RED FOGS AND SEA DUST. 99 



days, more or less. To account for this sort of periodical occur- 

 rence of the falls of this dust, Ehrenberg thinks it *' necessary to 

 suppose a dust-cloud to he held constantly swimming in the atmos- 

 phere by continuous currents of air, and lying in the region of the 

 trade-winds, hut suffering partial and periodical deviations''^ 



It has already been shown (^ 128) that the rain or calm belt 

 between the trades travels up and down the earth from north to 

 south, making the rainy season wherever it goes. The reason of 

 this will be explained in another place. 



159. This dust is probably taken up in the dry, and not in the 

 wet season ; instead, therefore, of its being " held in clouds suf- 

 fering partial and periodical deviations," as Ehrenberg suggests, 

 it more probably comes from one place about the vernal, and from 

 another about the autumnal equinox ; for places which have their 

 rainy season at one equinox have their dry season at the other. 



160. At the time of the vernal equinox, the valley of the Lower 

 Oronoco is then in its dry season — every thing is parched up with 

 the drought ; the pools are dry, and the marshes and plains arid 

 W'astes. All vegetation has ceased ; the great serpents and rep- 

 tiles have buried themselves for hibernation ;* the hum of insect 

 life is hushed, and the stillness of death reigns through the valley. 



Under these circumstances, the light breeze, raising dust from 

 lakes that are dried up, and lifting motes from the brown savan- 

 nas, will bear them aw^ay like clouds in the air. 



This is the period of the year when the surface of the earth in 

 this region, strewed w^ith impalpable and feather-light remains of 

 animal and vegetable organisms, is swept over by whirlwinds, 

 gales, and tornadoes of terrific force ; this is the period for the 

 general atmospheric disturbances which have made characteristic 

 the equinoxes. Do not these conditions appear sufficient to afford 

 the *' rain dust" for the spring showers ? 



161. At the period of the autumnal equinox, another portion of 

 the Amazonian basin is parched with drought, and liable to winds 

 that fill the air with dust, and with the remains of dead animal 

 and vegetable matter ; these impalpable organisms, whicli each 

 rainy season calls into being, to perish the succeeding season of 

 drought, are perhaps distended and made even lighter by the gases 

 of decomposition which has been going on in the period of drought. 



* Humboldt. 



