snow-season (May-August) is the dry season, and the snowfalls are not heavy enough to interfere with breeding. [50] This anomaly is doubtless due to the proximity of the sea and the respite of the westerly winds in winter. The coast, with its cold waters and the land-winds causing the deeper water to rise, has a special climate of fogs and mists. These, which remind us of the _garuas_ of the coast of Peru, do not penetrate into the interior. The surface of the Patagonian tableland is very uneven, though it bears traces of having been much worn by the agencies of its desert climate, which seems to have lasted through the whole Tertiary Era. Going up the Rio Negro, one sees the grey sandstones and Tertiary tufas which form the cliffs, on both sides of the lower valley. They give place higher up to the variegated marls and red sandstones of the Cretaceous which form the tableland at the foot of the first Andean chains. The core of ancient granites and porphyries crops up at places from under the mantle of Cretaceous and Tertiary sandstones. The horizon of the peneplain passes from the Tertiary and Cretaceous tableland to level masses of crystalline rock, the contour of which has been almost entirely effaced. Volcanic eruptions have occurred until quite recent times, and so eruptive areas are the salient features of the tableland, at Anecon and at Somuncurra, south of the district of the Rio Negro, in the ridge on the left bank of the middle Senguerr, in the Chubut province. The basalts have spread out in sheets, the surface of which seems to have cooled not long ago. Basalt flows are found as far as northern Patagonia, south of Valcheta and Maquinchao; but their chief seat is in eastern Patagonia. They cover the inhospitable tablelands to the east of Lakes Buenos Aires and Pueyrredon. The Rio Chico and the Santa Cruz cross them for the upper two-thirds of their course, South of Coile and Gallegos they spread almost to the coast, and the Tertiary Pampas in this part are dominated by an archipelago of small volcanic cones. The tableland is crossed from west to east by deep and broad valleys, enclosed between high cliffs, often strangled by ridges of basaltic or crystalline rock, and very little ramified. The ravines (_canadones_), which make breaches in their cliffs on both sides, go only a little way into the sandstone Pampa or the lava tableland. Only a certain number of these valleys are occupied by important rivers (the Rio Negro and the Santa Cruz, for instance) which are born in the Andes, but receive little addition from the light rains of eastern Patagonia. Most of the valleys have only intermittent streams (Sheuen, Coile) or are altogether dry and sown with salt lakes (Deseado). The west wind is now the ruler of this network of fossil valleys. It carves their slopes, and brings into them sand, with which it makes dunes.