TIME SCALE OF OUR UNIVERSE — OPIK 209 



and lavas — are still older ; and, what is more remarkable, remains of 

 primitive plants — algal structures — are found there in graphitic lime- 

 stone (Macgregor 1940, 1941). This provides "indubitable evidence 

 that life has existed for at least 2,600 million years and probably for 

 considerably longer than 2,700 million years" (10). Similarly, signs 

 of organic remains are either foimd or strongly suspected in the rocks 

 of Lake Superior and Manitoba, which are 2,000 to 2,500 million years 

 old (11, 12). 



Thus, direct measurements set the minimum age of the earth's crust 

 at 2,000 million years, the oldest specimens being found in Africa. 

 This confirms also a long-maintained belief that Africa was the first 

 continent to be formed. 



It is but natural to expect that the oldest rocks have not yet found 

 their way into man's laboratories, and that the age of the earth's 

 crust is greater than the presently known oldest sample. 



An ingenious method, based on data for rocks of widely different 

 ages from 25 to 1,330 million years as determined by Nier, led Holmes 

 (14) to the calculation of the true age of the earth, or the time during 

 which radiogenic lead has been produced in its materials. The method 

 is one of extrapolation, consisting in the study of the observed relative 

 isotopic abundances within one age group and their theoretical varia- 

 tion with time. The moment when the isotopic ratios, calculated back- 

 ward for various age groups, become equal is the "beginning." Plolmes 

 found in such a manner 3,350 million years for the age of the earth. 

 The oldest analyzed sample used in his calculation was 2,000 million 

 years younger than this figure — a gap which might have caused con- 

 siderable error in the extrapolation, as was pointed out bv Jeffreys 

 (15). 



Recent data as quoted above push the directly observed age limit 

 much farther back in time, and nearer the beginning. Applying the 

 method of extrapolation to modern data, the probable age of the earth 

 results as 3,500 million years ( 16) , in excellent agreement with Holmes' 

 former figure, but more reliable, the range of extrapolation being 

 now only a few hundred million years. 



The figure of 3.5 thousand million years can at present be accepted 

 as a close approximation to the age of the earth — the time elapsed 

 since its elements w^ere uniformly mixed, probably in a molten state. 

 The same figure, or one perhaps only slightly greater, can be con- 

 sidered the age of the solar system ; the formation of the planets and 

 of the earth's crust must have taken relatively short intervals of 

 time (17). 



The uranium and thorium content of iron meteorites is so small 

 that their lead can be assumed to be of primeval isotopic composition, 

 no radiogenic lead having been added in the course of time. If tliis 



