Prof. John Milne — On the Form of Volcanos. 513 



miles in thickness, and equally expansible, to have its temperature 

 raised 600° or 800°, this might produce an elevation of between 

 1,000 and 1,500 feet. The cooling of the same mass might after- 

 wards cause the overlying rocks to sink down again and resume 

 their original position." 



In the case we have been considering, we see that temperatures 

 are produced far greater than those spoken of by Lyell, and conse- 

 quently the effect will be correspondingly augmented. 



But the effect produced by simple expansion is not all. The rocks 

 distant from the heating surface will offer resistance to the expanding 

 mass which lies between them, and cause it to elevate itself towards 

 the centre, the action being not unlike that which is exhibited by 

 the iron arch of a bridge when expanding on a hot day. In the 

 bridge the rise is only perhaps one or two inches. To us this is 

 almost imperceptible. But if the same fact could be communicated to 

 a creature no larger than a fly which lived upon the bridge, it would 

 seem to be immense. Similarly if we could only learn the height to 

 which a volcano is elevated by the heat developed during its period 

 of activity, we might be equally astonished ; for, when reckoned in 

 feet, it is probably very great. 



So far we have only considered the heat beneath a volcano as an 

 agent producing expansion, and consequently elevation. If we 

 imagine the heat to be much greater than that which I have taken, 

 it is possible that in certain tracts it might pi'oduce fusion or at least 

 plasticity, the result of which might be that the pressing downwards 

 of the superincumbent weight would produce depression. However, 

 if such conditions have any existence, I fancy that their extent is 

 limited; and anyhow, outside the plastic area there will be expansion 

 and consequent elevation. 



Results of elevation or depression. — As elevation or depression 

 beneath a volcano, such as those of which I have been speaking, will, 

 in all probability, take place so slowly, the effect cannot be expected 

 to materially alter the form of a volcano, degrading influence tending 

 to produce a natural form compensating sufficiently rapidly to out- 

 weigh the influence of these motions. The effect which will be pro- 

 duced is rather a change in height. 



We have here perhaps in part the means of explaining why the 

 measurements of the heights of certain volcanos when made at long 

 intervals apart should have given diffei'ent results. 1 



We may carry these ideas from a single volcano to a volcanic 

 district. During periods Of activity we should expect to find the 

 heat gradients steeper than during periods of repose, and therefore 

 during such periods of maximum temperature, elevation might be 

 expected. Also the rate at which the steepness of a heat gradient 

 in a district varies might indicate the approach or recession of 

 volcanic agency, and perhaps may even yet give us a measure of 

 the time by which such actions are removed. 



1 I think that these discrepancies have heen noticed and written upon in the 

 " Geographical Magazine," October, 1877, but I have not the means of making 

 reference. 



DECADE II. — VOL. VI. — NO. XI. 33 



