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, but 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 h3^^e their dry season at the other. 
160. At the time of the vernal equinox, the valley of the Lower 
Oronoco i's then in its dry season — every thing is parched up with 
the drought ; the pools are dry, and the marshes and plains arid 
wastes. All vegetation has ceased ; the great serpents and rep- 
tiles have burie^i themselves for hibernation ;* the hum of insect 
life is hushe(3, 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 away like clouds in the air. 
This is the period of the year when the surface of the earth in 
this region, strewed with 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 ? 
1.61. 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, which 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. 
