232 GEOLOGY. 



or bluff into the river, or settle away slowly from their former positions. 

 This is a common phenomenon along streams which have cut valleys 

 in drift, and along shores on which waves are encroaching. The same 

 phenomenon is common on a larger scale on the slopes of steep moun- 

 tains.^ Considerable terraces are sometimes developed on their slopes 

 in this way, but they are usually irregular and discontinuous (Figs. 208, 

 and 209). The loose debris on steep slopes sometimes assumes a sort 

 of flowing motion and descends the slope with some such form and at 

 some such rate as a glacier. Such bodies of debris are sometimes called 

 ''talus glaciers" (Fig. 210). In many such cases, snow and ice have 

 had some part in their development. 



In creep and in landshdes gravity is the force involved, and the 

 ground-water only a condition which makes gravity effective. Gravity 

 alone accomplishes similar results, as illustrated by Fig. 211. 



j: Su7nmary. 



All in all, ground-water is to be looked upon as a most important 

 geological agent. When it is remembered that a very large part of 

 all the water which falls on the surface of the earth, either in the form 

 of rain or snow, sinks beneath the surface; that much of it sinks to a 

 great depth; that nmch of it has a long underground course before it 

 reappears at the surface; that it is everywhere and always active, 

 either in subtracting from the rock through which it passes, in adding 

 to it, in effecting the substitution of one mineral substance for another, 

 or in bringing about new chemical combinations; and when it is re- 

 membered that this process has been going on for untold millions of 

 years, it will be seen that the total result accompHshed must be stu- 

 pendous. The rock formations of the earth to the depths to which 

 ground-water penetrates are to be looked upon as a sort of chemical 

 laboratory through which waters are circulating in all directions, 

 charged with all sorts of mineral substances. Some of the substances 

 in solution are deposited beneath the surface, and some are brought 

 to the surface where the waters issue. Much of the material brought 

 to the surface in solution is carried to the sea and utilized by marine 

 orga,nisms in the making of shells. Without the mineral matter brought 

 to the sea by springs and rivers, many shell-bearing animals of great 



^ Russell has emphasized this point in 20th Ann. U. S. Geol. Surv., Pt. II, pp. 

 193-202, and Cross, 21st Ann. U. S. Geol. Surv., Part II, pp. 129-150. 



