GENERAL FEATURES OF THE EARTH. 15 



occur on the continent of Asia, which has also its great depressed Cas- 

 pian area. In America, below the ocean's level, Death's Valley, east 

 of the Sierra Nevada, California, about latitude 36°, is 100 to 200 feet 

 below the ocean's level. 



(5.) Subdivisions of the surface, and character of its reliefs. — ■ 

 The surfaces of continents are conveniently divided into (1) low lands; 

 (2) plateaus, or elevated table lands; (3) mountains. The limits be- 

 tween these subdivisions are quite indefinite, and are to be determined 

 from a general survey of a country rather than from any specific defi- 

 nitions. 



The low lands include the extended plains or country lying not far 

 above tide-level. In general they are less than 1,000 feet above the 

 sea ; but they are marked off rather by their contrast with higher 

 lands of the mountain-regions than by any precise altitude. The Mis- 

 sissippi Valley of the great interior region of the North American 

 continent is an example ; also the plains of the Amazon ; the pampas 

 of La Plata ; the lower lands of Europe and Asia. The surface is 

 usually undulating, and often hilly. Frequently the surface rises so 

 gradually into the bordering mountain-declivities that the limit is alto- 

 gether an arbitrary line, as in the case of the Mississippi plains and 

 the Rocky Mountain slope. 



A mountain is either an isolated peak, as Mount Etna, Mount 

 Washington, Mount Blanc ; or a ridge ; or a series of ridges, some- 

 times grouped in many more or less parallel lines. 



A mountain-range is made up of a series of ridges or elevations, 

 closely related in position and direction, as the Green Mountain range, 

 or, simply, the Green Mountains ; the Sierra Nevada, the Ozark 

 Mountains, etc. A sierra is, in Spanish, the name of a ridge or group 

 of ridges of serrated or irregular outline. 



A mountain-chain consists of two or more mountain ranges, which 

 belong to a common region of elevation, and are generally either par- 

 allel or in consecutive lines, or consecutive curves, with often inferior 

 transverse lines of heights. Thus, the Blue Ridge or range, the Alle- 

 ghanies, and the Green Mountains, are parts of the Appalachian Chain, 

 — a chain of heights that reaches from Canada to Alabama. So the 

 Rocky Mountain chain includes many different ranges over a common 

 region of elevation, the ranges composing it having been made at sev- 

 eral different epochs. 



A cordillera includes all the mountain-chains in the whole great belt 

 of high land that borders a continent. Thus the Western Cordillera 

 of North America comprises the Rocky Mountain chain, the Wash- 

 ington chain (Sierra Nevada and Cascade ranges), the Coast ranges, 

 and other ranges of heights on the Pacific side of the continent. The 



