CONVERGENCE OF EVIDENCE 875 



virtue of the large vohime of material, and the quantity of helium should 

 approach more nearly the theoretic amount. 



AGE POINTS GIVEN BY URANIUM MINERALS 



By diligent search the evidence of age based on uranium minerals cau, 

 without doubt, be greatly increased. With that which is at present avail- 

 able it is desirable, however, to construct a tentative table of geologic 

 time. 



In regard to the table of ages given on page 846 by the helium ratio 

 and quoted from Holmes after Strutt, it should be noted that certain of 

 the ages are open to question. In the Eifel, New Zealand, and Auvergne 

 occurrences the zircons may have been derived from older rocks. ^^^ The 

 Pliocene age of the Campbell Island, N'ew Zealand, formation is, further- 

 more, open to question. The hematite from County Antrim, Ireland, 

 offers a maximum age, 30,800,000 years, for a Tertiary occurrence of a 

 helium mineral, and, furthermore, occurs associated with leaf beds which 

 serve to fix the age within comparatively narrow limits. Heer originally 

 classified these beds as Miocene. With the separation of the lower Miocene 

 as Oligocene they become Oligocene. Gardner, however, maintained later 

 that they were lower Eocene. In order to obtain the latest opinion tlie 

 Avriter submitted the question of their age to Professor E. W. Berry, who 

 kindly replied as follows: 



"Regarding the age of the Antrim basalts with the intercalated leaf beds— 

 Heer's Miocene was of course 'old Miocene' — that is, Oligocene. I am <iuite 

 sure that their age is not Lower Eocene, as J. Stanley Gardner held, and it is 

 mj- opinion that they are either Upper Eocene or Lower Oligocene, and by that 

 I mean either Bartonien (as restricted to the Upper Bartonien after segregat- 

 ing the Auversien below) or Sannoisien (Lattorfien). It is increasingly diffi- 

 cult to paleobotanically differentiate the temperate floras of late Eocene and 

 early Oligocene times, although it is easy enough in lower latitudes, as, for 

 example, in southern Europe or southeastern North America. 



"I think the evidence is rather good for considering all of the far northern 

 basaltic sheets associated with the plant beds of the 'Arctic Miocene flora' 

 (Mull, Iceland, West Greenland, etcetera) as essentially synchronous and as 

 contemporaneous with the most tropical floras and marine faunas of lower 

 latitudes, as, for example, those of the Jackson and Vicksburg of our Southern 

 States. Correlation by old-fashioned methods is not possible for a variety of 

 reasons, chiefly since Heer, who was the original describer of most of the 

 Arctic floras, brought to that study a profound acquaintance with the Miocene 

 floras of central Europe, and in subsequently dealing with these more frag- 

 mentary northern floras he often saw resemblances that did not exist. Both 



131 R. J. strutt : The accumulation of helium in geological time. III. Proc. Royal 

 Soc. London, vol. 83, ser, A, 1910, p. 301. 



