THE ATMOSPHERE AS A GEOLOGICAL AGENT. 23 



soon settled; but, caught by the currents of the upper atmosphere, 

 many of the finer particles were transported incredible distances. 

 Through all their long journey, the particles of dust were gradually 

 settling from the atmosphere, but not until the dust had traveled 

 repeatedly round the earth did its amount become so small as to cease 

 to make its influence felt in the historic red sunsets which it occasioned.^ 

 Some of this dust completed the circuit of the earth in 15 days. 



In various parts of Kansas and Nebraska^ there are very con- 

 siderable beds of volcanic dust, locally as much as 30 feet thick, which 

 must have been transported from volcanic vents by the wind, though 

 there are no known centers of volcanic action, past or present, within 

 some hundreds of miles of some of the localities where the dust occurs. 

 These beds of volcanic dust, so far from its source, may serve as an 

 illustration of the importance of atmospheric movements as a geo- 

 logical force. 



Volcanic dust is shot into the atmosphere rather than picked up 

 by it. Dust picked up by the wind is perhaps transported not less 

 widely than volcanic dust, but, after settling, its point of origin is less 

 readily determined. It would perhaps be an exaggeration to say 

 that every square mile of land surface contains particles of dust brought 

 to it by the wind from every other square mile, but such a statement 

 would probably involve much less exaggeration than might at first be 

 supposed. 



Examples of extensive deposits of dust other than volcanic are 

 also known. In China there is an extensive earthy formation, the 

 loess, sometimes reaching 1,000 feet in thickness, which von Richtofen 

 believes to have been deposited by the wind.^ This conclusion has, 

 however, not passed unchallenged.'' The loess of some other regions 

 has been referred to the same origin, and some of it is quite certainly 

 eohan.^ 



The transportation of dust is important wherever strong winds 

 blow over dry surfaces, free or nearly free of vegetation, and com- 

 posed of earthy matter. Its effects may be seen in such regions as 

 the sage-brush plains of western North America. The roots of the 



* A brief account of the influence of the dust on sunsets is found in Davis's Ele- 

 mentary Meteorology, pp. 85 and 119. 



' Science, New Ser., Vol. IV, p. 816, 1896. 

 3 Von Richtofen. ''China." 



* Sketcherley and Kingsmill. Quar. Jour. Geol. Soc, Vol. LI, 1895, pp. 238-254. 

 «Chamberlin. Jour, of Geol., Vol. V, p. 795. 



