338 THE AGE OF THE EARTH. 



plieres; tlience steadily augiiieiitiii.i;- until at the center it readies the 

 impressive figure of 3,01 8, 0(K) atmospheres. 



Since we are to look to heat and i>ressnre for the keys totlie 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 oppos- 

 ing and overcoming the results of i^ressure must be limited to superficial 

 earth-depths not exceeding 20^ miles for an earth of the Kelvin 

 assumptions; secondly, that below this depth and onward to the center 

 there is a complete reversal of relations and a great and continual 

 increase of i^ressure available to oppose and destroy the volumetric 

 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 ov^r 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 xvii, but it will be shown later 

 that these figures are, upon the criterion of solidity, far more probable 

 than very hot or very old earths. 



Out of the infinite number of jiossible earth-temperature gradients 

 to discriminate the i^robably trne case, is of critical imi)ortance in any 

 attemi)t to determine the earth's thermal age or to delimit the period 

 of active geological dynamics. 



PRESSURE AND TEMPERATURE TAHLES. 



The following tables offer figures for the construction of the pressure 

 and some of the temperature gradients on both Plates xvii and xviii. 

 Data for the distribution of earth pressure may be obtained either from 

 the formula of Laplace or that of G, 11. Darwin for radial earth density, 

 combined with tlie known decrease of terrestrial gravitation from 

 center to surfjice. 



In table 1, Laplace's law is used as giving the most conservative 

 values of density at great depths. For the superficial 0-2 of radius, 

 however, the two density laws are near together, and as the thermal 

 phenomena which determine the earth's age are probably wholly in the 

 surface tenth, either law might be applicable to the present purpose. 

 As, however, Darwin's law requires a surface density of 3-7, while 

 Lajilace only 2*75, the latter accords better with the average si)ecific 

 gravity of superficial rocks and is therefore here preferred. 



Tables 2, 3, and 4 give data for three temperature gradients derived 

 by mechanical quadrature from the well known Fourier ecjuation in the 

 manner given by Lord Kelvin, and are considered assutticieut in num- 

 ber and variety to indicate the character of the data; figures for the 

 other gra<lients shown on Plate ii are therefore omitted. 



Table 2 presents data for the Kelvin gradient, 3 900° 0. initial excess, 

 surface rate 0.O3G00 in degrees centigrade per meter of depth, and secu- 

 lar cooling 100 xlO" years. Earth temperatures in ° C. are4j;iven for 

 depths that are expressed both in miles and fractious of radius and 



