404 Captain F. W. Hutton on the Phenomena of the 



surfaces called isogeothermal surfaces^ and that at great depths 

 these isogeothermal surfaces '^ will be spherical ; but as they ap- 

 proach the surface, they will by degrees conform themselves to 

 the solid portion " (Herschel, Proc. Geol. Soc. vol. ii. p. 548) 

 and to the mean temperature of its surface. Any change,, there- 

 fore, in the temperature or outline of the surface must necessa- 

 rily cause them to move ; so that if a surface of rock be covered 

 by deposits 100 feet in thickness, and the new surface have the 

 same mean temperature as that formerly belonging to the old 

 surface, all the isogeothermals below that area will move out- 

 wards 100 feet ; and if we assume that the internal tempera- 

 ture increases 1° F. for 50 feet of depth, the old surface will 

 have its temperature raised 3° F. The same result would of 

 course follow if, instead of being covered up, the old surface had 

 its mean temperature raised by 2° F. by a warm current passing 

 over it. If its temperature were lowered 2°F., or if 100 feet of 

 rock were denuded off, the effects would be just the reverse; the 

 isogeothermals would recede 100 feet, and all the region below 

 would be 2° colder. The effects of this increase or decrease 

 of temperature would be to increase or decrease the volume of 

 the rocks. 



From experiments made by Colonel Totten (LyelPs Pr. of Geol. 

 vol. ii. p. 235) and Mr. Adie (Trans. Roy. Soc. of Edinb. vol. xiii. 

 p. 354), it appears that rocks expand from '0000045 to -0000095 

 with an accession of temperature of 1° F. If, now, the heated rock 

 consisted of loose, incoherent particles which could move among 

 themselves, the increase in volume would, owing to lateral re- 

 sistance, all take place upward ; and if we suppose the rate of 

 expansion to be the maximum just given, we find that we should 

 have an elevation of '00034 inch for every foot in depth raised 

 1^ F. in temperature*. This effect is so small that it would be 

 quite imperceptible for all thicknesses of deposit and increase of 

 temperature that we are warranted in assuming. If, however, 

 the particles of w'hich the rock was composed were rigidly fixed 

 together, the case would be very different; the expansion in 

 height would still take place upward; but those in length and 

 breadth could not be so diverted, and they would give rise to an 

 irresistible pressure, which could only be relieved by the rock 

 rising up in one or more anticlinal ridges. Now sands and clays 

 are always deposited in the incoherent state first mentioned; 

 but deposits of carbonate of lime are indurated almost immedi- 



* It must be remembered that in this case the lateral thrust would not 

 have to support the arch entirely. As soon as sufficient lateral thrust 

 was developed to overcome the rigidity of the crust, movement would com- 

 mence, and the remainder of the force necessary for supporting the arch 

 would be borne by the expansible underlying rocks. 



