8 GEOLOGY. 



ting rapidly would act toward a transient external force as though 

 it were a solid. 



Notwithstanding these answers, the opposing considerations have 

 greatly reduced the number of adherents of a molten interior, among 

 whom were once embraced the great body of geologists. The view is 

 still held, however, by eminent geologists, and — what is more needful 

 to note — it remains embodied unwittingly in many doctrines relating 

 to the interior and to its dynamics which are held by those who have 

 ceased to hold the parent view. A striking example of this is found 

 in the thermal discussions of Lord Kelvin and others in which a uni- 

 form temperature in the deep interior is assumed. This is really a 

 corollary of the conception of surface cooling and convection, and 

 apparently can have no other basis. Yet it has entered profoundly 

 into nearly all the fundamental doctrines of deformation. Its bear- 

 ings have been indicated in Vol. I, pp. 559, 560. 



Supposed solidification from the center outward. — For substances 

 which shrink in solidifying, as do most rocks, it is a general law that 

 the melting-points are raised by pressure. Out of this has grown 

 the doctrine that solidification due to pressure would begin at the center 

 of the earth while yet the outer part was liquid. This view has been 

 much strengthened by the experiments of Barus and others. 1 Under 

 such pressure as could be applied, Barus found the rise of the melting- 

 point of a typical rock (diabase) to be directly as the pressure. Extended 

 by computation to the center of the earth, the melting-point would 

 be 76,000° C. It is of course uncertain whether the law would hold 

 good throughout the extraordinary conditions of the deep interior, 

 and little weight is to be attached to the precise figure given, but the 

 general deduction is of fundamental importance. As this doctrine 

 of solidification under pressure, in spite of heat, seems to have the 

 support of the best available experimental data, and as it meets the 

 astronomical and topographic arguments for a solid earth, it has Come 

 to have wide acceptance. Under this general view, it is held by some 

 that the solidification continued outward from the center until the 

 surface was reached, and that the surface cooled last of all, a conclusion 

 precisely the opposite of the old view. 



Supposed middle molten zone. — By others it is thought that crust- 

 ing over at the surface took place before the central solidification 



1 U. S. Geol. Surv., Bull. 103. See also Vol. I, this work, pp. 562-564. 



