EROSION IN ARIZONA BOLSON REGION 143 
material is pried off and made ready for the attack of torrential 
stream by daily temperature change, and the fine material is brought 
up by the wind. There is no grading in size between the two, the 
deficiency of material of intermediate size being marked. 
Under the well-known erosion analysis in the case of ‘‘a homo- 
geneous island with one depression” let any flood encounter a depres- 
sion of any kind accelerating its flow, then there must be a gullying 
started which will work up stream. Now the sheet flood described 
below often flows over irregular ground, and even then its gullying 
action is almost entirely lacking. The observation of this phe- 
nomenon led to an investigation of the manner in which material is 
transported by flood action. It has not yet been possible to check 
observation with experimental data, but on account of the bearing 
on the problems at hand, I present the following inspection of certain 
phases of this transportation. 
Sediments are carried forward by running water in the following 
three ways: (1) The large material is rolled on the bottom; (2) 
Finer material is thrown up and down by subcurrents, drifting for- 
ward with the current; (3) The superfine is held in suspension by the 
kinetic action of inter-impact. 
t. As is well known the mass of the largest particle that can be 
rolled on the bottom varies approximately as the sixth power of the 
velocity of the stream at its bottom. The material rolled comprises 
the cutting machinery of the stream, and the fact that the size increases 
so rapidly with quickening velocity, gives to the stream its great power 
of concentration of energy at declivities and at the down-stream side 
of hard strata. If a stream is in the rather unusual position of having 
no coarse material to roll on its bottom, it will not only not be able 
to cut hard rock, but it will have nothing with which to stir up the 
finer material in its channel, and therefore its cutting power is reduced. 
2. The intermediate and fine material is danced upward by the 
secondary currents, eddies, etc., of the stream, and during its up- 
and-down journeys it is carried forward by the current. The forma- 
tion of these eddies is generally recognized’ and they are shown in an 
impressive way in the eddies and whirlpools of the sullen mud-laden 
floods of the southwest. The whirls are formed by friction both 
t See Chamberlin and Salisbury, Geol., Vol. I, pp. 111, 112. 
