63 



as it rests on red sandstone, or on compact lime- 

 stone and gypsum ; it varies according as peri- 

 odical inundations have accumulated mud on 

 the lower grounds, or as the shock of the waters 

 has carried away from the small elevations the 

 little soil that 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 considerably, if an increase of the 

 trees and shrubs be effected. 



The aridity and the excessive heat of the 

 mesas* do not depend solely on the state of 

 their surface, and the local reverberation of the 

 soil ; their climate is modified by the adjacent 

 regions, by the whole steppe of which they 

 form a part. In the deserts of Africa or Arabia, 

 in the llanos of South America, in the vast 

 heaths that reach from the extremity of Jutland 

 to the mouth of the Scheldt, the stability of 

 the limits of the desert, the savannahs, and the 

 downs, depends for the most part on their im- 

 mense extent, and the nakedness these plains 

 have acquired from some revolution destruc- 

 tive of the ancient vegetation of our planet. 

 By their extent, their continuity, and their mass, 



* Little table-lands, banks, parts more elevated than the 

 rest of the steppe. 



