18 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



surface gradient arc assumed. He also found that the corresponding 

 initial temperature of such a globe would be 1950° C. 



Kelvin's last paper on a cooling earth ' was read in 1897, and he there 

 stated that after having worked out the problem of conduction of heat 

 outwards from the earth by an elaborate method, he was not led to differ 

 much from Clarence King's estimate. This he adopted as the most prob- 

 able age and reduced his limits to between 20 X lO*' and 40 X lO*' years. 



While King's earth is tidally stable, I confess that his solution of the 

 problem seems to me to be fatally defective. He himself gives a tempera- 

 ture curve for the same earth at an age of 15 million years, and this earth 

 shows a couche at a temperature above the melting point of diabase, this 

 layer extending from a depth of 34 miles below the surface to 66 miles. 

 According to Laplace's law of densities these two levels correspond re- 

 spectively to densities of 2.85 and 2.93, and it seems certain that the 

 material must consist chiefly of basaltic rocks. Thus the 15-million-year 

 earth would be unstable and this instability would only just disappear at 

 24 million years. I am obliged to conclude that if an earth could cool in 

 this way — if the crust could be prevented from breaking — the 24-million.- 

 year earth would only just have reached the " consistentior status " or the 

 epoch of solidity. 



The real earth, however, has been in a condition of tidal stability at 

 least since the beginning of the Cambrian : for the strata are full of ripple 

 marks, sands and pebbles rearranged by tidal currents, beach footprints 

 and similar evidence of tides. Now oceanic tides would not exist upon a 

 tidally unstable earth and, therefore, the consistentior status occurred long 

 ago. It was the remoteness of this epoch that Kelvin attempted to 

 calculate. 



Kins' ffives data for onlv one earth which is satisfactorv from this point 

 of view. It had an initial temperature of 1230° C. and reached a surface 

 gardient of 1° F. in 50.6 feet in 10 million years. It was solid almost 

 from the beginning. But apart from the excessive brevity of the age, it 

 seems to me that this earth must likewise be rejected. The temperature 

 was insufficient to melt even diabase a few miles below the surface, much 

 less andesites and rhyolites, while there is a mass of well-known evidence 

 that the earth has been fluid at least to depths of many miles from its 

 growing surface. This is shown by the general dependence of gravity on 

 latitude, the nearly spheroidal shape of the earth, the oblateness of the 

 interior layers of equal density and the conclusion reached by Kelvin," 

 Eoche' and Wiechert' that a nucleus of constant high density (approxi- 



1 Trans. Victoria Institute, voL 31, 1809, p. 11. 



2 " Natural Philosophy," Pt. U, p. 420. This article also appeared in the first edition of the 

 " Natural Philosophy," 1867. 



3 Mf'm. Acad. Montpellier, 1882. 



* Gottingen Nachrichten, 1897, p. 221. 



