270. _Volcanic rocks are composed of such mineral substances as have been ejected from volcanoes, or have been formed by the agency of subterraneous fires, and have undergone certain changes in such fires._ They are of two kinds; the one called _pseudo volcanic_, such as burnt clay, porcelain jasper, and earth─slag, which have been altered in consequence of the burning of beds of coal in their neighbourhood; and the other, called _true volcanic_ minerals, such as stones, ashes, and lava, which have been thrown out of real volcanoes. 271. It will somewhat tend to illustrate the history of the mineral kingdom, to state, in conclusion, under a tabular form, the relative heights of the principal mountains, or masses of rocks, which occur in the different countries of the world; previously remarking, that the most lofty and magnificent of these, respecting which any account sufficiently authentic has hitherto been obtained, are the mountains of Nepaul and Thibet, in Asia, one of the former being 27,667, and the highest of the latter measuring at least 23,262 feet, or from 4½ miles to 5¼ miles in perpendicular height above the level of the sea. Previously to the knowledge that has lately been attained respecting the Asiatic mountains, those of the Andes, on the continent of South America, had been considered by far the highest in the world. One of them, Chimborazo, is 20,900 feet in height. Of the European mountains, the highest is Mont Blanc, in Switzerland, which measures 15,680 feet, or about 2¾ miles. The loftiest summit within the British islands is Ben Nevis, in Inverness─shire, Scotland, which does not exceed 4,380 feet, or somewhat more than three quarters of a mile; and the great pyramid of Egypt, the loftiest work of human art and industry with which we are acquainted, and which will serve as a point in the scale, measures only 477 feet. 272. It has been remarked that the greatest altitude at which _bananas_ and other _palm─trees_ grow in America is about 3280 feet above the level of the sea (Frontispiece, Fig. 48): that in the torrid zone, the superior limits of _oaks_ is about 10,500 feet (49), of _pines_ 12,000 feet (50), and of _lichen plants_ 18,225 feet (51). The American travellers, Messrs. Humboldt and Bonpland, on the twenty─third of June, 1802, ascended the mountain of Chimborazo to the height of 19,400 feet (52). The highest flight that has been remarked of the South American vulture, called the _condor_, was 21,000 feet (53). M. Lussac, on the 16th of September, 1804, ascended in a balloon from Paris, to the height of 22,900 feet. In Switzerland, the limit of perpetual snow is above the altitude of 9000 feet (54). =Height of the Principle Mountains.= _Frontispiece._ Ft. above