30 DYNAMICAL GEOLOGY 



not be left out of account in estimating the forces which have 

 made the earth what we find it. 



Much as may be learned by the study of the operation of the 

 forces which are still at work in modifying the earth, this method 

 of study is yet insufficient to solve all geological problems. Many 

 of the changes which have indisputably taken place are such as 

 no man has ever observed, because they are brought about so 

 slowly or so deep down within the crust that no direct observation 

 is possible, and we can only infer the mode of procedure by 

 examining the result. No human eye has ever witnessed the 

 birth of a mountain range, or has seen the beds of solid rock 

 folded and crumpled like so many sheets of paper, or observed 

 the processes by which a rock is changed in all its essential charac- 

 teristics \ " metamorphosed," as it is technically called. All such 

 problems must be discussed in connection with structural geology. 



The dynamical agencies may, primarily, be divided into two 

 classes : I, the Subterranean Agencies, which act, or at least origi- 

 nate, at considerable depths within the earth ; and II, the Surface 

 or Superficial Agencies, whose action takes place at or near the 

 surface of the earth. The former are due to the inherent energy 

 of the earth, and their seat "is primarily subterranean, though their 

 effects are very frequently apparent at the surface. These agen- 

 cies are also called igneous (fromignis, fire), which is a misnomer; 

 but the term is nevertheless in common use. 



The logical order of treatment of these subjects is to begin with 

 the subterranean agencies, because the most ancient rocks of the 

 earth's crust were doubtless formed by these forces, and the circu- 

 lation of matter upon and through the crust started originally 

 from igneous rocks, made by cooling at the surface of a molten 

 globe. 



