The range of one and the same cause varies infinitely with the circumstances. The limit of the prairie, as of the scrub (_monte_) which surrounds it on every side, and keeps it at a distance of 320 to 440 miles from Buenos Aires, had no decisive influence on primitive colonization. Whether covered with grasses or brushwood, the plain is equally suitable for extensive breeding. The ranches are the same on both sides of the border. At the end of the nineteenth century, however, when the area of cultivation increased, the prairie was at once found to be superior. The labour required for clearing the brushwood before the plough can work is enough to divert from it, at least for some time, the stream of agricultural colonization. While the population of the _monte_, wood-cutters and breeders, are indigenous, the prairie has absorbed the immigrants from Europe, and the border of the scrub has become in many places an ethnographical frontier.[2] [2] See E. A. S. Delachaux, "Las regiones fisicas de la Republica Argentina," _Rev. Museo Plata_, XV, 1908, pp. 102-131. [Illustration: THE ARID ANDES. PUNTA VACAS, ON THE TRANS-ANDEAN RAILWAY. _The bottom of the valley is 8,000 feet above sea-level; the sides buried under rubbish. It is especially in this latitude, above a height of 10,600 feet, in the zone where the moisture falls as snow even in summer, that the rock is everywhere buried under its own rubbish. This is Keidel's_ Schuttzone. _It extends to the foot of the Alpine peaks, carved by glaciers._ Photograph by Moody, Buenos Aires.] [Illustration: 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. To face p. 22.] 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 deg. 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.