THE AFRICAN ISLANDS 205 



the history of agriculture : a number of products of the 

 Old World, such as the sugar-cane and the banana, only 

 became American after a stage of acclimatization in 

 those islands. The native populations, now wiped out, 

 were mainly agricultural, and reached a fairly high 

 standard of civilization. 



Madeira, with very much the same plant- life, enjoys 

 a moister climate, while, except for a few fertile spots, 

 the Cape Verde Islands remain huge naked blocks of 

 lava. 



Sahara. Three main aspects have been distinguished 

 in the Great Desert. They are produced not by climatic 

 differences but by the nature of the surface. 



The Hamada, or rocky desert, is due to the cracking 

 and splitting of the rocks by alternate expansions and 

 contractions brought about by abrupt changes of tem- 

 perature. It occurs as bare hills, as tablelands intersected 

 by waterless river-beds, as undulating expanses and 

 broken outcrops, or as isolated blocks, vestiges of the prim- 

 itive surface, rising above the accumulated detritus. Low 

 bushes are sparsely scattered on the bare floor, anchoring 

 their roots in the fissures and cracks. Despite the scarcity 

 of the vegetation, it is surprising to find in the hamada 

 a large variety of species, among which are the alfa, the 

 white artemisia, and several sage-like shrubs, or again 

 various loose and leafless undershrubs. When the waste 

 rocks of the ruined hamada are worn down to pebbles 

 and gravel, or where the surface is clayey and stony, the 

 desert is known as ' reg ' or ' areg '. This gives the im- 

 pression of a flat, boundless, almost plantless waste of 

 pebbles, where sheep and camels can find practically no 

 food. The immense barren is dotted with what at first 

 sight appear like rounded, low, whitish boulders, but, 

 on inspection, turn out to be extremely dense tussocks 



