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earth. From a study of these curves Mr. King is led to conclude that in order to satisfy the conditions of tidel effective rigidity there can be no considerable fused couche within the upper .06 of radius. And since for a given initial temperature the temperature gradient decreases as the period of refrigeration lengthens, an excessive period produces too low a gradient at the surface to satisfy observed rates of heat-augmentation.

To meet both these requirements Mr. King selects a gradient which falls below the diabase line of fusion, and emerges at the earth's surface at a rate not less than the mean rate of 64 ft. to 1° F. This corresponds to an initial excess of 1950° C. and a period of 24✕10⁶ years.

Corrections which should be made to the assumed rate of refrigeration are considered, and found to modify the result but slightly.

In comparing this method of determining the age of the earth with that by Kelvin, based on tidal retardations, King contends that, from abundant geological observation plasticity must be admitted for slow deformations, enormously in excess of the small change of figure which the stress of tidal attraction would produce but for elastic resistance. And although rigidity prevents a sudden tidal deformation of five feet, it does not prevent a slow radial deformation of five miles of the surface matter. Hence it appears that no time measure can be deduced from the supposed fixing of the present ellipticity at some past date.

A very significant comparison of the earth's age is made with that of the sun, which, according to Helmholtz and Kelvin, is 15✕10⁶ to 20✕10⁶ years. It is remarked by Newcomb that the period during which the heat received by the earth from the sun has been of a temperature which would permit water to exist in a liquid state upon the earth is probably not more than 10,000,000 years. King calls attention to the fact that all we know of the earlier strata indicates a water mechanism for their deposition, and that the evidences that life was continuous in them necessitates a climate continuously suitable for the circulation of waters. All of this period therefore must have fallen within Newcomb's limits. And the earth's age, about 21,000,000 years, accords with the 15,000,000 or 20,000,000 found for the sun.

Mr. Upham reviews the estimates which have been made concerning the age of the earth. These range from 10,000,000 to 14,000,000,000 years. The most reliable means, the writer argues, for estimating the age of the earth is through comparing the present rate of denudation of continental areas with the greatest determined thickness of the strata referable to the successive time divisions. He assumes the rate adopted by Wallace of a continental reduction of one foot in 3000 years. Taking Houghton's estimate