142 THE PHYSICAL GEOGRAPHY OF THE SEA. 



dust from lakes that are dried up, and lifting motes from tlie 

 brown savannas, 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 af- 

 ford the "rain dust" for the spring showers? 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. May not, 

 therefore, the whirlwinds which accompany the vernal equinox, 

 and sweep over the lifeless plains of the Lower Orinoco, take up 

 the ''rain dust" which descends in the northern hemisphere in 

 April and May? and may it not be the atmospherical disturb- 

 ances which accompany the autumnal equinox that take up the 

 microscopic organisms from the Upper Orinoco and the great 

 Amazonian basin for the showers of October ? 



326. The Baron von Humboldt, in his Aspects of Nature^ thus 

 Humboldt's descrip- coutrasts the wct and the dry seasons there: 



tion of the dust-whirl- ,, ^,,., , , . , r« i 



winds of the Orinoco. "When, uudcr the vertical rays oi the never- 

 clouded sun, the carbonized turfy covering falls into dust, the in- 

 durated soil cracks asunder as if from the shock of an earthquake. 

 If at such times two opposing currents of air, whose conflict pro- 

 duces a rotary motion, come in contact with the soil, the plain as- 

 sumes a strange and singular aspect. Like conical-shaped clouds, 

 the points of which descend to the earth, the sand rises through 

 the rarefied air on the electrically-charged centre of the whirling 

 current, resembling the loud water-spout, dreaded by the experi- 

 enced mariner. The lowering sky sheds a dim, almost straw-col- 

 ored light on the desolate plain. The horizon draws suddenly 

 nearer, the steppe seems to contract, and with it the heart of the 

 wanderer. The hot, dusty particles which fill the air increase its 

 suffocating heat, and the east wind, blowing over the long-heated 



