16 PHYSIOGRAPHIC GEOLOGY. 



Eastern Cordillera of North America includes the Appalachian chain, 

 the Adirondacks, and the Nova Scotia range. The term is thus used 

 by J. D. Whitney. 



The ridges of a common chain, and even those of a range, are not 

 generally of the same age, as regards origin. Even the Green Moun- 

 tains contain ridges that existed long before the main range had been 

 elevated. The Appalachian chain has, in the Highlands of New Jer- 

 sey, and the Blue Ridge, ridges of pre-Silurian age; in the Green 

 Mountains, others which are mostly of middle-Silurian age ; in the 

 Alleghanies, others that are post-Carboniferous. Ridges, in topog- 

 raphy, are grouped according to their relations in position and some 

 related method of origin, but not according to time of origin. 



A plateau is an extensive elevated region of flat or hilly surface, such 

 as often occurs in mountainous regions. Any extensive range of coun- 

 try 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 re- 

 gion between distant chains. The " Great Basin " between the Salt 

 Lake and the Sierra Nevada is a plateau of the Rocky Mountain chain, 

 4,000 to 5,000 feet in elevation: the Salt Lake lies in its northeast 

 corner, 4,200 feet above the sea. The plateau or table-land of Thibet 

 lies between the Himalayas and the Kuen Lun 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, hav- 

 ing a mean elevation of 4,000 feet. The State of New York is an 

 elevated plateau, 1,500 to 1,700 feet in altitude north of the Mohawk 

 Valley (an east-and-west valley), and 2,000 to 2,500 feet south of it; 

 it lies in the course of the Appalachian Mountains. 



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



Mountains. — The form of an isolated mountain-peak depends on 

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

 the crest, and (3) the course or arrangement of the consecutive 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 Loa (Ha- 

 waii), 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 exam- 

 ples 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 60(? 



