144 SNAKE EIVER PLAINS OF IDAHO. [bull. 199. 
frequently calcium carbonate) which are sparingly soluble and which 
most probably owe their accumulation to direct precipitation of 
material previously held in solution, while the deposits produced 
on exposed surfaces result from chemical precipitation and are not 
dissolved by beating rain. These two varieties of surficial deposits, 
due most conspicuously to the action of atmospheric agencies, fre- 
quently occur together, but the most characteristic examples arc 
found in separate localities. The far-reaching process here touched 
upon, by which a concentration of sparingly soluble substances is 
brought about on rock surfaces or in their surficial portions while 
more soluble substances are carried away in solution, produces vari- 
ous changes dependent on the resultant, so to speak, of the several 
conditions or agencies involved. Not only do soils and flat-rock sur- 
faces experience the changes referred to, but talus slopes and the faces 
of precipices are similarly acted on. In some instances along the bor- 
der of the canyon of Idaho the aprons formed of dislodged fragments 
at a depth of a foot or two below the surface are firmly united with a 
calcareous cementing material, which includes dust particles. The 
faces of bold escarpments of tuff, consisting largely of fragments of 
glassy basalt, which occur on each side of Snake River from 1 to 2 
miles above the mouth of Salmon River exhibit a surface hardening 
which has led to conspicuous changes through the erosive action of 
dust- and sand-charged winds. The resistant surface has been broken 
in many places, and the softer material beneath scooped out so as to 
form alcoves and grotto-like recesses of various shapes and sizes. 
The appearance of the rugged grotto cliffs, as they might be termed, 
is most peculiar and interesting. 
Possibly a suggestion in reference to the wide influence of the process 
of subaerial precipitation just referred to may not be out of place. 
The formation of subsurface deposits, desert varnishes, etc., depends, 
as has been stated, on atmospheric conditions, and occurs, as might 
easily be shown, on a large variety of rocks. Probably there is no 
rock that is sufficiently resistant not to yield to the action of percolat- 
ing water and, under the requisite climatic conditions, become hardened 
at the surface. The marked difference in results produced in arid and 
humid climates has been referred to. In arid climates absorbed water 
is drawn to the surface on account of capillary attraction induced by 
surface evaporation, and precipitation results, but when there is free 
underdrainage the more soluble portions of such precipitates are 
removed in solution. In humid climates all the material dissolved out 
of the rocks is normally carried away in solution, and rock decay 
progresses with omparative rapidity. In regions having an arid cli- 
mate, as is well known, rock decay progresses slowly, and the principal 
method by which denudation is initiated is rock disintegration, or the 
breaking of the rocks into fragments through the influence of changes 
