Other considerations seem to place a limit to the descent of water. Rock, solid and unyielding as it seems, is yet plastic when under sufficiently great pressure. The cracks and cavities affecting it are believed to descend a distance which is but slight in comparison with the radius of the earth. Even if openings were once formed at greater depths, they could not persist, for the adjacent rock, under the pressure which there exists, would “flow” in, in effect (though perhaps not in principle) much as stiff liquid might, and close them. The outer zone of the earth where cavities may exist is known as the _zone of fracture_.[94] The depth of the zone of fracture differs for different rocks, but is not believed to extend below some such depth as five or six miles, even for the most resistant.[95] It is to be noted that these depths are less than those at which the critical temperature of water would be reached under most of the conditions, including all the more probable ones, specified in the above table. Let it be assumed that water descends through openings in the rock to a depth of six miles. At this depth it would, under the various assumptions specified in the first and second columns of the following table, have the temperature indicated in the third column: Initial Rate of Increase Temperature at Temperature. of Temperature. Depth of Six Miles. 50° 1° for 50 feet 683° Fahr. 50° 1° for 60 „ 578° „ 50° 1° for 70 „ 502° „ 80° 1° for 50 „ 713° „ 80° 1° for 60 „ 608° „ 80° 1° for 70 „ 532° „ In two of these cases, namely, those in which the assumed rate of increase of temperature is highest, the temperature of the water at the assumed lower limit of the zone of fracture is above the critical temperature of water. If the assumptions involved in these two cases be correct, water might descend to the point where it would be converted into water-gas, and in this condition it might be occluded by the hot rock. In the other cases, involving the more probable assumptions, the critical temperature is not reached at a depth of six miles. If pores and cracks do not extend to greater depths, liquid water could not; and since the water at this depth has probably not reached its critical temperature, it cannot exist as water-gas. If it does not exist in the form of water-gas, its occlusion by the hot rock substance would not be probable. It would seem, therefore, that the descent of water under ordinary conditions is much more likely to be limited by the _zone of fracture_, than by temperature.