Mont Blanc And Mont D'Or Will Not Be Ranged In One And The Same Class.

Even the partisans of the Huttonian or volcanic theory make a distinction between the lavas melted under the mere pressure of the atmosphere at the surface of the globe, and those layers formed by fire beneath the immense weight of the ocean and superincumbent rocks.

They would not confound Auvergne and the granitic valley of Caracas in the same denomination; that of a country of extinct volcanoes.

I never could have pronounced the opinion, that the Silla and the Cerro de Avila, mountains of gneiss and mica-slate, were in dangerous proximity to the city of Caracas because they contained a great quantity of pyrites in subordinate beds of primitive limestone. But I remember having said, during my stay at Caracas, that the eastern extremity of Terra Firma appeared, since the great earthquake of Quito, in a state of agitation, which warranted apprehension that the province of Venezuela would gradually be exposed to violent commotions. I added, that when a country had been long subject to frequent shocks, new subterranean communications seemed to open with neighbouring countries; and that the volcanoes of the West India Islands, lying in the direction of the Silla, north-east of the city, were perhaps the vents, at the time of an eruption, for those elastic fluids which cause earthquakes on the coasts of the continent. These considerations, founded on local knowledge of the place, and on simple analogies, are very far from a prediction justified by the course of physical events.

On the 30th of April, 1812, whilst violent commotions were felt simultaneously in the valley of the Mississippi, in the island of St. Vincent, and in the province of Venezuela, a subterranean noise resembling frequent discharges of large cannon was heard at Caracas, at Calabozo (situated in the midst of the steppes), and on the borders of the Rio Apure, over a superficies of four thousand square leagues. This noise began at two in the morning. It was accompanied by no shock; and it is very remarkable, that it was as loud on the coast as at the distance of eighty leagues inland. It was everywhere believed to be transmitted through the air; and was so far from being thought a subterranean noise, that in several places, preparations were made for defence against an enemy, who seemed to be advancing with heavy artillery. Senor Palacio, crossing the Rio Apure below the Orivante, near the junction of the Rio Nula, was told by the inhabitants, that the firing of cannon had been heard distinctly at the western extremity of the province of Varinas, as well as at the port of La Guayra to the north of the chain of the coast.

The day on which the inhabitants of Terra Firma were alarmed by a subterranean noise was that of the great eruption of the volcano in the island of St. Vincent. That mountain, near five hundred toises high, had not thrown out lava since the year 1718. Scarcely was any smoke perceived to issue from it, when, in the month of May 1811, frequent shocks announced that the volcanic fire was either rekindled, or directed anew to that part of the West Indies. The first eruption did not take place till the 27th of April, 1812, at noon. It was merely an ejection of ashes, but attended with a tremendous noise. On the 30th, the lava overflowed the brink of the crater, and, after a course of four hours, reached the sea. The sound of the explosion is described as resembling that of alternate discharges of very large cannon and musketry; and it is worthy of remark, that it seemed much louder to persons out at sea, and at a great distance from land, than to those within sight of land, and near the burning volcano.

The distance in a straight line from the volcano of St. Vincent to the Rio Apure, near the mouth of the Nula, is two hundred and ten leagues.* (* Where the contrary is not expressly stated, nautical leagues of twenty to a degree, or two thousand eight hundred and fifty-five toises, are always to be understood.) The explosions were consequently heard at a distance equal to that between Vesuvius and Paris. This phenomenon, in conjunction with a great number of facts observed in the Cordilleras of the Andes, shows that the sphere of the subterranean activity of a volcano is much more extensive than we should be disposed to admit, if we judged merely from the small changes effected at the surface of the globe. The detonations heard during whole days together in the New World, eighty, one hundred, or even two hundred leagues distant from a crater, do not reach us by the propagation of the sound through the air; they are transmitted by the earth, perhaps in the very place where we happen to be. If the eruptions of the volcano of St. Vincent, Cotopaxi, or Tunguragua, resounded from afar, like a cannon of immense magnitude, the noise ought to increase in the inverse ratio of the distance: but observations prove, that this augmentation does not take place. I must further observe, that M. Bonpland and I, going from Guayaquil to the coast of Mexico, crossed latitudes in the Pacific, where the crew of our ship were dismayed by a hollow sound coming from the depth of the ocean, and transmitted by the waters. At that time a new eruption of Cotopaxi took place, but we were as far distant from the volcano, as Etna from the city of Naples. The little town of Honda, on the banks of the Magdalena, is not less than one hundred and forty-five leagues* (* This is the distance from Vesuvius to Mont Blanc.) from Cotopaxi; and yet, in the great explosions of this volcano, in 1744, a subterranean noise was heard at Honda, and supposed to be discharges of heavy artillery. The monks of San Francisco spread a report that the town of Carthagena was besieged and bombarded by the English; and the intelligence was believed throughout the country.

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