Alkalis In* Colorado 
45 
t 
this purpose as we knew that there was an excess of silicic acid present 
sufficient to expel all of the other acids. 
When such waters as these pass into the soil which differs from 
the rocks and sands with which they previously have been in contact, 
changes take place which vary with the character of the soil. In the 
case of the Poudre water these changes, in their rougher features, are 
that the silicic acid is almost completely removed, and the calcic, mag- 
nesic and sodic carbonates are exchanged for sulfates, the sodic car¬ 
bonate less completely than the others. Potassic salts are, as a rule, 
completely removed. Other changes may also take place and give rise 
to special forms or types of alkalis. These changes are often complex 
and their course not self evident. 
The waters previously referred to are collected from the same gen- 
eral watershed as those of the Rio Grande, though they do not find 
their way into it but are delivered into the valley by other smaller 
streams. I have stated that the strata, even the soil of the valley, are- 
made up of the same minerals that form the rock masses surrounding 
the valley. The principal differences between its condition as the 
water of mountain streams and when spread out in the strata of the 
valley is the change from a flowing mass to a resting one spread over 
many times the area, with changing conditions of pressure and con¬ 
centration. The effect of these differences is to permit the calcic and 
magnesic compounds to be removed probably as carbonates and the 
silicic acid also either as such or as newly formed silicates, leaving in 
solution the sodic salts as sulfates, chlorid and carbonate. If a solu¬ 
tion containing these salts, sulfate, chlorid and carbonate, be allowed to- 
run through the soil, the carbonate is permitted to pass most readily 
while the sulfate and chlorid are retained to a very much larger degree. 
The evaporation of the water and consequent concentration of the 
salts held in solution is a matter of fact and of great importance. I 
hold that this, in a rough way, is the manner in which we have to ac¬ 
count for the difference in the character of the alkalis in the two sec¬ 
tions which I have presented. The process of formation of the alkalis 
in the twx) sections is essentially the same and the source of the alkalis 
is the same. The fundamental cause of the difference is that the one 
section has always enjoyed drainage enough to keep the underground, 
the artesian water constantly removed from the surface, and evapora¬ 
tion has played a very minor part. Whereas in the other section the 
supply of fresh mountain water has, in fact, been less abundant and 
there has been no drainage, or one wholly insufficient to cause the re¬ 
newal of the water frequently enough to prevent accumulation of the 
sodic carbonate. In this case the discharge of water into the area is- 
not enough to replace more than one-half of the water that would evap¬ 
orate from a free water surface and scarcely more than enough to re- 
