ADDRESS SECTION A, B.A. 1876. 243 



not less than about 4,000 cent. It would not 

 take much " hurrying up " of the actions with 

 which they are concerned to satisfy geologists 

 with the more moderate estimate of 50,000,000 of 

 years. This would imply at least about 3,000 

 cent, for the limiting temperature at great depths. 

 If the actual substance of the earth, whatever it 

 may be, rocky or metallic, at depths of from 60 

 to 100 kilometres, under the pressure actually 

 there experienced by it, can be solid at tempera- 

 tures of from 3,000 to 4,000, then we may hold 

 the former estimate (90,000,000) to be as probable 

 as the latter (50,000,000), so far as evidence from 

 underground temperature can guide us. If 4,000 

 would melt the earth's substance at a depth of 

 100 kilometres, we must reject the former esti- 

 mate though we might still admit the latter ; if 

 3,000 would melt the substance at a depth of 

 60 kilometres, we should be compelled to con- 

 clude that 50,000,000 of years is an over-estimate. 

 Whatever may be its age, we may be quite sure 

 the earth is solid in its interior ; not, 1 admit, 

 throughout its whole volume, for there certainly 



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