Geology and Natural History. 161 



could not have taken place less than 20 million, nor more than 

 400 million years ago. With respect to this he adds : 



" During the 35 years which have passed since I gave this wide- 

 ranged estimate, experimental investigation has supplied much of 

 the knowledge then wanting regarding the thermal properties of 

 rocks to form a closer estimate of the time which has passed since 

 the consolidation of the earth, and we have now good reason for 

 judging that it was more than 20 and less than 40 million years 

 ago; and probably much nearer 20 than 40." 



Remarking upon the conclusion reached by Mr. Clarence King 

 (this Journal, Jan., 1893), based largely upon the experimental 

 data supplied by Dr. Carl Barus, in the laboratory of the U. S. 

 Geological Survey, that " we have no warrant for extending the 

 earth's age beyond 24 million of years," he adds that the results 

 of his own recent calculations lead to an estimate not differing 

 much from this. 



Going on to discuss the course of events following the first 

 solidification of the earth's surface, the author says : 



"§21. I have given strong reasons* for believing that immedi- 

 ately before solidification at the surface, the interior was solid 

 close up to the surface : except comparatively small portions of 

 lava or melted rock among the solid masses of denser solid rock 

 which had sunk through the liquid, and possibly a somewhat 

 large space around the center occupied by platinum, gold, silver, 

 lead, copper, iron, and other dense metals, still remaining liquid 

 under very high pressure." 



"§22. I wish now to speak to you of depths below the great 

 surface of liquid lava bounding the earth before consolidation ; 

 and of mountain heights and ocean depths formed probably a 

 few years after a first emergence of solid rock from the liquid 

 surface (see § 24, below), which must have been quickly followed 

 by complete consolidation all round the globe." . . . 



" § 23. To prepare for considering consolidation at the surface 

 let us go back to a time (probably not more than twenty years 

 earlier as we shall presently see — § 24) when the solid nucleus was 

 covered with liquid lava to a depth of several kilometers ; to fix 

 our ideas let us say 40 kilometers (or 4 million centimeters). At 

 this depth in lava, if of specific gravity 2 -5, the hydrostatic pres- 

 sure is 10 tons weight (10 million grammes) per square centime- 

 ter, or ten thousand atmospheres approximately. According to 

 the laboratory experiments of Clarence King and Carl Barusf on 

 diabase, and the thermodynamic theory| of my brother, the late 



*"On the Secular Cooling of the Earth," vol. iii, Math, and Phys. Papers, 

 §§19-33. 



f Phil. Mag., 1893, first half-year, p. 306. 



% Trans. Roy. Soc. Edinburgh, Jan. 2, 1849; Cambridge and Dublin Mathe- 

 matical Journal, Nov., 1850. Reprinted in Math, and Phys. Papers (Kelvin), vol. 

 i, p. 156. 



Am. Jour. Sci.— Fourth Series, Yol. VII, No. 38.— February, 1899. 

 11 



