This natural inequality was not felt at first. For a long time the spread of cultivation and the progress of wealth were restricted only by the scarcity of population, the difficulties of transport, and the inadequacy of the markets. The best endowed oases paid no attention to the surplus supply of water, for which they had no use. We have to come down to the close of the nineteenth century to find men reaching the limits which nature has set to colonization, and mapping out their domain. It is not until then that La Rioja ceases to compete with Mendoza, or Catamarca with Tucuman. While large industrial enterprises develop at Mendoza and Tucuman, strong centres of urban life arise, the population increases, and immigrants stream in, the oases of the interior scarcely change. Their population does not keep its level. Life has an archaic character that one finds nowhere else in Argentina. The physical conditions have retarded, one would almost say crystallized, the economic development. The living generation exploits the soil in ways that to some extent go back as far as the indigenous tribes, the masters of their Spanish conquerors in the art of irrigation. The industry of fattening and convoying cattle, which was once the chief source of wealth of the whole country, is still alive in those districts. The zone of the elevated tablelands of the Andes without drainage toward the sea--the Puna--has still, below 22 deg. S. latitude on the northern frontier of Argentina, a width of about 250 miles. This breadth steadily contracts southward as far as 28 deg. S. latitude, where the Puna ends about the level of the road from Tinogasta to Copiapo. To the east and south of the Puna the Argentine Andes are cut from north to south by a series of long gullies and large basins, between which there are lofty and massive chains with steep flanks. Some of these lie in the heart of the mountains, while others often open like gulfs upon the edge of the plain. These depressions with rectilinear contours are a common feature of the topography of the Andes in this latitude. The central plain of Chile is closely related to them. In the Argentine speech they are called _valles_: Valle de Lerma, Valle Calchaqui, Valle de Iglesias, de Calingasta, d'Uspallata. They are, however, not "valleys" in the sense of hollows made by erosion by running water. They owe their formation to tectonic movements, subsidences of the surface. The scanty rivers of the arid Anacs are not capable of doing work of that kind. When they enter the already formed bed of a _valle_, they seem to be lost in the immense space. Often they dry up in it, leaving behind the sediment and salts with which the water was laden. In other places they cut at right angles across the _valle_, escaping by narrow breaches in it, while the depression continues its course on either side, taking in sections of a number of independent streams.