IRRIGABLE LANDS OF THE VALLEY OF THE SEVIER. 147 
sometimes of sulphates. The reactions from which they are derived are 
many, and it will be proper here to give only a few illustrations. A portion 
of the salts of magnesia and soda are derived from the decomposition, by 
atmospheric influences, of volcanic, granitic, and other crystalline rocks. 
Where these materials exist in the form of felspar, hornblende, and pyroxene, 
the great decomposing agent is water charged with the carbonic acid of 
the atmosphere, by the action of which soda, magnesia, and lime are, with 
inconceivable slowness, dissolved out of the constituents of these rocks. 
There is no stream, however pure it may apparently be, which does not 
carry more or less of chlorides and carbonates in solution. The sulphates 
are derived mainly from subterranean sources. In the Rocky Mountain 
Region, one of the most common forms of sulphate is found very abun- 
dantly in the rocks of the Carboniferous, Triassic, Cretaceous, and Tertiary 
Ages, in the forms of gypsum and selenite, which are sulphates of lime 
Whenever waters containing carbonate of soda are filtered through strata. 
containing these sulphates, a double decomposition takes place, by which 
rarbonate of lime and sulphate of soda are formed. The carbonate of 
lime is very slightly soluble in water, while the sulphate of soda is highly 
so, and it is well known that waters emanating from the sedimentary rocks 
just spoken of are very frequently highly charged with it. Such, doubtless, 
is the origin of this mineral in the so called alkaline waters of the west, 
and of all the soluble minerals which pass under the name of alkali it is 
one of the most common. Carbonate of soda is also abundant in the 
soils. It is frequently found in the summer time, coating the surface of 
bottom lands which earlier in the season have been submerged by the 
augmented streams. Common salt (chloride of sodium) is even more 
abundant than the sulphate. It is well known, however, that many of the 
sedimentary rocks, particularly those of the Triassic and Jurassic Age, 
contain an abundance of it, and there are many localities in the west 
where a very fair article of common salt is obtained by the lixiviation 
of the detritus of the red Triassic rocks. Incrustations of these soluble 
saline materials occur most abundantly in the vicinity of the rivers and 
in the bottom lands. This may at first seem somewhat strange, but it is 
susceptible of a ready explanation. In order that these salts may accumu- 
