russell.] RECENT EROSION OF SOILS. 145 
of temperature, freezing- of absorbed water, etc. The conspicuous 
differences resulting from the same process when the conditions are 
varied, so frequently exhibited in the delicate adjustment observable 
throughout nature, are here well illustrated. Under humid climates 
rock solution leads to widespread and frequently deep rock decay, 
and surface terranes are rendered soft and readily crumble. The ulti- 
mate result, so far as most rock species are concerned, is a reddish clay 
or terra rossa. Under an arid climate most rocks become hard at the 
surface and not only do not crumble on account of the removal of 
material in solution, but by its reprecipitation at or near the surface 
are rendered resistant to mechanical agencies, such as the wind, which 
tend to wear them awa}'. The colors of the rocks are not generally 
conspicuously altered by this process, and the color of the fragmental 
products are such as are inherent to the parent rocks. The soils are 
light colored, usually light yellow, and terra rossa is absent. 
RECENT SOIL EROSION. 
Reference has been maae on a previous page to small valleys in the 
area underlain by the Boise granite, which are so deeply tilled with 
debris that they have no surface streams, all of the water that comes 
to them being absorbed by the unconsolidated material forming their 
floors. The valle} T s of this description were, up to within about 15 or 
20 years, much more numerous than at present. A recent change has 
occurred which has caused fresh stream channels to appear in pre- 
viously streamless depressions, gulches, etc., and even on hillsides 
formerly completely soil-covered. The change referred to is well 
known to ranchers and others, and is said to have been begun about 
J.880. At present there are numerous lateral depressions and gulches, 
branching from the larger valleys, which have fresh channels cut in 
their floors and in the sides of the adjacent hills, that are from 10 to 
15 or more feet deep and a mile or more long, with perhaps several 
branches, but which previous to the date just given are known to 
have had smooth, unbroken contours. In several instances well- 
traveled roads have been crossed by these gullies, necessitating a wide 
detour to get about a steep-sided gulch from 10 to 15 feet deep. It 
does not require historical evidence, however, to show that the gullies 
referred to are recent. They are fresh cuts, with nearly vertical walls 
of loose earth, and at their heads begin abruptly. Each rain that 
comes is assisting in their extension and enlargement. At certain 
localities, which are unfortunately numerous, cultivated fields have 
been cut across by gulches formed during the last few years, which 
are still being enlarged. A photograph of one of these is presented 
on PI. XXV, A. One conspicuous result of the more complete drain- 
age of valley bottoms by modern rill channels is the dying out of 
Bull. 199—02 10 
