GENERAL FEATURES OP THE EARTH. 21 



in 12 it is still straight, but complex ; in 13 the parallel parts are 

 so arranged as still to make a straight composite range ; while in 

 14 and 15 the succession forms a curve ; and in 16 there are trans- 

 verse ridges in a complex series. In ridges or ranges thus com- 

 pounded, the component parts may lie distinct, or they may 

 so coalesce as not to be apparent. 



These several conditions of uninterrupted and overlapping lines, constituting 

 straight and curving chains, are illustrated among the islands of the oceans, 

 the direction of coast-lines, and the courses of all the reliefs of the earth's sur- 

 face, as is explained in the following pages. Figure 28 on page 35, representing 

 the positions of the Australasian islands from New Hebrides to Sumatra, finely 

 exhibits the system of structure, — also fig. 27, giving the courses and relative 

 positions of the central groups of the Pacific, and fig. 29, representing the 

 Azores in the Atlantic ; for the courses of islands are the courses of mountain- 

 chains. The South Atlantic and North Atlantic are two overlapping lines 

 parallel in course, and on a still grander scale, one of them being much in 

 advance or to the westward of the other, and each several thousand miles 

 long. 



The preceding map of the trap ridges of Connecticut, from PercivaFs Report, 

 presents the structure finely. The narrow bands running nearly north-and- 

 south represent the ridges ; they are in many nearly parallel lines ; each con- 

 sists of subordinate parts ; and in several the parts lie in advancing or receding 

 series. The extent of the series is small compared with a mountain-chain j 

 and the ridges, few of which exceed 600 feet in height, are ejections through 

 fissures beneath. But the parallelism in structure is perfect. The curves in 

 some of the subordinate ridges have arisen from the fact that the fissures come 

 up through a tilted sandstone, and the ejected rock escaped partly direct from 

 the fissure and partly between the lifted strata of sandstone, and hence in a 

 direction different from that of the fissure, the two directions together making 

 the curve. 



25. Solid dimensions of mountains. — The modes of calculating the 

 mass of a mountain are the same that are given in treatises on 

 mensuration. By a careful system of averaging, based on deter- 

 minations of the slopes and altitudes, as far as practicable, the 

 mountain-mass is reduced to one or more cones, pyramids, or 

 prisms ; and then the solid contents of the cones or pyramids are 

 obtained by multiplying the area of the base into one-third the 

 altitude ; or, for a triangular prism lying on one of its sides, the 

 area of that side into half the length of a line drawn vertical to it 

 from the opposite edge. 



26. Elevated Plateaus, or table-lands. — Some examples of these 

 plateaus have been mentioned ($ 22). The Llano Estacado 

 (staked plain) in New Mexico and Upper Texas, southeast of 

 Santa Fe, is another, of great extent, about 4500 feet in elevation. 

 The great Mexican plateau, in which the city of Mexico lies, has 



