GENERAL FEATURES OF THE EARTH. 21 



These several conditions of interrupted 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 surface, as is explained in 

 the following pages. Figure 28 on page 34, representing the positions of the Australa- 

 sian islands from Kew Hebrides to Sumatra, well exhibits the system of structure, — 

 also Fig. 27, giving the courses and relative positions of the central groups of the Pa- 

 cific, 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 over- 

 lapping 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 Percival's Report, pre- 

 sents well the structure. The narrow bands running nearly north and south represent 

 the trap-ridges ; they are in many nearly parallel lines ; each consists 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 ; and the ridges, few of which exceed 

 900 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. 



Solid dimensions of mountains. — The modes of calculating the 

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

 suration. By a careful system of averaging, based on determinations 

 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 ar^a of that side into half the length of 

 a line drawn vertical to it from the opposite edge. 



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

 plateaus have been mentioned (p. 16). The Llano Estacado (Staked 

 Plain) in New Mexico and Upper Texas, southeast of Santa Fe, is 

 another, of great extent, averaging 4,000 feet in elevation. The 

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

 that city a height of 7,482 feet, and slopes from this to 5,000 on the 

 east and 4,000 on the west ; and it stretches on north beyond the Mexi- 

 can territory, blending with the plateaus of New Mexico. Above it 

 rise many lofty volcanic cones, among which Popocatepetl is 17,799 

 feet high, Orizaba 17,373 feet, and Ixtaccihuatl 17,083. South Park in 

 Colorado is in its northern part 9,500 to 10,000, and in its southern 

 about a thousand less ; and the average height of Middle Park is 8,500 

 feet. 



The plateau of Quito, in the Andes, has a height of 10,000 feet; Quito itself 9.540 

 feet; and around it are Cotopaxi, 18,775 feet, Chimborazo, 21,421, Pichincha, 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 lllimani, 23,868 feet, Sorata, 25,290, Huayna Potosi, 20,260. In Europe, Spain 

 is for the xnost part a plateau about 2,250 feet in average elevation ; Auvergne, in 



