tree with thick trunk and naked roots which is found scattered over the prairie in the Parana region as far as south of Buenos Aires. [76] On the left bank of the Salado, west of the Resistencia railway, a great gulf of low prairie penetrates into the forest of the Chaco in the north, almost as far as 28 deg. S. lat., but it has rather the character of one of the floodable clearings of the Chaco (_esteros_) than of the temperate Pampa. In the west, between San Luis and the mouth of the Colorado, the transition from the Pampa to the _monte_ is gradual. Just as at Santa Fe, the approach of the _monte_ is announced by the appearance of _chanares_, in the south-west corner of the Cordoba province and on the southern slope of the Sierra de la Ventana. The _monte_, properly so called, though impoverished, invaded by the _jarilla_, and mainly composed (as in northern Patagonia) of dwarf mimosas, covers the area of the Pampean sierras on the left bank of the Chadi Leuvu and the Colorado. Between this area and a line passing through Rancul, Anguil, Atreuco, and Bernasconi, where the naked prairie begins, there is a mixed zone which one may call the _calden_ zone. This mimosa, a near relative of the _algarroba_, which has a wider range than the other plants of the _monte_ in this latitude, forms woods at intervals in the south of the San Luis province and on the flanks of the parallel valleys of the central Pampa. Between these woods the tableland is generally covered by the prairie, with occasional patches of _chanares_. About twenty-five miles east of Buena Esperanza the line from San Rafael touches the far corner of a forest of _caldenes_, which stretches south-westward, and reaches the Rio Salado about 35 deg. 30' S. lat. Beyond Buena Esperanza it keeps on the prairie as far as the crossing of the Salado, which here marks the limit of the _monte_. The Rio Negro line passes directly from the prairie to the Patagonian scrub mid-way between Bahia Blanca and the Colorado. Within these limits the prairie extends without a break. The sierras of the Buenos Aires province have no arborescent vegetation.