THE AGE OF THE EARTH. 337 



not twice that of diabase, whence it appears that no probable chemical 

 distribution of material could result in a surface couche of 0-05 of 

 radius liaviuj;' a greater specific gravity than 3- to 3-3. 



\Valtershausen,t in his interesting scheme of chemical distribution, 

 attempts to account for the augmentation of density chiefly by the 

 increase of the heavy bases, but leaves the whole surface tenth of radius 

 iu silicates. Eruptions of alkaline earths or metals are unknown. In 

 fact, with the exception of carbonates of superficial origin the whole 

 visible body of the crust is of silicates, and the earliest rocks are seen 

 to be made of the debris of still older ones. All that can be said is 

 that there is absolutely no known reason why the surface tentli of radius 

 may not be of silicates, nor why specific material of widely different 

 thermal properties from diabase should be postulated. 



The two principal conditions within the interior of the earth, upon 

 which physical state and all purely i)hysical reactions of the specific 

 materials depend, are the distributions from center to surface of pres- 

 sure and heat. Secular or sudden variations of either or both have the 

 power, if carried sufficiently fiir, to disturb chemical and physical equi- 

 librium and produce changes of volume, rigidity, viscosity, and con- 

 ductivity, as well as changes of state from liquidity to solidity, and the 

 reverse. Before proceeding to consider in detail some of the results of 

 heat and pressure as existing in the surface 0-05 of radius, it is desira- 

 ble to glance at the relations of these two great antagonistic energies 

 in the whole radius. Plate xvii gives earth pressures from Laplace's law 

 expressed in a gradient of which the ordinates are 100,000 atmospheres 

 (larger divisions 1,000,000 atmospheres) and the abscissaj tenths of 

 radius. Upon the same diagram are delineated two hypothetical cases 

 of earth temperature, the abscissie remaining as for the pressure line, 

 tenths of radius, and the ordinates corresponding in interval to the 

 100,000 atmospheres lines, are taken as each 1,000° C. The left verti- 

 cal boundary of the plate represents the center of the earth and the 

 right one the surface. The upper heat gradient corresponding to a tem- 

 perature of 3,000° C. at the earth's center is the 100 x 10^ curve of Kel- 

 vin. The lower is computed for a central temi)eratuie of 1,711° C, 

 about the melting point of platinum, and a secular cooling in 20x10^ 

 years. Data for the construction of these gradients are given in the 

 tables a few paragraphs later. The feature here called attention to is 

 the exceedingly slight change of temperature from very near the sur- 

 face downward to the center. In the Kelvin gradient even after the 

 lapse of 100 x 10^ years the original maximum temi)erature is reached 

 within 0-05 of radius and remains thence unchanged to the center. 

 Pressure, on the other hand, augments with one downward sweep 

 through theentire radius. On plate xvii its line is seen cutting both tem- 

 perature gradients near the surface, passing the 1,741° C. line at a 

 pressure of 175 000 atmospheres, and the Kelvin line at 390,000 atmos- 



" "Rocks of Sicily and Iceland," 

 SM 93 23 



