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IOWA ACADEMY OF SCIENCE 
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 helium 
associated with each gram of uranium. Obviously if we divide the total amount 
of helium per gram of uranium by the above constant, 10.7x10-®, we obtain 
the number of years during which the uranium has been depositing helium, 
i.e., the age of the rock containing the uranium. 
The greatest chance for error in the above methods of calculation lies in 
the possible escape of helium from the rock containing the uranium. Therefore 
the age of the rock as calculated might be too small. The method would there- 
fore 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 radioactive disintegration of the uranium series of 
elements, and that one gram of uranium gives rise to 1.88x10-^^ gram of lead 
per year, we have a check upon the results obtained with the helium deposit. 
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. 
Using the method outlined 'above, Rutherford 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 1.81cc. of helium was taken from one gram of the mineral 
which contained about 7 % uranium. 
Strutt by the same method found two stones from Quebec of the Archaean 
age to be 222 and 715 million years old, and two of the same kind from Norway 
to be 213 and 449 million years. He also found the average minimum value 
for Haematite of the Eocene period to be 31 million, the same from the car- 
boniferous period limestone to be 150 million years, while for the Archaean 
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: 
Period. 
Age. 
Carboniferous . . . 
Deconian 
Pre-carboniferous 
Siluvian 
340.000. 000 years 
370.000. 000 years 
410.000. 000 years 
430.000. 000 years 
Pre-Cambrian 
Sweden .1,025,000,000 years 
U. S 1.310,000,000 years 
Ceylon 1,640,000,000 years 
The above results show that the age of the earth in its present form must be 
many times a million years old. 
However, if we take the evidence as based on the result that is obtained by 
dividing the total amount of sodium in the ocean by the annual output of all the 
rivers of the globe, we find that the age of the ocean can not be more than 
one hundred million years old. Two of the mpst eminent geologists, F. W. 
Clarke and J. Joly, think that 70,000,000 years is much more nearly the correct 
age based upon the solvent denudation of sodium. 
There is therefore a discrepancy between the age of the earth as deduced 
by the two methods. Joly, in the Philosophical Magazine for September, 1911, 
