CLASSIFICATION OF MOUNTAIN RANGES. 667 



1. Folded Mountain Ranges. Foremost in their geographic 

 importance, and in the intricacy and significance of their geologic 

 structure and origin, are the mountain belts which consist of 

 folded rock formations. The strata forming the upper part of 

 the earth's crust are bent up and down in long, nearly straight or 

 curving, wave-like ridges and troughs, and where their disturb- 

 ance was greatest the successive ridged folds are closely pressed 

 together. The waves of the rock structure are then pushed to 

 such steepness that their sides become parallel with each other, 

 and the entire fold is driven forward into an inclined position. 

 The order of the strata on the lower side of the appressed fold is 

 thus inverted ; the originally highest and last formed deposits 

 there lie beneath older beds, in an overturned series. Subaerial 

 erosion then wears down the undulations and the crests of the 

 closely folded strata, often planing them off until a long section, 

 crossing mountain ranges, passes from older to newer beds, and 

 onward from newer to older, in several alternations, having 

 throughout the whole a nearly constant steep dip. Owing to the 

 interbedding of hard and enduring sandstone, quartzite, gneiss, 

 and other rock formations, with more easily eroded limestone, 

 shales, incoherent sandstones, or schists, the erosion commonly pro- 

 duces a new topography, making hollows and long valleys where 

 the more erosible beds have been removed, and leaving ridges and 

 mountain ranges of the harder rocks. More than this, when ero- 

 sion has been continued through very long periods, it tends toward 

 the ultimate result of removing the upward curved or anticlinal 

 portions of the great folds and sparing the originally lower down- 

 ward curved or synclinal portions, until valleys take the places 

 which were originally occupied by the highest upheavals, while 

 the original troughs, where the rocks were most compacted by 

 pressure, remain now as the principal mountain ridges. Under 

 denudation, the folded mountainous belt fulfills the prophecy, 

 " Every valley shall be exalted, and every mountain and hill shall 

 be made low." 



The most perfect type which the world affords of this structure, 

 or at least the example which has been most fully studied as to 

 the age of its strata, the dates of their foldings and upheavals, and 

 the effects of erosion, is the Appalachian mountain system. As 

 made known by the brothers W. B. and H. D. Rogers and by later 

 geologists, a vast series of Pakeozoic strata, representing continu- 

 ous deposition from the early Cambrian to the close of the Car- 

 boniferous period, is thrown into many long, steep folds in the Ap- 

 palachian ranges of Pennsylvania and the Virginias, making the 

 southeast part of this mountain system, and into plateaus and 

 gentle undulations in the Catskill, Alleghany, and Cumberland 

 Mountains, which are its northern and western portions. After 



