Igneous rocks are those which have formed from material that has been melted, which involves temperatures around 1300° C.; or, if there is water in the original material, temperatures as low as 800° C. will suffice. Considering the increase of temperature to be a degree for every 50 feet downward, this involves the rocks having been at depths of 5 to 10 miles below the surface. While at such depths the temperature must be high enough to melt rocks, the great pressure of the overlying rocks seems to keep them solid; for we know that the center of the earth is solid, as is shown by a variety of observations, such as the rate at which earthquake waves are transmitted through the earth, the lack of tidal effects, etc. However, there is every reason to believe that if the pressure is removed from the rocks which are five to ten miles below the surface, there is heat enough at those depths to melt them. When the crust of the earth is folded, as when mountain ranges are formed, the areas under the arches or upward folds are relieved of pressure. Then those rocks, which are under the arches and are relieved, become molten. The molten magma may well up and fill the space beneath the arch where it would cool again very slowly; or, if there is fissuring during the folding, some of the molten material may be forced out through the fissures and pour out over the surface as lava. Another area in which pressures may be locally relieved is in the region of faulting, where the crust of the earth is broken into blocks, between which there are readjustments, some being tipped one way, some another, some uplifted. Here again there would be areas of relieved pressure and molten magmas would form, some of them solidifying in place, others rising to the surface. The molten material is termed the magma, and when it reaches the surface, great quantities of water vapor and other gases escape: or these gases may even escape from magmas which do not reach the surface, rising through fissures. As these hot vapors pass through the fissures, they are cooled, and may deposit part or all of their dissolved compounds in the fissure, making veins. Lava is the magma minus the vapors. Magmas vary greatly from place to place, indicating that they are formed locally and do not come from any general interior reservoir, as has sometimes been suggested.