QUEBRADA DE IRUYA.

Eastern slope of the Sierra de Santa Victoria, 65 miles from the Bolivian frontier, in the zone of summer rain. The valleys have been filled with an enormous mass of torrential alluvia. The water afterwards made a course through the mobile deposits.

Photograph by Keidel, Mines Division.

Plate II.

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The changes which man has made in the floral landscape are, as a rule, slight. The limits of the forest zone have scarcely been altered. The beech forest of the southern Andes seems to be less tenacious than the monte which surrounds the Pampa, and it has been ravaged by fire along the whole edge of the southern steppe at 37° S. lat. The work of man is generally confined to changing the primitive complexion of the natural formations, without altering their general appearance. Thus valuable essences are disappearing from the forest and the scrub, the larch and the cypress from the district of the Patagonian Lakes, and the red quebracho from Santiago del Estero.

A change that is scarcely visible, but is of considerable economic importance, thus takes place in the vegetation of the prairie owing to the presence of herds. The pasto fuerte, composed of rough grasses, which is the natural vegetation, is being succeeded by the pasto dulce, in which annual species, soft grasses, leguminous plants, etc., predominate. It is mainly composed of plants of European origin. The difference between the pasto dulce and the pasto fuerte or duro is so important for the farmer that there is hardly a single work on Argentina which does not dwell on it. The idea, however, that the pasto dulce has advanced steadily westward, starting from the vicinity of Buenos Aires and constantly enlarging its domain, is not strictly accurate. In 1895 Holmberg[3] traced the western limit of the zone of the pasto dulce through Pergamino, Junin, Bragado, Azul, Ayacucho, and Mar Chiquita. When we compare this with earlier observations, we see that in the course of the nineteenth century the zone of the pasto dulce has extended by about a hundred miles on the southern Pampa. When Darwin travelled from Bahía Blanca to Buenos Aires in 1833, he found no pasto dulce except round Monte, on the right bank of the Salado. Further north, on the other hand, the extent of the pasto dulce does not seem to have altered appreciably. The expedition to the Salt Lakes in 1778 found that there were already thistles beyond the line of the ranches, and these are characteristic of the pasto dulce in the Chivilcoy region on the Salado, which was then abandoned to herds of wild cattle. "There was thistle enough to cook," says the journal of the expedition. The difference is connected with the history of colonization in the province of Buenos Aires, where ground was gained only toward the south between 1800 and 1875. Since 1895 the pasto duro has been eliminated by agriculture rather than by the feet of the herds. Hence the advance of the pasto dulce is no longer in a continuous line moving toward the west. It is sporadic, depending upon the construction of new railways which open up the plain to the plough.[4]