The name _vegas_, which in the Puna and at San Juan means alpine pasture, is applied here to tilled patches in the Andean valleys. They are found in the north in the valley of the Neuquen, round Chosmalal. In the south, the valley of the Rio Pico marks the limit of cultivation. Irrigation is almost always necessary north of Lake Nahuel Huapi, where the _vegas_ have, as a rule, a soil of coarse alluvia or permeable tufa, which dries up quickly. Water is plentiful, it is true, and increases in quantity rapidly as one travels southward. The chief obstacle to the extension of cultivation is the frequency of frost in spring and summer. The deep hollows of the sub-Andean depression south of Lake Nahuel Huapi, the height of which drops to 1,000 feet at the Bolson, and 1,600 feet at Diez y seis de Octubre, have no frosts in summer, and they sustain small agricultural communities. At higher levels, in the basin of the lake or on the _vegas_ of the Traful and Lake Lacar, at an altitude of about 2,600 feet, the distribution of the summer frosts is closely related to the contour and lie of the land, which may facilitate or impede the circulation of the layers of cold air, and the play of what has been called atmospheric drainage. The valleys which are very open from west to east, at the outlet of the lakes, where the west winds have a free passage, are little liable to frost. Wherever frost is frequent, cultivation has to be restricted to fodder plants. The more favoured cantons, which grow wheat, rye and potatoes, help to feed the local pastoral population, and export part of their produce to some distance on the tableland.