DESTRUCTIVE ACTION OF WATER 



out of the mineral matter contained in the water consumed 

 yearly by the townspeople. 



Rocks and minerals are not all equally soluble in water ; 

 some are so little soluble that it is years before any change 

 becomes apparent, and the substances are said to be insolu- 

 ble, yet in reality they are slowly dissolving. Other rocks, 

 like limestone, are so readily soluble in water that from the 

 small pores and cavities eaten out by the water, there may 

 develop in long centuries, caves and caverns (Fig. 30). Most 

 rock, like granite, con- 

 tains several sub- 

 stances, some of which 

 are readily soluble and 

 others of which are not 

 readily soluble; in such 

 rocks a peculiar ap- 

 pearance is presented, 

 due to the rapid disap- 

 pearance of the soluble 

 substance, and the per- 

 sistence of the more re- 

 sistant substance (Fig. 



SO- 



We see that the 

 solvent power of water 

 is constantly causing 

 changes, dissolving 

 some mineral substan- 

 ces, and leaving others 

 practically untouched ; 

 eating out crevices of various shapes and sizes, and by gradual 

 solution through unnumbered years enlarging these crevices 

 into wonderful caves, such as the Mammoth Cave of Kentucky. 



FIG. 31. The work of water as a solvent. 



