(_f._) _Access of Surface Water to Molten Lava during Eruptions._--The existence of water during certain stages in eruptions is too frequent a phenomena to be lost sight of; but its presence may be accounted for in other ways, besides proximity to the sea or ocean. Certain volcanic mountains, such as Etna and Vesuvius, are built upon water-bearing strata, receiving their supplies from the rainfall of the surrounding country, or perhaps partly from the sea. In addition to this the ashes and scoriæ of the mountain sides are highly porous, and rain or snow can penetrate and settle downwards around the pipe or throat through which molten lava wells up from beneath. In such cases it is easy to understand how, at the commencement of a period of activity, molten lava ascending through one or more fissures, and meeting with water-charged strata or scoriæ, will convert the water into steam at high pressure, resulting in explosions more or less violent and prolonged, in proportion to the quantity of water and the depth to which it has penetrated. In this manner we may suppose that ashes, scoriæ, and blocks of rock torn from the sides of the crater-throat, and hurled into the air, are piled around the vent, and accumulate into hills or mountains of conical form. After the explosion has exhausted itself, the molten lava quietly wells up and fills the crater, as in the cases of those of Auvergne and Syria, and other places. We may, therefore, adopt the general principle that in volcanic eruptions _where water in large quantities is present, we shall have crater-cones built up of ashes, scoriæ, and pumice; but where absent, the lava will be extravasated in sheets to greater or less distances without the formation of such cones; or if cones are fanned, they will be composed of solidified lava only, easily distinguishable from crater-cones of the first class_. (_g._) _Nature of the Interior Reservoir from which Lavas are derived._--We have now to consider the nature of the interior reservoir from which lavas are derived, and the physical conditions necessary for their eruption at the surface. Without going back to the question of the original condition of our globe, we may safely hold the view that at a very early period of geological history it consisted of a solidified crust at a high temperature, enfolding a globe of molten matter at a still higher temperature. As time went on, and the heat radiated into space from the surface of the globe, while at the same time slowly ascending from the interior by conduction, the crust necessarily contracted, and pressing more and more on the interior molten magma, this latter was forced from time to time to break through the contracting crust along zones of weakness or fissures.