VERTEBRATE PALAEONTOLOGY 139 



non-radiogenic lead present in the material. Where this is 

 high there is a corresponding high error in the estimation. This 

 can lead to an error of 700 million years in the exceptional case 

 of the Caribou Mine, Colorado, where the deposit contained as 

 much as 97% lead. The correct age of the deposit was 25 million 

 years old. 



From the stratigraphic point of view the radiogenic data is a 

 little disappointing. Thus Kulp (1955) states, " The only 

 thoroughly satisfactory sample from a stratigraphic point of view 

 is the Swedish kolm which contains Upper Cambrian fossils." 

 Unfortunately the isotopic ages obtained by Nier for this sample 

 do not agree. Thus: 



U 238 /Pb 206 gave 380 million years 

 Tj235/pb>2 07 gave 440 million years 

 Pb 207 /Pb 206 gave 800 million years 



The correct age appears to be 440 million years and it is probable 

 that the other values are in error due to radon loss. 



There are two other locations of fossiliferous rocks that have 

 also been accurately radiogenically dated. A pitchblende from 

 Colorado has been dated as 60 million years old. This had been 

 placed at the beginning of the Eocene. It will be remembered 

 that the dating of the Eocene was tentatively done by Matthew 

 (1914) by estimating the time required for the evolution of the 

 horse. Matthew decided that it took 45 million years, i.e. he 

 differed by 15 million years from the radiogenic dating. 



All these dates are based on pitchblendes which might have 

 percolated through into the examined strata and they could in 

 fact have been derived from some other strata than that in which 

 they have been discovered. Kulp in discussing the uranium 

 contents of rocks states that the high uranium concentration is 

 often associated with a carbonaceous deposit and it is conceivable 

 that the uranium was accumulated by some biochemical process 

 before the rock systems became molten. 



The fact that pegmatites are few and far between makes it 

 improbable that the uranium method will have extensive use in 

 dating fossils. It is more likely that some other method will be of 

 greater use. 



