Page:The Journal of geology (IA journalofgeology11893univ).pdf/218

The object of Mr. King's paper is to advance the method of determining the earth's age which was employed by Lord Kelvin (Sir William Thomson), and which was based on a consideration of its probable rate of refrigeration, by applying it to new criteria. The criteria are derived from the tidal effective rigidity of the earth, and the further argument for rigidity based upon the periodic variation of latitude, and also from the researches of Dr. Carl Barus upon the latent heat of fusion of the rock diabase, and upon its specific heats when melted and solid, and upon its volume expansion between the solid and melted state. This rock was considered to represent the probable density and composition of the surface .03 or .04 of the earth's radius.

The two principal conditions within the interior of the earth upon which physical state and all purely physical reaction of the specific materials depend, being the distribution from center to surface of pressure and heat, Mr. King points out the relative values of earth-pressures deduced from Laplace's law, and two hypothetical cases of earth-temperatures. These are expressed by a diagram which shows that the temperatures maintain an almost maximum value from within .05 of radius of the surface to the center of the earth, while pressure increases steadily throughout the entire radius. Near the surface the rate of increase of heat is greater than that of pressure, and hence its effect is to overcome the results of pressure. But this relation obtains only for earth-depths of 200 miles for an earth of the Kelvin assumption. Below this the relations are completely reversed.

The results of Barus's researches furnish the means of fixing the melting points of diabase at pressures corresponding to increasing depth within the earth. These points fall in a straight line when plotted on a chart in which the coördinate axis represent temperatures and parts of the earth's radius. By plotting on the same chart curves expressing the temperature gradients of the earth for different assumptions regarding the initial excess of heat and period of cooling, it is possible to determine the extent of the couche which must remain fused in certain cases, and also the temperature gradient at the surface of the