16 PHYSIOGRAPHIC GEOLOGY. 



the plural is used, as the Green Mountains, the Ozark Mountains. A 

 mountain-chain is a system of ridges with the included high land. 



A sierra is, in Spanish, a ridge of mountains, and alludes to the saw-like out- 

 line. A cordillera, in South America, is a mountain-chain. 3Iauna, as in 

 Mauna Loa, of Hawaii, signifies mount. 



An elevated plateau is an extensive elevated region of flat or hilly 

 surface, such as often occurs in mountainous regions. Any exten- 

 sive range of country that is over a thousand feet in altitude would 

 be called a plateau. It may lie along the course of a mountain- 

 chain, or occupy a wide region between distant chains. The 

 " Great Basin" between the Salt Lake and the Sierra Nevada is a 

 plateau of the Kocky Mountain chain, 4000 to 5000 feet in eleva- 

 tion : the Salt Lake lies in its northeast corner, 4200 feet above the 

 sea. The plateau or |able-land of Thibet lies between the Himalayas 

 and the Kuen-Luen Mountains next to the north, and is 11,500 

 to 13,000 feet in altitude ; and the plateau of Mongolia (Desert of 

 Gobi) occupies a vast region farther north, having a mean eleva- 

 tion of 4000 feet. The State of New York is an elevated plateau, 

 1500 to 1700 feet in altitude north of the Mohawk (an east-and- 

 west valley), and 2000 to 2500 feet south of this river: it lies in the 

 course of the Appalachian Mountains. 



Plateaus often have their mountain-ridges, like low lands. 



23. Mountains. — The form of an isolated mountain-ipe&k. depends 

 on its general slopes ; that of a ridge, on (1) its slopes, (2) the out- 

 line of the crest, and (3) the course or arrangement of the consecu- 

 tive parts of the ridge ; that of a chain, on all these points, and in 

 addition (4) the order or arrangement of the ridges in the chain. 



(a.) Slopes of mountains. — The mountain-mass. — The slopes of the larger 

 mountains and mountain-chains are generally very gradual. Some 

 of the largest volcanoes of the globe, as Etna and Mount Loa 

 (Hawaii), have a slope of only 6 to 8 degrees. The mountains are 

 low cones having a base of 50 miles or more. 



The Rocky Mountains, Andes, and Appalachians are three 

 examples of mountain-chains. The average eastern slope of the 

 Rocky Mountains seldom exceeds 10 feet in a mile, which is about 

 1 foot in 500, equal to an angle of only 7 minutes. On the west 

 the average slope is but little less gradual. The rise on the east 

 continues for 600 miles, and the fall on the other side for 400 to 

 500 miles ; the passes at the summit have a height of 6000 to 10,000 

 feet ; and above them, as well as over different parts of the slopes 

 (especially on the west), there are ridges carrying the altitude to 

 12,000 or 14,000 feet. The mountain-mass, therefore, is not a nar- 

 row barrier between the east and west, as might be inferred from 



