76 



PHYSICAL GEOLOGY 



deeply eroded before the sand was spread over it (Why?). 

 Likewise an irregular weathered surface between two parallel 

 beds indicates that deposition was interrupted after the first 

 layer was deposited and that erosive processes carved the 



FIG. 59. An unconformity in Wyoming. (Fisher, U.S. Geol. Surv.) 



irregular surface before deposition was resumed in forming the 

 upper layers. Although not so conspicuous, these also are 

 cases of unconformity. 



It is obvious that unconformities give important evidence 

 of changes that have taken place in previous ages. Their 

 significance is further discussed in later chapters. 



VULCANISM 



Rise of lava through the crust. The rise of molten 

 rock from unknown depths into the outer crust may be ob- 

 served and is proved also by many other facts. How it 

 becomes molten, how far down it originates, why it rises to the 

 surface, and how it makes its way, are difficult problems, none 

 of which has thus far been satisfactorily solved. 1 



1 The many theories which have been suggested to answer these ques- 

 tions are discussed in some of the larger textbooks of Geology. 



