332 THE EEGOLITH 



which owe their origin and present structural features mainly 

 to wind action, though, as is made apparent in the discussion of 

 the loess and adobe, sharp lines cannot in all cases be drawn 

 between such and those of alluvial origin. 



The efficacy of the wind as an agent of transportation was 

 dwelt upon in considerable detail on pp. 163 and 280. The 

 material thus carried into the air, often to great heights, is 

 brought to the surface again by gravity, though the normal rate 

 of descent is frequently greatly accelerated by rain or snow. 

 Indeed, the clearness, limpidity, of the atmosphere after a rain- 

 fall is due simply to the fact that it has been washed, is cleansed 

 of its suspended impurities. 



The amount of dust brought down even from moderately clear 

 atmospheres is often sufficiently abundant to attract the at- 

 tention of the most casual observer. Professor H. L. Buner of 

 Irvington, Indiana, has stated^ that during a storm in Febru- 

 ary, 1895, a layer of snow about one-fourth of an inch in thick- 

 ness was colored distinctly brown by the dust it contained. 

 One sample of snow collected yielded .37% of dust, by weight, 

 and it was calculated that the material was thus deposited at the 

 rate of 30.7 pounds avoirdupois for each acre. Another observer 

 calculated the fall as taking place at the rate of 12.77 pounds 

 per acre. 



From a gallon of water melted from a snowfall of but 4 

 inches, which fell in London in January, 1895, there was obtained 

 10.65 grains of solid matter, 5.75 grains being inorganic and 

 4.90 grains carbonaceous. Water from a snow collected near 

 the centre of the city, January 30 of this same year, gave 6.25 

 grains of mineral and 11,07 grains of carbonaceous matter. It 

 was also found that 75% of these impurities were brought down 

 with the first 2 inches of the snowfall. 



Dr. Whitney examined samples of the black earth brought 

 down near Eockville, Indiana, during a snowfall of the win- 

 ter of 1895 and reported it as consisting of material almost 

 identical with the prevailing loess of that region, from whence 

 it was doubtless derived. The individual particles varied in size 

 between .10 and .05 millimetre. The results of a mechanical 

 analysis of the dust are tabulated with those of loess on p. 319. 

 Samples of the same dust submitted to microscopic examination 



1 Monthly Weather Review, tJ. S. Dept. of Agneultiire, January, 1895. 



