VULCANISM. 73 



been heated to high temperatures, the deposition of the solutes from any 

 penetrating water should have sealed the capillary tubes and fissures at 

 a distance from the lava such that the latter cannot absorb them and 

 keep the water-way open. Kemp has stated in a recent paper 1 that at the 

 contacts with eruptives, limestone rocks, instead of being porous, are 

 prevailingly dense and compact, and often very hard to drill, as if due to 

 deposition within their interstices. However, the author assigned this 

 supposed deposition to magmatic waters from the intrusion. This brings 

 up a widely established view that magmas, instead of absorbing water from 

 the intruded rocks, give it off, depositing matter in solution to form veins in 

 the zone of fracture. 

 To quote Van Hise : 2 



In the belt of cementation, in consequence of the porosity of that zone, the material 

 of the magma, both by direct injection and by transmission through water, may pro- 

 foundly affect the average chemical composition of the intruded rock for great distances 

 from the intrusive mass. 



Geikie cites a case in Bohemia, where certain Senonian marls, invaded 

 by a mass of Tertiary dolerite, begin to get darker in color and harder 

 in texture at a distance of 800 meters from the contact, while, as the intru- 

 sive mass is approached, the interstratified beds of sandstone have been 

 indurated to the compactness of quartzite. 3 



But considering only meteoric waters at depths greater than 6,900 

 feet, where water remains liquid up to the critical 'temperature, it is less 

 probable that the pore spaces will be filled up in this manner. Nor does 

 it seem likely that Daubree's theory that water may penetrate rocks 

 against a steam-pressure can operate at these depths, since that principle 

 is dependent upon a marked difference between the capillarity of water 

 and of steam, while at the critical point, the density of water-gas being 

 the same as that of water, this force should be absent. The problem then 

 becomes a question of equilibrium between the hydrostatic column of 

 water and that of the lava, in which the pressure of the lava at a depth of 

 7,000 feet should be in the neighborhood of 2.7 times that of the water, 

 though this preponderance steadily diminishes as the water-gas becomes 

 condensed, with increasing depth, at a rate higher than lava. Whether 

 under these conditions lava can absorb water-gas, is an open question. 



Water can only penetrate from 25,000 to 30,000 feet below the surface 

 on account of the closure of all crevices by pressure. But on the assumption 

 that the temperature gradient in the outer part of the earth's crust is 1 C. 

 for each 100 feet of descent (which is probably too high) the critical tem- 

 perature will not be reached, except in the neighborhood of volcanic intru- 

 sions, until at a depth of about 36,000 feet. Hence, over the greater part 

 of the earth, water will remain in the liquid state as far down as fractures 

 and fissures will allow it to seep, and no appeal can be made to the more 

 rapid and potent gaseous diffusion to carry it beyond 30,000 feet. But 

 because of their heat, lavas must originate at much greater depths below 

 the surface, and hence far beyond the reach of surface-waters, which can 



1 Kemp, Economic Geol., vol. 2, p. 11. 



2 Van Hise, Monograph 47, U. S. G. S., p. 714. 



8 Hibsch, cited by Geikie, Textbook of Geology, vol. 2, p. 774. 



