12 PHYSIOGNOMY OF PLANTS. 



The deserts to the south of the Atlas, and the immense 

 plains or steppes of South America, must be regarded as 

 only local phenomena. The latter, the South American 

 steppes, are clothed, in the rainy season at least, with grass, 

 and with low-growing almost herbaceous mimosas. The 

 African deserts are, indeed, at all seasons devoid of .vegeta- 

 tion ; seas of sand, surrounded by forest shores clothed with 

 perpetual verdure. A few scattered fan-palms alone recall 

 to the wanderer's recollection that these awful solitudes be- 

 long to the domain of the same animated terrestrial creation 

 which is elsewhere so rich and so varied. The fantastic 

 play of the mirage, occasioned by the effects of radiant heat, 

 sometimes causes these palm trees to appear divided from 

 the ground and hovering above its surface, and sometimes 

 shews their inverted image reflected in strata of air undu- 

 lating like the waves of the sea. On the west of the great 

 Peruvian chain of the Andes, on the coasts of the Pacific, I 

 have passed entire weeks in traversing similar deserts 

 destitute of water. 



The origin of extensive arid tracts destitute of plants, in 

 the midst of countries rich in luxuriant vegetation, is a 

 geognostical problem which has hitherto been but little 

 considered, but which has doubtless depended on ancient 

 revolutions of nature, such as inundations or great volcanic 

 changes. When once a region has lost the covering of 

 plants with which it was invested, if the sands are 

 loose and mobile and are destitute of springs, and if the 

 heated atmosphere, forming constantly ascending currents, 

 prevents precipitation taking place from clouds ( 9 ), thou- 



