THE MAJOR SOIL-FORMING MINERALS. 37 



gether with more or less of potash and soda, more rarely 

 magnesia and baryta. The water is easily expelled by heat- 

 ing, but is present in the basic form, not merely as water of 

 crystallization. All zeolites are readily decomposed by chlor- 

 hydric and other stronger acids. 



The zeolites proper are not original rock ingredients, but are formed 

 in the course of rock decomposition by atmospheric agencies, heated 

 water, and other processes not fully understood. They are therefore 

 usually found in the cavities and crevices of rocks that have been sub- 

 ject to the influence of atmospheric or thermal waters, most frequently 

 in eruptive rocks, particularly in the vesicular cavities characterizing 

 what is known as amygdaloids. They are also found in the crevices of 

 sandstones and shales percolated by water, as well as in nodules of 

 infiltration (geodes), in which they are frequently associated with quartz. 

 Those found in the cavities of rocks are usually well crystallized wherever 

 room is afforded, and are readily recognized by their crystalline form; 

 they are mostly colorless, sometimes yellow or reddish. 



Exchange of bases in Zeolites. Although zeolites rarely 

 form a large proportion of rock masses and therefore do not 

 enter directly into the soil minerals to any great extent, their 

 interest in connection with soil-formation is very great, because 

 of the continuation, within the soil, of the same processes that 

 bring about their formation in rocks. Under the conditions 

 existing in soils they will naturally rarely form crystals, but 

 will appear in the pulverulent or gelatinous form, leaving the 

 zeolitic nature of the material to be inferred from its chemical 

 behavior. Among these characters the ready decomposability 

 by acids has already been mentioned ; another of special import- 

 ance in the economy of soils is the fact that when a pulverized 

 zeolite is subjected to the action of a solution containing either 

 of the stronger bases usually present (potash, soda or lime), 

 such base or bases will be partially or wholly taken up by the 

 zeolitic powder, while corresponding amounts of the bases 

 originally present will pass into solution. 



Thus when a hydrosilicatc of soda and alumina is digested with a 

 solution of potassic chlorid or sulphate, the soda may be partially or 

 wholly replaced by potash, while the corresponding sodium salt passes 

 into solution. In the case of zeolites containing lime or magnesia or 



