On the Agency of Water in Volcanic Eruptions. 121 



2. The insufficiency of the elevatory force. 



3. The want of agreement in time and proportion between the dis- 



charge of steam and the discharge of lava. 



On the hypothesis of Mr. Poulett-Scrope it has to be assumed 

 either that the water formed part of the original molten magma, 

 or that the surface-waters find a passage by percolation or by 

 fissures through the crust of the earth to the molten mass beneath. 

 That water does penetrate to great depths, there can be no doubt, and 

 if nothing interfered to check its descent, the extent and range of the 

 percolation could hardly be limited. But various stratigraphical 

 causes interfere with this transmission, such as the thinning out of the 

 strata, faults, and uncomformity of superposition. These causes 

 especially impede the flow of water in the more frequently disturbed 

 Palaeozoic strata, but in the less disturbed newer strata it is compara- 

 tively little affected by them. At the greatest depths in Tertiary 

 and Secondary strata water is always, or almost always, met with in 

 the permeable beds. It is otherwise with the Palaeozoic strata 

 generally ; where,, for example, as is so common in Coal Mines, the 

 faulting of the measures divides them into separate segments or com- 

 partments, in each of which the supply of water from adjacent areas 

 is cut off,, and no. fresh supplies being received from the surface, there 

 is no further accession of water. Again, where Tertiary or Secondary 

 strata overlie uncomformably Palaeozoic strata, the interstices on the 

 surface of the older rocks are so effectually plugged by the basement 

 bed of the superimposed strata, that they are often rendered perfectly 

 watertight. There is a remarkable instance of this in the coal-field 

 of Mons, # where the Coal-measures are in places worked under a depth 

 of 600 to 900 feet of loose sandy Cretaceous strata full of water, and 

 yet are found to be perfectly dry, owing to the circumstance that the 

 basement bed of the overlying strata has formed a sort of puddle 

 cover, sealing up, as it were, the edges of the underlying strata. 

 The percolation of the surface-water to great depths is, in consequence 

 of these interruptions, far from being so general as might be supposed. 



Admitting, however, the possibility of water descending in certain 

 cases, — as through the fissures and crevices of crystalline rocks, or, in 

 the absence of any mechanical conditions to stop its descent, through 

 permeable strata, — it is a question whether its descent would not be 

 stayed by the increase of heat at great depths, although under the 

 enormous pressure it may remain liquid at high temperatures. 



It is known experimentally that the pressure of steam which at 

 212° P. equals one atmosphere, is, at a temperature of 432° F., equal to 

 that of nearly 24 atmospheres ; and also that the rate increases with 

 the rise of temperature, and faster at high than at low temperatures, 

 as the following reduction from Regnault's tables shows : — 



* The author in " Proc. Inst. Civil Engineers," vol. xxxvii, p. 129. 



