8 Mr. It. W. Fox on Pseudomorphous Crystals of Quartz. 



ceive, be accounted for but by supposing the water existing in 

 the fissures of the earth to have been changed by circulation 

 from time to time, and to have been charged with different 

 ingredients at different periods. 



I have on former occasions alluded to various causes which 

 would produce circulation in the subterranean waters, such as 

 the opening or closing of any portions of fissures ; the ascent 

 of warm and the descent of cooler currents of water, in conse- 

 quence of the differences in their specific gravities; or in some 

 instances by the pressure of the sea- water acting on the fresh*. 

 Nearly two years ago I stated in this room my views in refer- 

 ence to the operation of this latter cause on land springs, and 

 at the same time I attempted to show the possibility, not to 

 say probability, of steam existing in fissures below the water 

 at a very great depth. I may perhaps be permitted to refer 

 again to this subject, because it appears to me to be one of 

 some interest. I then took it for granted that the tempera- 

 ture of the earth increases in some proportion to the increase 

 of depth below its surface, and that if the ratio be taken at 1° 

 Fahr. for every forty-eight feet, as found in our deep mines, 

 and if Le Roche's data for calculating the elastic force and 

 density of steam be adopted, the forces of steam and of water 

 pressure would balance each other at rather more than nine 

 miles deep, each being equal to the pressure of more than 

 1400 atmospheres. The density of the steam would there be 

 about one-fourth that of water at 60° Fahr., and its tempera- 

 ture above 1050° Fahr. But the temperature may probably 

 not increase so rapidly as this at great depths, and the equi- 

 librium in the pressures of the column of water and of steam 

 may occur much further below the surface, where the density 

 of steam under an augmented pressure of water would, of 

 course, be still greater. However this may be, it would seem 

 that, under any probable circumstances in regard to the ratio 

 of increase in the earth's temperature, the increase in the 

 pressure of the lengthened column of water would not keep 

 pace with the rapidly increasing tendency of the water in de- 

 scending into more heated parts of the earth to expand into 

 steam, the elasticity of which at very high temperatures, when 

 confined and in contact with water, is greatly augmented by 

 very small increments of sensible heat. 



No water could long remain unchanged into steam below 

 the line of division between them, and there the steam would 



* Columns of sea and spring water, about five feet high, balanced against 

 each other in a U-shaped tube, more than a year ago, still remain un- 

 mixed, showing nearly the same difference of level as at first (exceeding 

 an inch). 



