PLAINS AND MOUNTAINS. 



Various names are given to such tracts according to the language 

 of the country and their condition in respect to vegetation. Thus 

 an extensive sandy plain, destitute of all vegetation, is called a 

 Desert ; an example of such a plain on an immense scale is pre- 

 sented by the Desert of Sahara, in the North of Africa. Such 

 plains are called Landes in France, Steppes in Russia, and 

 Llanos, Pampas, Selvas, Savannahs, and Prairies, according as 

 they are more or less covered with vegetation, in North and South 

 America. 



Fig. 1. Forms of Plateaux, Hills and Mountains. 



14. Plateaux and Tablelands are extensive level tracts, 

 placed at considerable elevations above the level of the sea, or 

 the general level of the surrounding country, a I, fig. 1. 



15. Hills are elevations not exceeding about 1000 feet in 

 height above the plain at their base, and having an outline 

 variously formed ; rounded, e, fig. 1. ; ridged, d, fig. 1, or peaked, 

 as c, fig. 1, and fig. 3. 



Fig. 2. Groups of Mountains. 



16. Mountains are elevations generally exceeding 1000 feet 

 in height, and likewise subject to a similar variety of forms, 

 as shown in fig. 2. 



The application of these terms " hills" and " mountains" is very 

 arbitrary, elevations which receive the name of mountains in one 

 place being lower than those called hills in another. 



The forms of mountains are very various, and have an im- 

 portant relation to their external structure. Geologists are often 

 able to determine the character of the rocks of which they consist 

 by their outline. Thus, when the outline is characterised by 

 needles rising to considerable elevations, as in fig. 4, the moun- 

 tainous mass consists of the rocks called Gneiss. Such peaks, 

 which are frequent upon the chain of the Alps, are called needles, 

 teeth, and horns. Mountains are sometimes columnar in their 



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