Another source of power behind volcanic action is steam pressure. We have already mentioned the fact that vast amounts of water in the form of steam escape from volcanoes or even from streams of molten lava. The violent volcanic explosions are quite certainly all, or nearly all, direct results of sudden giving way of volcanoes to steam pressure which accumulates during greater or less periods of time, and with little or no possibility of escape, without rupturing the mountain. Steam alone, or combined with some of the other gases so common as volcanic products, may also aid in forcing out molten rock. What is the source of the steam and other gases or vapors? According to one view they were originally in the earth, while according to another view the water at least has been absorbed by the molten rocks from surface waters which worked their way downward. At least two arguments oppose the second hypothesis: first, that not a few volcanoes are really many miles from the sea or other bodies of water, while downward percolation of rain water would fall far short of supplying the tremendous quantities of water ejected, and second, any water taken up by molten rock must be absorbed within a very few miles of the surface because, as we have learned, farther down there are no openings large enough to permit the downward passage of water, but, as a matter of fact, the very upper part of the earth's crust is just the place where molten rocks begin to give up their water, often with terrific violence. We may now turn to a consideration of the other very important kind of igneous activity, namely, the rise and transfer of molten materials within the earth's crust, but not to the surface. The quantity of such deep-seated (so-called "plutonic") igneous rock material which has been intruded into the earth's crust within known geologic time, is far greater than that which has been forced to surface, that is the so-called "volcanic" material. The plutonic rocks are always thoroughly crystallized, and they are generally coarser grained than the volcanic rocks.