14 Clarence King — Age of the Earth. 



by the thickening- crust and therefore would have played no 

 very important part in accelerating the loss of heat, and thus 

 for this particular initial excess is of small effect in shortening 

 the estimate of earth's age. 



b. The direct effects of heat and pressure upon the conduc- 

 tivity of matter under such high temperatures and pressures 

 are also beyond laboratory investigation, and again we are 

 driven to use the determined conductivity value unmodified, or 

 seek for some other property which may be considered as its 

 approximate measure. Such an index is found in viscosity 

 which if not of high quantitative significance in defining the 

 changing values of terrestrial conductivitj in depth, neverthe- 

 less affords data applicable at least for determining the sign of an 

 important correction. 



Dr. Barus has lately determined that at least 200 atmospheres 

 of pressure are required per one degree Centigrade in order 

 that viscosity may remain constant. Examining several tem- 

 perature distributions of the chart and applying the com- 

 puted augmentation of earth-pressure, it appears that the 

 required relation (200 atm. to 1° C.) is found at successively 

 lower depths for successively higher values of initial excess and 

 age. In the 1741° C. case the relation after 20xl0 6 years' cooling 

 is found at about 016 of radius counting from the surface, 

 where the vertical broken line v. v. of the chart intersects the 

 gradient and marks the locus of stationary viscosity. As 

 above this point temperature relatively to pressure has aug- 

 mented more rapidly than the ratio required for constant 

 viscosity it follows that viscosity has been diminished by tem- 

 perature more than it has been raised by pressure. Below the 

 stationary point, on the other hand, an excess of pressure 

 above the required ratio is available for increase of viscosity. 



For the gradient of 3900° C. excess the transitional depth is 

 indicated by the intersection of the broken line V V. In both 

 cases the transitional points occupy positions in their respective 

 gradients not far below their full initial temperatures, and pres- 

 sure having been most stationary the transitional points have 

 moved but little during the whole period of secular cool- 

 ing, and the earth shells passing through them have divided 

 radius into a lower solid of higher viscosity and a surface couche, 

 partly liquid partly solid of lower viscosity. So far therefore as 

 viscosity indicates the behavior of conductivity, that also should 

 have been systematically diminished (relatively to the surface 

 value obtained at normal pressure and temperatures and used 

 in the construction of the gradients) from the surface downward 

 for a small fraction of radius, till at the appropriate depth for 

 each excess and age of cooling, it reaches a transitional value 

 and thence increases. 



