22 PHYSIOGRAPHIC GEOLOGY. 



about that city a height of 7482 feet, and slopes from this to 5000 

 on the east and 4000 on the west ; and it stretches on north 

 beyond the Mexican territory, blending with the plateaus of New 

 Mexico. Above it rise many lofty volcanic cones, among which 

 Popocatepetl is 17,884 feet high, Orizaba 17,373 feet, and Istacci- 

 huatl 15,704. 



The plateau of Quito, in the Andes, has a height of 10,000 feet, — Quito itself 

 9540 feet; and around it are Cotopaxi, 18,775 feet, Chimborazo, 21,421, Pichin- 

 cha, 15,924, Cayambe, 19,535. The plateau of Bolivia is at an elevation of 

 12,900 feet, with Lake Titicaca, 12,830 feet, and the city of Potosi at 13,330 feet ; 

 and near are the volcanic peaks Illimani, 23,868 feet, Sorata, 25,290, Huayna 

 Potosi, 20,260. In Europe, Spain is for the most part a plateau about 2250 feet 

 in average elevation; Auvergne, in France, another, at about 1100 feet; Bavaria 

 another, at 1660 feet. Persia is a plateau varying in elevation between 3800 

 and 4500 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 ; that of the interior of Africa south of the equator, about 2500 feet. 



27. River-Systems. — Plateaus and mountains are the sources of 

 rivers. They pour the waters along many channels into the basin 

 or low country towards which they slope ; and the channels, as 

 they continue on, unite into larger channels, and finally into one or 

 more trunks which bear the waters to the sea. The basin and its 

 surrounding slopes make up a river-system. The extent of such 

 a region will vary with the position of the mountains and ocean. 

 It may cover but a few hundred square miles, like the river-regions 

 on a mountainous coast, or it may stretch over the larger part of 

 a continent. 



The interior of the United States belongs to one river-system, — 

 that of the Mississippi ; its tributary streams rise on the west 

 among the snows of the Rocky Mountains, on the north in the central 

 plateau of the continent, west of Lake Superior, near lat. 47° and 

 beyond long. 93°-96°, 1680 feet in elevation, and on the east in the 

 Appalachians, from western New York to Alabama. Besides the 

 Mississippi, there are other rivers rising in the Rocky Mountains 

 and flowing into the Gulf of Mexico ; and, in a comprehensive 

 view of the continent, these belong to the same great river-system. 



The St. Lawrence represents another great river-region in North 

 America, — a region which commences in the head-waters of Lake 

 Superior about the same central plateau of the continent that 

 gives rise to the Mississippi, and embraces the great lakes with 

 their tributaries and the rivers of Canada, and flows finally north- 

 eastward into the Atlantic, following thus a northeast slope of the 

 continent. North of Lake Superior and the head-waters of the 

 Mississippi, as far as the parallel of 55°, there are other streams, 



