114 PHYSICAL GEOLOGY 



At irregular intervals the expansion of steam formed at 

 some point below the surface of the water of the geyser, 

 supposedly by contact with hot volcanic rocks, forces the 

 overlying column of water into the air, like a great fountain. 

 As the lava which heats the water cools, eruptions become 

 less and less frequent, and finally cease. All existing geysers 

 will therefore become extinct. They may, however, be re- 

 placed by others elsewhere. 



THE WORK OF GROUND WATER 



Mechanical work. In general, ground water transports 

 very little material in a solid state, and this material wears 



FIG. 106. Shows the results of creep. The hare rock is sandstone, resting 

 on a sloping surface of shale which is often made wet and more or less slip- 

 pery by water sinking through the joints and other openings of the rocks 

 above. The detached mass of sandstone has slowly settled away from 

 the rock wall on the left, the line of division being determined by a large 

 joint plane. 



the rock surfaces it encounters but slightly. This is because 

 ground water is rarely concentrated in well-defined channels 

 to form streams, but usually moves very slowly (a few feet 



