GENERAL FEATURES OF THE EARTH. 



19 



24. (b.) Composition of mountain-chains. — (1.) Mountain-chains have 

 been stated to include several mountain-ridges ; and even the 

 ridges often consist of subordinate 

 parts similar in arrangement. In 

 the great chain of western North 

 America, — the Eocky Mountains, 

 — about the summit there are, in 

 general, two prominent ranges ; 

 then, west of the summit, within 

 100 to 150 miles of the coast, there 

 is the Washington Eange, includ- 

 ing the Cascade of Oregon and the 

 Sierra Nevada of California, each 

 with peaks over 12,000 feet in 

 height ; between this range and the summit there are in many 

 parts several ridges more or less important ; and between it and 

 the coast other ridges make up what has been called the Coast 

 Eange. The Appalachians also, although but a small chain, con- 

 sist of a series of nearly parallel ridges. In Virginia there are, 

 beginning to the eastward, the Blue Eidge, the Shenandoah Eidge, 

 and the Alleghany, besides others intermediate. 



(2.) The ridges of a chain vary along its course. After continu- 

 ing for a distance, they may gradually become lower and disap- 

 pear ; and while one is disappearing another may rise to the right 

 or left ; or the mountain may for scores of leagues be only a pla- 

 teau without a high ridge, and then new ranges of elevations 

 appear. The Eocky Mountains exemplify well this common cha- 

 racteristic, as is seen on any of the recent maps. The Sierra Ne- 

 vada dies out where the Cascade Eange begins ; and each has 

 minor examples of the same principle. The Andes are like the 

 Eocky Mountains ; only the parts are pressed into narrower corn- 



Figs. 11 to 16. 



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pass, and the crest-ranges are hence continuous for longer dis- 

 tances. The Appalachian ridges are rising and sinking along the 



