But for this volcano to continue in eruption, it is necessary that the conditions shall continue that force the molten rock upwards from great depths. It is not enough for the lava to fill the crevice that exists upwards to the surface, it must continue to be forced upwards until it escapes. If it is permitted to remain in the fissure for any time, it hardens, and only great dikes are formed. It would seem, therefore, that some other force must be called into action to keep the fissure open or, in other words, to prevent the chilling of the lava. Now, this force is generally believed to be the expansive force of steam or the vapor of water. As Dana points out, by far the greater part of the vapor which escapes from the craters of volcanoes consists of steam or the vapor of water. Indeed, it can be shown that for every hundred parts of different vapors, at least ninety-nine of such parts consist of water vapor. It is for the greater part, to the pressure of steam or water vapor that the escape of lava from the tube near the top of the crater is due. Of course, the question arises as to where the water comes from that produces this steam. There are three possible sources. From the rains; from leakage at the bed of the ocean; and from vapors existing at great depths below the surface. It is not probable that either rain water, or water from the ocean, penetrates through the earth's crust for distances much greater than a few thousand feet. It is, however, very well known that in all parts of the earth, except in desert regions, whether they are near or far from the ocean, the rocks are always found fully charged with water. When, therefore, the slowly rising lava passes through the moist rocks that everywhere form the crust of the earth, there must be formed in them great quantities of steam under very high pressure. Moreover, many substances, especially those forming lava, possess the power of absorbing large quantities of steam and other gases. Therefore, as the molten material reaches the moist rocks in the earth's crust, it becomes highly charged with steam, and as the lava rises towards the surface this steam expands. Where the lava is in a very fluid condition the steam quietly escapes, as does the steam from the surface of boiling water. But where the lava is viscous, like tar or pitch, great bubbles are formed, which, on their explosion, throw the lava upwards for great distances into the air. We can, therefore, account in this manner for both the non-explosive as well as the explosive type of volcanoes. It must not be supposed, however, that it is the explosive power of steam which is the principal cause of the lava rising upwards from great depths. This is caused by the great pressure or stress set up by the contraction of a cooling crust. The pressure of this steam is added to this pressure which keeps the lava flowing upwards from great depths below.