direction of traffic. By this route the long _quechua_ passed amongst the Diaguites populations. The conquerors followed the Indian guides. Almagro, in going from Peru to Chile, passed through the _valles_ at the eastern edge of the Andes.