The use of irrigation in this district raised different technical problems from those of the north-western provinces. In this latitude the torrents of the Andes are formidable when the snows melt, at the beginning of summer. The flood is all the greater and more sudden as the heat is late. From all the ravines of the mountains the muddy waters then converge toward the valley. The flood scours the bed of the river, erodes its banks, and threatens to find a way amongst the estates. Even the towns of Mendoza and San Juan have more than once been in danger. The fear of diverting the flood and of bringing it upon themselves compelled them to be content with raising only light and frail dams in the path of the torrent. At San Juan they used, for a long time, the waters of the Arroyo del Estero, a small brook fed by infiltration from the Valle de Zenda, and it was some time before they ventured to draw upon the river itself. [Illustration: THE OASIS DEL RINCON, BELOW SAUJIL (ANDALGALA LINE, PROVINCE OF CATAMARCA). _The dejection-cone, at the foot of which is the very small oasis, is seen resting against the Sierra d'Ambato._ Photograph by the Author.] [Illustration: THE MONTE AT EL YESO. _Zone of clay hills at the foot of the Sierra de San Antonio, at the edge of the Chaco. Corral (cattle park) made from tree-trunks._ Photograph by the Author. PLATE VII. To face p. 82.] Another problem, which the smaller oases of the north-west hardly know--the problem of drainage--is of paramount importance at San Juan and Mendoza, as far as a large part of the irrigated surface is concerned. The water infiltrating into the soil forms a subterranean sheet which approaches more or less to the surface according to the topography. It comes to the surface at the foot of the cone, where the slope diminishes and the cone gradually passes into the plain. Hence the cone has, at its base, a belt of marshes (_cienagas_), and sometimes a line of good springs (_barbollon_). At San Juan, if you move far enough away to get a comprehensive view of the whole of the estates, you see that they occupy the middle belt, half-way down the cone, the top of which is composed of coarse shingle, while the bottom is too wet. The advance of the plots upward and the steadily increasing use of the available water tends to raise the level of the underground sheet and enlarge the area of marsh.