36 STEPPES AND DESERTS. 



forests both to a very restricted field of view, and to the 

 aspect of a rich and highly luxuriant vegetation. In- 

 effaceable is the impression which I received on our return 

 from the Upper Orinoco, when, from the Hato del Capuchino, 

 on a mountain opposite to the mouth of the Rio Apure, we 

 first saw again the distant Steppe. The sun had just set ; 

 the Steppe appeared to rise like a hemisphere ; and the light 

 of the rising stars was refracted in the lowest stratum of air, 

 The excessive heating of the plain by the vertical rays of 

 the sun causes the variations of refraction, occasioned by 

 the effects of radiation, of the ascending current, and of the 

 contact of strata of air of unequal density, to continue 

 through the entire night. 



( 4 ) P- 2- " The naked stony crust." 

 Immense tracts of flat bare rock form peculiar and 

 characteristic features in the Deserts both of Africa and 

 Asia. In the Schamo, which separates Mongolia and the 

 mountain chains of Ulangom and Malakha-Oola from the 

 north-west part of China, these banks of rock are called 

 Tsy. They are also found in the forest-covered plains of 

 the Orinoco, surrounded by the most luxuriant vegetation 

 (Eelation Hist. t. ii. p. 279). In the middle of these flat 

 tabular masses of granite and syenite of some thousand feet 

 diameter, denuded of all vegetation save a few scantily dis- 

 tributed lichens, we find small islands of soil, covered with 

 low and always flowering plants which give them the ap- 

 pearance of little gardens. The monks of the Upper Orinoco 

 regard these bare and perfectly level surfaces of rock, when 



