[16] This series, stretching from the Permian to the Tertiary, also includes, especially in the region of the sub-Andean chains, on the fringe of the Chaco, a number of marine strata (see Bonarelli, _Las sierras subandinas del Alto y Aguarague y los yacimientos petroliferos del distrito minero de Tartagal_ "Ann. Min. Agric.," Seccion Geologia, Mineralogia, y Mineria, viii. No. 4: Buenos Aires, 1913). [17] G. Bodenbender, _Parte meridional de la Provincia de la Rioja y regiones limitrofes_ (Ann. Min. Agric., Seccion Geol., Minerol., y Mineria, vii. No. 3: Buenos Aires, 1912). Hence the _valles_ are by no means wholly productive. The oases represent only a limited portion of them. It would be impossible to imagine a more striking contrast than that of the freshness and life of the oases compared with the surrounding desert. Screens of poplars shelter them from the _zonda_. The water runs along trenches paved with round pebbles under the spreading vines, at the foot of which, to economize water and space, lucerne is sown. Each garden feeds a family. Near the raw-brick houses there are large earthenware vessels, as tall as a man, in which the corn is kept. The hammering of the cooper fills the air. In places the oasis is watered by a stream. In those cases there is on each side of the bed of the stream a narrow fringe, a continuous ribbon, of smiling gardens, which hide the path. Above and below Santa Maria a trench is opened every mile in the wet sands of the Rio. The water rises in it and fills it, and is directed by it toward one of the banks, where it is jealously collected and distributed. The water which flows from the irrigated fields and returns to the river, as well as that which the porous side of the trench has permitted to escape, goes to fill another trench and supply other fields farther on. The region of Los Sauces, in the northern part of the province of La Rioja, to the south of Tinogasta, shows a different type of irrigated cultivation, on account of the sandy course of the stream. The fields follow the feeding artery for about fifty miles. It is bled at the beginning of each bend, the waters remaining underground like hidden wealth.