VULCANISM. 71 



meteoric waters grow relatively scant, as a rule, below the uppermost 

 1,500 to 1,800 feet of the earth's crust. 1 This shallowness of meteoric water 

 increases the difficulties encountered by the hypothesis that the lava beds 

 are supplied from this source, since they rise from far greater depths and 

 only the upper portions of their conduits would be exposed to these waters. 



It is in this portion of the zone of fracture that DaubreVs much quoted 

 experiment upon the Strasbourg sandstone 2 finds its application, if any- 

 where, since numerous capillary pores with plenty of water are requisites 

 for the operation of this principle. This famous experiment demonstrated 

 that, owing to its force of capillarity, boiling water will pass through a 

 disk of sandstone, 2 centimeters in thickness, against a slight steam-pres- 

 sure on the other side. But it was only necessary for the steam-pressure 

 to reach 685 millimeters, or nine-tenths of an atmosphere, in order to 

 prevent any more water from passing through the sandstone. It is a long 

 jump from this trivial capillary force, equal to less than one atmosphere 

 of steam-pressure, to the great pressures which would have to be overcome 

 in the depths of the earth's crust in order to reach the hot lavas, even though 

 it be allowed that the water-vapor, if it came in contact with the lava, 

 would be absorbed. Capillary force seems quantitatively inadequate. 



To reach the critical pressure of water due to the hydrostatic column, 

 it is necessary to penetrate the earth to a depth of about 6,900 feet. At 

 depths less than this, water passing into the vaporous condition, in the 

 neighborhood of hot volcanic conduits, at temperatures below the critical 

 point, should leave behind more or less of the matter held by it in solution, 

 since the condensation, and hence molecular attraction of the vapor for 

 solutes, is less than that of the water. Thus even if vapor from underground 

 waters should enter the lavas, as Daubree has suggested, in the outer 

 6,900 feet of the earth's crust, much of the chlorides, sulphates, carbonates, 

 and silicates, dissolved in the water, would have been left behind. At 

 depths between 6,900 feet and 25,000 feet, beyond which water can not 

 penetrate, owing to the closure of all pores by the pressure of superin- 

 cumbent rock, mineral matter dissolved in the water would probably still 

 remain in solution when the liquid passed into the gaseous state at the 

 critical temperature, since the density of the gas is equal to, or greater 

 than, that of the liquid. 



The lava, being under considerable pressure, may be supposed to occupy 

 all the cracks and crevices in the adjacent rocks, except those of capillary 

 dimensions. If, therefore, in the passage of underground water into vapor, 

 preparatory to entering lavas in the outer 6,900 feet of the earth's crust, 

 much of the dissolved mineral matter be deposited in the minute pores 

 leading to the lava, they should quickly become sealed, preventing any 

 further access, even of water, to the lava. To test this principle experi- 

 mentally, a cylinder of medium-grained Potsdam sandstone from Wis- 

 consin, 40 millimeters in diameter and 28 millimeters in thickness, was 

 soldered into a short piece of iron piping, fitted at one end with an elbow 



1 Kemp, Economic Geol.. vol. 2 (1907), p. 3; Finch, Proc. Col. Sci. Soc., vol. 7 (1904), 

 pp. 193-252. 



2 Daubrde, Etudes SynthStiques, t. 1, pp. 236-246. 



