i8 THE STORY OF PLANT LIFE 



carries the particles away, by sorting them out, 

 elutriating them, and redepositing the fine particles 

 of sand, etc., in a looser form ready for use by 

 plants, as soil on which they can live, in place of 

 hard rock surfaces upon which they cannot. 



Frost, ice, glaciers, alternate heat and cold are 

 similarly important agents in breaking up rock 

 masses into soft and loose soils, as sands, or in closer 

 form, as clays, etc. 



Out of all these many different rock formations, of 

 which fifty might be named, there are about six 

 types, differing chemically and physically, clay or 

 loam, affording aluminium ; sand, yielding silicon ; 

 chalk, yielding calcium ; marl, yielding aluminium 

 and calcium ; humus, yielding carbonic acid, phos- 

 phorus, etc.; and humous acids in the form of peat. 



Not only do soils differ in chemical characters, but 

 in regard to the water they contain — a most important 

 feature. 



Rainfall is one source of the soil water. Another 

 is dew. The slope of a tract determines the drainage, 

 and mountainous tracts have to depend largely upon 

 continuous rain, dew, and mist, which they, more- 

 over, help to precipitate. The water which lies 

 deep in the soil or telluric water derived from below 

 depends upon the character of the soil, its perme- 

 ability, etc, and the available underground sources. 



The water thus collected from the clouds or 

 underground springs or reservoirs may not be avail- 

 able, however, to the plant. The water in mountainous 



