ERUPTIVE SURFACES 125 



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. 1 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. 



On the eruptive flows of recent date the rock is naked. 

 The wind carries away the products of its decomposi- 

 tion, and the dust accumulates only in the fissures. 

 Traffic is difficult, sometimes impossible. 



Toward the west the tableland is separated from 

 the Cordillera by a longitudinal depression, though 

 the continuity of this has been exaggerated. This 

 depression, which outlines the contact between the 

 folded zone of the Andes and the flat zone of the table- 

 land, is very important from the point of view of 

 colonization. Just at the frontier of the steppe and 

 the forest, it is the most hospitable part of Patagonia, 

 the richest in natural resources. Amidst the glacial 

 lacustrine deposits which are accumulated on it there 

 rise masses of different kinds of rock which break it 

 up into compartments, granitic ridges of laccolites 

 exposed to view, eruptive structures that have been 

 dismantled. In the south the sub-Andean depression 

 forms a broad passage between Lake Maravilla and 

 Punta Arenas, about two hundred miles long, enclosed 

 between the basalt cliffs of the tableland on the east 

 and the mountains of the Brunswick Peninsula and 



1 G. Rovereto, " Studi di geomorfologia argentina : la valle del 

 Rio Negro," Bull. Soc. Geol. ItaL, xxxi. 1912, pp. 101-142 and 181-237 



