INORGANIC GEOLOGICAL DYNAMICS. 555 



alteration in the length of the day had taken place, even to the amount 

 of one two-hundredth of a second ; and thus, there was here no con- 

 6rmation of the hypothesis of a primitive heat of the earth. 



Though we find no evidence of the secular contraction of the earth 

 in the observations with which astronomy deals, there are some geolo- 

 gical facts which at first appear to point to the reality of a refrigeration 

 within geological periods; as the existence of the remains of plants 

 and shells of tropical climates, in the strata of countries which are now 

 near to or within the frigid zones. These facts, however, have given 

 rise to theories of the changes of climate, which we must consider 

 separately. 



But we may notice, as connected with the doctrine of central heat, 

 the manner in which this hypothesis has been applied to explain vol- 

 canic and geological phenomena. It does not enter into my plan, to 

 consider explanations in which this central heat is supposed to give 

 rise to an expansive force,* without any distinct reference to known 

 physical laws. But we may notice, as more likely to become useful 

 materials of the science now before us, such speculations as those of 

 Mr. Babbao-e : in which he combines the doctrine of central heat with 



O ' 



other physical laws ; 7 as, that solid rocks expand by being heated, but 

 that clay contracts ; that different rocks and strata conduct heat differ- 

 ently ; 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 principles, 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 observed, may produce explosion by the con- 

 version 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. Ly ell's reasons for this rejection belong 



8 Scrope On Volcanoes, p. 192. 



T On the Temple of Serapis, 1834. See also Journal of the Royal List. vol. 

 ii., quoted in Conyb. and Ph. p. xv. Lyell, B. ii. c. xix. p. 883, (4th ed.) en 

 Expansion of Stone. 



