But there is another hypothesis which is, to say the least, equally probable. Water containing various gases in solution is continually finding its way downwards by infiltration into the earth's crust. Much of this water, after passing through pervious beds, reaches some impervious stratum and is returned to the surface in the form of springs. But that some of this percolating water penetrates to enormous depths is shown by the fact that the deepest mines and borings encounter vast underground supplies of water. When we remember that nearly three-fourths of the earth's surface is covered by the waters of the ocean, and that the average depth of these oceanic waters is more than 10,000 feet, we may easily understand how great a portion of the earth's crust must be penetrated by infiltrating waters which can find no outlet in springs. The penetration of the waters of the ocean into the earth's crust will be aided, too, by the enormous pressure amounting to not less than several tons to the square-inch upon the greater part of the ocean-floor. It might be thought that this downward penetration of water would be counteracted by the upward current of steam that would be produced as these subterranean waters reach the hotter portions of the earth's crust. But the experiments of Daubrée have conclusively shown that the penetration of water through rocks takes place in opposition to the powerful pressure of steam in the contrary direction. Hence, we may assume that certain quantities of water, containing various gases and solids in solution, are continually finding their way by capillary infiltration from the surface to the deeply seated portions of the earth's crust, there to undergo absorption by the incandescent rock-masses and to produce oxidation of some of their materials. [Sidenote: POSITION OF THE ISOGEOTHERMS.] The deep-sea soundings of the 'Challenger' have shown that the floor of the ocean is constantly maintained at a temperature but little above that of the freezing point of water. This low temperature is probably produced by the absorption of heat from the earth's crust by the waters of the ocean, which distribute it by means of convection currents on the grandest scale. Hence, the isogeotherms, or lines indicating the depths at which the same mean temperature is found within the earth's crust, are probably depressed beneath the great ocean-floors, and rise towards the land-masses. It is to this circumstance, combined with that of the enormous pressure of water on the ocean-beds, that we must probably ascribe the general absence of volcanoes in the deep seas and their distribution near coast-lines.