Peru

Between sierra and montana lies a second transitional zone, ceja de la montana (eye-brow of the montana). The contrast between sierra and montana on the east, in rock structure, topog raphy, climate and vegetation is one of the most abrupt and spec tacular on earth. The sierra is dry and cold. The montana is covered with tropical forest, impenetrable except for rivers and a few short trails to gold or rubber camps.

Geology.

The conception of the structural geology of the Andes has been revolutionized during the past few years. Instead of the notion of three separate, structurally unlike chains, Orien tal, Central and Occidental, of which the eastern was the oldest, the Andes of Peru are now considered a single broad plateau, a peneplain uplifted to 13,000-15,000 ft., whose original character has been largely destroyed by erosion. Deep canyons cut by streams tributary to the Amazon divide the plateau in a general way into three longitudinal strips, designated in the past as Eastern, Central and Western Cordilleras, and still popularly so called. Crest-lines and major valleys trend with the axes of the mountain folds, showing that the streams follow broader structural depressions and that residual mountain masses survived be cause of greater resistance. Isolated volcanic peaks in southern Peru are not survivals, but are superimposed on the plateau sur face. Erosion of these peaks has played an important part in levelling mountain basins, many of which are the result of drop faulting accompanying the general elevation. The Cordillera Ori ental has a central core of granite and gneiss, flanked by shales, slates and schists, with infolded limestones and sandstones. The surface of the plateau, where not covered by volcanic extrusives or alluvium, is sedimentary, largely limestone and sandstone deposits dipping west. The same formations in the Cordillera Occidental have been so deformed and metamorphosed by igneous intrusions, andesite, porphyry and diorite, as to be almost unrecognizable. In places they are covered by volcanic lavas and ash, and are much dissected by entrenched streams. The coast area consists of sedi mentaries and late volcanic products; most traces of sub-strata are obscured except in river valleys. The maturely dissected coast range between 14° and 16° S. is composed of granitic rock

of uncertain age. Other outcrops exist at frequent intervals along the coast. As the zone has been recently uplifted, the sea cliffs are largely composed of stratified conglomerate, sands and clays, with broken shells and detrital material from igneous rocks interbedded with sand. The eastern lowlands do not resemble the structure of adjacent mountains, being unlike the western low lands in this respect. The foot-hills are composed of upturned beds of sandstone rising above the level of the plain. Surface conglomerates are somewhat dissected and covered by present river gravels (see ANDES).

Coast Zone.

The coast zone varies in width and topography. At the north it is a plain more than ioo m. wide, gently sloping toward the sea, where it terminates in a bluff. At the south it is a more elevated plain, separated from the coast by a range of low hills, differing in topography and climate from the rest of the coast. Here fog prevails much of the year, causing intermittent vegetation, enough at times for pasture. Though excellent crops, such as olives and grapes, are raised in the deeply entrenched valleys, comparatively little is exported, as there are few ports.

Mollendo, the largest, serves only the sierra. Valleys north of Lima are given over to industrial crops, mostly sugar and cotton. They monopolize the water, have eliminated food crops and re stricted pasture. Dispossessed Indians take refuge in the valley heads. Valleys near Lima are intensively cultivated in order to supply the capital with food ; its white towers are surrounded by market-gardens and plains of alfalfa with grazing cattle. The coastline is remarkably straight, usually steep, with occasional stretches of beach and but few real harbours; those which are most sheltered are too shallow or too far from productive areas. A thick mantle of wind-blown sand covers the desert surface, forming in some localities crescent-shaped sand-dunes (medanos). The islets are barren and rocky. From north to south the more important are Lobos de Tierra and Lobos de Afuera ; San Lorenzo (1,05o ft. high, 1 by 41 m. in extent), which protects the harbour of Callao, and near Pisco bay rugged San Gallan and the three Chincha islands, famous for their guano.