AGE OF THE EARTH RAYLEIGH. 259 



springs would be increased; and as denudation is now acting on land 

 in which sodium chloride has been produced in unusual quantities 

 by volcanic action the estimated age of the earth must be again ex- 

 tended. The rhythmic acceleration of geological processes lengthens 

 the estimates based on sedimentation, but would affect the biological 

 argument inversely, since at periods of rapid physical change bio- 

 logical change would have been quickened, and thus the occasional 

 abrupt introduction of a new fauna does not necessitate so long an 

 interval as has been thought. 



The best-known geological estimates of the age of the earth re- 

 quire to be multiplied ten or twenty fold in order to agree with the 

 physical estimates, but this increase is consistent with the geological 

 evidence. 



By Dr. Hakold Jeffreys. 



The rate of denudation must have varied very considerably dur- 

 ing the earth's history, for it depends on both the height of the land 

 and on the meteorological conditions, both of which have certainly 

 changed very much from time to time. The consistency of the 

 various geological methods among themselves does not prove that 

 there has been no change in the rate of denudation, for such a 

 change would affect them all in the same ratio. 



Professor Eddington's argument shows that there must be an 

 unknown source of energy in the Cepheid variables. It is possible, 

 however, to infer from the condition of the earth that its own age 

 must be much greater than the Kelvin theory allows, and therefore 

 the sun itself must have such a source of energy. The rate of increase 

 of temperature downwards in the earth's crust is the sum of three 

 parts, one depending on the original temperature at the surface, 

 one on the original increase of temperature downward, and one on 

 the radio-active emission of heat. With the best data available, sup- 

 posing the time elapsed since solidification to be 1.6X10 9 years, all 

 the known facts regarding the earth's thermal condition can be 

 coordinated. If the age is supposed to be 1.6 X10 7 years, how- 

 ever, the data can not be reconciled; the part of the increase of 

 temperature downwards depending on the initial temperature at 

 the surface is by itself greater than the present rate. 



If we force an approximate agreement by supposing that the origi- 

 nal temperature was uniform and that radio-activity does not exist at 

 depths greater than a kilometer, we can calculate the amount of sur- 

 face compression available for mountain building, the thickness of 

 the layer of the crust which has cooled considerably and therefore 

 become geologically strong, and the depth to which compressive 

 movements in the crust extend. In each case the results are incon- 

 sistent with the geological and geodetic evidence, while the greater 



