Hutton.—On the Formation of Mountains. xxvii 
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 takot 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, read 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,t 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 temperature 
of the earth increases as we descend into it at the rate of about 1° Fahr. for 
every 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° Fahr., the mean temperature of the old surface would be raised 2°, or to 
52°, while at 100 feet below it would be 54°, and so on, so that the covering 
of the surface by a deposit 100 feet thick would raise the temperature of the 
whole underlying rocks 2°. If the deposit was thicker, the temperature would 
of course be more raised in proportion. Now we know that rocks expand on 
being heated and contract on being cooled, and Colonel Totten and Mr. Mp 
have shown that this expansion for each degree of temperature is from Zm to 
m of the whole, according to the nature of the rock. If, however, the 
deposit was unconsolidated, like clay or sand, and the particles were free to. 
move among themselves, this expansion would have very little effect in raising 
the surface ; but if the deposit was rigid, like limestone, the effect would be 
totally different, and the irresistable pressure, caused by the expansion of the 
*Q J. G:S.,. 10, 186. + Pro. G. S., IL, 548., 596. 
t Clay contracts on heing heated, but this does not affect the theory. 
