The Waters oe the Rio Grande 
37 
first flow, while the maximum is 108 grains, in water from a depth of 
880 feet. The two great differences in these classes of artesian waters 
are, the very marked disappearance of the silicic acid from the brown 
waters, and the very marked increase in the total solids. The minor 
changes consist of an acquired color and the lessened amounts of cal¬ 
cium and magnesium. The color is accounted for by the presence of 
humus dissolved from peaty matter in the strata traversed by the 
water. We accept the fragments of wood appearing in refuse washed 
out in sinking the wells as indicating the source of the brown matter 
held in solution. 
ORIGIN OF THE CARBONATES 
I shall later attempt to explain the concentration of the sodic car¬ 
bonate. Its original source may be given correctly by tracing it back 
to a mineral origin, but this does not account for the concentration 
in these waters. The “Soda Weir 1 , now closed, carried 97 grains of 
this salt per gallon. One can scarcely appeal successfully to the 
masses of organic matter that may be indicated by the humus as the 
source, for our most alkali-tolerant plants would scarcely yield such 
pure solutions of soda, unless we assume that the plants of that time 
were different from those of the present, which is not probable. 
The source of the soda and its predominance in the area north of 
the Rio Grande may far more reasonably be explained by the action 
of water on the igneous rocks, fragments of which constitute a very 
large percentage of the sand occurring in the strata. The remnants 
of trees (wood) which are met with in making these wells, and the 
peaty or brown color of the water, indicate that lake or marshy con¬ 
ditions existed for probably long periods in this section. At the pres¬ 
ent time the lowest portion of the valley lies within this area of arte¬ 
sian water rich in sodic carbonate. This area has probably been the 
lowest portion of the valley in former times, and for the same or simi¬ 
lar reasons as now, i. e., because the bed of what we call the Rio 
Grande was higher than this portion of the valley, perhaps enough 
higher to prevent drainage southward, and what was then lake or 
swamp waters were removed by evaporation. 
The action of water on the plagioclase felspars present in the sand, 
formed largely of small, more or less worn fragments of igneous 
rocks, would give rise in the first instance to carbonates of soda and 
calcium with the liberation of silicic acid. That the carbonate of calcium 
was formed and separated is evidenced by the presence of very nu¬ 
merous kidney-shaped concretions of this substance in the sand as it 
was washed up in sinking the artesian wells. Sample available was 
from a depth of 550 feet 
The character of the white, artesian and spring waters, at the 
present time, is exactly such as is produced by the action of water on 
