G. K. Gilbert—The Colorado Plateau Province. 89 
Disintegration is naturally divided into two parts. So much 
of it as is accomplished by running water is called corrasion, 
and that which is not, is called weathering. 
Stated in their natural order, the three general divisions of 
the process of erosion, are (1) weathering, (2) transportation, and 
(3) corrasion. The rocks of the general surface of the land are dis- 
integrated by weathering. The material thus loosened is érans- 
ported by streams to the ocean or other receptacle. In transit 
it helps to corrade from the channels of the streams other ma- 
terial, which joins with it to be transported to the same goal. 
1.) In weathering the chief agents of disintegration are solu- 
tion, change of temperature, the beating of rain, and vegetation. 
The great solvent of rocks is water, but it receives aid from 
some other substances, of which it becomes the vehicle. These 
substances are chiefly products of the formation and decompo- 
sition of vegetable tissues. Some rocks are disintegrated by 
their complete solution, but the great majority are divided into 
grains by the solution of a portion ; and fragmental rocks usu- 
ally lose by solution the cement merely, and are thus reduced 
to their original, incoherent condition. 
The most rigid rocks are cracked by sudden changes of tem- 
perature ; and the crevices thus begun, are opened by the freez- 
ing of the water within them. The coherence of the more 
porous rocks is impaired and often destroyed by the same ex- 
pansive force of freezing water. 
The beating of the rain overcomes the feeble coherence of 
earths, and assists solution and frost by detaching the particles 
Which they have partially loosened. ; 
Plants often pry apart rocks by the growth of their roots, but 
their chief aid to erosion is by increasing the solvent power of 
percolating water. 
(2.) A portion of the water of rains flows over the surface 
and is quickly gathered into streams. A second portion is ab- 
sorbed by the earth or rock on which it falls, and after a slow 
underground circulation reissues in springs. Both transport 
the products of weathering, the latter carrying dissolved min- 
erals, and the former chiefly undissolved. ‘ 
ransportation is also performed by currents of air, and by 
the direct action of gravity; but in the present discussion it 
will not be necessary to consider these accessory agents. 
-) In corrasion’ the agents of disintegration are solution 
and mechanical wear. herever the two are combined, the 
Superior efficiency of the latter is evident; and in all fields of 
rapid corrasion the part played by solution is so small that it 
= be disregarded. : 
_ the mechanical wear of streams is performed by the aid of 
hard mineral fragments which are carried along by the current. 
