INORGANIC GEOLOGICAL DYNAMICS. 611 



combines the doctrine of central heat with other- 

 physical laws 6 ; as, that solid rocks expand by being 

 heated, but that clay contracts ; that different rocks 

 and strata conduct heat differently; that the earth 

 radiates heat differently, or at different parts of its 

 surface, according as it is covered with forests, with 

 mountains, with deserts, or with water. These prin- 

 ciples, applied to large masses, such as those which 

 constitute the crust of the earth, might give rise to 

 changes as great as any which geology discloses. 

 For example : when the bed of a sea is covered by 

 a thick deposit of new matter worn from the shores, 

 the strata below the bed, being protected by a bad 

 conductor of heat, will be heated, and, being heated, 

 may be expanded ; or, as Sir J. Herschel has ob- 

 served, may produce explosion by the conversion of 

 their moisture into steam. Such speculations, when 

 founded on real data and sound calculations, may 

 hereafter be of material use in geology. 



The doctrine of central heat and fluidity has 

 been rejected by some eminent philosophers. Mr. 

 Lyell's reasons for this rejection belong rather to 

 Theoretical Geology ; but I may here notice M. 

 Poisson's opinion. He does not assent to the con- 

 clusion of Fourier, that since the temperature in- 

 creases in descending, there must be some primitive 

 central heat. On the contrary, he considers that 



6 On the Temple of Serapis, 1834. See also Journal of the 

 Royal Inst. vol. ii., quoted in Conyb. and Ph. p. xv. Lyell, 

 B. ii. e. xix. p. 383, (4th ed.) on Expansion of Stone. 



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