PATAGONIAN SCENERY 123 



and the hajos of Gualicho and Valcheta have wrongly 

 been regarded as the former bed of the Rio Negro 

 and the Limay. Erosion by wind seems to have 

 had something to do with these depressions. Their 

 persistence, at all events, is one of the effects of the 

 aridity which prevents normal erosion from moulding 

 the surface of the tableland. The chief of them are 

 centres for collecting running water. There is a group 

 of valleys all round them, and alluvial beds accumulate 

 in them. 



The climate determines the character of the soil 

 in Patagonia. The rounded pebbles of granite and 

 eruptive rock, so often described since the time of 

 Darwin, sometimes free and sometimes embedded in 

 red sand or limestone, ^ are spread over the tableland 

 like aureoles round the masses of rock, and they are 

 particularly abundant in the coast region. On the 

 Rio Negro they seem to be confined to the vicinity of 

 the valley ; they disappear as one goes away from it. 

 The progressive reduction in the volume of the Rio 

 Negro gravels, as one goes downward, has been observed 

 to begin in the Andean zone, and it is from the Andes 

 that they come. South of Santa Cruz, in a moister 

 climate, in which the circulation of the water is less 

 localized, the bed is more continuous, and it covers 

 the Tertiary sandstones and clays. It is of fluvio- 

 glacial origin, and comes from the destruction of the 

 old moraines, before the excavation of the actual valleys. 

 But it is the wind that explains the concentration 

 of the gravel at the surface. It separates the pebbles 

 from the more mobile material about them. Wherever 

 the outcrop-strata contain pebbles, the wind eventu- 

 ally converts the place into a field of shingle. It has 



' The calcareous flag-stone of La Tosca, which is characteristic of 

 the south-west province of the plain of the Pampa, stretches in the 

 south as far as the Rio Negro in the coast-district. On the other 

 hand, it is almost entirely abseiit a hundred miles to the west, between 

 the Colorado and the Rio Negro, along the line of the railway from 

 Fortin Uno to Choele Choel. 



