POTASH SALTS AND OTHER SALINES IN THE GBEAT BASIN REGION. 15 

 Chemical composition of the more important basic and acidic rocks. 



Constituents. 



Type, 

 acid. 



Type, 

 basic. 



Constituents. 



Type, 

 acid. 



basic' 



Base: 



MgO 



Per cent. 

 1.05 

 2.15 

 3.35 

 4.10 



Per cent. 

 4.52 

 6.33 

 3.29 

 2.09 



Acid: 



S 



Per cent. 

 0.370 

 .015 

 .160 

 .035 

 .145 



Per cent. 

 0.068 



CaO 



CI 



.068 



Na20 



CO2 



.326 



KjO 



SO3 







P2O5 



Total 



.240 



Total 



10.65 



16.23 





.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 leas 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 ampMboles yield most readily to waters; then follow the 

 plagioclase 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 



