168 Notices of Memoirs — Hutton, on Mountains. 



any point,, the underlying- superheated rocks, being thus relieved 

 from the pressure above them, would expand and rise up, and fill the 

 hollow; but this expansion would spread through the mass, and 

 would therefore lessen the outward pressure of the interior in all the 

 surrounding areas, which would consequently subside, and equilibrium 

 would only be once more restored when the mass of the subsided 

 areas equalled the mass of the elevated areas. Consequently eleva- 

 tion implies subsidence, and vice versa. "Where now must we look 

 for the causes that are in operation to disturb this equilibrium ? The 

 most obvious is the radiation of heat into space by the earth, and the 

 consequent cooling and contraction of the superheated interior. This 

 is at present almost universally accepted by geologists as the cause of 

 the movements of the surface and the upheaval of mountain chains, 

 but many arguments have been urged against it, and although I am 

 willing to allow that it must have some effect in producing move- 

 ments, these effects are, I think, completely absorbed by the much 

 larger ones that flow from causes that I shall presently describe ; and 

 it is quite impossible that it can be the only cause of movement, 

 partly because some effect must be produced by the other causes that 

 I have yet to describe, and partly because since the Glacial epoch the 

 earth has been warming instead of cooling, and consequently no 

 contraction, can have taken place since then, while we know not only 

 that extensive movements have taken place, but that they are still 

 taking place on the surface of the globe. 



The other cause of disturbance of the equilibrium, to which I have 

 alluded, is the removal of matter from one portion of the earth by 

 running water and its deposition on another portion. It is now 

 nearly forty years ago since Mr. C. Babbage, in his celebrated paper, 

 I'ead before the Geological Society of London,^ on the temple of 

 Jupiter Serapis, proposed a theory to account for oscillations of the 

 surface of the earth, which he called the theory of " the change of 

 isothermal surfaces." At about the same time, Sir J. Herschel, in a 

 letter to Sir C. Lyell,^ proposed to account for the same phenomena 

 by a theory which he called " the alteration of the incidence of 

 pressure." Both these theories are founded on the same fact, viz. 

 the removal of matter from one portion of the earth's surface and its 

 deposition on another ; but while Mr. Babbage laid the most stress 

 on the changes of internal temperature that would be thus brought 

 about, Sir J. Herschel laid the most stress on the change of direct 

 pressure, or weight. These theories have never been taken up by 

 geologists, but I hope to be able to show to you that, when combined, 

 they are capable of explaining all, or nearly all, of the observed 

 phenomena. I have already told you that, owing to its internal 

 heat, the mean temperatui'e of the earth increases as we descend into 

 it at the rate of about 1° Fahr. for eveiy fifty feet. If, therefore, the 

 mean temperature of the surface at any place was 50° Fahr. the mean 

 temperature 100 feet below would be 52° Fahr. If now the surface 

 was covered up by a deposit of clay or sand 100 feet thick, and if its 

 surface retained the same mean temperature as the old one, viz. 50° 



1 Quart. Jouin. Geo. Soc, iii., p. 186. - Proc. Geol. Soc, ii., pp. 548, 596. 



