Only one of these temperatures reaches the boiling-point of water _at sea-level_. It is therefore clear that at this depth water has not even closely approached the boiling temperature _for this depth_, and since this is the depth of the critical pressure, it is clear that it cannot boil at any greater depth. The descent of water is therefore not stopped, under normal conditions of crustal temperature, because it reaches its boiling-point. Locally, as in the vicinity of active or recently extinct volcanoes, the case may be different. It is conceivable that water may descend until it reaches its critical temperature (somewhere between 610° and 635° Fahr.). The depth at which the critical temperature would be reached, under various assumptions, is shown in the following table: Rate of Increase Depth of Critical Initial Temperature. of Temperature. Temperature. 50° 1° for 50 feet 28,000 to 29,250 feet 50° 1° for 60 „ 33,600 to 35,100 „ 50° 1° for 70 „ 39,200 to 40,950 „ 80° 1° for 50 „ 26,500 to 27,750 „ 80° 1° for 60 „ 31,800 to 33,300 „ 80° 1° for 70 „ 37,100 to 38,850 „ There is good reason, in the increasing density beneath the surface, for believing that the rate of increase of temperature decreases with depth, and therefore that the rate of 1° for 50 feet for the depths concerned is too high. The greater depths of the table above are therefore believed to more nearly represent the truth than the lesser ones. (See discussion of underground temperature in Chapter XI.) If descending water attained its critical temperature, the extent to which the resulting _water-gas_ might be absorbed is not known. _So far as limited by temperature_, therefore, it is not possible to assign a limit to the descent of water under average conditions of crustal temperature.