SOIL-FORMATION, WATER 21 



porous. Soils have a porosity anywhere from 30 to 75 

 per cent of their volume. Sandstone may have as much 

 as 25, limestone from .1 to .01. marble .008, and granite 

 .01 per cent. If this spore space is filled with water, as 

 is generally the case in nature, and the rock is cooled 

 below the freezing-point, it is evident that it will be 

 shattered. As the process is repeated, the fractures 

 become larger and more numerous. 



8. Water. -The chemical and mechanical action of 

 water in rock-decay may be discussed separately. 



(1) The chemical action may be divided into: (a) 

 The changes due to pure water, (b) Changes due to 

 material in solution in the water. Owing to the porosity 

 of rocks, water is distributed through all the earth's 

 crust to a depth of many thousand feet. 



The first direct result of the presence of water is the 

 assumption of its elements by many of the minerals. 

 This is hydration. It may be the direct imbition of 

 water, as when calcium sulfate in crystallizing takes 

 into its constitution several molecules of water; or it 

 may be the substitution of the elements of water for 

 some elements already in the mineral. The alterations 

 in the mineral orthoclase feldspar may be taken as the 

 type of this kind of changes as follows: 



K 2 0, A1 2 3 , 6Si0 2 + 6 H 2 = 2KOH + H 2 0, A1 2 3 , 6Si0 2 , 4H 2 



Since water is so widely diffused, this process of 

 hydration is an especially important one. The signifi- 

 cant chemical effect of hydration is that it alters the 

 solubility of the mineral, and particularly of the elements 

 composing the mineral. 



