562 GEOLOGY. 



lies at a very slight depth, and that the thrust zone above is, therefore, 

 very shallow. This should be kept constantly in mind in all deductions 

 drawn from this hypothesis. If the thickness of the thrust zone be 

 taken at 8 or 10 miles, it will apparently be conceding to the view all that 

 can legitimately be claimed for it. 



2. Thermal distribution on the hypothesis of central solidification. — 

 When the previous conception was first formed, the effect of pressure on 

 the melting-points of lavas was neglected, as little or nothing was known 

 on the subject. Experiment, however, has shown that pressure, as a 

 rule, raises the melting-points of lavas, and out of this has grown the doc- 

 trine that the earth solidified first at the center, where the pressure was 

 greatest, and gradually congealed outwards. Barus has sho^vn that the 

 melting-point of diabase, ^ selected as a representative rock, rises directly 

 with the pressure. If this rate holds good to the center of the earth, 

 the melting temperature of diabase there would be 76,000° C. (136,- 

 800° F.). The range of the experiment is, however, very small com- 

 pared with the range of the application, and little confidence can be felt 

 in the special numerical result reached. The rate of rise of the fusion- 

 point may be much changed as the extraordinary conditions of the 

 deep interior are invaded. Still there is good ground for the hypothesis 

 that solidification took place at some very high temperature at the 

 center, because of the very great pressure there. The inference then 

 is that when the temperature of the center of the supposed molten globe 

 reached the appropriate point, solidification began there, and that it 

 took place at lesser depths in succession as the appropriate tempera- 

 tures Avere reached. This view excludes convection in the successive 

 zones from the center outward after the time when their temperatures 

 of solidification were reached, or after these were approached sufficiently 

 near to develop prohibitive viscosity. Some loss of heat from these 

 horizons would be suffered while the outer parts were solidifying, but 

 !on account of the exceedingly slow conductivity of rock, it is improbable 

 ithat the amount of loss would be sufficient to change the general char- 

 acter of the internal distribution of heat previous to solidification at 

 the surface, the time when the existing phase of the earth's history 

 by hypothesis began. Fig. 451 shows the theoretical distribution of 

 heat under this view. The consequences of this assumption are very 

 important to geological theory and, carried out to their logical conse- 



' Amer. Jour. Sci., 1893, 3d series, Vol. 45, p. 7. 



