AGE OF EAETH AND OCEAN KNOPF 



197 



tions are based upon the value of the half-life period of uranium 

 {Tu = 4:.5QX10^ years) that appears on critical scrutiny to be the most 

 trustworthy yet determined. There is some uncertainty as to which 

 of two is the better value for the half-life period of thorium, and so 

 both were used in preparing the following table and the results are 

 given in parallel columns. 



Geologic age determinations based on the lead method {in years) 



Except for the mineral high in thorium (the thorite from Brevik, 

 Norway) , it is essentially immaterial which of the two values for the 

 disintegration constant of thorium is taken. Only one of the sub- 

 stances whose age has been accurately determined bj' atomic disinte- 

 gration, the kolm of Sweden, is precisely dated by the geologic evi- 

 dence; distinctive fossils occur in the kolm and prove it to be of 

 latest Cambrian age. The age determination of the kolm is there- 

 fore by far the most important yet made. As to the pre-Cambrian 

 minerals, they can be only vaguely located as to their positions in 

 the pre-Cambrian. The broggerite from Karlhus, Raade, near Moss, 

 Norway, is believed on a tenuous correlation to be middle or late 

 pre-Cambrian, and the uraninite from Keystone, S. Dak., on which 

 some of the finest of chemical work has been done, is thought by 

 Paige to be late pre-Cambrian, and by Wright and Hosted to be 

 early pre-Cambrian, but the geologic evidence adduced for either of 

 these positions within the pre-Cambrian is inconclusive. 



The uraninite from Sinyaya Pala, Carelia, Russia, appears to be 

 the most ancient yet found, 1,852,000,000 years. As this uraninite, as 

 well as that from Keystone, S. Dak., occurs in pegmatite dikes that 

 are intrusive into older rocks, it must be concluded that the age of 

 the earth is, in round numbers, at least 2,000,000,000 years. As to 

 how much older it is, there is no substantial evidence either from 

 geology, radioactivity, or astronomy. By a method first used by 

 H. N. Russell, Holmes, with the aid of better geochemical data, re- 

 duces Russell's estimate of 11,000,000,000 years to 3,000,000,000 



