510 GEOPHYSICS 



rhenius's opinions, the matter requires further consideration. In 

 particular it is most important to know whether the earth is sub- 

 stantially a crystalline solid or an amorphous substance, for many 

 modern physical chemists consider amorphous matter as liquid. 

 This opinion is far from being established, however, and recent 

 experiments by Mr. Spring show that mere deformation at ordinary 

 temperatures, attended by only a very small absorption of energy, 

 suffices to convert crystalline metals into substances exhibiting 

 characteristics of amorphous bodies. Since Nordenskiold's great 

 discovery of large masses of terrestrial iron, or rather nickel steel, 

 in Greenland, and the wide distribution since proved for similar 

 metal imbedded in igneous rocks, a great amount of evidence has 

 accumulated that a large part of the earth is composed of material 

 indistinguishable from that of metallic meteorites. Meteoric iron is 

 of course a highly crystalline material. 



It is a very striking fact that the mean rigidity of the earth is about 

 that of steel, for the only substance likely to occur in extensive con- 

 tinuous masses and displaying such rigidity at ordinary temperatures 

 and pressures is steel itself. Nevertheless, the conclusion cannot yet be 

 drawn from the resistance to deformation displayed by the earth, that 

 it is chiefly composed of steel. Elastic resistance is known to be a func- 

 tion both of pressure and of temperature, and until this function has 

 been determined by theory and experiment, the bearing of the evalu- 

 ation of rigidity by tidal action cannot be ascertained. 



Having shown the earth to be a solid globe, Lord Kelvin calculated 

 its age from one of Fourier's theorems, assuming for purposes of com- 

 putation an initial temperature of 7000 F. (nearly 3900 C.) and that 

 the thermal diffusivity of the earth is that of average rock. These 

 assumptions, with the observation that the temperature near the 

 surface of the earth increases at the rate of 1 F. for every 50 feet 

 of depth, lead to an age of 98,000,000 years; but on account of the 

 uncertainty as to conductivities and specific heats in the interior, the 

 conclusion drawn by Lord Kelvin was only that the time elapsed 

 since the inception of cooling is between 20 and 400 million years. 



Clarence King subsequently took a further important step on the 

 basis of data determined at his request by Professor Carl Barus on the 

 volume changes which take place in diabase during congelation, and 

 on the effects of pressure in modifying the melting and solidifying 

 points. Assuming that the earth can never have had a crust floating 

 on a liquid layer of inferior density, computation leads him to 24 

 million years as the maximum period for the time since superficial 

 consolidation was effected, provided that the superficial temperature 

 gradient and conductivity are correctly determined. 



These researches, together with Helmholtz's investigation on the 

 age of the solar system, which is incomplete for lack of knowledge of 



