According to the grand views of Elie de Beaumont, we must ascribe a relative age to each system of mountain chains* on the supposition that their elevation must necessarily have occurred between the period of the deposition of the vertically elevated strata and that of the horizontally inclined strata running at the base of the mountains.
[footnnote] *Leop. von Buch, 'Ueber die Geognostischen Systeme von Deutschland', in his 'Geogn. Briefen an Alexander von Humboldt', 1824, s. 265-271; Elie de Beaumont, 'Recherches sur les Revolutions de la Surface du Globe', 1829, p. 297-307.
The ridges of the Earth's crust — elevations of strata which are of the same geognostic age — appear, moreover, to follow one common direction. The line of strike of the horizontal strata is not always parallel with the axis of the chain, but intersects it, so that, according to my views,* the phenomenon of elevation of the strata, which is even found to be repeated in the neighboring plains, must be more ancient than the elevation of the chain.
[footnote] *Humboldt, 'Asie Centrale', t. i., p. 277-283. See, also my 'Essai sur le Gisement des Roches', 1822, p. 57, and 'Relat. Hist.', t. iii., p. 244-250.
The main direction of the whole continent of Europe (from southwest to northeast) is opposite to that of the great fissures which pass from northwest to southeast, from the mouths of the Rhine and Elbe, through the Adriatic and Red Seas, and through the mountain system of Putschi-Koh in Luristan, toward the Persian Gulf and the Indian Ocean. This almost rectangular intersection of geodesic lines exercises an important influence on the commercial relations of Europe, Asia, and the northwest of Africa, and on the progress of civilization on the formerly more flourishing shores of the Mediterranean.*
[footnote] *'Asie Centrale', t. i., p. 284, 286. The Adriatic Sea likewise follows a direction from S.E. to N.W.
Since grand and lofty mountain chains so strongly excite our imagination by the evidence they afford of great terrestrial revolutions, and when considered as the boundaries of climates, as lines of separation for waters, or as the site of a different form of vegetation, it is the more necessary to demonstrate, by a correct numerical estimation of their volume, how small is the quantity of their elevated mass when compared with the area of the adjacent continnents. The mass of the Pyrenees, for instance, the mean elevation of whose summits, and the real quantity of whose base have been ascertained by accurate measurements, would if scattered over p 301 the surface of France, only raise its mean level about 115 feet. The mass of the eastern and western Alps would in like manner only increase the height of Europe about 21 1/2 feet above its present level. I have found by a laborious investigation,* which from its nature, can only give a maximum limit, that the center of gravity of the volume of the land raised above the present level of the sea in Europe and North America is respectively situated at an elevation of 671 and 748 feet, while it is at 1132 and 1152 feet in Asia and South America.
[footnote] *'De la hauteur Moyenne des Continents', in my 'Asie Centrale', t. i., p. 82-90, 165-189. The results which I have obtained are to be regarded as the extreme value ('nombres-limites'). Laplace's estimate of the mean height of continents at 3280 feet is at least three times too high. The immortal author of the 'Mecanique Celeste' (t. v., p. 14) was led to this conclusion by hypothetical views as to the mean depth of the sea. I have shown ('Asie Centr.', t. i., p. 93) that the old Alexandrian mathematicians, on the testimony of Plutarch ('in Aemilio Paulo', cap. 15), believed this depth to depend on the height of the mountains. The height of the center of gravity of the volume of the continental masses is probably subject to slight variations in the course of many centuries.
These numbers show the low level of norther regions. In Asia the vast steppes of Siberia are compensated for by the great elevations of the land (between the Himalaya, the North Thibetian chain of Kuen-lun, and the Celestial Mountains), from 28 degrees 30' to 40 degrees north latitude. We may, to a certain extent, trace in these numbers the portions of the Earth in which the Plutonic forces were most intensely manifested in the interior by the upheaval of continental masses.
There are no reasons why these Plutonic forces may not, in future ages, add new mountain systems to those which Elie de Beaumont has shown to be of such different ages, and inclined in such different directions. Why should the crust of the Earth have lost its property of being elevated in the ridges? The recently-elevated mountain systems of the Alps and the Cordilleras exhibit in Mont Blanc and Monte Rosa, in Sorata, Illimani, and Chimborazo, colossal elevations which do not favor the assumption of a decrease in the intensity of the subterranean forces. All geognostic phenomena indicate the periodic alternation of activity and repose;* but the quiet we now enjoy is only apparent.