EROSION IN ARIZONA BOLSON REGION 147 
matter of fact the whirl must produce upward and return currents. 
Therefore the effect of increased subwhirl velocity in shooting up a 
larger number of particles will be in part balanced by a more rapid 
downward journey of the same particles due to the increased velocity 
of the complementary return currents. ‘Therefore 
(4) L c less than directly with v. 
Will, however, this moderate increase in power to suspend material 
be effective under the conditions that obtain in the arid southwest ? 
Is there loose material to pick up? Is equilibrium easily established 
between the material in the stream and the material on the ground 
over which it passes? The flood is advancing over a surface that 
(1) may be a baked mud flow, or (2) is cemented by desert salts, 
or (3) is protected by pavements described later; therefore it cannot 
cut, for its tools have been laid down above, and it cannot pick up 
loose material, for that has been swept away by the wind. 
3. It has been suggested to me by Dr. A. E. Douglass, of the 
University of Arizona, that the finest of the material is not suspended 
by the subcurrents, but that due to the difference in velocity in the 
various layers of the water, the numerous superfine particles are in 
constant collision, and the kinetic bombardment (like that of solutions 
ascribed to heat) causes a diffusion of particles upward from the 
region of greatest velocity difference, viz., the bottom. Further, it is 
to be suspected that the attached air bubbles aid in the suspension of 
the emulsion, for after boiling a mixture of very fine loess, Dr. Douglass 
found that the cloud of particles did not rise as readily upon rotating 
the containing vessel as before. The increasing power to suspend 
superfine material from the increase in declivity will not cause 
noticeable erosion, however, on account of a lack of loose fine 
material of this nature. 
We may conclude therefore that a flood sheet is far less responsive 
to moderate increase of gradient, than an ordinary stream. It is a 
depositing and not an eroding agency and will not develop into the 
former even when subject to a moderate increase in current velocity. 
As the flood sheet advances its velocity is checked (1) by evapora- 
tion, to a minor extent, (2) by absorption in the ground, and (3) 
mainly by spreading out to a thin sheet, thus greatly increasing surface 
