THE SURFACE OF THE EARTH. 



the contrary, liable to frequent changes of direction, are in many 

 places discontinuous, and are subject to still greater irregularity 

 in both height and width. 



191. Spurs Each main ridge also throws out lateral ramifica- 

 tions at various angles with its general direction, called spurs. 

 Those are often themselves considerable chains of mountains, 

 throwing off secondary spurs parallel to the primary chain, or nearly 

 so. These subordinate ranges, intersecting each other at various 

 angles, and broken by valleys and ravines, form a reticulation, 

 which usually covers a vast extent of country stretching out at 

 either side from the base of the principal chain. They decrease 

 gradually in their dimensions, both horizontal and Yertical, with 

 their distance from the. parent chain. Every one who has visited 

 the country on either side of the ranges of the Alps, the Pyrenees, 

 or Mount Atlas, will be familiar with these features. 



192. Relief of the Earth's surface. Considering the Earth as 

 a globe formed of solid matter, subject to superficial inequalities, 

 and partially covered with water, the land must be regarded 

 merely as the more elevated parts rising out of the waters, which, 

 according to the common law of gravitation, are lodged upon the 

 lowest levels. The level parts of the land must be considered as 

 plateaux, table-lands, or terraces formed upon vast mountains, 

 the bases of which are at the bottom of the Ocean. 



The mountains, in like manner, which rise from the surface of the 

 land, must be considered as only the highest peaks of those whose 

 bases are established upon the bottom of the deep, and the plains 

 around them, upon which they rest, as merely terraces or 

 plateaux forming steps, as it were, in ascending their acclivities. 



In accordance with these views, it is found that systems of 

 mountains, whether they form continued chains, detached groups, 

 or isolated peaks, seldom have their bases upon a low level. On 

 the contrary, the plains upon which they stand are almost 

 always plateaux or tablelands at a considerable altitude above 

 the level of the sea. The heights of mountain- summits being 

 always expressed with relation to the level of the sea, it must 

 therefore be remembered, that in estimating heights above the 

 general level of the plains on which the mountains stand, it is 

 necessary to deduct the heights of such plains from the tabulated 

 heights of the mountains. 



That this correction is of importance, will be comprehended 

 when it is remembered, that the heights of the tablelands upon 

 which the great chains of mountains stand, above the level of the 

 sea, varies from 2000 to 12000 feet. Thus, for example, the 

 tabulated altitude of Kunchinjunga, the highest peak of the 

 Himalayas, and indeed the most lofty point hitherto discovered 

 186 



