IOWA ACADEMY OP SCIENCE 
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fore carried far before it is redeposited. Hence it may be carried 
out of the region in a single season. Witness the muddiness of 
the Missouri and White rivers. Hence also the periphery of the 
stream is kept clear for corrasion by the small amount of coarse 
material which may be present and for the free wash of the stream. 
Over the whole surface, likewise, the plash of the rain and the 
wash of the rill easily produces perceptible erosion. Therefore 
the whole of a basin underlaid with clay is denuded rapidly. This 
is well illustrated by the Cheyenne which has a deeper valley and 
has captured the head waters of its neighbors so that it monop- 
olizes the drainage of the Black Hills. 
The two characteristics already considered are much the more 
prominent in effect, but another may be noticed which also some- 
times has considerable effect. We may call it: 
3. Expansibility by moisture. Sands and loams and consoli- 
dated rocks show but little if any expansion when moistened, but 
clays and earths often exhibit it in a high degree. This is seen in 
the slaking of lime, the similar heaving and cracking of the super- 
ficial layers of dry masses of clay and the converse is shown in 
mud-cracks. 
The influence of this on erosion is obvious. A slight rain may 
open up the surface of a compact clay so that the winds or the 
next hard rain, or a more vigorous stage of the same shower, easily 
carries away the loosened particles. It tends to a kind of spher- 
oidal weathering, resembling that produced by heat and frost in 
more consolidated rocks. This property is particularly efficient 
in producing “bad lands.” It of course is most efficient where there 
are frequent and extreme changes from moisture to drouth, as 
where showers visit regions which are habitually dry. Absence 
of vegetation is both a result and an adjunct of this characteristic. 
In leaving this suggestive study, a word should be said of White 
and Bad rivers which lie between the two specifically considered. 
The latter works almost entirely on Pierre clay like the Cheyenne, 
but lacks efficiency because of deficiency of water, but it shows 
great erosion and is subject to severe floods. 
White River works very largely in Oligocene beds which are 
mostly clays and marly clays which are impervious and expand 
greatly with moisture. A limited amount of coarse material has 
on the whole rather helped erosion or compensated for rather 
greater hardness of rocks than is found in the Pierre. The valley 
has been greatly and deeply excavated. The presence of the Mio- 
cene sands and loams above the clays has complicated the results. 
