1890. ] . Erosive Agents in the Arid Regions. 457 
days in succession strong winds blow clouds ‘of dust along the 
mesa, and these are often so dense that the Oregon Mountains, 
ordinarily plainly visible, are completely obscured from view. On 
the upper Rio Grande, at Embudo, in March, 1889, clouds of 
dust not uncommonly swept up the cañon, obscuring the high 
canyon walls from view, although they were only a mile distant. 
In February of the same year I encountered a dust storm in the 
pass among the White Mountains, near Fort Stanton, New 
Mexico, which lasted for several hours, The wind was blowing 
at a velocity of sixty miles an hour, and not only sand but small 
bits of gravel were blown with such force as to produce a painful 
blow onthe face. The road could not be seen for a distance of 
twenty feet. Fine particles of dust penetrated to the works of 
my watch and caused it to stop. 
Another common phenomenon of aerial erosion is the forma- 
tion of small whirlwinds, to be seen on all sides on the plateau 
during the summer time. These sometimes gather force enough 
to carry away small bushes. The creeping action of blowing 
sand I have often had forcibly illustrated, when during a windy 
night the sand has blown upon my blankets, and formed a thin 
coating of sand. 
The direct effect of change in temperature must be an import- 
ant geological agent. On the plains the temperature of the sand 
is often 115° during mid-day, while in the early morning it may 
be as low as 60°. During the winter a black bulb thermometer 
registered 105° at one o'clock, while the minimum temperature at 
night was 27°. Such an excessive thermometric range must aid 
gréatly in breaking up the rocks, especially the black basalts so 
common in the west. 
Organic aids to erosion are also important. Plants serve very 
slightly in breaking up the soil. They grow with extreme slow- 
ness and great difficulty. They are important more in a con- 
servative than a destructive way. The vegetation is thin and 
scanty, and hence does not act as a protective covering to the 
extent that plants of a moister climate do. The conservative 
effect is well shown, however, in the case of the mesquite, which 
catches and holds down the blowing sand, and as a result often 
