2 ag ELEMENTARY GEOLOGY 
rock into a compact mass, though here and there joint 
planes cross it in several directions. It seems to be so 
hard that water could not enter it; yet, if our eyes 
were powerful enough, they would detect many minute 
crevices, through which water is constantly passing. 
Indeed, water is in every rock, and everywhere in the 
earth is slowly journeying along. Some beds (like the 
* 
Fic. 48. 
Weathering of a rock of variable hardness. Soft layers removed, leaving harder 
parts standing in unstable position. 
sandstones) are fairly porous, and in them wells will 
find an inexhaustible supply of water (see p. 147); but 
granites have fewer and smaller crevices, though even 
in these, water is always present in the very hearts 
of the blocks. 
Mechanical Agents. — Most Action. If our granite 
ledge is in a cold climate, like that of the northern 
