568 Hillocks or Mound-Formations of San Diego, Cal. [September, 
Dust set in motion and borne along by the winds is arrested 
by the shrub and, together with its fallen leaves, accumulate 
within and around it, and, as is seen in thousands of instances in 
this vicinity, an elevation of many inches is produced in this 
manner alone, in many cases covering the lower branches, and in 
case of the Simmondsia especially, nearly enveloping the whole 
plant. The gopher, subsisting upon roots and preferring for its 
operations the loose soil about them, is, in exceptional cases, an 
adjunct of the wind in heaping up material about the plant. Of 
the thousands of these clusters of shrubbery which have come 
under my observation, a very large proportion show unquestion- 
able evidences of these agencies in elevations more or less marked 
about them, the surface portions of them at least being generally 
composed of a light loam of dust and decaying leaves. While 
the loose earth of which the deposit is composed is protected by 
the branches and foliage of the plant, the more solid earth 
beneath is also protected from the wash of rain by its massive 
roots, while all around erosion goes slowly on, facilitated by the 
peculiar susceptibility of the soil to wash, a quality familiar to the 
casual observer. 
Instances doubtless exist in which the mounds have been more 
or less fully developed without the aid of those forces which ele- 
vate the earth above its original level, but the shrub and the rain 
wash have been constant factors. ; 
In the course of time the plant dies—is smothered by the drift 
which nearly covers it, or is destroyed by the fires which annually 
sweep over extensive tracts of country. Thus deprived of its 
protection, the winds in turn, and the rains which fall upon it wear 
down the top of the loose deposit, and to some extent widen its 
base. While this is going on the surrounding earth, or inter- 
spaces, are being continually lowered by the action of water. 
The wash always being greater at the base than at its summit, its 
tendency is to perpetually maintain or increase the prominences. 
The presence of beds of roots, well preserved as well as in the 
different stages of decay, within many of the more modern fully 
formed structures, upon the surfaces of which it is known from 
observation that no vegetation has grown for many years, 18 
strongly suggestive of a relation between them of cause and 
effect. In the oldest ones all traces of the original roots have 
long since disappeared. ne 
_ A well known effect of timber and shrubbery everywhere isto 
