The sugar district of Tucuman is not, properly speaking, an oasis; that is to say, it is not an irrigated canton in the midst of a desert, but a moist patch in the heart of a less favoured region. The traveller who comes from the Chaco finds that the dust disappears from the moister air as he approaches Tucuman. The rainfall approaches 974 millimetres at Tucuman. Irrigation is a valuable aid to the farmer, but it is not indispensable. Maize is generally raised without watering, and part even of the sugar-cane crop is raised on land that is not irrigated. It is not the relatively heavy rainfall that has led to the development of the sugar-cane estates at Tucuman, but the evenness of the temperature, together with the atmospheric moisture and the rareness of frost. The mists which develop at the foot of Aconcagua form a protecting mantle above Tucuman which prevents nocturnal radiation. The nearer one gets to the mountain, the later, rarer, and lighter the frosts are. If, on the contrary, one goes out some distance westward toward the plain, the frost becomes more severe, and it is impossible to grow sugar-cane. Not only the humidity, but the contour also, has some influence on the changes of temperature and the distribution of frost. The depressions in which the cold air accumulates, in virtue of the well-known meteorological phenomenon of inversion of temperature, are more exposed than sloping districts, where the air circulates regularly and freely. The eastern limit of the zone spared by the frosts passes about thirty-five miles from the foot of Aconcagua. It has only been made clear by experiment, and one can still see there the traces of abandoned plantations.