THE FORMATION OF SOIL. 55 



formerly acted as a cement to bind its different 

 ingredients together. "When charged with either 

 oxygen or cabonic acid gas, derived generally 

 directly from the air, the chemical action of these 

 dissolved gases greatly aids the water in break- 

 ing up, and thus rendering partly porous, even the 

 hardest of the igneous rocks. 



Let us take, for example, some of the com- 

 monest minerals of the igneous rocks, such as 

 quartz, feldspar, and mica. 



None of these ingredients are very soluble in 

 pure water ; but if the water contains oxygen and 

 carbonic acid gas in solution, the feldspar will be 

 gradually broken up, and the hard granites or 

 gneisses, which form so large a portion of the 

 igneous rocks, will be gradually disintegrated. 

 From the feldspar will be derived the kaolins or 

 clays, and the water percolating through them 

 will be charged with a small quantity of potash, 

 so necessary for the growth of plants. 



Limestones are readily disintegrated by water 

 which contains carbonic acid in solution, and from 

 such rocks are derived the calcareous matter so 

 necessary to plant-growths. 



The carbonic acid dissolved in water sometimes 

 acts to considerably change the character of the 



