CONVERGENCE OF EVIDENCE 879 



* 



sion of the Glastonbury, Connecticut, uraninites. The Brevig, N'orway, 

 rocks are indicated as only 15,000,000 to 30,000,000 years older than the 

 Glastonbury uraninites; but according to the geological positions which 

 have been assigned to the one by Brogger and to the other by Emerson, 

 all of the Pennsylvanian and Mississippian periods and parts of the Per- 

 mian and Devonian lie between the two. Taking the ratios of ages of 

 these periods to the whole length of time as given by stratigraphical 

 studies, there should be from 100,000,000 to 150,000,000 years between 

 them. This discrepancy may be reconciled by finding that some original 

 lead is in the Glastonbury uraninites, or the stratigraphic ratios may be 

 seriously in error, or, what is more probable, the geological position of 

 one or both is in considerable error. As has been pointed out, but little is 

 positively kno\^ai regarding the age of the Glastonbury uraninites, further 

 than that they are Upper Paleozoic. In regard to the geological position 

 of the Brevig, Norway, uranium minerals, they belong to the second series 

 of igneous rocks in a total of six series. The first series is interbedded as 

 extrusives in the Middle Old Red sandstones, and Brogger regards the 

 second series as related and but slightly younger. The actual age of the 

 younger series is, however, not fully proven. ^^^ It appears, nevertheless, 

 that greater weight should be attached to the Brevig than to the Glaston- 

 bury occurrences. The column of minimum ages may be regarded as 

 having greater weight than the column of maximum ages. 



Of course, the question might also be raised here if the discrepancy 

 does not throw doubt on the fundamental assumption of the uniform rate 

 of decay of radioactive substances through geological time. The whole 

 body of evidence is, however, sufficiently in accordance to indicate that 

 this is the more improbable explanation. 



The next older series of minerals comes from Branchville, Connecticut, 

 and consists of uraninites giving in four analyses a lead-uranium ratio of 

 .053 to .054,^^^ the mean corresponding to an age of 390,000,000 years. 

 The atomic weight of the lead is not known, but the ratio is highly ac- 

 cordant in the several analyses, suggesting that adventitious lead is prob- 

 ably low. The associated granites can not be older than the end of the 

 Ordovician nor younger than the close of the Permian. By assigning 

 them to the Taconic disturbance at the end of the Ordovician, a dias- 

 trophic movement Avhich is known to have greatly disturbed the rocks 

 immediately to the west, in the Hudson Valley, they are brought into this 

 place in the column of maximum ages. In the column of minimum ages 

 the Branchville uraninites fall in the early Devonian. If the presence of 



136 w, C. Brogger : Die Mineralien der Syenitpeginatitgiinge der Sudnorwegischen. ' Au- 

 gitund Nephelin-syenite. Leipzig, 1890, pp. 43-81. 



"7 B. B. Boltwood : Am. Jour. Sci., vol. xxiii, 1907, p. 79. 



LXV — Bull. Geol. Soc. Am., Vol. 28, 1916 



