238 STRUCTURAL GEOLOGY. 



SECTION III. MOUNTAINS : THEIK ORIGIN AND 

 STRUCTURE. 



Mountains are the glory of the earth the culminating 

 points of scenic grandeur and beauty. But few perceive 

 that they are so only because they are also the culmi- 

 nating points of all geological agencies. This is but 

 one illustration of the general truth, that there is an in- 

 dissoluble and necessary connection between truth and 

 beauty, between science and fine art. It is evident, then, 

 that the study of mountains is the key to dynamical and 

 structural geology. 



The difficulty which meets us at the threshold of this 

 subject is the loose use of the term mountain. The term 

 is used to express every conspicuous elevation above the 

 general level, whatever be its extent, and in whatever way 

 it may have been formed. Thus an isolated eminence 

 produced by circum-erosion, or a peak formed by volcanic 

 ejection, a ridge between two stream-gorges, a great bulge 

 produced by the folding of the earth's crust, or a series of 

 such foldings parallel to each other in the same general 

 region are all called by the same name, mountain. 

 Qualifying terms are indeed often used, such as moun- 

 t&in-peak, mountain-nW^e, mountain-ra^e, etc., but 

 these also are used loosely and interchangeably. It is 

 necessary, therefore, first of all, to define our terms. 



Definitions. A mountain-#m, or, better, moun- 

 tain-system, is an assemblage of ranges parallel to each 

 other in the same general region, but usually formed at 

 different times (poly genetic). All the great mountain- 

 chains of the world are of this nature. For example : 

 The Appalachian system consists of the Blue Ridge, the 

 Alleghany, and the Cumberland ranges. The Rocky 

 Mountain system, or North American Cordilleras, consists 

 of the Colorado range, the Park range, the AVahsatch, 



