208 AKNTJAL REPORT SMITHSONIAN mSTITUTION, 1955 



is expected to come nearest to that of the earth's crust or the earth 

 itself. 



The most suitable method of age determination of rocks consists 

 in a comparison of the abundance of radioactive isotopes, such as 

 those of uranium, with the abundance of their end products, e. g., 

 lead isotopes. Pure minerals in the form of crystals are chosen for 

 samples; there must be a guaranty that no exchange of substance 

 between the sample and its surroundings has taken place, in which 

 respect only individual crystals can be considered as reliable. Know- 

 ing the rate of decay of the radioactive substance, the time during 

 which decay has been going on can be calculated from the amount of 

 end product accumulated. The determinations are accurate to within 

 8 to 10 percent, much more accurate than the other astrophysical age 

 estimates referred to below. 



Radioactive age determinations yielded, indeed, high values for 

 some mineral samples from the shields. Pegmatites from northern 

 Karelia, in the so-called Baltic shield, gave an average age of 1,950 

 million years according to the lead method, but without isotope anal- 

 ysis (7). In the Canadian shield, lead isotope determinations for 

 pegmatites from southeastern Manitoba resulted in an average age of 

 2,100 million years (8) ; for the same an average of 2,240 million 

 years was derived by the radioactive rubidium-strontium method (7), 

 in good agreement with the former value. At that remote epoch, un- 

 like the present conditions, volcanic activity was prominent in the 

 Canadian shield. The Manitoba pegmatites are associated with 

 granitic intrusions into older rocks which reveal traces of a long pre- 

 vious geological history, and whose age can be estimated at 2,550 

 million years (12, 22) . Recent very consistent lead-isotope age deter- 

 minations (9) have yielded still higher values of age for some samples 

 from the continental shields : 



LOCALITY OF SAMPLE AGE 



Millions of years 

 Ivigtut, Greenland 1, 830 



Yellowknife Area, N. W. T., Canada 2, 140 



Horseshoe Island, Great Slave Lake, Canada 2, 180 



Phoenix Mine, Norseman, W. Australia 2, 190 



Borderline Mine, Busia, E. Province, Uganda 2,220 



Risks Mine, Kenya 2, 220 



Copperhead Mine, W. Australia 2,300 



Ingiiladhal, Mysore, India 2, 300 



Sioux Lookout, Ontario 2,310 



Steel Rock Lake, Ontario 2, 360 



Rosetta Mine, South Africa 2,860 



Sierra Leone, Br. W. Africa (13) 2,930 



Pegmatites from the Rhodesian shield, near Bulawayo, yield an 

 age of 2,640 million years (10), yet the surrounding rocks — sediments 



