142 SNAKE RIVER PLAINS OF IDAHO. [bull.199. 
the upper portions of basaltic outcrops when covered with loose stones, 
sand, soil, etc. These incrustations are perhaps most commonly only 
an inch or two thick, but at times have a depth of several inches, and 
not infrequently a layer of rock fragments 6 to 8 or more feet thick 
will be solidly cemented with similar material, forming a breccia. In 
some localities the surface portion of a bed of pebbles is similarly 
cemented into a conglomerate. Less frequently the white deposit is 
largely free of other material, visible to the eye, and forms a subsur- 
face layer of hard, yellowish- white rock 3 to 4 or more feet thick. In 
certain instances, again, the cavities in the surface portion of a sheet 
of lava will be filled with similar material. 
The most striking peculiarity of these deposits, aside from their white 
or yellowish-white color, is their occurrence just below the surface, and 
their absence at a depth of a few feet. As they occur on loose stones. 
and in surface deposits generally, it is obvious that they are of recent 
origin, and are probably still forming. This material, as shown by 
partial analysis, consists essentially of calcium carbonate, and agrees 
both chemically and in mode of occurrence with similar deposits widely 
distributed in arid regions. 
As to the way in which these Lime deposits are formed, it is evident 
from their mode of occurrence, composition, etc., that they are of the 
nature of efflorescences. Rain water on percolating downward and 
more or less completely saturating the soil and upper portion of the 
rocks beneath is drawn upward again by capillary attraction as the 
surface dries. Why the deposit formed should consist principally of 
calcium carbonate is less clear, but the fact seems to be due to the 
removal and carrying to a distance of the more soluble salts during 
the downward passage of water. The lime incrustations occur where 
there is free underdrainage, and the water drawn to tin 4 surface is the 
portion supplied by the occasional rains, which was absorbed by the 
rocks. The lime deposit is thus washed during each rain and a part, 
no doubt, removed, but in arid regions a residue remains. Similar 
incrustations occur, especially in gravel deposits in humid regions, but 
they are thin and by no means of as common occurrence as in arid 
regions. There is thus a delicate balancing of conditions: With slack 
underdrainage, alkaline efflorescences are formed when surface evapo- 
ration is active; with free underdrainage, the more soluble salts are 
removed, and if surface evaporation is active, lime incrustations are 
deposited; with more abundant rainfall and less surface evaporation, 
all the material dissolved by percolating waters is carried away. 
That the lime deposits referred to are derived in part from the soil 
is shown by the presence of cemented grains, as has been described in 
connection with the two soil samples, of which mechanical analyses 
have been given. In fact deposits of lime 3 or 4 feet thick, which 
seem to be homogeneous throughout, when digested in acid leave a 
