268 
IOWA ACADEMY OF SCIENCE 
the Black Hills to the mouth, may be partly due to the greater vol- 
ume of water in the Cheyenne, though that remains to be shown, 
and if shown it may still be referred as we shall see to the dif- 
ference of rock in the two basins. Yet aside from this the character 
of the rock is argued to be the main cause for the apparent anom- 
aly. The valley of the first stream is mostly in sand, of the second 
almost exclusively in clay, both little consolidated, or soft. 
Two physical characteristics of these kinds of rocks seem to 
conspire to produce the results mentioned: 
1. Degree of porosity . Sand is very porous; clay is very im- 
pervious, taking little water except near the surface. The former 
absorbs promptly a large proportion of the rainfall so that little 
runs off unless the rainfall is very copious. This tends to reduce 
the maximum of floods, the wash upon the surface and the vigor 
of the stream and moreover favors vegetation, which also tends to 
check erosion. 
On the other hand clay absorbs little water; the rainfall nearly 
all runs off, producing sudden and severe floods, causing not only 
vigorous corrasion of water-courses but also frequent washing of 
the general surface. 
2. Coarseness of grain or size of particles. Sand may be quite 
coarse and a fair proportion of gravel is found in the Miocene (Ari- 
karee and Ogalalla), which constitutes most of the basin of the 
Niobrara. Even when the channel is cut from the underlying 
Pierre, as below Valentine, its course is largely lined with rear- 
ranged sands. 
This feature affects the erosion in different ways. The coarse- 
ness of grain may aid the simpler or earlier stages of erosion, but 
seriously hinders indirectly, because it necessitates slower trans- 
portation; a fact which is now generally recognized. In time of 
flood, the coarser the material the greater the velocity before the 
material is moved, then it rapidly joins the stream and overloads 
it but is as rapidly dropped at the first checking of velocity, hence 
the tendency for streams dealing with such material to exhibit 
steeper slope, wider and shallower channels, such as is well illus- 
trated in the Platte river of Nebraska. Therefore we find the Nio- 
brara belonging to this class. 
On the other hand, the Cheyenne works mainly in the clay shales 
of the Benton and Pierre formations and illustrates the results of 
erosion in clayey rocks. These are of very finely comminuted 
material which is easily mixed with water having the slightest 
motion, but when once mingled it separates very slowly, and is there- 
