876 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



ill America and in Europe we commence to get a northward spread of plants 

 from the equatorial regions in the Lower Eocene (for example, in the Wilcox). 

 This is emphasized in the Middle Eocene (Claiborne and the corresponding 

 Upper Lutetien and Auversien ) and culminates in the Upper Eocene and Lower 

 Oligocene. Several of the Lower and Middle Eocene types of our Gulf States 

 appear in the flora of west Greenland and in the Kenai flora of Alaska, etcet- 

 era, and, granting the time necessary for dispersal and considering the forested 

 condition of the country north to latitude 75°, it seems to me that this should 

 make these northern floras correspond to the time when the most tropical floras 

 and faunas flourished farther south." 



Let it be assumed that the beds are Lower Oligocene and that but little 

 helium has been lost, or that they are Up^er Eocene and somewhat more 

 helium has been lost. The length of the whole of Cenozoic time may 

 then be regarded as 50 per cent longer, giving 55,000,000 to 65,000,000 

 years. This is obtained by considering from stratigraphic evidence that 

 the Eocene constitutes at least one-third of the Tertiary period. The 

 mean figure of 60,000,000 years is twenty times as long as the con- 

 ventional estimate of 3,000,000 years, but a discussion has been given in 

 a preceding section which goes to show that the geologic evidence demands 

 that this figure of 3,000,000 should be increased to the order of magni- 

 tude set by the helium measurement. The two independent lines of 

 investigation and the nature of the helium measurement are in sufficient 

 accord to warrant accepting the round figure of 60,000,000 years as a 

 provisional estimate for the length of Cenozoic time. 



We come next to a group of Upper Paleozoic occurrences in which the 

 precise geologic position, or the possible presence of some original lead, 

 is in most cases undetermined. 



First come the Glastonbury uraninites, previously discussed in this 

 article. Emerson regards the age of the associated igneous rocks as late 

 or post-Carboniferous. They are clearly LTpper Paleozoic, but the gra- 

 nitic intrusions in New England and the Maritime Provinces are known 

 to range from the Devonian onward, and some of them may be dated 

 from the Taconic disturbance at the close of the Ordovician. So far as 

 the writer is aware, the schists cut by the Glastonbury pegmatites might 

 possibly be as early as sub-Carboniferous, the Mississippian period of 

 present American nomenclature, but there is no real evidence for Holmes* 

 positive statement that the granite intrudes Lower Carboniferous strata. 

 Caution and latitude are desirable in this regard. The lead content 

 averages about 3 per cent, giving reliable determinations, and the lead- 

 uranium ratio is .041, giving an age of 300,000,000 years. N"o deter- 

 minations of the atomic weight of the lead have been made, but the lead- 

 uranium ratio is very constant in the several analyses, suggesting that 



