CAUSES OF LABGE PLAINS. 97 



un red sandstone, or on compact limestone and gypsum ; it 

 varies according as periodical inundations accumulate mud 

 on the lower grounds, or as the shock of the waters carries 

 away from the small elevations the little soil that has covered 

 them. Many solitary cultivated spots already exist in the 

 midst of the pastures, where running water, and tufts of 

 the mauritia palm, have been found. These farms, sown 

 with maize, and planted with cassava, will multiply con- 

 siderably if trees and shrubs be augmented. 



The aridity and excessive heat of the mesas do not depend 

 solely on the nature of their surface, and the local reverbera- 

 tion of the soil ; their climate is modified by the adjacent 

 regions ; by the whole of the Llano of which they form apart. 

 In the deserts of Africa, or Arabia, in the Llanos of South 

 America, in the vast heaths extending from the extremity ot 

 Jutland to the mouth of the Scheldt, the stability of the 

 limits of the desert, the savannahs, and the downs, depends 

 chiefly on their immense extent, and the nakedness these 

 plains have acquired from some revolution destructive of the 

 ancient vegetation of our planet. By their extent, their 

 continuity, and their mass, they oppose the inroads of culti- 

 vation, and preserve, like inland gulfs, the stability of their 

 boundaries. I will not enter upon the great question, whe- 

 ther in the Sahara, that Mediterranean of moving sands, the 

 germs of organic life are increased in our days. In propor- 

 tion as our geographical knowledge has extended, we have 

 discovered in the eastern part of the desert islets of verdure, 

 oases covered with date-trees, crowd together in more nume- 

 rous archipelagos, and open their ports to the caravans ; but 

 we are ignorant whether the form of the oases have not 



:ined constantly the same since the time of Herodotus. 

 Our annals are too incomplete to enable us to follow Na- 

 ture in her slow and gradual progress. From these spaces 



Ay bare, whence some violent catastrophe has swept 

 away the vegetable covering and the mould ; from those de- 

 of Syria and Africa, which, by their petrified wood, 

 attest the changes they have undergone ; let us turn to the 

 BMs-covered Llanos and to the consideration of phenomena 

 that come nearer the circle of our daily observations. Re- 

 specting the possibility of a more general cultivation of the 

 tcppes of America, the colonists, settled there, concur in the 



VOL. III. 11 



