Patagonia

PATAGONIA, a regional name applied to the southern part of South America south of approximately 40° S. lat. Physio graphically, it is divided into two longitudinal sections—the cor dilleras of the Andes (including the islands of the Chilean archi pelago) and the Patagonian table-land between the Andes and the Atlantic ocean. It is as a regional name for this table-land region that the term Patagonia is now most commonly used. The term, in this sense, is usually applied to that section of the Argen tine Republic between the Negro river and its tributary the Limay, on the north, and the Beagle channel, on the south, together with that section of southern Chile in the region of the Strait of Magellan which lies east of the Andes. Actually the Patagonian table-land extends considerably north of the Negro river and, in addition to the Argentine territories of Tierra del Fuego, Santa Cruz, Chubut and Rio Negro, includes the southern part of the territories of Neuquen and La Pampa. There is no sharp line of division, but the chief characteristics of the table-land (a surface cover of rounded pebbles or shingle, and volcanic materials in place of the clays of the pampa) appear a short distance north of the Colorado river.

General Description.

The general aspect of the table-land is that of vast steppe-like plains rising terrace-fashion from high coastal cliffs to the foot of the Andes; but their true aspect is by no means so simple as such a general description would imply. Along the Negro river they rise by a series of fairly level plains from 30o ft. at the coast to about 1,300 ft. at the junction of the Limay and Neuquen rivers and 3,000 ft. at the base of the Andes. South of the Negro river they are much more irregular. There volcanic eruptions have occurred down to fairly recent times.

Basaltic sheets, apparently only recently cooled, cover the table land east of Lake Buenos Aires and Lake Pueyrredon. On the Chico and Santa Cruz rivers they have spread to within about 5o m. of the coast and reach almost to the coast south of the Coyle and Gallegos rivers. Basaltic massifs like Anecon Grande east of Lake Nahuel Huapi and those along the railway south of Maquinchao, in the Somuncurra region west of the Gulf of San Antonio, and south of Lake Musters, are the salient features of the landscape. The coast consists in large part of high cliffs

separated from the sea by a narrow coastal plain.

Deep, wide valleys bordered by high cliffs cut the table-lands from west to east. All are the beds of former rivers which flowed from the Andes to the Atlantic, but only a few now carry per manent streams of Andean origin (the Colorado, Negro, Chubut, Senguerr, Chico, Santa Cruz). The majority either have inter mittent streams like the Chalia, Coyle and Gallegos, which have their sources east of the Andes, or, like the Deseado, are com pletely dry except for salt ponds in the deeper depressions and so altered by the combined effect of wind and sand as to afford little surface evidence of the rivers that once flowed in them. In the past many observers have classed with these true river valleys the numerous other long depressions without outlet, known as bajos, that are scattered over the table-land, but more recent observers have found no evidence that they were once river channels. A part of them at least are considered as caused by local faulting.

Wind erosion has undoubtedly contributed to their present form and it is due .to the aridity of the region that they have not been filled in by the normal processes of erosion. They serve an im portant purpose in the collection of the scanty surface water. Alluvial soils of considerable depth have been built up in them.

In the larger of them ranches are located. Along the main routes they are of great value as pasturing grounds for droves of cattle and sheep on their way to seaport or railhead. The shingle (rounded pebbles of granite and eruptive rock), although of glacial origin, is the product of the destruction of old moraines carried out over the table-land before the transverse valleys were cut and the circulation of water from the glaciers localized in them. Its concentration on the surface is due to the action of the wind in sorting the pebbles from the finer and more easily moved materials.