12 JOHN JOHNSTON AND L. H. ADAMS 



pores by the plastic rock should not occur. Indeed it is hard to 

 conceive of the existence, at any considerable depth, of continuous 

 spaces, unless they be very small ;^ and if they be small and traversed 

 by water, it would appear that they must in a very short time 

 become choked up with material deposited by the water when it 

 evaporates. This point has already been discussed by R. T. 

 Chamberlin,^ who agrees in thinking that capillary force is quantita- 

 tively inadequate; and after adducing various lines of evidence 

 writes: ''All of these facts and deductions lead to the general con- 

 clusion that our surface-waters have been derived from the interior 

 of the earth, and oppose the idea that to explain the presence of 

 hydrogen, or water, in magmas and rocks, we have merely to appeal 

 to the penetration of surface-waters." 



In order to show the quantitative significance of capillarity 

 we present in Table II calculated values of the pressure producible 

 by capillarity at various depths, assuming a temperature gradient 

 of (i) 1° C. per 30 meters, which is about the normal (so far as one 

 can judge from the present very faulty data) (2) 1° C. per meter, 

 which must be nearly the maximum gradient possible, even in the 

 vicinity of volcanoes. In making these computations we have 

 taken into account the variation of surface tension (cr) with tempera- 

 ture, but have neglected the (unknown) effect of pressure; we have 

 also neglected the influence of temperature and pressure {a) on the 

 angle of contact a (&) on the density of water, which enters as a 

 factor (i) in the value of k proper to each temperature and (2) in 

 the calculation of the hydrostatic pressure. In calculating the 

 pressure due to the overlying rock, a mean rock density of 2.7 

 was assumed. The values given in Table II are therefore approxi- 

 mate only, but nevertheless are amply accurate for the present 

 purpose. Such figures can be used to support geological specula- 

 tions with regard to the penetration of water into deep-seated rocks 

 only if both of the following restrictive conditions can be considered 

 to be fulfilled: (i) that pores persist to the depth in question; 

 (2) that the rock mass adjoining the mouth of the pore shall be 



^ As to the depth to which spaces may persist, see F. D. Adams, Joiir. Geology, XX 

 (191 2), 97-118, and L. V. King, ibid., 119-38. 



^ "The Gases in Rocks" {Carnegie Inst. Publication No. 106, 1908), 70-75. 



