AGE OF THE LLANO SERIES 



863 



THE LEAD-URANIUM RATIOS OF THE LLANO MINERALS 



The original analytical data. — The lead-uranium ratios should be widely 

 different for the Connecticut and Texas regions, but they should be ac- 

 cordant among themselves. Such accordance exists among the Glaston- 

 bury analyses, though the force of the accordance is perhaps somewliat 

 weakened by the determinations being all from one species of mineral, 

 rfraninite. For the Llano region a variety of analyses and from different 

 mineral species are available, but in the published lists no constancy is 

 shown in the lead-uranium ratio. Becker from the varying ratio obtains 

 ages as cited, ranging from 1,671 million years to 11,470 millions, and 

 takes the variation, the maximum being about seven times the minimum. 

 as evidence of the unreliability of the method for measuring age. Holmes; 

 gives a list of values of the lead-uranium ratio ranging from 0.102 to 

 1.130. He ascribes the variations in ratio to the altered character of the 

 minerals, riddled with secondary products, and speaks of them as eloquent 

 of the uselessness of this series of minerals as an index of geological time. 



Uranium Minerals of the Llano District, Texas 



Analyses giving Lead-Uranium Ratios 



Used by Boltwood (1907) 



Arranged in order of increasing ratio; minor constituents omitted 



No. 15. 



Uraninite. 



Mackintosh. 



No. 14. 

 Uraninite. 

 Hillebrand 



No. 16. 



Mackintoshite. 



Hillebrand- 



No. 17. 

 Yttrocrasite- 

 C- H. Warren. 



UO3 



UO2 



ThO^ 



ZrOa 



Rare earths 



PbO 



FOaOs 



FeO 



SiO^ 



TiOa 



H,0 



Total 



Ref . : Am. Jour. Sci. 



volume and page. 



Year 



Pb 



U 



Pb/U 



46.75 

 19.89 



7.57 



ii*.22 



10.16 



0.58 



2.54 



44.17 



20.89 



6.69 



0.34 



12.16 



10.08 



1.48 



99.9^ 



98.74 



22.40 



45.30 



.88 



1.86 



3.74 



13.90 

 '4.31 



96.50 



0.64 

 1.98 



8.75 



28.59 

 0.48 

 1.44 



49! 72 

 4.36 



100.57 



38.481. 

 1889 



42.391. 

 1891 



46.101. 

 1893 



22.516. 

 1906 



9.43 

 56.45 

 .167 



9.35 

 55.18 

 .170 



3.47 

 19.75 

 .176 



0.45 



2.28 

 .195 



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



