of the earth, may also be regarded as the subsidiary action of a non-telluric cause, by which an increased pressure must be produced, either immediately against a solid, superimposed rocky arch; or indirectly, when the solid mass is separated, in subterranean basins, from the fused, fluid mass by elastic vapours.

The nucleus of our planet is supposed to consist of unoxidised masses, the metalloids of the alkalies and earths. Volcanic activity is excited in the nucleus by the access of water and air. Volcanoes certainly pour forth a great quantity of aqueous vapour into the atmosphere; but the assumption of the penetration of water into the volcanic focus is attended with much difficulty, considering the opposing pressure" of the external column of water and miles (21, 25 English) below the surface, a heat capable of melting granite prevails. Nearly the same number (45,000 metres=24 geographical miles) was named by Elie de Beaumont (Geologie, edited by Vogt, 1846, vol. i, p. 32), as the thickness of the solid crust of the earth. Moreover, according to the ingenious experiments of Bischof on the fusion of various minerals, of which the importance to the progress of geology is so great, the thickness of the unfused strata of the earth is between 122,590 and 136,448 feet, or on the average 21 geographical (24 English) miles; see Bischof, Wärmelehre des Innern unsers Erdkörpers, pp. 286 and 271. This renders it the more remarkable to me to find that, with the assumption of a definite limit between the solid and fused parts, and not of a gradual transition, Hopkins, from the fundamental principles of his speculative geology, establishes the result that "the thickness of the solid shell cannot be less than about one-fourth or one-fifth(?) of the radius of its external surface" (Meeting of British Association, 1847, p. 51). Cordier's earliest supposition was only 56 geographical (72 English) miles, without correction, which is dependent upon the increased pressure of the strata at great depths, and the hypsometrical form of the surface. The thickness of the solid part of the crust of the earth is probably very unequal.

11 Gay Lussac, Reflexions sur les Volcans, in the Annales de Chimie et de Physique, tome xxii, 1823, pp. 418 and 426. The author, who, in company with Leopold von Buch and myself, observed the great eruption of lava from Vesuvius in September, 1805, has the merit of having submitted the chemical hypotheses to a strict criticism. He seeks for the cause of volcanic phenomena in a "very energetic and still unsatisfied affinity between the substances, which a fortuitous contact permits them to obey" in general he favours the hypothesis of Davy and Ampère, which is now given up, "supposing that the radicals of silica, alumina, lime, and iron are combined with chlorine in the interior of the earth," and the penetration of sea water does not appear to him to be improbable under certain conditions (pp. 419, 420, 423, and 426).

of the internal lava; and the deficiency, or, at all events, very rare occurrence of burning hydrogen gas during the eruption, (which the formation of hydrochloric acid,12 ammonia, and sulphuretted hydrogen, certainly does not sufficiently replace) has led the celebrated originator of this hypothesis to abandon it of his own accord.

13

According to a third view, that of the highly endowed South American traveller, Boussingault, a deficiency of coherence in the trachytic and doleritic masses which form the elevated volcanoes of the chain of the Andes, is regarded as a primary cause of many earthquakes of very great extent. The colossal cones and dome-like summits of the Cordilleras, according to this view, have by no means been elevated in a soft and semifluid state, but have been thrown up and piled on one another when perfectly hardened, in the form of enormous, sharp-edged fragments. In an elevation and piling of this description, large interstices and cavities have necessarily been produced; so that by sudden sinking, and by the fall of solid masses which are too weakly supported, shocks are produced.14

Upon the difficulty of a theory founded upon the penetration of water, see Hopkins, Brit. Assoc. Rep. 1847, p. 38.

12 According to the beautiful analyses made by Boussingault, on the margins of five craters (Tolima, Purace, Pasto, Tuqueras, and Cumbal), hydrochloric acid is entirely wanting in the vapours poured forth by the South American volcanoes, but not in those of Italy (Annales de Chimie, tome lii, 1833, pp. 7 and 23).

13 Cosmos, vol. i, p. 234, Bohn's edition. Whilst Davy, in the most distinct manner, gave up the opinion that volcanic eruptions are a consequence of the contact of the metalloid bases with water and air, he still asserted that the presence of oxidizable metalloids in the interior of the earth might be a co-operating cause in volcanic processes already commenced.

14 Boussingault says:-"I attribute most of the earthquakes in the Cordillera of the Andes to falls produced in the interior of these mountains by the subsidence which takes place, and which is a consequence of their elevation. The mass which constitutes these gigantic ridges has not been raised in a soft state; the elevation did not take place until after the solidification of the rocks. I assume, therefore, that the elevated masses of the Andes are composed of fragments heaped upon each other. The consolidation of the fragments could not be so stable from the beginning as that there should be no settlements after the elevation, or that there should be no interior movements in the fragmentary masses" (Boussingault, Sur les Tremblemens de Terre des Andes, in

The effects of the impulse, the waves of commotion, may be reduced to simple mechanical theories with more distinctness than is furnished by the consideration of the nature of the first impulse, which indeed may be regarded as heterogeneous. As already observed, this part of our knowledge has advanced essentially in very recent times. The earth-waves have been represented in their progress and their propagation through rocks of different density and elasticity;15 the causes of the rapidity of propagation, and its diminution by the refraction, reflection, and interference of the oscillations have been Annales de Chimie et de Physique, tome lviii, 1835, pp. 84-86). In the description of his memorable ascent of Chimborazo (Ascension au Chimborazo le 16 Déc. 1831, loc. cit. p. 176), he says again :-" Like Cotopaxi, Antisana, Tunguragua, and the volcanoes in general which project from the plateaux of the Andes, the mass of Chimborazo is formed by the accumulation of trachytic débris, heaped together without any order. These fragments, often of enormous volume, have been elevated in the solid state by elastic fluids which have broken out through the points of least resistance; their angles are always sharp." The cause of earthquakes here indicated is the same as that which Hopkins calls a shock produced by the falling of the roof of a subterranean cavity," in his "Analytical Theory of Volcanic Phenomena" (Brit. Assoc. Report, 1847, p. 82).

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15 Mallet, Dynamics of Earthquakes, pp. 74, 80, and 82; Hopkins, Brit. Assoc. Report, 1847, pp. 74-82. All that we know of the waves of commotion and oscillations in solid bodies shows the untenability of the older theories as to the facilitation of the propagation of the movement by a series of cavities. Cavities can only act a secondary part in the earthquake, as spaces for the accumulation of vapours and condensed gases. "The earth, so many centuries old," says Gay Lussac very beautifully (Ann. de Chimie et de Phys. tome xxii, 1823, p. 428), "still preserves an internal force, which raises mountains (in the oxidized crust), overturns cities and agitates the entire mass. Most mountains, in issuing from the bosom of the earth, must have left vast cavities, which have remained empty, at least unless they have been filled with water (and gaseous fluids). It is certainly incorrect for Deluc and many geologists to make use of these empty spaces, which they imagine produced into long galleries, for the propagation of earthquakes to a distance. These phenomena, so grand and terrible, are very powerful sonorous waves, excited in the solid mass of the earth by some commotion, which propagates itself therein with the same velocity as sound. The movement of a carriage over the pavement shakes the vastest edifices, and communicates itself through considerable masses, as in the deep quarries below the city of Paris."

16 Upon phenomena of interference in the earth-waves, analogous to those of the waves of sound, see Cosmos, vol. i, p. 211, Bohn's edition, and Humboldt, Kleinere Schriften, Bd. i. p. 379.

mathematically investigated. Attempts have been made to reduce to a rectilinear" standard the apparently circling (rotatory) shocks of which the obelisks before the monastery of San Bruno, in the small town of Stephano del Bosco (Calabria, 1783), furnished such a well-known example. Air, water, and earth-waves follow the same laws which are recognized by the theory of motion, at all events in space; but the earth-waves are accompanied, in their destructive action, by phenomena which remain more obscure in their nature and belong to the class of physical processes. As such we have to mention,-discharges of elastic vapours, and of gases; or, as in the small, moving Moya-cones of Pelileo, grit-like mixtures of pyroxene crystals, carbon, and infusorial animalcules with silicious shields. These wandering cones have overthrown a great number of Indian huts.18

In the general Delineation of Nature many facts are narrated concerning the great catastrophe of Riobamba (4th of February, 1797), which were collected on the spot from the lips of the survivors, with the most earnest endeavours after historic truth. Some of them are analogous to the occurrences in the great earthquake of Calabria in the year 1783; others are new, and especially characterized by the mine-like manifestation of force from below upwards. The earthquake itself was neither accompanied nor announced by any subterranean noise. A prodigious explosion, still indicated by the simple name of el gran ruido, was not perceived until 18 or 20 minutes afterwards, and only under the two cities of Quito and Ibarra, far removed from Tacunga, Hambato, and the principal scene of the destruction. There is no other event in the troubled destinies of the human race, by which in a few minutes, and in sparingly peopled mountain lands, so many thousands at once may be overtaken by death, as by the production and passage of a few earth-waves, accompanied by phenomena of cleavage!

17 Mallet on vorticose shocks and cases of twisting, in Brit. Assoc. Report, 1850, pp. 33 and 49, and in the Admiralty Manual, 1849, p. 213 (see Cosmos, vol. i, p. 199, Bohn's edition).

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18 The Moya-cones were seen by Boussingault nineteen years after I saw them. Muddy eruptions, consequences of the earthquake, like the eruptions of the Moya of Pelileo, which have buried entire villages" (Ann. de Chim. et de Phys. t. lviii, p. 81).

In the earthquake of Riobamba, of which the celebrated Valencian botanist, Don José Cavanilles, gave the earliest account, the following phenomena are deserving of special attention :- fissures which alternately opened and closed again, so that men saved themselves by extending both arms in order to prevent their sinking; the disappearance of entire caravans of riders or loaded mules (recuas), some of which disappeared through transverse fissures suddenly opening in their path, whilst others, flying back, escaped the danger; such violent oscillations (non-simultaneous elevation and depression) of neighbouring portions of the ground, that people standing upon the choir of a church at a height of more than 12 feet, got upon the pavement of the street without falling; the sinking of massive houses,19 in which the inhabitants could open inner doors, and for two whole days, before they were released by excavations, passed uninjured from room to room, procured lights, fed upon supplies accidentally discovered, and disputed with each other regarding the probability of their rescue; and the disappearance of such great masses of stones and building materials. Old Riobamba contained churches and monasteries amongst houses of several stories; and yet, when I took the plan of the destroyed city, I only found in the ruins heaps of stone of 8 to 10 feet in height. In the south-western part of Old Riobamba (the former Barrio de Sigchuguaicu) a minelike explosion, the effect of a force from below upwards, was distinctly perceptible. On the Cerro de la Culca, a hill of some hundred feet in height, which rises above the Cerro de Cumbicarca situated to the north of it, there lies stony rubbish mixed with human bones. Translatory movements, in a horizontal direction, by which avenues of trees become displaced, without being uprooted, or fragments of cultivated ground of very different kinds mutually displace each other, have occurred repeatedly in Quito, as well as

19 Upon the displacement of buildings and plantations during the earthquake of Calabria, 'see Lyell's Principles of Geology, vol. i, pp. 484 -491. Upon escapes in fissures during the great earthquake of Riobamba, see my Relation Historique, tome ii, p. 642. As a remarkable example of the closing of a fissure it must be mentioned that, according to Scacchi's report, during the celebrated earthquake (in the summer of 1851), in the Neapolitan province of Basilicata, a hen was found caught by both feet in the street pavement in Barile, near Melfi,