568 



GEOLOGY. 



age in the outer parts than in the inner. Neither of these conceptions 

 can be affirmed, as actually taking place. They merely lie within the 

 range of reasonable hypothesis in the present state of experimental 

 data. What the real truth is must be left to further research. Present 

 effort may be regarded as temporarily successful if it forms consistent 

 conceptions of the applicable hypotheses, and of their consequences. 



Recombination of material. — One other peculiarity of the accretion 

 hypothesis must be recalled here. The incoming bodies must probably 

 be assumed to have fallen in promiscuous order, and hence to have been 

 indiscriminately mingled in the growing earth. As they became buried 

 deeper and deeper and their temperatures and pressures were raised, 

 much recombination, chemical and physical, may be presumed to have 

 followed. As already noted, these changes would probably give in- 

 creased density in the main. The material being, however, in a solid 

 state, the rearrangement would be slow and its persistence in time 

 indeterminate, and it may yet be far from complete. It is not improb- 

 able, therefore, under this hypothesis, that some notable part of the 

 recent shrinkage of the earth has been due to the continued rearrange- 

 ment of its heterogeneous internal matter. This would not be equally 

 so in an earth derived from a molten mass, for the required adjustments 

 of the material should have taken place while in the fluid state before 

 solidification. 



Comparison of the hypotheses. — By comparing the three hypotheses 

 of the earty states of the earth's temperature, it will be seen that there 

 is a radical difference, thermally, between the first and the last two. 

 The first assumes a nearly uniform distribution of internal temperature, 

 and hence, owing to the exceedingly slow rate of conduction, limits 

 the movements and deformations of the crust, so far as dependent on 

 heat, to very superficial horizons. The second and third views agree 

 in postulating changes of temperature in the deep portions, as well as 

 in the superficial, and hence involve the central portion of the earth in 

 the great movements and deformations. It is not to be supposed that 

 this of itself necessarily increases the sum-total of the effects of con- 

 traction, for, given a certain loss of heat from the surface, it may be 

 relatively immaterial whether this loss arose from a large reduction of 

 temperature in a shallow zone, or a small reduction of temperature in 

 a deep zone, for, except as the coefficient of expansion varies, the total 

 shrinkage would be the same. But the difference in distribution makes a 



