GEOLOGY. 663 



But these healthful stations, scattered over the sand, to use 

 the expression of Ptolemy, like the black spots on the yel- 

 low hide of the panther, are sometimes large spaces, abun- 

 dantly supplied with springs and sheltered by a vigorous 

 vegetation. In the Sahara there are even some which form 

 small but populous kingdoms, and which the caravans take 

 several days to cross. 



When reviewing the steppes, those living deserts, other 

 pictures unfold themselves to our gaze. In them we see 



246. The Great Desert of Korosko. Tremaux, Soudan Oriental. 



diversity of vegetation sharply defined, so that we might 

 fancy each zone had at first its own special sheet of ver- 

 dure. As Humboldt says, " The history of the vegetable 

 envelope of our planet, and of its gradual propagation over 

 the naked surface of the earth, has its epochs, like the 

 most ancient history of the human race." 



In some places we find steppes which display only an 

 attempt at vegetable life ; extending over immense spaces, 

 and losing themselves in a boundless horizon, they open out 

 before the eye like the ocean, but without offering the 



