EARTH'S INTERIOR ADAMS AND WILLIAMSON 249 



THE ELASTIC CONSTANTS OF TYPICAL ROCKS 



The elastic properties of a series of rock types are shown in Table 

 2, which is taken (with slight changes) from a previous paper by 

 the authors. 14 The compressibilities are based on direct measure- 

 ments in the laboratory. From the compressibility is' calculated the 

 rigidity, and also the velocities with which earthquake waves are 

 transmitted through the given kind of rock. 



EFFECT OF TEMPERATURE 



This is a disturbing and uncertain factor. From the known tem- 

 perature gradient at the surface it follows that the temperature at 

 100 km. depth must be considerably above the melting point of 

 ordinary rocks; and it seems unlikely that the central temperature 

 can be less than several thousand degrees. The effect of this high 

 temperature on the density is not easily estimated, and might con- 

 ceivably be very large, but it so happens that the problem is 

 simplified by the fact that at high pressures the expansion coeffi- 

 cient becomes less than at low pressures. Now, the pressure halfway 

 down to the center of the earth is more than a million atmospheres, 

 and it is not at all improbable that at this pressure the total thermal 

 expansion and the effect of temperature on elastic constants would be 

 relatively small. For the present, at any rate, we shall ignore the 

 effect of temperature, but with the belief that it is a minor factor. 



14 Adams and Williamson. Journ. Franklin Inst., 195, 520. 1923. 



