388 SOILS. 



the arid California soils, as large an amount was found in 

 the sand-grain sizes between .12 and .50 millimeters as in the 

 fine silt .016 to .025 mm. in Mississippi. 



Hydrous Silicates are More Abundant in Arid than Humid 

 Soils. This predominance of hydrous silicates in the soils of 

 the arid regions should not be a matter of surprise when we 

 consider the agencies which are brought to bear upon these soils 

 with so much greater intensity than can be the case where the 

 solutions resulting from the weathering process are continu- 

 ally removed as fast as formed, by the continuous leaching 

 effect of atmospheric waters. In the soils of regions where 

 summer rains are insignificant or wanting, these solutions not 

 only remain, but are concentrated by evaporation to a point 

 that, in the nature of the case, can never be reached in humid 

 climates. Prominent among these soluble ingredients are the 

 silicates and carbonates of the two alkalies, potash and soda. 

 The former, when filtered through a soil containing the carbon- 

 ates of lime and magnesia, will soon be transformed into com- 

 plex silicates, in which potash takes precedence of soda, and 

 which, existing in a very finely divided (at the outset in a 

 gelatinous) condition, serve as an ever-ready reservoir to 

 catch and store the lingering alkalies as they are set free from 

 the rocks, whether in the form of soluble silicates or carbonates. 

 The latter have another important effect : in the concentrated 

 form at least, they, themselves, are effective in decomposing 

 silicate minerals refractory to milder agencies, such as calcic 

 carbonate solution ; and thus the more decomposed state in 

 which we find the soil minerals of the arid regions is intelligi- 

 ble on that ground alone. 



It must not be forgotten that lime carbonate, though less effective 

 than the corresponding alkali solutions, nevertheless is also known to 

 produce, by long-continued action, chemical effects similar to those 

 that are more quickly and energetically brought about by the action of 

 caustic, lime. In fact, the agricultural effects of "liming" are only in 

 degree different from those produced by marling with finely pulverized 

 carbonate ; and in nature the same relation is strikingly exemplified in 

 the peculiarly black humus that is characteristic of calcareous lands, 

 but which can be much more quickly formed under the influence of 

 caustic lime on peaty soils. 



