512 Prof. John Milne — On the Form of Volcanos. 



rocks beneath the base of a volcano, which at their central parts are 

 buried beneath a mass of material equal to the height of the volcano, 

 which will usually be many thousands of feet. And just as the 

 water at the bottom of an Icelandic geyser is hotter than that near 

 the surface, so we may expect the rocks of which we speak to have 

 an augmented temperature. 



Another condition is that of the heated plane. From what is 

 known of the internal structure of a volcano, a central column termi- 

 nating in a crater above, and provided with many branches ramifying 

 through the mass of the volcano, would be a truer representation of 

 the actual conditions than those which have been taken. These 

 ramifications would probably be continuous into the ground beneath, 

 and deep down it is not at all unlikely that they converge towards a 

 highly heated nucleus from which all the molten matter emanates. 

 If these are anything like the conditions beneath a volcano, and I 

 see no reasons why they should not be, then the plate of heated 

 matter is but a very feeble representative of the heating surfaces 

 which actually exist. 



The only point about which I think any doubt can be expressed 

 will be relative to the length of time during which such a heating 

 surface may be imagined to retain a constant temperature. Whilst 

 a volcano is in actual activity, the temperature deep down beneath 

 the crater probably remains constant far above 2000° Fahr., and 

 during the intervals which fall between the eruptions, whilst the 

 volcano is only perhaps giving out wreaths of vapour, it does 

 not seem at all improbable that, if it were possible to take the 

 temperature some thousands of feet below the crater from which 

 the steam is issuing, we should also find a temperature far above 

 2000° Fahr. 



It must be also remembered that the heating effect which takes 

 place is over and above any original temperature which the rocks 

 may be supposed to have had before any volcanic eruption took place. 



On the whole, therefore, I think the conditions of the given case 

 have by no means been overestimated, but rather, were we able to 

 take the conditions which actually exist, we should find temperatures 

 far above 2000° Fahr., acting over periods longer than 2000 years, and, 

 what is more important, instead of heat being given off from a single 

 plane, it would be found to be given off from a multitude of sur- 

 faces, and the effect dependent on these temperatures enormously 

 exaggerated. 



Confining one's self to the example which has been taken, it now 

 only remains for us to consider what these effects might be. 



In vol. ii. p. 238, Lyell, in his " Principles of Geology," says, 

 " It was ascertained that fine-grained granite expanded with 1° Fahr. 

 at the rate of -000004825 ; white crystalline marble -000005668 ; 

 and red sandstone -000009532, or about twice as much as granite. 

 Now, according to this law of expansion, a mass of sandstone a mile 

 in thickness, which should have its temperature raised 200° Fah., 

 would lift a superimposed layer of rock to the height of ten feet 

 above its former level. But suppose a part of the earth's crust fifty 



