454 



A TREATISE ON METAMORPHISM. 



well known that plants abstract certain ingredients from the soil, and there- 

 fore exert a soluble effect. The inorganic materials which are most abun- 

 dantly abstracted are potash, soda, magnesia, lime, phosphoric acid, and 

 silica. The relative proportions of these substances in grain and straw, as 

 given by Johnson, are as follows: ° 



Besides these substances, the plants also abstract such compounds as 

 iron, sulphur, and chlorine in small amounts. The sulphur is absorbed as 

 a sulphate. 



Finally, living plants have a further effect upon the rocks. Fresh roots 

 give acid reactions, and as a consequence, where they are in contact with 

 the rocks they corrode them chemically. This is illustrated by the action 

 of lichens and creepers and by the corroded surfaces of rocks where roots 

 are in contact with them. In some cases where the rocks are rather readily 

 soluble, as, for instance, limestone, actual furrows may be produced in the 

 rocks as a result of the solution.'' In places where lichens occur the rocks 

 may be softened to a depth of 2 to 4 centimeters. Johnson states that "on 

 the Schwalbenstein, near Grlatz, in Silesia, at a height of 4,500 feet [1,368 

 meters], the granite is disintegrated under a covering of lichens, the feldspar 

 being converted into kaolin or washed away, only the grains of quartz and 

 mica remaining unaltered." d 



Positive evidence that the chemical action of the plants is important is 

 furnished by experiments showing that soils upon which plants have been 

 grown are more soluble after all the parts of the plants have been remoA^ed 

 than are similar soils in which no plants have been grown. 



« Johnson, S. W., How crops feed, Orange Judd & Co., New York, 1870, p. 364. 

 ^Storer, F. H., Agriculture in some of its relations with chemistry, Charles Scribner's Sons, 

 New York, vol. 1, 1887, pp. 187-188. 

 ''Storer, cit, pp. 130, 131. 

 ''Johnson, cit., p. 142. 

 * Storer, cit., p. 186. 



