268 REPORT UNITED STATES GEOLOGICAL SURVEY. 



water, and as being thus one of the efiScieut causes of the liquefaction 

 of deeply-buried sedimentary rocks." 



Conceding these views, we have a double action producing a decrease 

 of specific gravity. We liave the mechanical action produced by pres- 

 sure, and we have the result of chemical changes of hydration. The 

 pressure produced within the limits of the reservoir will tend to increase 

 its size. As iresh, undisturbed zones are reached by the spreading ac- 

 tion, new acquisitions of chemical solvent agents will be encountered. 

 Heat will be lost in the conversion of this fresh supply from a rigid to 

 a plastic or liquid condition. It is evident, therefore, that the enlarge- 

 ment of the reservoir will eventually reach a point beyond which it will 

 progress but very slowly. Should the effects of superimposed sedimen- 

 tation, which have gradually wrought this condition, cease, then, too, 

 will the process of liquefaiction progress no further. 



In case the reservoir thus formed is sufficiently near the surface of 

 the earth, the strain incident upon its generation will be entirely suffi- 

 cient to produce seismic phenomena. These may be manifested simply 

 by vibrations and concussions, or they may result in the formation of 

 fissures. In addition to this method of forming fissures we are ac- 

 quainted with many other modes. Perhaps one of the most striking is 

 that suggested by Sir John Herschel, which has been given above. 



At the time of the opening of a fissure or series of fissures we have, 

 within the reservoir, certain definite conditions ; a more or less homo- 

 geneous magma has been formed by the liquefaction of the original 

 rocks; the mass is in the condition of maximum tension ; water is held 

 there at very high temperatures and under great pressure ; gaseous 

 vapors which have been reduced, even at such great heat, to liquidity 

 may be retained within the limits of the reservoir. 



Upon the opening of the fissure the viscous mass will rise into it by 

 hydrostatic pressure. Should the lava then reach points where the con- 

 ditions are no longer such as to retain water and liquefied gases in the 

 same state as within the reservoir, these will suddenly expand, and, 

 under favorable circumstances, produce the phenomenon of catastrophic 

 volcanic eruptions. Where either th"^. configuration of the fissure or 

 the teusion of the material within the reservoir are such as to preclude 

 an occurrence of this character, we will have different results. Either 

 the lava will ascend into the fissure until it flows over, with the demon- 

 stration of violent force, at the surface, or the lava will rise within the 

 fissure until hydrostatic pressure and expansion of gases can no longer 

 propel it upward. By the agency of heat, decomposition of water may 

 take place, and the additional quantities that may be encountered on 

 the way upward will be utilized to a certain extent. This feature seems 

 to account for the presence of hydrogen and chlorine during volcanic 

 eruptions of the present day, while nitrogen would be derived from the 

 decomposition of atmospheric air which was enclosed within the fused 

 masses. 



Should the eruption be essentially a slow one, it may easily occur 

 that the liquid masses will be cooled down sufficiently, and the conditions 

 of pressure may be favorable to the sudden conversion of superheated 

 water to steam, and liquefied gases to vapors at very considerable depths. 

 This in turn would tend to restore the original conditions of equilibrium, 

 which now would be abnormal. Continued deposition of sediment, the 

 everchauging absolute position of the zone of liquefaction, and the at- 

 tempted restoration of static conditions would eventually result in a 

 repetition of the same occurrences. Fissures would again be formed at 



