We shall now consider the igneous agents that act on the earth's surface. These agents are chiefly volcanoes and earthquakes, and we find that both usually occur in particular parts of the world. At various times and at various places within historical times volcanic eruptions and earthquakes have both proved their potency to alter the face of the earth. The principal geological facts and theories with regard to volcanoes and earthquakes are as follows. The primary causes of the volcano and the earthquake are to a great extent the same, and connected with the development of heat and chemical action at various depths in the interior of the globe. Volcanic heat has been supposed to be the result of the original high temperature of the molten planet, and the planet has been supposed to lose heat by radiation. Recent inquiries, however, suggest that the apparent loss of heat may arise from the excessive local development of volcanic action. Whatever the original shape of our planet, it must in time have become spheroidal by the gradual operation of centrifugal force acting on yielding materials brought successively within its action by aqueous and igneous causes. The heat in mines and artesian wells increases as we descend, but not in uniform ratio in different regions. Increase at a uniform ratio would imply such heat in the central nucleus as must instantly fuse the crust. Assuming that there are good astronomical grounds for inferring the original fluidity of the planet, yet such pristine fluidity need not affect the question of volcanic heat, for the volcanic action of successive periods belongs to a much more modern state of the globe, and implies the melting of different parts of the solid crust one after the other. The supposed great energy of the volcanic forces in the remoter periods is by no means borne out by geological observations on the quantity of lava produced by single eruptions in those several periods. The old notion that the crystalline rocks, whether stratified or unstratified, such as granite and gneiss, were produced in the lower parts of the earth's crust at the expense of a central nucleus slowly cooling from a state of fusion by heat has now had to be given up, now that granite is found to be of all ages, and now that we know the metamorphic rocks to be altered sedimentary strata, implying the denudation of a previously solidified crust. The powerful agency of steam or aqueous vapour in volcanic eruptions leads us to compare its power of propelling lava to the surface with that which it exerts in driving water up the pipe of an Icelandic geyser. Various gases also, rendered liquid by pressure at great depths, may aid in causing volcanic outbursts, and in fissuring and convulsing the rocks during earthquakes.