Page:Popular Science Monthly Volume 24.djvu/532

516 of the globe; although the celebrated catastrophe at Lisbon on the 1st of November, 1755, extended over some 17° or 18°, into Africa and the two Americas, or over a surface equal to about four times that of Europe.

The detailed examination of many earthquakes has enabled us to determine the center of the shocks as well as the contours of the disturbed areas. From the manner in which the latter surfaces agree with the lines of pre-existing dislocations, several of the most distinguished geologists, including Mr. Dana, M. Suess, and Albert Heim, have considered the shocks in question as connected with the formation of chains of mountains, of which they may be a kind of continuation.

In fact, the crust of the earth everywhere shows the enormous effects exercised by the lateral pressures that have been in operation at all epochs. The strata, bent and bent over again many times through thousands of metres of thickness, as well as the great fractures that traverse them, are the eloquent witnesses of these mechanical actions. Notwithstanding the apparent tranquillity now reigning on the surface of the globe, equilibrium does not exist in the earth, and commotions have not been arrested in its depths. The proof of this is found, not only in earthquakes, but also in the slow movements of the soil, of elevation and depression—a kind of warping, which has continued to manifest itself within historical times in all parts of the globe. It is conceivable that slow actions of this kind, after more or less prolonged strains, may end in abrupt movements, as Élie de Beaumont supposed. We can see, also, in experiments intended to imitate the bending of strata, how gradual inflections lead all at once to fractures and outbursts. Simple cavings-in, in deep cavities, have also been regarded as possibly giving rise to earthquakes; and this opinion has been adopted by M. Boussingault after the well-known observations he made in the Andes. There is, in fact, nothing to prove that disturbances of these different kinds do not take place in the interior of the globe; but we may certainly consider them as the general cause of earthquakes. These shocks are, however, most commonly in evident connection with volcanoes; and it is in the neighborhood of the latter that they are especially frequent. As is well known, every volcanic eruption is announced by precursory earthquakes, the violence of which is stilled when an outlet is opened for the vapor of water which is successively the cause of the subterranean agitations and the projecting agent of all the eruptions. The tension of the vapor in the volcanic reservoirs must be very high. Thus, that pressure which forces the lava up to more than 3,000 metres above the sea, to the top of Etna, can not be less than a thousand atmospheres.

An attentive study of the phenomena confirms the attribution of the cause of the shocks, however violent they may be, to the vapor of water. It is sufficient for this to be the case for vaporization to take