556 HISTORY OF GEOLOGY. 



rather to Theoretical Geology ; but I may here notice M. Poissona 

 opinion. He does not assent to the conclusion of Fourier, that since 

 the temperature increases in descending, there must be some primitive 

 central heat. On the contrary, he considers that such an increase mav 

 arise from this ; that the earth, at some former period, passed (by the 

 motion of the solar system in the universe,) though a portion of space 

 which was warmer than the space in which it now revolves (by rea- 

 son, it may be, of the heat of other stars to which it was then nearer). 

 He supposes that, since such a period, the surface has cooled down by 

 the influence of the surrounding circumstances ; while the interior, for 

 a certain unknown depth, retains the trace of the former elevation of 

 temperature. But this assumption is not likely to expel the belief in 

 the terrestrial origin of the subterraneous heat. For the supposition 

 of such an inequality in the temperature of the different regions in 

 which the solar system is placed at different times, is altogether arbi- 

 trary ; and, if pushed to the amount to which it must be carried, in 

 order to account for the phenomenon, is highly improbable. 8 The 

 doctrine of central heat, on the other hand, (which need not be con- 

 ceived as implying the universal fluidity of the mass,) is not only 

 naturally suggested by the subterraneous increase of temperatures, but 

 explains the spheroidal figure of the earth ; and" falls in with almost 

 any theory which can be devised, of volcanoes, earth quakes, and great 

 geological changes. 



Sect. 5. Problems respecting Elevations and Crystalline Forces. 



OTHER problems respecting the forces by which great masses of the 

 earth's crust have been displaced, have also been solved by various 

 mathematicians. It has been maintained by Von Buch that there 

 occur, in various places, craters of elevation ; that is, mountain-masses 

 resembling the craters of volcanoes, but really produced by an expan- 

 sive force from below, bursting an aperture through horizontal strata, 



8 For this hypothesis would make it necessary to suppose that the earth has, 

 at some former period, derived from some other star or stars more heat than 

 she now derives from the sun. But this would imply, as highly probable, that 

 at some period some other star or stars must have produced also a mechanical 

 effect upon the solar system, greater than the effect of the sun. Now such a 

 past operation 01 forces, fitted to obliterate all order and symmetry, is quite 

 inconsistent with the simple, regular, and symmetrical relation which the whole 

 "solar system, as far as Uranus, bears to the present central body. 



