THE OASIS OF ANDALGALA.

At the western foot of Aconcagua, the snowy crest of which can be seen.

Photograph by the Author.

Plate VI.

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On the eastern slope of Aconcagua is the limit of the crescent of tropical forest, which begins about three thousand miles away, on the flank of the Venezuelan and Colombian Cordilleras, and is connected in the centre, in the equatorial zone, from Guaviare to Mamore, with the forests of the Amazon region. At its two ends it is reduced to a narrow belt which does not reach, in the east, the alluvial plains, the savannahs of the Orinoco and the scrub of the Chaco. The humid forest of the Argentine Andes is nowhere more luxuriant than near its southern limit, above Tucumán. There are no palms or tree-ferns, but the convolvulus abounds, and the evergreen trees are covered with epiphytes. Aconcagua is one of the sharpest climatological limits in the world. In the latitude of Salta one has only to go about 150 miles to pass from the moist forests of the sub-Andean chain of the Lumbrera to the arid valley of Cachi. On both sides of Aconcagua there are less than fifty-five miles between the sugar-cane fields won from the forest and the oasis of Andalgala, or that of Santa Maria, which are right in the desert zone. According as one approaches Aconcagua from the east or the west, one finds, from base to summit, either the successive stages of vegetation of the humid Andes—from forest to grain-sown prairie (paramo or pajonal)—or those which are characteristic of the arid Andes, from the spiny scrub of the valleys to the fields of resinous tola of the Puna. The contrast of climates is repeated in the character of the soils. Aconcagua contains in itself the entire Andes in miniature. At the foot of the narrow zone of Alpine crests, in the few square miles of the elevated valleys of Tafi and Pucara, there is a small agricultural and pastoral world, in a temperate climate, that has nothing quite like it elsewhere, narrowly confined between the forest and the desert.[32]

The sugar district of Tucumán 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 Tucumán. The rainfall approaches 974 millimetres at Tucumán. 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 Tucumán, 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 Tucumán 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.