254 



Prof. J. Prestwich. On the 



[Apr. 1C>, 



reached in mines, decreases, as a rule, with the depth, and is less in 

 the Palaeozoic than in the Mesozoic and Kainozoic strata. 



The main difficulty is thermo-dynamical. As the elastic vapour of 

 water increases with the rise of temperature, and faster at high than 

 at low temperatures, the pressure, — which at a depth of about 7,500 

 feet and with a temperature (taking the thermometric gradient at 

 48 feet per 1° F.) of 212° P., would be equal to that of one atmo- 

 sphere only, — would at a depth of 15,000 feet and a temperature of 

 362°, be equal to 10J atmospheres, and at 20,000 feet and tempera- 

 ture of 467°, would exceed 25 atmospheres. Beyond this temperature 

 the pressure has only been determined by empirical formula?, which, 

 as the increase of pressure is nearly proportional to the fifth power 

 of the excess of temperature, would show that the pressure, in 

 presence of the heat at greater depths, becomes excessive. Thus, if 

 the formulas hold good to the critical point of water or 773°, there 

 would at that temperature be a pressure of about 350 atmospheres. 



At temperatures exceeding 1000° F. and depth of about 50,000- 

 feet, the experiments of M. H. St. Claire Deville have shown that 

 the vapour of water, under certain conditions, probably undergoes 

 dissociation, and, consequently, a large increase in volume. It 

 would follow also on this that if the water-vapour had been subject 

 to the long-continued action of the high temperatures of great 

 depths, we might expect to meet with a less amount of steam and a 

 larger proportion of its constituent gases than occurs in the erup- 

 tions. Capillarity will assist the descent, and pressure will cause 

 the water to retain its fluidity to considerable depths, but with the 

 increasing heat, capillarity loses its power. 



Taking these various conditions into consideration, the author doubts- 

 whether the surface waters can penetrate to depths of more than seven 

 to eight miles, and feels it impossible to accept any hypothesis based 

 upon an assumed percolation to unlimited depths. That there should 

 be open fissures through which water could penetrate to the volcanic 

 foci, he also considers an impossibility. 



But the objection to which the author attaches most weight against 

 the extravasation of the lava being due to the presence of vapour in 

 the volcanic foci, is, that if such were the case, there should be a 

 distinct relation between the discharge of the lava and of the vapour., 

 whereas the result of an examination of a number of well-recorded 

 eruptions, shows that the two operations are in no relation and are 

 perfectly independent. Sometimes there has been a large discharge 

 of lava and little or no escape of steam, and at other times there have 

 been paroxysmal explosive eruptions with little discharge of lava. 



There are instances in which the lava of Vesuvius has welled out 

 almost with the tranquillity of a water-spring. A great eruption of 

 Etna commenced with violent explosions and ejection of scoriee, 



