250 STRUCTURE COMMON TO ALL ROCKS. 



the strata the quantity is so small as to be undetectable ; it is thence 

 carried and concentrated in veins in a more available form ; it is next 

 set free along the backs of these veins in a still more available form ; 

 it is last carried down by currents along with other materials, neatly 

 sorted, and deposited iu placers in a form the most available of all. 



Section 3. — Mountains: their Origin and Structure. 



Mountains are often regarded as types of permanence. We speak of 

 the everlasting hills. The first lesson taught by geology is that all 

 things, even the most stable, are slowly changing. In this section we 

 treat of the origin, growth, maturity, decay, and death — in a word, the 

 whole life-history of mountains. 



Mountains are the glory of our earth, the culminating points of 

 its scenic grandeur and beauty. But few recognize the fact that they 

 are so, only because they are also the culminating points, the theatres 

 of greatest activity, of all geological agencies. The study of mountains 

 is therefore of absorbing interest not only to the poet and painter, but 

 also and especially to the geologist, because it furnishes the key to 

 many of the obscurest problems of dynamical geology. 



But we are met at the very threshold of the subject by a difficulty 

 arising from the loose use of the term mountain. This term is used 

 for every conspicuous elevation above the general level of the surround- 

 ing country, whatever may be its dimensions or its mode of origin. 

 Thus we apply it to a whole system of ranges, such as the Rocky- 

 Mountain system, or the Andes, or the Himalayas ; or to each compo- 

 nent range of such a system, such as the Sierra or the Wahsatch ; or to 

 each prominent peak on such a range, as, for example, Mount Lyell, 

 Mount Dana, or Mount Shasta. It is necessary, therefore, first of all, 

 that we should define what we are going to discuss. 



Definitions of Terms. — A Mountain- System is a great complex of 

 more or less parallel ranges in the same general region, but born at 

 different times (polygenetic). It is a family of mountains. In the 

 Rocky- Mountain system, or, as it is better called, the North American 

 Cordilleras, we have the Colorado range (Front range), the Wahsatch 

 range, the Basin ranges, the Sierra range, and several others. Simi- 

 larly the Andes and Himalayas consist of several ranges. 



A Mountain- Range is a single mountain-individual produced by 

 one birth-throe (monogenetic), although both the origin and the subse- 

 quent growth is a slow process. The Sierra, the Wahsatch, the Uinta, 

 and the Colorado Mountains are good examples. 



A Mountain- Ridge is a subordinate part of a range, produced either 

 by separate folds made at the same time, or by faulting, or by erosion. 

 The Blue Ridge, the Alleghany, and the Cumberland Mountains are ex- 



