878 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



to 355,000,000 years. If, however, the anomalous pyrochlore with P!)/IT 

 ratio of .062 is omitted for reasons stated by Holmes^^^ and the analyses 

 with less than 0.14 per cent of nraninm are rejected because of lessened 

 analytical reliability, a lesser age is obtained. By weighting the remain- 

 ing six analyses according to the content of nraninm a mean lead-nranimn 

 ratio of .043 is obtained by the writer, or from the whole list of analyses 

 thus weighted .045. These ratios correspond to ages from 315,000,000 to 

 330,000,000 years. It would seem safer to take the age as no higher than 

 the latter of these figures. 



Regarding the geological position, Holmes makes the following state- 

 ment : 



"In this area there Is a nearly complete sequence of early Paleozoic rocks. 

 Above these strata there are a few beds of red sandstone of Lower Devonian 

 age. Over these beds and intercalated with them are lava flows ; and, finally, 

 penetrating the whole mass, representing a later phase of this period of igne- 

 ous activity, are great intrusions of plutonic rocks. Among the earliest of the 

 intrusions is a series of thorite-bearing nepheline syenites. Brogger believes 

 them to be of Middle or Lower Devonian age, most probably the latter. The 

 minerals occurring in them are, in many instances, notably radioactive, and 

 thus they afford an admirable series in which to investigate the consanguinity 

 of lead and uranium. Several of these minerals were obtained from Brevig. 

 and estimations of these elements in each case were made." ^^* 



Arthur Holmes adds later : 



"A recent discovery of Ostenlcpis proves that the sediments upon which the 

 igneous rocks are intruded belong to the Middle rather than the Lower De- 

 vonian, and the age assigned to the nepheline syenites from which our min- 

 erals were taken is not far removed from this, being almost certainly Middle 

 Devonian." "' 



It should be said, however, that Osteolepis is not clearly Middle De- 

 vonian, but belongs more precisely to the Middle Old Red or Orcadian 

 deposits. These are probably somewhat older than the marine division 

 of Middle Devonian, the greater break, corresponding to Middle Devonian 

 time, occurring between the Middle and Upper Old Red formations. The 

 minimum table of geologic time, on page 885 of this article, is arranged 

 so that 330,000,000 years falls early in the Middle Devonian; 315,000,000 

 years falls early in the Upper Devonian. In the column of maximum 

 ages this age of 315,000,000 to 330,000,000 years falls into the first half 

 of the Pennsylvanian, a period of great diastrophism. This shift in age 

 brings to attention a discrepancy in the evidence noted also in the discus- 



133 pvoc. Royal Soc. London, vol. 85, ser. A, 1911, p. 254. 

 131 Proc. Royal Soc. London, vol. 85, ser. A, 1911, p. 251. 

 135 Philosophical Mag., 6 ser., vol. 28, 1914, p. 831. 



