26 PHYSIOGRAPHIC GEOLOGY. 



border, and Mono Lake in California, at the foot of the lofty Sierra, is 

 another on the western border. The eastern half of the plateau south of 

 the Colorado River extends south into Mexico, and there has similar arid 

 features, with saline lakes and inside drainage. 



The plateau of Tibet is an intermont plateau between the main range of 

 the Himalayas and the Kuen-Lun Mountains. It is about 13,000 feet in 

 altitude, but is overlooked by mountains having an altitude of 25,000 to 

 29,000 feet, and has its own ridge of 20,000 feet. It is 1200 miles from east 

 to west, and half this in mean breadth ; but its eastern half is much encum- 

 bered by ridges. 



The plateau of Quito, about 300 miles long, 40 miles wide, and 10,000 feet 

 above tide level, is situated between two parallel cordilleras of the Andes, 

 the eastern of which contains among its snow-capped cones or domes, 

 Cayambe (19,535, and on the equator), Antisana, Cotopaxi (19,613), Sangay; 

 and the western, including Chimborazo (20,498 feet), Pichincha (15,924 feet), 

 and others. The plateau of Bolivia has an elevation of 12,900 feet, with 

 Lake Titicaca at 12,830 feet, and the city of Potosi at 13,330 feet. 



In Europe, Spain is for the most part a plateau about 2250 feet in average elevation; 

 Auvergne, in France, another, of about 1100 feet ; Bavaria, another, of 1660 feet. Persia 

 is a plateau varying in elevation between 2000 and 4000 feet, with high ridges in many 

 parts. The Abyssinian plateau, in Africa, has an average elevation of more than 7000 

 feet ; the region of Sahara about 1500 feet, except the southern part, which lies mostly at 

 a greater altitude than 650 feet ; that of southern Africa south of the parallel of 10° S. 

 from 3000 to 4000 feet in mean altitude, and rising into many high summits, with the ele- 

 vation least to the west. 



Mountains. — (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 (Sicily) and 

 Loa (Hawaii), have a slope of only six to eight degrees: such mountains are 

 broad cones, having a base of 40 miles or more. The higher volcanic cones 

 of western America are mostly 25° to 35° in angle of slope. 



The average eastern slope of the Rocky Mountains seldom exceeds 10 

 feet a mile, which is about one foot in 500, equal to an angle of only 7'. 

 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 4944 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 above 14,000 feet. The highest 

 part of the range is in Colorado, where the passes are 11,000 to 13,000 feet 

 high ; while in latitude 32° the passes are about 5200 feet ; on the Central 

 Pacific Railroad, 6184 feet high ; in Canada, 5264 to 7100 feet high ; and on 

 the Canadian Pacific (the Kicking Horse Pass) 5300 feet high. The moun- 

 tain mass, therefore, is not a narrow barrier between the east and west, as 

 might be inferred from the ordinary maps, but a vast yet gentle swell of the 

 surface, having a base 1000 miles in breadth, and the slopes diversified with 

 various mountain ridges, or spreading out in plateaus at different levels. 



