880 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



original lead should reduce the age to 360,000,000 or 370,000,000 years, 

 the intrusion of the associated granites would fall into the Middle or 

 Upper Devonian, times when granitic intrusions are known to have oc- 

 curred in Nova Scotia and which were marked by notable diastrophism 

 in the northern Appalachians, as reflected by the greatly stimulated sedi- 

 mentation. 



The next older granite whose age is accurately measured by the lead- 

 uranium ratio is that of Morogoro, German East Africa, giving a ratio 

 of .093 and an age of 660,000,000 years. The atomic weight of the lead 

 is 206.04, showing it to be wholly uranium lead. The age relations are, 

 unfortunately, not definitely known. 



The next determination in the sequence of age is that from a series of 

 analyses from the Moss district of south Norway. The lead is wholly 

 uranium lead^ as proved by its atomic weight, and the large per cent of 

 uranium gives high reliability to the result. The weighted lead-uranium 

 ratio is .132, corresponding to an age of 925,000,000 years, the age of the 

 post-Kalevian granites. 



At a period still more remote in time occurs the most distinctive group 

 of Precambrian granites. The percentages of lead and uranium from an 

 analysis of a uraninite from Villeneuve, Ontario, have been recomputed 

 by the writer, allowing for the two states of oxidation of the uranium. 

 The result is somewhat different from that given by Holmes, the following 

 figures being obtained : 



Pb = 10.46, U = 64.74, Pb/U = 0.162 



in contrast to the ratio of 0.17 given by Holmes. From the Llano district 

 in Texas a lead-uranium ratio of 0.167 to 0.170 was obtained as the most 

 reliable. From the Arendal district in Norway a mean ratio of 0.168 to 

 0.169 has been derived by the writer by a reinspection of the original 

 data. Holmes gives ratios for two other regions, but they depend on 

 much smaller amounts of uranium. From Ytterby, Sweden, two ratios of 

 0.17 and 0.15 were obtained and two ratios of the same values were found 

 for two localities in Mozambique. 



These constitute the oldest reliable group of determinations and they 

 show a remarkable concordance among themselves. A ratio of .165, cor- 

 responding to an age of between 1,100,000,000 and 1,200,000,000 years- 

 more exactly, 1,125,000,000 years — satisfies the data from Canada, Texas, 

 Norway, Sweden, and Africa. The geological relations show in Canada 

 that the intrusions at Villeneuve belong to one of the Middle Precambrian 

 granites — post-Sudburyan or post-Huronian. In the Arendal district the 

 indications are similar, the granites being either post-Bottnian or post- 



