146 LANDS OF THE ARID REGION OF THE UNITED STATES. 
immediate surface. By capillary action, this water always tends to diffuse 
itself throughout the loose materials which make up the overlying soils. 
As fast as it is evaporated at the surface, more water from below rises by 
capillary action to take its place. When the air is exceedingly dry, as it 
jnvariably is in summer throughout the whole Rocky Mountain Region at 
moderate altitudes, the evaporative power becomes so great and extends 
to such a depth below the immediate surface, that we are unable to recog- 
nize the slightest traces of moisture indicating that evaporation is going on 
The water which may have accumulated beneath has gradually risen by 
percolation through the interstices of the unconsolidated materials of the 
soil, bringing with it whatever soluble salts it may have taken into solution 
during its sojourn beneath the surface. These soluble salts are left at the 
surface by the final evaporation of the water, and, as the process is contin- 
uous until the reservoir beneath is exhausted, the salts accumulate. Contrast 
this now with the action going on ina moist country. Here the copious 
waters wash the soils as rapidly as the salts come up from below, and carry 
them in solution into the drainage channels. During the greater part of 
the year the movement of the waters is partly from the surface downward 
into the subterranean water courses, from which they emerge in springs ; 
partly by surface drainages into rills, and thence into living streams. By 
both movements, any tendency to accumulate soluble salts at the surface 
during the relatively brief periods of dryness is prevented. In a dry 
country the periods of dryness are very much longer, and the rainfall is 
seldom sufficient to wash the accumulated salts from the soil. There is, 
however, usually a limit to this accumulation, since at long intervals rains 
occur sufficient to remove a large portion of the salts. The difference 
between a dry and wet country in this respect is therefore one of degree 
rather than of kind. In a dry country the periods of accumulation of 
salts at the surface are long and continuous, while the washings of the soil 
are rare and imperfect. In a wet country the periods of accumulation are 
short and rare, while the washings are frequent, copious, and thorough. 
The saline materials vary widely in character and constitution. They 
are, however, chiefly salts of soda, lime, potash, and magnesia. Sometimes 
they exist in the condition of chlorides, sometimes of carbonates, and 
