Page:The Journal of geology (IA journalofgeology21894univers).pdf/339
| Average diameter of particles. |
Behavior of the particles when thrown into the air. |
|---|---|
| .75 mm. | Described a path diverging about 10° from a vertical line. |
| .37 mm. | Described a path diverging about 45° from a vertical line. |
| .18 mm. | Described a path diverging but a few degrees from a
horizontal line, were blown upward by eddies. |
| .08 mm. | Could scarcely be noticed to settle in transport. |
| .04 mm. | Apparently completely borne up by the wind. |
| .007 mm. | Completely borne up by the wind. |
| .001 mm. | Completely borne up by the wind. |
It is hardly necessary to add that the average size of the largest particles carried varies greatly with the velocity of the wind. Sand grains will occasionally be found to have been thus carried, which have a diameter many times larger than the average maximum here stated. The presence of such large grains can readily be accounted for by the chances for becoming entangled in specifically lighter objects, such as fragments of leaves and other vegetation, and thus to be carried by them. It will be understood, also, that the statement made above does not apply to that phase of wind-transportation which takes place on the surface of a sand-dune, where the sand is as if rolled forwards, nor to that in the very lowest part of the atmosphere generally, where materials are thrown forwards short distances at a time by eddies due to the contact of the atmosphere with the more or less irregular surface of the land.
The capacity of the atmosphere for transporting particles of quartz below the size of .1 mm. in diameter, is very great.
Disregarding the occasional transference of matter by volcanic forces and by living organisms, there are only three principal agents known to be at work removing materials from place to place on the surface of the globe. These are water, ice, and air. It is believed that, with the above limitation as to the fineness of the material, the transporting power of the atmosphere, as compared with that of water and ice, is very great. The transporting capacity of the water in our continental rivers is better known than that of glaciers or of ice fields, and it makes our best