Clarence King — Age of the Earth. 13 



ously it cannot be correct to apply the rock-conductivity value 

 obtained at air temperatures and normal pressure to even 

 so young and cool an earth with its conche of an initial 

 temperature of 1,741° C. and a pressure difference be- 

 tween the top and bottom of 22,000 atm. The probable 

 method of cooling the couche into solidity, involves three 

 corrections of the accepted rate of refrigeration : a, accelera- 

 tion of the process by possible convection ; b, the direct effect 

 of heat and pressure upon conductivity, and c, the relative 

 conductivity of matter at the same temperature, liquid and 

 solid. 



a. Convection. Leaving out of present consideration possi- 

 ble polymerization of the magma, or the descent of solid 

 bodies of crust, vertical transfers of the liquid matter in 

 the fused couche depend upon differences of density and this 

 upon the ratio of the rates at which density is raised by pres- 

 sure and lowered by heat. Isometrics of melted rock under 

 high pressure are of course beyond the reach of experimenta- 

 tion, hence we are forced to look to those of the available 

 materials. Isometrics from high pressure observations have 

 been found to slope as follows : 



Ether 8*7 atm. per ° C. 



Alcohol 10-5 " " 



Thymol... 13'9 " " 



Dyphenylamine 15*4 " " 



Paratoluidine 13*9 " " 



Glass, computed ]0'0 " " 



Since ether boils at 34° C. and dyphenylamine at 310° C, 

 the range here given is wide. It is reasonable, therefore, to 

 take the mean value, 12*5 atm. per ° C, as an index of the 

 slope sought for. In the Kelvin earth this rate occurs 

 between *010 and "015 of radius, the crust being -0065 of 

 radius thick. In so far, therefore, as the isometrics can be 

 regarded as parallel straight lines with a slope of the order of 

 the value given above, convection can only have taken place 

 in the first 52 miles of the initial couche of fusion and in the 

 present residual couche of 200 miles only the upper 26 miles 

 would be subject to convection. In younger earths the 

 above value per ° C. will be found much nearer the surface 

 so that in them convection would be confined to a shell which 

 is shallow in proportion as the earth is young. Initially when 

 the whole earth was at one temperature there could have been 

 no convection, since the change of temperature in depth was 

 nil, but the change of density due to pressure was always pro- 

 nounced. In the case of the 1,741° C. earth the zone of 

 convection would have early been covered and extinguished 



