Patagonia

The line of contact between the Patagonian table-land and the folds of the Andes is marked by a line of lakes. From Lake Alumine northward these lakes are not of glacial origin but have been dammed up by lava flows. South of Lake Alumine they are long lakes of Alpine form that reach back fiord-like into the cor dillera. Their beds have been deepened by glacial action and in most cases are continued across the table-land by the broad transverse valleys described above, through which all the lakes at some former time drained to the Atlantic. From Lake Nahuel Huapi northward the lakes, except for Lake Lacar, still drain to the Atlantic. South of Lake Nahuel Huapi all of the lakes Viedma and Argentino now drain to the Pacific through deep canyons cut across the cordillera; and the interoceanic divide follows the terminal moraines of the old glaciers to the east of the present lake beds, leaving the majority of the transverse valleys without streams. Lakes Viedma and Argentine, which drain to the Atlantic by the Santa Cruz river, lie opposite that part of the cordillera which is still effectively covered by inland ice. The lakes from Nahuel Huapi southward are frequently de scribed as lying wholly or in part in a longitudinal depression that separates the table-land from the cordillera. This de p:ession, however, is by no means continuous, and, in many places, the table-land butts directly against the cordillera.

In the Argentine Territory of Neuquen the Andes lower rapidly toward the south, Cerro Domuyo east of the upper basin of the Neuquen river being the last peak that exceeds 12,000 feet. The cordillera there consists of a broad zone of mountain chains and narrow valleys. The crests that have a fairly uniform elevation have been carved by erosion from the older Andean rocks. Above them rise isolated volcanic peaks. From Lake Buenos Aires south ward the Andes are known in detail only on their eastern border. Two great fields of inland ice fill all the central part of the cordillera from about 46° S. lat. to 51° S. lat., separated only at the 48th parallel by the Baker canal where the Baker river, drain ing Lake Buenos Aires from the north, and the Pascua river, draining Lake San Martin from the south, break through the cordillera. From these ice-fields great glaciers flow down to the lake region on the eastern border of the cordillera and to the fiords of the western border. At the time of writing this article these ice-fields have never been crossed, although the southernmost field has recently been penetrated for some distance by expeditions working in from Lake Argentino and Lake Viedma. On the

Chilean side the coast and the fiords that thrust far into the cor dillera have been examined and mapped by British, Chilean and other hydrographic surveys and the snouts of numerous glaciers that flow down to the fiord heads have been noted.

Geology.

The deep sedimentary cover of the table-land con tains a Triassic Series, an Upper Cretaceous and a Tertiary, through which outcrops in places (particularly along the Atlantic coast in the territory of Rio Negro) the core of ancient granites and porphyries. North of the basaltic massif of Aliecon Grande the granites rise to over 5,000 feet. On the lower Negro river, Tertiary tufas and grey sandstones form the cliffs that border the river valley, while at the foot of the Andes the table-land is of variegated marls and red sandstones of Cretaceous age. Here and there throughout the whole table-land are evidences of an extensive peneplain which dates from the end of the Pliocene. This peneplain has not been base-levelled uniformly throughout but is warped and broken by local faulting. The basalt flows and intrusions that are the characteristic features of so many sections of the table-land have been mentioned.

The central part of the Andes of Patagonia consists of a granite core which thrusts through the folded sedimentary cover to form the crests of the cordilleras. Recent volcanic rocks (andesites and basalts) cover the sedimentaries and the granitic core over large areas throughout this section of the Andes and have spread far out over the eastern table-land. In the north the andesites are of Miocene age and covered by Pliocene basalts. Farther south the andesite intrusions continued well into and probably through the Pliocene. The folded sedimentaries do not reach across to the western slopes of the Andes north of 52° S. lat. Fiords and valleys, longitudinal and transverse, some of which cut across the cordillera and drain the lakes of the eastern side to the Pacific, divide the Patagonian Andes into blocks that have their lowest average altitude (about 6,50o ft.) in the north and in Tierra del Fuego and rise 'to 13,000 ft. between 46° and 50° S. lat. A line of active volcanoes stands along the Chilean shore in the north completely separated from the main ranges of the cordillera, and isolated cones, all of which are of contemporary origin, while some are of recent eruption, form the outstanding peaks throughout the Patagonian Andes. (For geological history of Patagonia see