116 PHYSICAL GEOLOGY 



many expansions and contractions is an appreciable move- 

 ment toward the base of the slope. Landslides, too, may be 

 due to a combination of causes. Rock masses may be in an 

 unstable condition, and traversed by fissures and joint 

 planes in such a manner as to favor landslides. Among 

 other things, earthquake tremors and sudden and consider- 

 able changes in temperature (especially if they involve the 



FIG. 108. Landslide in the San Juan Mountains, Colo. (Howe, U.S. 

 Geol. Surv.) 



freezing point) may determine the moment when such masses 

 slump. 



Although the mechanical work of ground water is rela- 

 tively unimportant, it does a vast amount of chemical work 

 of several sorts, to which attention is now directed. 



Solution by ground water. Pure water dissolves only a 

 few minerals and rocks readily, but practically all ground 

 waters are impure. In general, water dissolves mineral 

 matter more easily when its temperature is high rather than 

 low, and when it is under great, rather than little, pressure. 

 Its power to dissolve certain rock material is also increased 



