2»4 



NATURE 



[October 27, 192 1 



mated age of the earth must be again extended. 

 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 biological 

 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 require 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. Harold Jeffreys. 



''F'HE rate of denudation must have varied very 

 -'- considerably during 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. 



Prof. 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 down- 

 wards, and one on the radio-active emission of 

 heat. With the best data available, supposing 

 the time elapsed since solidification to be i-6x lo^ 

 years, all the known facts regarding the earth's 

 thermal condition can be co-ordinated. If the 

 age is supposed to be i-6xio~ years, however, 

 the data cannot 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 sup- 

 posing that the original temperature was uniform 

 and that radio-activity does not exist at depths 

 greater than a kilometre, we can calculate the 

 amount of surface compression available for 

 mountain building, the thickness of the layer of 

 the crust which has cooled considerably and there- 

 fore become geologically strong, and the depth 

 to which compressive movements in the crust ex- 

 tend. In each case the results are inconsistent with 

 the geological and geodetic evidence, while the 

 greater estimate of the age of the earth agrees well. 

 We have, in fact, the following comparison : — 



Calculated. 



Actual. 



Assumed age of earth 

 i'6x lo** years, i ^x lo" years. 

 Area compressed (km.-) ... 49 x 10' 5 x 10* > 19 x 10^ 

 Greatest depth of consider- 

 able cooling (km.) ... 300 30 100-400 

 Depth of compressive move- 

 ments (km.) ... ... 70 I > 10 



I do not agree with Lord Rayleigh's suggestion 

 that the earth must be becoming hotter. That 

 hypothesis is not acceptable on cosmogonical 

 grounds, and Dr. Holmes has shown that it is im- 

 possible to reconcile it with the existence of vol- 

 canic temperatures, and that there must be a con- 

 centration of radio-active matter in the upper 

 layers of the crust. Dr. Holmes has told me 

 privately that there is reason to believe that in a 

 fluid magma the radio-active materials will be con- 

 centrated in the upper layers on account of the 

 volatility of their compounds, but I do not know 

 whether this argument has been published. The 

 numerical estimates here given rest on the sup- 

 position of such a concentration. 



An alternative estimate of the age may be 

 made from the tidal theory of the origin of the 

 solar system, the only theory which is not unsatis- 

 factory on dynamical grounds. The planets 

 must, on this theory, have moved originally in 

 highly eccentric orbits, and have had their eccen- 

 tricities gradually reduced by the action of a 

 gaseous resisting medium. If the density of the 

 medium near Mercury was p, the time needed to 

 reduce the eccentricity to its present value would 

 be of the order of 4000//?, C.G.S. units being 

 use^. On the other hand, the time it would take 

 the medium to be dispersed by viscosity and 

 diffusion would be of the order of 16 x \o^p. 

 These must be equal ; for if the former was the 

 greater the medium would have dispersed before 

 doing the work, and if the latter was the greater 

 the medium would still be a conspicuous object. 

 This shows that the time needed was of the order 

 of 8 X lo^^sec. or 2-5x10® years, agreeing with 

 the estimate given by the uranium-lead ratios. 



Obituary. 



Dr. a. S. F. Levton. 



BY the death of Dr. Albert Sidney Frankau 

 Levton on September 21, at fifty-two years 

 of age, we lose a worker who, through his re- 

 searches in pathology, contributed much to medi- 

 cine. The value of these researches, though ap- 

 preciated by those who follow closely the advance 

 in scientific medicine, will come to be fully recog- 

 NO. 2713, VOL. 108] 



nised only when the history of the development of 

 that science during the last four decades is 

 written. Dr. Le\i;on, the son of Joseph 

 Griinbaurh, who early in life settled in this 

 country, had a brilliant scholastic career, first in 

 the City of London School and then at Cam- 

 bridge, where he was elected a scholar of Gon- 

 ville and Caius College, and proceeding to his 



