All this speaks so emphatically against the tectonic origin of earthquakes that it can not be considered as a general cause. Even the mighty disturbances and shocks of the times when such ranges as the Alps and Himalayas were lifted up can prove nothing for the present time; for the conditions, the mechanical work and acting forces, of the earth were quite different, and the latter much greater and more acute than in our time, as the number and magnitude of the volcanoes of those ages show, before which ours are almost as nothing. We have no adequate comprehension of the way that mechanical work was done. A depression like that of the plain of the Rhine could certainly not have taken place without severe earthquakes; but we do not know how they may have come to pass, for we have nothing analogous to them. The upper strata of the earth's crust are broken up, fissured, and cavernous; hence purely local minor earthquakes may undoubtedly be produced by cavings-in, landslides, and settlings of small extent. But this explanation, in view of the nature of the crust, is not possible for strong earthquakes, even in the upper layers, which send their waves far over the land; their origin must be, almost of necessity, in the greater deeps beneath the crust, far down where the immense gas globe of the interior is constantly forcing its way into the fluid band, and this into the solid stone; in those zones of changing conditions a mighty movement must be incessantly prevailing. The pressure upon the gases of the interior diminishes here, and the excessive temperature as well. This can not take place without changes. Temperature and pressure now fall, now rise again, but continue very high through it all. The dissociated gases unite and separate again, and most violent explosions are infallibly produced thereby. Water exists in the interior in immense masses, and that not solely in consequence of percolation from the surface. Vapor at very high pressure separates into its elements--hydrogen and oxygen--the reunion of which ensues with violent explosions, similar to our gas explosions, which must be very numerous in the interior of the earth, and accompanied with great development of force. The principal effect of such explosions is, of course, against the cooler and more weakly resisting sides, and therefore not toward the interior but toward the crust and the weakest parts of it, toward the rupture lines of the zones of disturbance, the synclinals. Such attacks, striking the earth's crust from within, occasion most earthquakes, especially violent, destructive, deep-seated outbursts like those of Lisbon and Charleston. The relation of the seismic and the volcanic phenomena is clearly to be seen.