The Hypothesis According To Which, In The Earthquakes Of Cumana, Elastic Fluids Tend To Escape From The Surface Of The Soil, Seems Confirmed By The Great Noise Which Is Heard During The Shocks At The Borders Of The Wells In The Plain Of Charas.

Water and sand are sometimes thrown out twenty feet high.

Similar phenomena were observed in ancient times by the inhabitants of those parts of Greece and Asia Minor abounding with caverns, crevices, and subterraneous rivers. Nature, in her uniform progress, everywhere suggests the same ideas of the causes of earthquakes, and the means by which man, forgetting the measure of his strength, pretends to diminish the effect of the subterraneous explosions. What a great Roman naturalist has said of the utility of wells and caverns* is repeated in the New World by the most ignorant Indians of Quito, when they show travellers the guaicos, or crevices of Pichincha. (* "In puteis est remedium, quale et crebri specus praebent: conceptum enim spiritum exhalant: quod in certis notatur oppidis, quae minus quatiuntur, crebris ad eluviem cuniculis cavata." - Pliny lib. 2 c. 82 (ed. Par. 1723 t. 1 page 112.) Even at present, in the capital of St. Domingo, wells are considered as diminishing the violence of the shocks. I may observe on this occasion, that the theory of earthquakes, given by Seneca, (Nat. Quaest. lib. 6 c. 4-31), contains the germ of everything that has been said in our times on the action of the elastic vapours confined in the interior of the globe.)

The subterranean noise, so frequent during earthquakes, is generally not in the ratio of the force of the shocks. At Cumana it constantly precedes them, while at Quito, and recently at Caracas, and in the West India Islands, a noise like the discharge of a battery was heard a long time after the shocks had ceased. A third kind of phenomenon, the most remarkable of the whole, is the rolling of those subterranean thunders, which last several months, without being accompanied by the least oscillatory motion of the ground.* (* The subterranean thunders (bramidos y truenos subterraneos) of Guanaxuato. The phenomenon of a noise without shocks was observed by the ancients. - Aristot. Meteor. lib. 2 (ed. Duval page 802). Pliny lib. 2 c. 80.)

In every country subject to earthquakes, the point at which, probably owing to a particular disposition of the stony strata, the effects are most sensibly felt, is considered as the cause and the focus of the shocks. Thus, at Cumana, the hill of the castle of San Antonio, and particularly the eminence on which stands the convent of St. Francis, are believed to contain an enormous quantity of sulphur and other inflammable matter. We forget that the rapidity with which the undulations are propagated to great distances, even across the basin of the ocean, proves that the centre of action is very remote from the surface of the globe. From this same cause no doubt earthquakes are not confined to certain species of rocks, as some naturalists suppose, but all are fitted to propagate the movement. Keeping within the limits of my own experience I may here cite the granites of Lima and Acapulco; the gneiss of Caracas; the mica-slate of the peninsula of Araya; the primitive thonschiefer of Tepecuacuilco, in Mexico; the secondary limestones of the Apennines, Spain, and New Andalusia; and finally, the trappean porphyries of the provinces of Quito and Popayan.* (* I might add to the list of secondary rocks, the gypsum of the newest formation, for instance, that of Montmartre, situated on a marine calcareous rock, which is posterior to the chalk. - See the Memoires de l'Academie tome 1 page 341 on the earthquake felt at Paris and its environs in 1681.) In these different places the ground is frequently agitated by the most violent shocks; but sometimes, in the same rock, the superior strata form invincible obstacles to the propagation of the motion. Thus, in the mines of Saxony, we have seen workmen hasten up alarmed by oscillations which were not felt at the surface of the ground.

If, in regions the most remote from each other, primitive, secondary, and volcanic rocks, share equally in the convulsive movements of the globe; we cannot but admit also that within a space of little extent, certain classes of rocks oppose themselves to the propagation of the shocks. At Cumana, for instance, before the great catastrophe of 1797, the earthquakes were felt only along the southern and calcareous coast of the gulf of Cariaco, as far as the town of that name; while in the peninsula of Araya, and at the village of Maniquarez, the ground did not share the same agitation. But since December 1797, new communications appear to have been opened in the interior of the globe. The peninsula of Araya is now not merely subject to the same agitations as the soil of Cumana, but the promontory of mica-slate, previously free from earthquakes, has become in its turn a central point of commotion. The earth is sometimes strongly shaken at the village of Maniquarez, when on the coast of Cumana the inhabitants enjoy the most perfect tranquillity. The gulf of Cariaco, nevertheless, is only sixty or eighty fathoms deep.

It has been thought from observations made both on the continent and in the islands, that the western and southern coasts are most exposed to shocks. This observation is connected with opinions which geologists have long formed respecting the position of the high chains of mountains, and the direction of their steepest declivities; but the existence of the Cordillera of Caracas, and the frequency of the oscillations on the eastern and northern coast of Terra Firma, in the gulf of Paria, at Carupano, at Cariaco, and at Cumana, render the accuracy of that opinion doubtful.

In New Andalusia, as well as in Chile and Peru, the shocks follow the course of the shore, and extend but little inland. This circumstance, as we shall soon find, indicates an intimate connection between the causes which produce earthquakes and volcanic eruptions.

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