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and the same internal force produces colossal craters of elevation, nay even the mighty, desolating, lava-pouring volcanoes of Etna and the Peak of Teyde, and the cinder-erupting Cotopaxi and Tunguragua.

Amongst the multifarious, mutually intensifying, phenomena of the reaction of the interior of the earth upon its external crust, I first of all separate those, the essential character of which is purely dynamical, namely, that of movement or tremulous undulations in the solid strata of the earth; a volcanic activity which is not necessarily accompanied by any chemical changes of matter, or by the expulsion or production of anything of a material nature. In the other phenomena of the reaction of the interior upon the exterior of the earth :-in gas and mud volcanoes, burning springs and salses, and in the large burning mountains to which the name of volcano was first, and for a long time exclusively, applied, the production of something of a material nature (gaseous or solid), and processes of decomposition and gas-evolution, such as the formation of rocks from particles arranged in a crystalline form, are never wanting. When most fully generalized, these are the distinctive characters of the volcanic vital activity of our planet. In so far as this activity is to be ascribed in great measure to the high temperature of the innermost strata of the earth, it becomes probable that all cosmical bodies which have become conglomerated with an enormous evolution of heat, and passed from a state of vapour to a solid condition, must present analogous phenomena. The little that we know of the form of the moon's surface, appears to indicate this._Upheaval and plastic activity in the production of crystalline rock from a fused mass, are conceivable even in a sphere which is regarded as destitute of both air and water.

The genetic connexion of the classes of volcanic phenomena here referred to is indicated by the numerous traces of the simultaneousness of the simpler and weaker with stronger and more complex effects, and the accompanying transitions of the one into the other. The

arrangement of the materials in the representation selected by me is justified by such a consideration. The increased magnetic activity of our planet, the seat of which, however, is not to be sought

l'usmos, vol. iii, p. 44; iv, pp. 426, 491, 495—498.

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-in the fused mass of the interior (even though, according to Lenz and Riess, iron, in the fused state, may be capable of conducting an electrical or galvanic current), produces evolution of light in the magnetic poles of the earth, or at least usually in their vicinity. We concluded the first section of the volume on telluric phenomena with the luminosity of the earth. This phenomenon of a luminous vibration of the ether by magnetic forces is immediately followed by that class of volcanic agencies, which, in their essential nature, act purely dynamically, exactly like the magnetic force:causing movement and vibrations in the solid ground, but neither producing nor changing anything of a material nature. Secondary and unessential phenomena (the ascent of flames during the earthquake, and eruptions of water and evolutions of gas following it) remind one of the action of thermal springs and salses. Eruptions of flame, visible at a distance of many miles, and masses of rock, torn from their deep seats and hurled about,' are presented by the salses, which thus, as it were, prepare us for the magnificent phenomena of the true volcanoes, which again, between their distant epochs of eruption, like the salses, only exhale aqueous vapour

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gases from their fissures. So remarkable and instructive are the analogies which are presented in various stages by the gradations of vulcanism.

a. Earthquakes.
(Amplification of the Picture of Nature.

Cosmos, vol. i. pp. 199–213). Since the appearance in the first volume of this work (1845) of the general representation of the phenomena of earthquakes, the obscurity, in which the seat and causes of these phenomena are involved, has but little diminished; but the excellent worksø of Mallet (1846) and Hopkins (1847)have thrown some light upon the nature of concussions, the connection of apparently distinct effects and the separa4 Cosmos, vol. i, P.

214. Cosmos, vol. i, p. 222. Compare Bertrand-Geslin, “ Sur les roches lancées par le Volcan de boue du Monte Zibio près du bourg de Sassuolo," in Humboldt, Voyage aux Régions Equinoxiales du Nouveau Continent (Relation Historique), t. iii, p. 566.

'Robert Mallet, in the Transactions of the Royal Irish Academy, vol. xxi (1848), pp. 51—113, and First Report on the Facts of Earth. quake Phenomena, in the Report of the Meeting of the British Association,

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tion of chemical and physical processes, which may accompany it or occur simultaneously with it. Here, as elsewhere, a mathematical mode of treatment, such as that adopted by Poisson, may have a beneficial effect. The analogies between the oscillations of solid bodies and the sound-waves in the ordinary atmosphere to which Thomas Young? had already called attention, are peculiarly adapted to lead to simpler and more satisfactory views, in theoretical considerations upon the dynamics of earthquakes.

Displacement, commotion, elevation, and formation of fissures indicate the essential character of the phenomenon. We have to distinguish the efficient force, which, as the impulse, gives rise to the vibration; and the nature, propagation, increase or diminution of the commotion. In the Picture of Nature I have described what is especially manifested to the senses; what I had myself the opportunity of observing for so many years on the sea, on the sea-bottom of the plains (Llanos), and at elevations of eight to fifteen thousand feet; on the margin of the craters of active volcanos, and in regions of granite and mica schist, twelve hundred geographical miles from any eruptions of fire ; in districts where at certain periods the inhabitants take no more notice of the number of earthquakes, than we in Europe of that of the showers of rain, and where Bonpland and I were compelled to dismount, from the restiveness of our mules, because the earth shook in a forest for 15 to 18 minutes without intermission. By such long custom, as Boussingault subsequently experienced even in a still higher degree, one becomes fitted for quiet and careful observation, and also for collecting varying evidence with critical care on the spot, nay, even for examining under what conditions the mighty changes of the surface of the earth, the fresh traces of which one recognises, have taken place. Although five years had already elapsed 1850, pp. 1–89; also Manual of Scientific Inquiry for the Use of the British Navy, 1849, pp. 196—223. William Hopkins, On the Geological Theories of Elevation and Earthquakes, in the Report of the British A880ciation for 1847, pp. 33–92. The rigorous criticism to which Mr. Mallet has subjected my previous work in his very valuable memoirs (Irish Transactions, pp. 99—101, and Meeting of the British Association at Edinburgh, p. 209), has been repeatedly made use of by me. 7 Thomas Young, Lectures on Natural Philosophy, 1807, pol. i,

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since the terrible earthquake of Riobamba, which, on the 4th of February, 1797, destroyed upwards of 30,000 people in a few minutes, we nevertheless saw the formerly advancing cone of the Moyao which rose out of the earth, and witnessed the employment of this combustible substance for cooking in the huts of the Indians. I might describe the results of alterations of the ground from this catastrophe, which, although on a larger scale, were exactly analogous to those presented by the famous earthquake of Calabria (February 1783), and were long considered to have been represented in an incorrect and exaggerated manner, because they could not be explained in accordance with hastily formed theories.

By carefully separating, as we have already indicated, the investigation of that which gives the impulse to the vibration, from that of the nature and propagation of the waves of commotion, we distinguish two classes of problems of very unequal accessibility. The former, in the present state of our knowledge, can lead to no generally satisfactory results, as is the case with so many problems in which we wish to ascend to primary causes. Nevertheless, whilst we are endeavouring to discover laws in that which is submitted to actual observation, it is of great cosmical interest that we should bear constantly in mind the various genetic explanations which have hitherto been put forward as probable. As with all vulcanicity, the greater part of these refer, under various modifications, to the high temperature and chemical nature of the fused interior of the earth; one of the most recent explanations of earthquakes in trachytic regions, is the result of geognostic suppositions regarding the want of cohesion in rocky masses raised by volcanic action. The following summary furnishes a more exact but

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brief indication of the variety of views as to the nature of the first impulse to the commotion :

The nucleus of the earth is supposed to be in a state of igneous fluidity, as the consequence of every planetary process of formation from a gaseous material, by evolution

8 I follow the statistical account communicated to me by the Corregidor of Tacunga in 1802. It rose to a loss of 30,000—34,000 people, but some twenty years later the number of those killed immediately was reduced by about one-third. 9 Cosmos, vol. I, p. 209, Bohn's edition.

of heat during the transition from fluidity to solidity. The external strata were first cooled by radiation, and were the first to become consolidated. The commotion is occasioned by an unequal ascent of elastic vapours formed (at the limit between the fluid and solid parts) either from the fused terrestrial mass alone, or from the penetration of sea-water into higher strata of rock, nearer to the surface of the earth, the sudden opening of fissures, and by the sudden ascent of vapours produced in the hotter and consequently more elastic depths. The attraction of the moon and suno on the fluid, fused surface of the nucleus

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Hopkins has expressed doubts as to the action upon the fused subjacent fluid confined into internal lakes," at the Meeting of the British Association for 1847 (p. 57), as Mallet has also done with regard to "the subterraneous lava tidal wave, moving the solid crust above it," at the British Association Meeting for 1850 (p. 20). Poisson also, with whom I have often spoken regarding the hypothesis of the subterranean ebb and flow, caused by the sun and moon, considers the impulse, which he does not deny, to be inconsiderable, "as in the open sea the effect. scarcely amounts to 14 inches.” Ampère, on the other hand, says :-“Those who admit the fluidity of the internal nucleus of the earth, do not appear to have sufficiently considered the action which would be exercised by the moon upon this enormous liquid mass; an action from which would result tides analogous to those of our seas, but far more terrible, both from their extent and from the density of the liquid. It is difficult to conceive how the envelope of the earth should be able to resist the incessant action of a sort of hydraulic ram(?) of 1400 leagues in length" (Ampère, Théorie de la Terre, in Revue des deux Mondes, July, 1833, p. 148). If the interior of the earth be fluid, which in general cannot be doubted, as, notwithstanding the enormous pressure, the particles are still displaceable, then the same conditions are fulfilled in the interior of the earth that give rise on the surface to the ocean tides; and the tide-producing force will constantly become weaker in approaching the centre, as the difference of the distances of every two opposite points, considered in their relation to the attracting bodies, constantly becomes less in receding from the surface, and the force depends exclusively upon the difference of the distances. If the solid crust of the earth opposes a resistance to this effort, the interior of the earth will only exert a pressure against its crust at these points ; as my astronomical friend, Dr. Brunnow, expresses himself, no more tide will be produced than if the ocean had an indestructible covering of ice. The thickness of the solid unfused crust of the earth is calculated from the fusing points of the different kinds of rock, and the law of the increase of heat from the surface into the depths of the earth. I have already (Cosmos, vol. i, p. 26), justified the assumption that, at somewhat more than twenty geographical

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