2 BULLETIN 180, U. S. DEPARTMENT OE AGRICULTURE. 
here is the expansion of the rock material due to changes in tempera- 
tures, including the expansion of the water contained in the rock at 
the freezing point. The most important chemical process is solution, 
while oxidation and hydration are of great importance. 
The layer of resulting material mixed with some organic matter 
and capable of supporting plant life is known as soil. Its composi- 
tion does not differ very markedly from the rocks from which it is 
derived. At best this process of accumulation is slow, the rate de- 
pending somewhat on the activity of the agencies producing it. 
TRANSPORTED SOILS. 
Soil materials formed in one place and deposited in another through 
the agency of wind, water, or ice constitute or develop into trans- 
ported soils. These, however, are not the eroding soils, but are the 
results of soil erosion. 
TRANSLOCATION OF SOILS BY WATER. 
The movement of soil material by water is limited by two inherent 
properties. Water can move only from a higher to a lower level, and 
it can affect only the surface with which it comes in actual contact. 
Since water moves over the surface of the soil only under the force 
of gravity, its action is always directed toward moving material from 
the hills and depositing it in the plains. If a stream is arrested in its 
movement down hill by the construction of an impediment, such as 
a dam, it produces a lake which acts as a settling basin. The stream 
gives up its burden of detritus and the lake is gradually filled. 
Eventually this results in the bottom of the lake reaching the level 
of the dam, when the stream will then carry its burden to some lower 
lake or to the ocean and deposit it. 
Streams can erode only those surfaces over which they flow, and 
this greatly restricts their power in this respect. Thus the larger 
part of the detritus of streams must be derived from the surface drain- 
age of adjoining areas. Here the amount of surface drainage is 
dependent on the absorptive power of the soil and on its drainage. 
If the soil is more or less loose and porous its absorptive capacity is 
high, so that it may absorb rain as rapidly as it falls unless the pre- 
cipitation be extraordinarily heavy. On the other hand, if the soil 
is close grained and compact the absorption is slow, even though the 
actual pore space within the soil be greater than in the case of the 
loose soil. In case of a gentle rain, absorption may be rapid enough 
to prevent surface drainage, but with any heavy and rapid rainfall 
the water runs off largely from the surface. 
WATER-TRANSPORTED MATERIAL. 
Physics. — The excess of water draining from the surface of a soil 
carries with it some of the material in suspension. The existing val- 
