16 I. C. RUSSELL CONCENTRATION AS A GEOLOGICAL PRINCIPLE 



Water in the zone of weathering is either under the dominant control 

 of gravity, as when it occurs in fissures, caverns, etcetera, or occupies tlie 

 interspaces between the pebbles or grains of gravel or sand and moves in 

 response to the pull of gravity exerted directly or indirectly under the 

 form of hydra,ulic pressure, or adheres to the solids present and is held 

 by them as surface films, owing to molecular attraction. In general and 

 as it appears imiversally, the free water content of cellular and porous 

 rocks is not saturated with any of the substances it contains in solu- 

 tion, and evaporation does not lead to precipitation. When, however, the 

 free water percolates away, leaving only films of water adhering to the 

 surfaces of the solids present, evaporation is favored and all the mineral 

 matter contained in the water films is frequently precipitated. This 

 process of subsurface evaporation goes on most actively adjacent to the 

 surface of land areas and decreases essentially in arithmetic ratio with 

 increase in depth. It is most active in open textured rocks, like beds of 

 boulders and pebbles, and decreases inversely with diminution in the cel- 

 lular condition of the containing terranes. By this process precipitates 

 are formed which incrust the surface of the solids present, and in the 

 case of terranes composed of sand grains, pebbles, etcetera, in many in- 

 stances increases in volume tmtil the fragments are cemented and under 

 favorable conditions the interspaces are completely filled. 



The process of subsurface concentration just outlined is modified in a 

 conspicuous manner by climatic conditions, and especially by the degree 

 of humidity and temperature. Its results are most conspicuous in re- 

 gions of small rainfall and of high temperature, and especially where 

 periodic or occasional rains occur in regions of high mean annual or sea- 

 sonable temperature, and are less common in cold, humid regions. Ex- 

 amples of the growth of this process are furnished by the cemented sub- 

 surface crusts or alkaline hard-pans of arid regions and by the lime 

 cemented gravels occurring near the surface in humid regions. 



A delicate adjustment between the conditions favoring subsurface con- 

 centration by evaporation and the removal of the precipitates formed in 

 solution is frequently manifest by the deposits in question. In regions 

 of small rainfall the precipitates which may be termed perennial contain 

 such readily soluble substances as sodium sulphate and carbonate and 

 calcium sulphate, but in more humid regions, where the volume of per- 

 colating water is greater, such salts are usually eliminated and the more 

 abundant calcium carbonate alone remains. A still more delicate adjust- 

 ment of the same nature is frequently illustrated in humid regions, 

 where in many instances incrustations of calcium carbonate occur prin- 

 cipally or wholly on the under surfaces of the pebbles and stones in 



