868 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



sufficiently abundant, so that this test should be applied. The analyses 

 which have been given, however, carry internal evidence that in the Llano 

 region such original lead must be very small in quantity. In the minerals 

 where the percentage of uranium lead is small and thorium is large the 

 original lead should notably modify the lead-uranium ratio. Now, in 

 analysis number 17, U is only 2.28 per cent, Pb is 0.45, and ThOg is 8.75. 

 The Pb/U ratio is nevertheless only .195, which is but one-sixth greater 

 than the most probable value. The presence of .08 per cent of original 

 lead would account for this increase in the ratio, which is but one-one- 

 hundred tli of the ThOg present. On the other hand, analytical errors 

 which come in with the determination of small quantities may be more 

 important than original lead. Furthermore, the agreement of the ratio, 

 as derived from four analyses and three different minerals which show 

 great ranges in the amounts of uranium and thorium, is another indica- 

 tion of the reliability of the ratio and the smallness of the amount of 

 original lead. 



On assigning weights to the several determinations, the greater weight 

 should be given to the minerals high in uranium, since this decreases the 

 errors due to original lead and to analysis. Some slight and variable 

 amount of original lead may be granted in all of these minerals. There- 

 fore the best value of the ratio for the Llano region may be taken as .165. 

 'J'his corresponds to an age of 1,125,000,000 years. 



Ea^planation of previous discordant results. — Having reached this con- 

 clusion as to the value of the ratio in the uraninite of the Llano region, 

 attention must be turned to an explanation of the entirely different con- 

 clusion reached by Becker and quoted on a previous page. In the six 

 analyses used by Becker no two give similar ratios and these range in 

 value from .167 to 1.147. As Becker does not state that Boltwood had 

 used four analyses giving accordant ratios from this region, the reader, 

 unless intimately familiar with Boltwood's article, would assulne that no 

 analyses from the Llano County minerals give accordant results. How 

 different this is from the facts has been shown on the previous pages. 

 Becker, for some unknown reason, avoided using two ratios of Boltwood's 

 list and apparently made an arithmetical blunder in recomputing the 

 lead-uranium ratio from a third. Instead of getting .18, as Boltwood did, 

 or .176, as computed by the writer to another significant figure, Becker 

 derives a ratio of .3894. Thus, if Becker had included correctly all of 

 Boltwood's list, there would have been four highly accordant ratios in a 

 total of eight — a very different result from no two similar ratios among 

 six analyses. 



