4 Clarence King — Age of the Earth. 



energies in the whole radius. Plate I gives earth-pressures 

 from Laplace's law expressed in a gradient of which the ordi- 

 nates are 100,000 atm. (larger divisions 1,000,000 atm.) and the 

 abscissa? tenths of radius. Upon the same diagram are delin- 

 eated two hypothetical cases of earth-temperature, the abscissae 

 remaining as for the pressure line, tenths of radius, and the 

 ordinates corresponding in interval to the 100,000 atm. lines, 

 are taken as each 1,000° C. The left vertical boundary of the 

 plate represents the center of the earth and the right one the 

 surface. The upper heat gradient corresponding to a tem- 

 perature of 3,900° C. at the earth's center is the 100x10* 

 curve of Kelvin. The lower is computed for a central tem- 

 perature of 1,741° O.j about the melting point of platinum, 

 and a secular cooling in 20xl0 6 years. Data for the construc- 

 tion of these gradients are given in the tables a few paragraphs 

 later. The feature here called attention to is the exceedingly 

 slight change of temperature from very near the surface down- 

 ward to the center. In the Kelvin gradient even after the 

 lapse of lOOxlO 6 years the original maximum temperature is 

 reached within *05 of radius and remains thence unchanged to 

 the center. Pressure, on the other hand, augments with one 

 downward sweep through the entire radius. On Plate I its 

 line is seen cutting both temperature gradients near the surface, 

 passing the 1,741° C. line at a pressure of 175,000 atm., and 

 the Kelvin line at 390,000 atm.; thence steadily augment- 

 ing until at the center it reaches the impressive figure of 

 3,018,000 atmospheres. 



Since we are to look to heat and pressure for the keys to the 

 physical condition of the matter of the earth, it is important 

 to realize from the relation of these gradients, first, that the 

 great effect of heat in opposing and overcoming the results of 

 pressure must be limited to superficial earth-depths not exceed- 

 ing 200 miles for an earth of the Kelvin assumptions ; second- 

 ly, that below this depth and onward to the center there is 

 a complete reversal of relations and a great and continual 

 increase of pressure available to oppose and destroy the vol- 

 umetric and other molecular effects of a temperature which 

 has ceased to increase. The empire of heat over pressure is 

 thus seen to be purely superficial, while that of pressure over 

 heat begins not far below the surface and extends more and 

 more powerfully to the center. This is obviously true only 

 for such moderate assumptions of heat and time as are given 

 in the gradients on Plate I, but it will be shown later that 

 these figures are, upon the criterion of solidity, far more proba- 

 ble than very hot or very old earths. 



Out of the infinite number of possible earth-temperature 

 gradients, to discriminate the probably true case, is of critical 



