HuTTON. — Oil the Formation of Mountains. xx^^i 



some effect must be produced by tlie 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, 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. Hei^schel, 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 Avhile 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 jDhenomena. 

 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 j: and contract on being cooled, and Colonel Totten and Mr. Adie 

 have shown that this expansion for each degree of temperature is from ^ to 

 of the whole, according to the nature of the rock. If, however, the 



10,000 



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., 111., 186. +Pro. G. S., II., 548., 596. 



% Clay contracts on being heated, but this does not affect the theory. 



