GROUND-WATER 45 



by variations of temperature, and the chemical and mechanical 

 changes caused by ground-water, all conspire to alter the surface 

 of exposed rock, so as to cause it to crumble and waste away. We 

 have already seen that the surfaces of the bowlders of the field are 

 often scaling off or crumbling. They are often discolored, even 



Fig. 30. Weathered lava, Yellowstone Park. (From photo. U. S. 

 Geol. Surv.) 



when they seem firm, and from the walls of stone buildings, from 

 monuments, and from other stone structures, flakes sometimes 

 scale off. The upper layers of stone in a quarry are in some cases 

 broken, and different in color from those below. Inscriptions on 

 some old tombstones are indistinct, and they have disappeared 

 completely from some stones which have been long exposed. In all 

 these cases some change has taken place in the rock whereby its 

 outer part is wasted, or weathered, away. 



It is to be noticed that weathering is not one process, but many 



