January 10, 1913] 



SCIENCE 



73 



annual output of these two elements by the 

 rivers. 



Different authorities give the age to range 

 between seventy and one hundred million 

 years. On the other hand, the data of radio- 

 activity require the age to be about ten times 

 as great as the figures above noted. The prin- 

 ciples of the radioactive method are based on 

 the determination of the amounts of helium 

 or lead associated with known quantities of 

 uranium found in rocks of different epochs. 

 The two principal assumptions that are in- 

 volved are that during the age in question the 

 amount of the uranium and its products which 

 give rise to helium shall have remained con- 

 stant and that the rate of production of he- 

 lium shall have remained unchanged. Nat- 

 urally these two assumptions can not be 

 proved. It can only be said that thei'e is no 

 evidence that casts much suspicion on these. 

 However, in all determinations by the radio- 

 active method some error may accrue owing 

 to a simultaneous deposition of uranium and 

 lead and helium at the time of formation of 

 the rock whose age is in question. As may 

 seem clear later in this discussion, the magni- 

 tude of this error is probably not greater than 

 the discrepancy between the age as determined 

 by the accumulation of helium and by the 

 accumulation of lead. 



According to experiments by Rutherford and 

 his colleagues one gram of uranium in equi- 

 librium with its products gives 10.7 XIO"" c.c. 

 of helium per year. Now if we examine the 

 rocks of the different geological epochs we 

 find the rare gas helium enclosed in the rock 

 wherever uranium is found, and further the 

 older the rocks the greater is the amount of 

 the helium associated with each gram of the 

 uranium. Obviously, if we divide the total 

 amount of helium per gram of uranium by the 

 above constant, 10.7 X 10 ^ '"'6 obtain the 

 number of years during which the uranium 

 has been depositing helium, i. e., the age of 

 the rock containing the uranium. It may be 

 mentioned that the diminution of the amount 

 of uranium during the age in question is so 

 small that it may be considered negligible in 

 comparison with other errors. 



Perhaps the greatest chance for error in the 

 above method of calculation lies in the possible 

 escape of helium from the rock containing the 

 uranium. If so the age of the rock as cal- 

 culated might be too small. The method 

 would therefore set a minimum limit on the 

 age of the earth. 



But if we accept Boltwood's conclusion that 

 the lead associated with uranium in rocks 

 resulted from the radio-active disintegration 

 of the uranium series of elements, and that 

 one gram of uranium gives rise to 1.88 X 10"" 

 gram of lead per year, we have a check upon 

 the results obtained based on the helium de- 

 posits. In general the lead deposits give a 

 somewhat larger age for a given rock than do 

 the helium deposits, which is what we should 

 expect if the helium may escape or if lead 

 might have been deposited with the uranium 

 originally. 



Using the method outlined above, Ruther- 

 ford, in 1906, found the age of a sample of 

 fergusonite to be 240,000,000 years. This was 

 deduced as outlined from the fact that 1.81 c.c. 

 of helium was taken from one gram of the 

 mineral known to contain about 7 per cent, 

 uranium. 



Strutt by the same method found two rocks 

 of the Archsean period from Quebec to be 222 

 and 715 million years old, and two of the same 

 kind from Norway to be 21.3 and 449 million 

 years old. He also found the average mini- 

 mum value for haematite of the Eocene period 

 to be 31 million years, the same for the car- 

 boniferous period limestone to be 150 million, 

 while for the Archssan age the average was 

 710 million years. 



Holmes" using as a basis the ratio of the 

 lead to the uranium in the rocks found the 

 values given in the following table : 



'Boy. Soc. Proc, Ser. A, 85, p. 248, 1911. 



