84(j 



J. BARKELL MEASUREMENTS 01*^ GEOLOGIC TIME 



begins to lose its helium. When it is powdered for analysis, still more 

 leaks away. Consequently the helium now found in a mineral can only 

 be a part, rarely as much as one-half, of the total amount which has been 

 generated within it during its lifetime. This must be carefully remem- 

 bered in interpreting the helium contents of minerals. 



Returning to our thorianite, it will now be clear that it would be 

 erroneous to suppose that its age is only 280,000,000 years. Its age must 

 be much greater than that. Tliorianite occurs in Ceylon in sands and 

 gravels, where it has been exposed to the action of the weather for thou- 

 sands of years — ever since it was broken away from its original home in 



Table A 



Geologfical 

 period- 



Mineral- 



Locality. 



Helium 

 ratio.^'i 



Millions of 

 years. 



Recent 



Pleistocene 



Pliocene 



Miocene 



Oligocene 



Post-Eocene 



Permian ? 



Upper Carbonif- 

 erous. 

 Carboniferous to 



Cambrian. 



Devonian 



Devonian 



Upper Precam 



brian. 

 U p p e r Precam 



brian. 

 Upper Precam 



brian. 

 Upper Precam 



brian. (?) 

 Middle Precam 



brian. 

 Middle Precam 



brian. 

 Lower Precam 



brian. 

 Lower Precam 



brian. 



Zircon 



Zircon 



Zircon . '. 



Zircon 



Siderite 



Hfematite. . . 

 Zircon 



Limonite . . . . 



Zircon 



Zircon 



Haematite . . . 

 Zircon 



Zircon 



Zircon 



Thorianite . . 



Sphene 



Sphene 



Zircon 



Spliene 



Mount Somma . . . . 



Mayen, Eifel 



Campbell Island, 

 New Zealand. 



Expailly, Auvergne 



Niederpleis, Rhine 

 Provinces. 



County Antrim 



Northeast Tasma- 

 nia. 



Forest of Dean . . . 



Green River, North 



Carolina. 

 Brevig, Norway . . . 



Caen 



Cheyenne Canyon, 



Colorado. 

 Miask, Urals 



Ceylon 



Ceylon 



Arendal, Norway... 



Twedestrand, Nor- 

 way. 



Renfrew County, 

 Ontario, Canada. 



Renfrew County, 

 Ontario, Canada. 



<0.01 

 0.09 

 0.223 



0.57 



0.76 



2.8 

 3.88 



13.3 



13.4 



4.94 

 13.2 



12.8 



19.0 

 26.0 

 27.9 

 36.8 

 40.8 

 56.6 

 65.0 



0.1 

 1.0 

 2.5 



6.3 



8.4 



30.8 



42.7 



146 (320)«2 



147 (330) 



54 (380) 

 145 

 141 



209 



286 



307 



405 (1,200) 



449 (1,200) 



623 



715 ri,500) 



91 The ratio of helium in cubic centimeters to an amount of uranium (U) equivalent in 

 its rate of helium generation to that of the uranium and thorium present in the mineral. 

 Age = I-Ie/U X 11 million years. 



»2 The figures in parentheses represent the age in millions of years based on lead ratios 

 for the corresponding periods. 



