608 Winchell and Miller— 



From all these lines of evidence it is believed that the 

 dust came from an arid region of the southwestern part 

 of this country, where siliceous feldspathic rocks are 

 abundant. Such areas are common in New Mexico and 

 Arizona. It is conjectured that the material was whirled 

 up from the surface on March 8 in the afternoon when 

 the convectional currents are most effective both in caus- 

 ing rapid vertical movements, and in increasing the 

 velocity of the surface air by mixture with the faster mov- 

 ing upper air. During vertical ascent the horizontal 

 component of velocity gradually increased, and the direc- 

 tion gradually veered, as shown by the dotted curve D 

 in fig. 3, until it coincided with the line of gradient veloc- 

 ity indicated by the continuous lines A and B in fig. 3. 

 The dust-bearing current then whirled around the storm 

 center, in contra-clockwise direction until it arrived at 

 the flank of the colder current flowing in from the east 

 over the Great Lakes and the St. Lawrence valley. The 

 warmer and lighter air from the southwest then rose over 

 the colder and denser air from the east, and the precipi- 

 tation of the moisture upon the dust particles as nuclei 

 came about through the mechanical cooling of the ascend- 

 ing air. The precipitated moisture was in the form of 

 rain at first, but froze to sleet as it fell through the cold 

 lower stratum. Higher ascent cooled the rising air below 

 the freezing point, and then the snow formed that fell 

 with the sleet formed lower down. The pure white snow 

 fell in the northwest winds, following the storm, and these 

 probably came from the snow-covered land to the north 

 or east. 



Conclusion. — The evidence here presented that a single 

 storm may transport a million tons of rock material a 

 thousand miles or more, emphasizes the importance of 

 the wind as a geological agent. "Water transports larger 

 rock fragments, and its work is readily seen on every 

 hand ; air transports much finer material and its work is 

 only rarely noticed at all; yet the air is constantly at 

 work over a much larger surface than that covered by 

 running water, and it is an open question whether the 

 total work done by the air in transporting rock material 

 is not of the same order of magnitude as the work of the 

 same kind accomplished by water. 



It is clear that arid regions will constantly lose rock 

 material by wind action and that the dust will be held by 



