CHAPTER XIX 

 MOUNTAIN RANGES — CYCLES OF EROSION 



The term mountain is somewhat loosely employed for any lofty 

 eminence, and the distinction between mountains and hills, as 

 ordinarily made, is principally a question of mere height. Some 

 so-called mountain peaks and ridges are merely the portions of 

 dissected plateaus left standing, such as Lookout Mountain and 

 Missionary Ridge in Tennessee, and the Allegheny Front in Penn- 

 sylvania. Such mountains usually have flat tops (table mountains), 

 are composed of strata which are nearly or quite horizontal, and 

 owe their existence either to their being composed of more re- 

 sistant rocks than the denuded parts of the plateau, or to their 

 favourable situation with reference to the drainage lines. Another 

 type of mountain is the volcanic, which is usually an isolated cone 

 and may be built up to great heights ; it is simply the accumula- 

 tion of volcanic material which has been piled up around the 

 vent. Typical mountain ranges and chains differ materially from 

 either of these classes, both in their structure and their mode of 

 origin. Before proceeding to discuss the origin and history of 

 mountains, it will be necessary to define the terms to be used. 



A Mountain Range is made up of a series of more or less par- 

 allel ridges, all of which were formed within a single geosyncline 

 (p. 236) or on its borders. The ridges are separated from one 

 another by longitudinal valleys and may be formed either by the 

 successive folds or by denudation within the limits of the folds. 

 In the latter case the outcropping harder strata make the ridges. 

 A mountain range is always very long in proportion to its width, 

 and its ridges have a persistent trend. These features distinguish 

 a true range from the ridges cut out of a plateau by denudation. 

 The Appalachian range, the Wasatch, the Coast Range, are ex- 

 amples of typical mountain ranges. 



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