Lavis— On the Structure of Rocks. 115 



Passing over the older and more crude theories relating to the 

 flowing 1 down of water by crevasses, and so coming in contact with 

 the molten lava, or over the fantastic hypothesis of Davy down to 

 that of Peacock, we find the question severely tackled in 1881 by 

 three eminent geologists : Professor Prestwich, 2 who attributes 

 eruptions to the percolation of water to the porosity and cleavage 

 planes of rocks, and not to fissures, but yet does not admit of that 

 intimate mixture of the water with the magma which anyone 

 accustomed to watch the lava in its fluent state soon becomes con- 

 vinced of. Professor Sollas 3 at the same time and place recognizes 

 the intermixture of water and lava, and supposes the former to 

 exist in the liquid state, but he fails to explain the variability of 

 eruptive action except by relief of pressure. The changes of pres- 

 sure, as explained by the author, are obviously insufficient to bring 

 about inactivity on the one hand, or explosive 4 eruptions on the 

 other. Besides, we should expect volcanoes in the same region to 

 act quite synchronously. It is to Professor Judd that merit is due 

 for the recognition of the fact that the igneous magma may, under 

 pressure, absorb gases such as water is at high temperatures, and he 

 gives in illustration a number of analogous examples, but does not 

 treat of the conditions of absorption and dispersion of such water. 



Just to this point we are provided with a demonstration of what 

 is really the motive power of volcanic eruptions, and it is here I 

 propose to take up the thread and discuss the conditions under 

 which this water is absorbed, retained, and expelled. 



Let us take an illustration, namely, the solution of carbonic 

 anhydride in water itself. Carbonic anhydride is, at the normal 

 temperature and pressure of the atmosphere, a gas ; but by either 

 increasing the pressure or lowering the temperature it may be 

 reduced to the liquid or solid state. The water of volcanoes, at 



1 Baron Dietrich in Ferber, Lettres sur la mineralogie, &c, de l'ltalie, 1776, 

 p. 207 ; also Braccini. 



2 " Some Observations on the Causes of Volcanic Action." — Reports, Brit. Assoc, 

 1881. 



3 "The Connexion between the Intrusion of Volcanic Action." — Reports, Brit. 

 Assoc, 1881. 



4 It may be here mentioned that I do not use paroxysmal in the same sense as Scrope, 

 but to indicate those increments of activity that occur from time to time during chronic 

 activity, always accompanied by the outpour of lava, leaving explosive for those erup- 

 tions with only fragmentary pumiceous or scoriaceous pumice ejectamenta. 



K2 



