234 
Here the agreement is unusually good 
and it can be concluded that about 
255 million years have elapsed since 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
the close of the Devonian period. 
In the following table some of the 
better-established dates are listed: 
Probable 
. . . age 
Mineral Locality Geological Age (ansllions 
of years) 
Pitchblende Colorado Beginning of Tertiary 58 
Pitchblende Bohemia Late Carboniferous 215 
Samarskite Connecticut End of Devonian 255 
Cyrtolite New York End of Ordovician 350 
Kolm Sweden Upper Cambrian 440 
Pitchblende Katanga, Belgian Congo Pre-Cambrian 580 
Uraninite Morogoro, ‘Tanganyika Pre-Cambrian 590 
Uraninite Besner, Ontario Pre-Cambrian 760 
Broéggerite Moss, S. Norway Pre-Cambrian 860 
Uraninite Wilberforce, Ontario Pre-Cambrian 1,035 
Cleveite Aust Agder, S. Norway Pre-Cambrian 1,075 
Pitchblende Great Bear Lake, Canada Pre-Cambrian 1,330 
Uraninite N. E. Karelia, U. S. S. R. Pre-Cambrian 1,765 
Uraninite Huron Claim, Manitoba Pre-Cambrian 1,985 
The uraninite from Manitoba is 
the oldest mineral so far investigated 
and its great age is roughly confirmed 
by analyses of another uraninite and 
of two monazites, all from the same 
pegmatite. The “apparent ages” of 
these, calculated from the total lead, 
are 1,950, 1,955, and 1,990 million 
years respectively. ‘The pegmatite rep- 
resents the closing phase of the plu- 
tonic activity of a typically Archaean 
orogenic belt. It followed a long 
series of granitic and other plutonic 
rocks which, in turn, are emplaced 
within a thick sequence of metamor- 
phosed volcanic and_ sedimentary 
rocks. The latter include still recog- 
nizable conglomerates, containing 
pebbles of preexisting granites and 
quartzites, which must therefore be 
well over 2,000 million years old. 
Since the earth must be older still, 
this figure can be regarded as a con- 
servative minimum for its age. 
To find a maximum for the age of 
the earth we may assume that when 
the earth began it was free from the 
lead isotope Pb’, and that all the 
Pb”? now present in the common 
granitic rocks of the continental crust 
has since been generated from U”*. 
Granitic rocks contain on average 
about 20 p. p. m. of lead and 3.5 
p. p.m. of U. The isotopic constitu- 
tion of granitic lead has not yet been 
determined directly, but isotopic an- 
alyses of several samples of lead from 
galena and other lead ores of Tertiary 
age have been made by Nier and his 
coworkers. ‘These ores represent con- 
centrations of the granitic lead of 
some 25 million years ago, which is 
near enough to the present for our 
purpose. The average isotopic am- 
bundances of Tertiary lead are 
Pb204 | Pph208 Pb207 Pb208 
Total 
il || ale}, SAE ay Sy |) Sieh, AAs} Wes 
corresponding in p. p. m. very nearly 
to 
5 AU baal 4,2 10.4 | 20 p. p. 
m. 
in the average granitic rocks of today. 
Inserting the values Pb®’/U=4.2/3.5 
in the appropriate formula given on 
page 233, the time required for the 
generation of all the Pb’ is found to 
be 5,400 million years. The age of 
the earth is therefore somewhere 
between 2,000 and 5,400 million years. 
Can we arrive at a closer estimate? 
I think we can, again by making use 
