POTASH SALTS AND OTHER SALINES IN THE GREAT BASIN REGION. 15 
Chemical composition of the more important basic and acidic rocks. 
Constituents. 
Type, 
acid. 
Type, 
basic. 
Constituents. ^ ! gg 
i 1 
Base: 
MgO 
Per cent. 
1.05 
Per cent. 
4.52 
Acid: Per cent. '■■ Per cent. 
S ■ 0.370 0.066 
CaO 
Na 2 
K 2 
2.15 6.33 
3.35 I 3.29 
4.10 2.09 
CI ■ .015 .068 
C0 2 : .160 ' .326 
S0 3 : .035 ! 
P2O5 .145 .240 
Total 
10 65 1fi 9a 
Total 1 .735 | .700 
The table gives the average percentage composition of acid and basic divisions of 
the igneous rocks. Only those constituents have been included which might be 
expected to contribute to the bases and acids of salines. 
Hydration and carbonation are the two important processes by which igneous 
rocks are decomposed. The rate at which decomposition proceeds is dependent 
upon the rate of disintegration, as well as upon the intensity of hydration and car- 
bonation. Hydration and carbonation are dependent for their intensity upon cli- 
matic conditions. Disintegration depends upon extremes of temperature, the physi- 
cal nature of the rock, the activity of erosion, and the rate of decomposition of the 
rock constituents. Disintegration and decomposition proceed simultaneously. 
Under arid climatic conditions, such as pertain in the basin region, disintegration is 
dominant and, decomposition is measurably less than under humid climatic condi- 
tions. This fact has been pointed out by a number of investigators — Van Hise, 
Merrill, Hilgard, Clarke. Further confirmation of this fact may be easily obtained 
by petrographic examination of the alluvial material taken from the aprons bordering 
the basin ranges. Comparatively fresh particles of feldspar may be found even in 
the finer silts of the central parts of the basin. 
The extent to which the igneous rocks of the basin region have been decomposed, 
and the constituents and proportion of each which might be expected to form acces- 
sions to the salines, have not been made the subject of special study. In a general 
way it might be said that the amount of rock decomposition in this region is nominal. 
Pre-Tertiary igneous rocks (in the main granites and diorites), where exposed, are 
noticeably decomposed. The older Tertiary volcanics (andesites) are also decom- 
posed to a considerable extent. This is particularly noticeable in the areas in which 
hydrothermal activity was once dominant. In such areas decomposition extends 
locally to comparatively great depths and the rock alteration is in many cases pro- 
found. In the basin region there are some 350 mining districts. Each of these may 
be considered to have been in the past the locus of more or less hydrothermal action. 
The aggregate altered rock area of these districts is not known, but it must constitute 
an extremely small part of the total basin area and be therefore relatively unim- 
portant as a source of saline material. Late Tertiary rhyolites and Quaternary igneous 
rocks are often only superficially decomposed, except in those regions where hot 
springs have continued their activities to comparatively recent times. 
Humid conditions exist only on the highest mountain ranges and consequently the 
areas exposed to weathering under the most favorable conditions for decomposition 
must constitute a relatively small part of the total. Over a large part of the area 
exposed to weathering influences the conditions in the Great Basin are such as to 
produce decomposition at a comparatively slow rate at the present time. That this 
was not always the case has been shown by the investigations of Gilbert and Russell. 
It is to be particularly noted that in Quaternary times climatic changes were numer- 
ous and humid conditions alternated with arid conditions. During the period of 
Quaternary lake development the rock decomposition must have proceeded at a 
very much more rapid rate than under present conditions. Consequently a greater 
amount of saline material must have been contributed and have been deposited in 
the basins. 
The minerals constituting igneous rocks are attacked at different rates. Clarke states: 
"The pyroxenes and amphiboles yield most readily to waters; then follow the 
piagioclase feldspars, then orthoclase and the micas, with muscovite the most resistant 
of all. Even quartz is not quite insoluble, and the corrosion of quartz pebbles in 
conglomerates has been noted by several observers. Among the common accesso- 
ries, apatite and pyrite are most easily decomposed, magnetite is less attacked, and 
such minerals as zircon, corundum, chromite, ilmenite. etc.. tend to accumulate with 
little alteration in the sandy rock residues. " 
This conclusion no doubt applies to conditions more nearly approaching humid 
than arid. We should expect under arid conditions, that the more insoluble minerals 
