78 B. B. Boltwood — Ultimate Disintegration Products 



amount of the parent substance with which it is associated. 

 And, in unaltered, primary minerals from different localities, 

 the proportion of each disintegration product with respect to 

 the parent substance must be greater in those minerals which 

 are the older and should correspond with the order of the 

 respective geological ages of the localities in which the min- 

 erals have been found. It also follows that in secondary min- 

 erals, namely, in minerals which have been formed by the 

 subsequent alteration of the original, primary minerals, the 

 relative amounts of the disintegration products must be less 

 than in the primary minerals from the same locality, provided, 

 however, that the disintegration products can not be considered 

 as original chemical constituents of the secondary mineral. 



It is the purpose of the present paper to show that the 

 above requirements are practically fulfilled by lead and by 

 helium also, in so far as the gaseous nature of the latter ele- 

 ment will permit of its retention in the minerals. The sug- 

 gestion that lead was one of the final (inactive) disintegration 

 products of uranium was first made by the writer in a paper 

 presented before the New York Section of the American Chem- 

 ical Society on February 10, 1905, and published later in the 

 Philosophical Magazine.* 



The amounts of uranium and lead present in a considerable 

 number of primary uranium minerals have been calculated 

 from the published analyses of these minerals. The number 

 of such analyses to be found in the literature is not large, and, 

 what is still more unfortunate, with the exception of those 

 made by Hillebrand and a few others, cannot be considered as 

 particularly accurate. Many of the analyses were made with 

 special objects in view, such as the identification of a given 

 specimen with a species already known or its recognition 

 as a new variety or species. There is also what is perhaps 

 an unfortunate tendency on the part of many mineralogists to 

 carry out an analysis merely for the purpose of assigning to 

 the mineral some definite chemical formula, which often leads 

 to the overlooking or ignoring of a number of the minor con- 

 stituents. And in addition to this there are also the actual 

 analytical difficulties to be taken into account, which may be 

 very considerable in the case of such minerals as samarskite, 

 fergusonite, euxenite and other minerals containing notable 

 proportions of niobium, tantalum and titanium. Notwith- 

 standing these objections, however, it is necessary to rely very 

 largely on these published analyses, for the simple reason that 

 the greater number of the uranium minerals are extremely 

 rare and the obtaining of suitable samples of the various spe- 

 cies and varieties is either extremely difficult or altogether 

 impossible. 



* April, 1905. 



