626 GEOLOGY. 



rial and rise in the lava-column; but to this it is answered that an experi- 

 ment of Daubree's has shown that water will penetrate the capillaries 

 of sandstone against high steam pressure and add itself to the steam 

 within. The fact is also cited that certain substances, when highly 

 heated, absorb gases which they give out when they cool. The absorp- 

 tion of hydrogen by platinum, and of oxygen by molten silver, are illus- 

 trations. It is certain that the lavas do contain large quantities of ab- 

 sorbed gases, and that these are partly, and in most cases largely, given 

 out in cooling, when the cooling takes place at the surface. The presump- 

 tion is that the lavas would take the gases up again on remelting under 

 similar conditions. If the lavas of actual volcanoes had the tem- 

 peratures of aqueo-igneous fusion (700°-1000° Faiir.) only, it would 

 strengthen this view; but as temperatures of lavas often exceed 2000° 

 Fahr., and probably sometimes reach 2500° Fahr., and perhaps 3000° 

 Fahr., it is not easy to account for such temperatures under this hypoth- 

 esis, because they would only be reached at levels far below those at 

 which aqueo-igneous fusion might be presumed to take place. Perhaps 

 this could be met by invoking pressure which might prevent even aqueo- 

 igneous fusion from taking place until these temperatures were reached, 

 but pressure brings in a grave difficulty in another line, as we shall 

 presently see. 



There is a phase of the water-penetration hypothesis which seeks 

 to account for an accession of heat. It is affirmed that the outer rocks 

 are oxidized, while the inner ones w^re not originally, or at least not 

 completely oxidized, and that air and water from the surface, reaching 

 the unoxidized zone, enter into combination and generate the necessary 

 heat. This view was pardonable before the development of modern 

 thermo-chemistry, but is now quite untenable, as may be shown by 

 working out the reactions thermally. 



All views which assign the penetration of surface air or water as a 

 cause meet with a grave, if not insuperable, difficulty in the condition 

 of the lower part of the earth's crust (see p. 218). The fractured con- 

 dition of the crust, which permits a ready penetration of water, is a very 

 superficial phenomenon. Below the first few thousand feet the crevices 

 and porosities of the rock are rapidly closed by the pressure of over- 

 lying rock, and all appreciable crevices and pores probably disappear 

 at a depth of five or six miles. The effective function of fissures is, 

 therefore, limited to the upper few miles of the crust, and even here 



