20 THE ORIGIN OF ALKALI 



The same authors (23) give a list of alkali-bearing min- 

 erals occurring in primary rocks as the ultimate source 

 of soil alkali. 



"Some of these alkali-bearing minerals are very generally 

 present in the primary rocks from which the soils have all 

 ultimately been derived, but they are of course usually 

 mixed with other- minerals, so that the total percentage of 

 alkalies in the rock is not so great as would appear from 

 these minerals." 



As to the method of separating these soluble substances 

 and transferring them to the surface, Cameron suggested 

 a hypothesis which is quoted by Dorsey (7) as follows: 



"The major part of the complex crystalline masses or 

 of rocks forming the earth's crust contain chlorine and 

 sulphur. F. W. Clarke gives as an average 0.07 per cent 

 chlorine and 0.108 per cent sulphur. As a result of the 

 hydrolyzing action of water and other decomposing agencies 

 probably all the chlorine and very much of the sulphur 

 is converted into hydrochloric acid and sulphuric acid, 

 which in turn form the corresponding salts of the alkalies 

 and alkaline earths. The aggregate amount which is 

 thus being constantly formed in the subsoils and under- 

 lying strata of any one area must be very large. As 

 evaporation proceeds at or in the surface soil, there is a 

 rise of the water in the underlying layers through the 

 capillary spaces toward the surface, bringing with it the 

 hydrochloric and sulphuric acids or their salts. 



"The sulphuric acid moves up more slowly than does 

 the hydrochloric acid; partly, perhaps, because the rock 

 masses and the soils have a greater absorbing action on 

 sulphuric than on hydrochloric acid, tending to withdraw 

 it from solution; partly, perhaps, because reducing con- 

 ditions may exist on some layers tending to the formation 



