On the other hand, the bedding action of the wind creates deposits consisting of small and uniform elements from the sands of the dunes to the finest dust. The lightest particles, caught up repeatedly by the squalls and carried to a great height in the atmosphere, go beyond the Patagonian region and reach the bottom of the Atlantic or the plain of the Pampa. Some of this, however, is deposited in the depressions of the tableland, where the moisture fixes it and prevents the wind from regaining it. These aeolian deposits in the depressions, a dark-grey clay, which hardens when it is dry, but is softened by water, form two entirely different kinds of soil. If the depression is closed in, or if the circulation of the water is too slight, there is a concentration of the mineral salts; this is the _salitral_, either naked or sustaining a halophytic vegetation, which the saline efflorescences cover with a white coat during the dry season. If on the other hand, the underground waters have a free course, the aeolian clay forms the _mallin_. Bushes and fine grasses grow on it, and, as they decay, gradually give it a darker shade and modify its composition. The soil above the _mallin_ is rich in organic elements. It covers the bottom of the valleys between low terraces, covered with faceted pebbles, and dominated by the vertical cliffs of tufa and lava. The contrast between the verdure of the _mallin_ and the arid, dusty, yellow steppe of the tableland is one of the most characteristic features of Patagonian scenery. The area in which _mallin_ has been formed coincides with the most humid districts in the vicinity of the Andes and round the higher hills. On the road that runs along the right bank of the Limay, at some distance from the river, on the surface of the tableland, the limit between the country of the _salitrales_ and that of the _mallinas_ passes between Tricaco and Chasico, a hundred miles south-east of Neuquen; it almost tallies with the curve of a 200 millimetres rainfall.[52] Though the word _mallin_ is not used at Santa Cruz, similar aeolian soils are found in the western part of the tableland up to this latitude. Further south glacial deposits, clays with moraine-blocks, fill the valleys, and from Gallegos onward, cover the greater part of the tableland. [52] G. Rovereto, "Studi di geomorfologia argentina: la valle del Rio Negro," _Bull. Soc. Geol. Ital._, xxxi. 1912, pp. 101-142 and 181-237. On the eruptive flows of recent date the rock is naked. The wind carries away the products of its decomposition, and the dust accumulates only in the fissures. Traffic is difficult, sometimes impossible. * * * * *