42 



PHYSIOGRAPHY 



it comes to the surface, some of the mineral matter in solution may 

 be deposited. (3) Warm spring-water often gives up what it held 

 in solution, when it cools. (4) Microscopic plants sometimes grow 

 in the waters which issue from hot springs, as in the Yellowstone 

 Park. These tiny plants, by some process not well understood, 

 extract mineral matter from the water, and cause it to be deposited 

 (Fig. 2, PL XI, p. 32). 



Solution and deposition may be going on at the same time, even 



in the same place; that is, 

 the water may be dissolving 

 one substance while it is 

 depositing another. Thus 

 the original material of a 

 buried shell may be dissolved 

 and carried away, at the 

 same time that other mate- 

 rial is left in its place, pre- 

 serving the form of the shell. 

 In the same way, wood may 

 be replaced by mineral mat- 

 ter, giving rise to petrified 

 wood, or wood " turned to stone " (Fig. 27). Such changes probably 

 take place slowly, the mineral matter which was in solution in the 

 water replacing the woody matter as it decays. 



Summary. From the preceding paragraphs, it will be seen 

 that ground-water brings about various changes in the rocks. 

 These changes take place slowly, but they are going on all the time. 

 In the long course of ages, they are so great that .an eminent geol- 

 ogist has said, " Given time enough, and nothing in the world is more 

 changeable than the rocks." 



Mechanical Work 



Abrasion, slumping, sliding, etc. Abrasion by ground-water 

 is slight, since ground-water rarely flows in strong streams. In- 

 directly, ground-water helps to bring about changes of another 

 sort. When the soil on a steep slope becomes full of water, its 

 weight is greatly increased, and the water in it makes it move 



Fig. 27. A petrified log near Holbrook, 

 Arizona. 



