middle of the summer the flocks collect round the permanent springs, and they scatter once more over the mountain pastures during the autumn rains, before they return to the plain. The second region is that of Valcheta. From Aguada Cecilia to Corral Chico the railway follows for sixty miles the edge of the outpour of lava from the south, which covers the Tertiary clays. In front of the basalt cliff the land dips in the north toward a closed depression, the Bajo de Valcheta, the bottom of which consists of clays impregnated with salt. Tertiary marine strata surround this hollow in the west and north, where they divide it from the Bajo del Gualicho, but here they form only a thin skin which covers the crystalline platform. The line of contact of the basalt and the Tertiary marls is marked by a series of good springs, and these give rise to permanent streams, such as the Arroyo Valcheta and the Nahuel Niyeu. At first they flow in a narrow valley crowned by basalts, with peaty prairies at the bottom, then over Tertiary marls, and, in the latitude of the railways, they pass into a gorge cut through the granites before losing themselves to the north in the _salitral_. A small agricultural oasis is sustained by the waters of the Valcheta. The site of Valcheta has an exceptional importance in the story of Patagonian colonization. It marks a necessary stage in the Indian track from the Atlantic to Nahuel Huapi, which is now followed by the line of the railway. Musters halted there. The track from Choele Choel, on the Rio Negro, to the southern coast and the Santa Cruz also passed by there. It was so much used, says Ezcurra, that the hoofs of the horses had hollowed it.[72] The Argentine village dates from 1890. At first it lived by supplying fodder to the convoys of wagons which carried the wool. The railway has suppressed this traffic, and the only outlet of the oasis to-day is the small port of San Antonio, where the wool is shipped, and where the district is unsuitable for any kind of cultivation. [72] Pedro Ezcurra, "Camino indio entre los rios Negro y Chubut: la travesia de Valcheta," _Bol. Instit. Geog. Argent._, xix. 1898, pp. 134-38. Like the coast region, the Valcheta district seems marked out by its moderate altitude to serve as winter pasture. In point of fact, it is used during the whole year. The springs do not dry up in summer. The streams which flow from the south toward the Bajo de Valcheta are permanent. In addition, a few wells have been bored in the Tertiary strata. Contrary to experience on the coast, therefore, cattle can be kept here during the summer. There is less chance for the grasses to reproduce, and the pasture tends to become impoverished.