16 



BULLETIN 61, U. S. DEPARTMENT OF AGRICULTURE. 



would be relatively less affected than the more soluble. Orthoclase and muscovite 

 are the two chief potash-bearing minerals. They are also the most resistant to weath- 

 ering. This, together with the fact that the acid rocks which contain these minerals 

 are relatively less abundant than the basic rocks which contain the plagioclaee feld- 

 spars, would lead one to conclude that potash would be found in the salines in much 

 smaller quantities than soda. Van Hise, in discussing the decomposition of ortho- 

 class, shows that this mineral may be altered into kaolin with the liberation of all of 

 the potash in the form of potassium carbonate, or into muscovite with the liberation of 

 only two-thirds of the total potash as potassium carbonate. His reactions are: 



2KAlSi308+2H20+C02=H4Al2Si209+4Si02-fK2C03. 

 2KAlSi308+H20+C02=KH2Al3Si30i2+6Si02+K2C03. 



The relative importance of these two reactions can not, for obvious reasons, be stated. 

 Both take place in nature. Probably in regions of hydrothermal activity the altera- 

 tion to kaolin is more often found, while in regions of simple weathering the reverse 

 is more often the case. 



From the foregoing table it is seen that the soda content of basic is only slightly less 

 than for acidic rocks. While the potash content of basic is about one-half that of 

 the acidic rocks, the greater susceptibility to weathering of the basic rocks would lead 

 us to conclude that the larger proportion of soda would be liberated from these rocka 

 rather than from acidic rocks. 



Lime and magnesia are liberated by decomposition but tend to pass into insoluble 

 compounds more quickly than either potash or soda; consequently, we should expect 

 to find them less abundant in salines. 



The acid constituents of igneous rocks are relatively less abundant than the basic. 

 Weathering would liberate these, and the abundance of oxygen present in the zone of 

 weathering would convert the sulphur into sulphuric anhydride. This is also indi- 

 cated by the comparative absence of reducing substances shown by the scanty vege- 

 tation of the basin. The chlorine, carbonic acid, and sulphuric acid would combine 

 with whatever bases were present to form chlorides, carbonates, and sulphates. 



The phosphoric acid, if liberated as soluble phosphate, would quickly pass into one 

 of the many insoluble phosphates. Phosphoric acid is found in the salines only in 

 small quantities and can not be considered as an important constituent of these sub- 

 stances. 



Merrill, in discussing the decomposition of igneous rocks, presents the results of a 

 number of studies and has endeavored to show what proportion of the original rock 

 has been lost in the form of soluble compounds. It is evident that a mere compari- 

 son of analyses of weathered versus fresh rocks is inadequate. While it is generally 

 true that the percentage of alkalies present in material resulting from weathering is 

 less than the percentage in the undecomposed rock, still we have many examples 

 where apparently the percentage composition has been unchanged, or the percentage 

 of the alkalies has been increased. This is due to the fact that as the rock weathers its 

 volume and weight change. If it were possible to determine the weight of fresh rock 

 and the weight of residual material (soil) resulting from weathering, we could deter- 

 mine the proportionate loss of the constituents. This, for obvious reasons, can not be 

 done. By assuming one constituent as constant, and that the most insoluble one, 

 Merrill 1 has calculated the proportional loss of constituents due to weathering. From 

 10 examples of igneous rocks given by this author I have calculated the average per- 

 centage losses. The following table gives these for alumina, ferric oxide, lime, mag- 

 nesia, potash, and soda: 



Percentage loss of constituents from igneous rocks caused by decomposition. 



Constituent. 



Mean loss 



Estimated 



humid 



for arid 



region.2 



regions.3 



Per cent. 



Per cent. 



14.17 



7.8 



32.84 



18.2 



66.9 



5.3 



64.7 



10.4 



62.1 



19.0 



72.0 



24.8 



RatiOjhumid- 

 arid.< 



AlsOj. 

 FejOs 

 CaO.. 

 MgO. 

 K2O.. 

 NajO. 



1.8 

 1.8 

 12.6 

 6.2 

 3.3 

 2.9 



1 Rocks, Rock Weathering, and Soils, p. 188. MerrUl. 



» Calculalod averages from examples of igneous rock decomposition given by Merrill. Merrill's examples 

 Include granites, phonoliies, syenite, diabases, basalts, diorites, and andesites. Ilocks, Rock Weathering, 

 and Soils, pp. 185-208, Merrill. 



' Calculated by dividing percentages of first column by ratios given in last column. 



* Calculated from data given by Clarke of average analyses of soils of humid and arid regions (Bui. 491, 

 U. S. Geol. Survey, p. 467). Ratio is percentage of constituent in acid-soluble portion of soils from arid 

 region divided by percentage of constituents in acid-soluble portion of soils from humid regions. 



