266 FIRST FORMS OF VEGETATION. 
dry season in the lifeless plains of the Orinoco, 
and the great Amazonian basin, when the soul is 
parched and triturated by the intense, long-con- 
tinued drought, dense clouds of diatomaceous dust 
are raised by the winds and wafted to great dis- 
tances. These showers happen most frequently 
in spring and autumn after the equinoxes, but at 
intervals varying from thirty to fifty days. From 
the nature of the species wafted by these winds, 
the region which originally produced them can be 
ascertained with tolerable accuracy; and hence 
they afford a clue to those mysteriously wayward 
aérial currents, and cyclical relations in the upper 
and lower atmosphere, which have hitherto per- 
plexed meteorologists. It has been observed that 
these storms in certain districts, amply compensate 
for the annoyance they occasion. The soil of the 
countries most subject to the visitation, when of a 
compact character, is loosened and lightened by 
the dust, and at the same time the lighter fertiliz- 
ing matters carried away by the great rivers are 
replaced by organic remains, so that an abundant 
harvest follows the devastations committed by 
these dust showers, Nearer home these curious 
meteoric phenomena have occasionally been ob- 
served. Black rain, composed of: portions of de- 
cayed plants, mixed with the skeletons of diatoms, 
fell in Ireland in April 1849, over a district of 700 
