AGE OF THE LLANO SERIES 869 



Let attention be given next to the four analyses used by Becker which 

 Boltwoocl did not use. On page 864 the six analyses which are the basis 

 of Beckei^s conclusions are listed, not in the order given by Becker, but 

 in the order of decreasing uranium content. It appears also that this is 

 the order of the increase in the lead-uranium ratio. As the uranium in 

 the four additional analyses is but 6.06 per cent in the first and ranges 

 in the remaining three from 1.45 down to the small quantity of 0.79 per 

 cent, the relations clearly indicate the worthlessness of these analyses as 

 tests, compared to minerals as high in uranium as are iiraninite and 

 mackijitosliite; yet, as Becker does not give in his article the per cents of 

 lead and uranium, but only their ratio, this conclusion could not be sus- 

 pected from his paper. The thorium in the last three analyses greatly 

 exceeds the (content of uranium. Some original lead is suggested by the 

 regular increase of ratio with decrease in uranium. The amount of origi- 

 nal lead implied, provided the analytical work is very accurate, would be 

 from approximately 0.5 to 1.0 per cent. Small errors in the analyses 

 would, however, greatly change the ratios between quantities so small, and 

 seems to the writer to be probably the more important factor. The small 

 changes which are likely to have resulted from the incipient alteration so 

 common in these minerals would also greatly affect the results. That we 

 may have to look to analytical errors and incipient alterations more than 

 to original lead for the explanation is suggested by the fact that gadolinite, 

 a silicate of the rare earths from the same region, shows in its analysis 

 by Mackintosh neither uranium nor lead. No thorium is present either, 

 but CcsOs occurs to the extent of 2.66 per cent. As cerium is very similar 

 to thorium in its chemical properties, original lead might be expected 

 to accompany it if it accompanies thorium. The same is true of row- 

 landite.^^®. Hess gives a complete list of the minerals found at tliis local- 

 ity.^^^ Although chalcopyrite, pyrite, spalerite, and molybderite are 

 listed, galena does not appear. It appears, then, that original lead must 

 have been very rare in the solutions which gave rise to these minerals. 



From this examination of the evidence it is seen that the four analyses 

 rejected by Boltwood but used by Becker are, as Boltwood stated, not 

 suitable for throwing any definite light on the lead-uranium ratio for 

 Llano County. 



Attention must next be given to the explanation of the discordant ratios 

 for the same region reached by Holmes, whose list is as follows :^^^ 



i^BuH. 419, U. S. Geol. Survey, 1910, p. 276. 



12' F. L. Hess : Minerals of the rare-earth metals at Barringer Hill, Llano County, 

 Texas. Bull. .340 D, IT. S. Geol. Survey, 1908, pp. 6.S-65. 



1^8 A. Holmes : Radioactivity and the measurement of geological time. Proc. Geolo- 

 gists Asso,, vol. xxvi, part 5, 1915, p. 303. 



