122 PATAGONIA AND SHEEP-REARING 



has been almost entirely effaced. Volcanic eruptions 

 have occurred until quite recent times, and so eruptive 

 areas are the salient features of the tableland, at 

 Afiecon and at Somuncurra, south of the district of 

 the Rio Negro, in the ridge on the left bank of the 

 middle Senguerr, in the Chubut province. The basalts 

 have spread out in sheets, the surface of which seems 

 to have cooled not long ago. Basalt flows are found 

 as far as northern Patagonia, south of Valcheta and 

 Maquinchao ; but their chief seat is in eastern 

 Patagonia. They cover the inhospitable tablelands 

 to the east of Lakes Buenos Aires and Pueyrredon. 

 The Rio Chico and the Santa Cruz cross them for the 

 upper two-thirds of their course, South of Coile and 

 Gallegos they spread almost to the coast, and the 

 Tertiary Pampas in this part are dominated by an 

 archipelago of small volcanic cones. 



The tableland is crossed from west to east by deep 

 and broad valleys, enclosed between high cliffs, often 

 strangled by ridges of basaltic or crystalline rock, 

 and very little ramified. The ravines (canadones), which 

 make breaches in their cliffs on both sides, go only a 

 little way into the sandstone Pampa or the lava table- 

 land. Only a certain number of these valleys are 

 occupied by important rivers (the Rio Negro and the 

 Santa Cruz, for instance) which are born in the Andes, 

 but receive little addition from the light rains of eastern 

 Patagonia. Most of the valleys have only intermittent 

 streams (Sheuen, Coile) or are altogether dry and sown 

 with salt lakes (Deseado). The west wind is now the 

 ruler of this network of fossil valleys. It carves their 

 slopes, and brings into them sand, with which it makes 

 dunes. 



We must not confuse with these dead valleys the 

 long depressions, with no outlet, which are scattered 

 over the granite and sandstone tableland (bajos, valles, 

 cuencas). Some have obstinately, but wrongly, sought 

 in these the traces of rivers that have disappeared ; 



