AGE OF THE LLANO SERIES 865 



Discussion of the value of the analyses. — In comment upon these 

 analyses it should be noted that nivenite of Becker^s list is the same 

 analysis as number 15^ uraninite, of Boltwood's list. 



Number 14 is a different lot of the same material as number 15. The 

 remarks of Hillebrand, thereto re ;, apply to both. Hillebrand states that 

 ^^a first glance sufficed to show that the specimens were not fresh, and 

 therefore analysis could throw no light on the ultimate composition of 

 the mineral. The cause of the considerable loss shown by the total of 

 98.74 for the analysis is not known." Notwithstanding Hillebrand's com- 

 ments on the lack of freshness, it would appear that these analyses are of 

 fair reliability — that is, that no appreciable amount of either lead or 

 uranium has been removed by solu'tion in meteoric waters, since the 

 amount of combined water is not abnormal for a uraninite and the large 

 amount of uranous oxide shows an absence of oxidizing weathering. 



Number 16, mackintoshite, shows from the presence of uranous and 

 ferrous oxides in large amount and the complete absence of uranic and 

 ferric oxides a lack of oxidation by descending waters. The material was 

 embedded in massive cyrtolite and associated with fergusonite. It is 

 opaque and black, and alters on the surface to dull yellowish brown thoro- 

 gummite, a hydrated and oxidized uranium thoro-silicatc liolding 7.88 

 per cent of combined water. An analysis is given of thoro^ummite show- 

 ing that the change is almost entirely one of oxidation and hydration. 

 In comment Hillebrand states : 



"It is in fact remarkable, considering the great molecular alteration that 

 must have taken place, as determined by the totally different appearance of 

 the two minerals, that so little loss of substance has taken place. Almost the 

 sole change has consisted in an oxidation of uranous oxide and an increase in 

 the hydration. These facts render not altogether safe the assumption above 

 made that all uranium and iron in the new mineral exist there in the lower 

 forms of oxidation, and they furthermore indicate that the black mineral itself 

 may have already undergone oxidation and hydration without this being mani- 

 fest to the eye — a supposition which is strengthened by the loss at 100° 0. of 

 half a per cent of water, and in fact by the intimate union which existed be- 

 tween the two minerals when received. Such material alteration without cor- 

 responding physical evidence of it seems to be common among uraninites." ^-- 



Notwithstanding this possibility of some oxidation and hydration, the 

 smallness of the losses in uranium and lead in tlie transformation of 

 mackintoshite into thorogummite indicates that no appreciable change in 

 the lead-uranium ratio has taken place in the mackintoshite, the parent 

 mineral. In fact, it is a rather general condition, as Merrill has shown, 



i^Am, Jour. Sci., vol. 46, 1893, p. 101. 



