In the source that has been pointed out as that from which volcanic heat itself is derived, viz., the secular cooling of our globe, and the effects of that upon its solid shell, we are enabled to point to that which is the surest test of the truth of any theory--that it not only enables us to account for all the phenomena, near or remote, but to predict them. We see here linked together as parts of one grand play of forces, those of contraction by cooling, producing by _direct_ mechanical action the elevation of mountain chains, and by their _indirect_ action, by transformation of mechanical work into heat, the production of Volcanoes; and both by direct and by indirect action, of Earthquakes, never previously shown to have thus the physical connection of one common cause, but merely supposed, more or less, to be connected by their distribution upon our earth's surface. We now discern thus the physical cause _why_ Volcanoes are distributed, viewed largely, linearly, and follow the lines of elevation; we see equally why their action is uncertain, non-periodic, fluctuating in intensity, with longer or shorter periods of repose, shifting in position, becoming extinct here, appearing in new activity or for the first time there. We have an adequate solution of the before inexplicable fact of their propinquity, and yet want of connection. We have an adequate cause for the fusion of rock at local points without resorting to the baseless hypothesis of perennial lakes of lava, etc. For the first time, too, we discern a true physical cause for earthquake movement, where volcanic energy does not show itself. The crushing of the world's solid shell, whether thick or thin, goes on _per saltum_ and at ever-shifting places, however steadily the tangential pressures producing it may act. Hence crushing _alone_ may be shown to develope amply sufficient impulse to produce the most violent Earthquakes, whether they be or be not at a given place or time connected with volcanic outburst or possible injection, or with tangential pressures, enough still, in some cases, to produce partial permanent elevation. When subterraneous crushing takes place, and the circumstances of the site do not permit the access of water, there may be Earthquake, but can be no Volcano; where water is admitted, there may be both. And thus we discern why there are comparatively few submarine Volcanoes, the floor of the ocean being, on the whole, water-tight--"puddled," as an engineer would say, by the huge deposit of incoherent mud, etc., that covers most of it, and probably having a thicker crust beneath it than beneath the land.