10 
Bulletin 65. 
going on continuously, for they are constantly removing salts in 
solution, the supply of which is, in part at least, maintained by the 
decomposition of the constituent minerals of the soil. If there were 
no such supply of these salts, they ought to have diminished in 
quantity to a very small fraction of that which we now find. The 
formation and continuous presence of highly reactive, hydrous 
silicates within the soil may be accepted as established, but back of 
this, as well as of that of clay, whether kaolinitic or colloidal, are 
the decompositions of which these compounds are themselves but 
products. In the case of the soils in question, the original minerals 
whose changes have furnished the materials for these zeolitic com¬ 
pounds are preeminently the feldspars of the granites of the front 
range. Traces of igneous rocks are found, but their part in the 
formation of these soils is so subordinate that they may be neglected. 
That the ground waters, already rich in salts, participate in pro¬ 
ducing these changes is very probable. The uniform presence of 
potash salts in the ground water is presumably due in part to this 
fact. The elimination of potash from the soil in the form of nitre, 
potassic nitrate, will account for the presence of some, but not for 
the whole, of the potash present in the ground water. The presence 
of silicic acid in all of the water, amounting in some instances to 
about 2 per cent, of the total solids, is likewise suggestive of the 
decomposition of silicates, yielding, among other products, new 
silicates soluble in water. The silicic acid present in the water 
analyses, which will subsequently be given, was in solution either 
as hydrated silicic acid or in the form of soluble silicates. 
A very striking instance of the presence of silicic acid in water 
came under my notice a few years ago. The water was a very 
excellent spring water, free from sediment, limpid, and had a 
temperature of 53° F. The silicic acid present amounted to 25 per 
cent, of the total solids held in solution. I do not know whence 
this silicic acid was derived, but most probably from the decompo¬ 
sition of some feldspar, as potash and soda were the next most 
abundant constituents of the residue obtained from the water. The 
chemical changes producing such a solution as this in a spring 
water are quite sufficient, even though they may be modified in 
many ways, to account for the silicic acid found in the residues 
obtained from ground waters. 
SOME SALTS ARE FORMED IN THE SOIL. 
§ 16. We cannot distinguish between the salts which have been 
dissolved out of adjacent localities and brought by the flowing 
waters into the area where it is found, and those which have been 
formed in the latter soil. In the former case they may contribute 
to bring about new changes, in the latter they will be products of 
the changes ordinarily going on in that soil, and aid in giving 
