EFFECT OF AN AEID CLIMATE. 
247 
and retarded by its diminution. When it is concentrated in one part of the 
year at the expense of the remainder, transportation and corrasion are accel- 
erated and weathering retarded. Weathering is favored by abundance of 
moisture. Frost accomplishes most when the rocks are saturated ; and solu- 
tion when there is the freest circulation. But when the annual rainfall is 
concentrated into a limited season, a larger share of the water fails to pene- 
trate, and the gain from temporary flooding does not compensate for the 
checking of all solution by a long dry season. 
“Transportation is favored by increasing water supply as greatly as by 
increasing declivity. When the volume of a stream increases it becomes at 
the same time more rapid, and its transporting capacity gains by the incre- 
ment to velocity as well as by the increment to volume. Hence the increase 
in ]Dower of transportation is more than proportional to the increase of 
volume. It is due to this fact chiefly that the transportation of a stream 
which is subject to floods is greater than it would be if its total water supply 
were evenly distributed in time. 
“The indirect influence of rainfall and temperature by means of vege- 
tation has different laws. Vegetation is intimately related to water supply. 
There is little or none where the annual precipitation is small, and it is pro- 
fuse where the latter is great — especially when the temperature is at the 
same time high. In proportion as vegetation is profuse the solvent power 
of percolating water is increased, and on the other hand the ground is shel- 
tered from the mechanical action of rains and rills. The removal of disin- 
tegrated rock is greatly impeded b}^ the conservative power of roots and 
fallen leaves, and a soil is thus preserved. Tran.sportation is retarded. 
Weathering by solution is accelerated up to a certain point, but in the end 
it suffers by the clogging of transportation. The work of frost is nearly 
stopped as soon as the deptli of soil exceeds the limit of frost action The 
force of rain drops is expended on foliage. Moreover, a deep soil acts as a 
distributing reservoir for the water of rains and tends to equalize the flow 
of streams. Hence the general effect of vegetation is to retard erosion ; 
and since the direct effect of great rainfall is the acceleration of erosion it 
results that its direct and indirect tendencies are in opposite directions. 
“In arid regions of which the declivities are sufficient to give thorough 
