russell.] CHEMICALLY FORMED DEPOSITS. 143 
residue consisting of fine grains of quartz sand, like that composing 
the bulk of the reolian soils. In such instances it seems that the 
deposit formed lias become so great that the fragmental character of 
the subsoil is obscured and it appears to be a compact, homogeneous 
bed of lime. 
Not all of the lime deposited as a surface incrustation, however, 
comes from the soil. The cavities in the outcrops of basalt are fre- 
quently tilled with similar material, as well as the spaces between 
blocks of basalt where no soil or but very little is present. Much of 
the lime in such situations must have come from the basalt itself. 
An interesting result of the concentration of lime near the surface, 
suggested b^.the occurrence of loosely cemented kernels and sand 
grains in the samples mechanically analyzed, is that by this process 
the soil is rendered more resistant to wind erosion, and therefore lias 
a greater tendency to remain in place. The chemical precipitates 
thus assist vegetation in holding the dust which is spread over the 
land by the wind. 
The surface incrustation just described is sometimes in sufficient 
abundance to be of commercial importance, and would yield a good 
lime on calcination, but would probably in all cases contain consider- 
able quantities of sand or rock fragments. Unless these foreign 
bodies were in considerable abundance, however, ' they would not 
detract from the value of the lime for all purposes for which it is 
ordinarily used. 
The deep-blue him described on a former page as occurring on the 
surface of one of the less recent lava flows among the Cinder Buttes 
may here be referred to as being due to the process just considered. 
The blue substance is apparently of the nature of a "desert varnish," 
and owes its presence on the surface of the lava to the evaporation of 
water drawn from within the lava itself. The evaporation of the 
water has seemingly brought about a chemical precipitation of some 
of the material leached out of the lava and produced a deposit that is 
practically insoluble in water and hence is not removed by rain. By a 
similar process the upper surfaces of many of the millions of smooth 
cobbles of quartzite strewn over the surface of the lake beds in the 
northern portion of Owyhee County have been rendered dark yellow 
or brown and occasionally black. This is also a desert varnish, which 
no doubt is due to a precipitation of salts of iron or manganese on 
the upper surfaces of the stones; their lower surfaces are not *' var- 
nished." 
It thus appears that two varieties of surface deposits resulting 
from the same general process must seemingly be recognized; one 
produced below the actual surface exposed to the sky and the 
other on the exposed surface. The deposits produced below the 
Surface, in many instances at least, consist of substances (most 
