CONVERGENCE OF EVIDENCE 873 



uranium minerals well adapted for age determination and of a well deter- 

 mined stratigraphic position, such as the Devonian, are found in different 

 regions to give notably different ages, it may be an indication of inhibition 

 of disintegration beyond certain limits of temperature and pressure which 

 have existed in these formations through a part of their existence. Unfor- 

 tunately the data are as yet too meager for this test to be so precisely 

 applied. Most of the uranium minerals come from formations whose 

 stratigraphic positions are not closely known, and they represent a wide 

 range of geologic occurrence. There is another test, however, which can 

 be reasonably well applied. That is the test of proper sequence. If 

 the rate of disintegration of uranium is altered by physical conditions 

 within a few miles of the surface, then, in a considerable number of 

 instances, some range in the lead-uranium ratio might be expected within 

 the minerals of a single district; but the more searching test is found 

 when the measurements of the age of minerals of widely different regions 

 and stratigraphic positions are assembled according to sequence of age as 

 determined by the radioactive transformations. It would be expected 

 that, if variable rates of disintegration prevailed, the geologically younger 

 mineral would in some instances show a greater amount of disintegration 

 products and indicate an apparently greater age. With a wide range 

 in age, with many points determined, and a large list of minerals, this 

 argument begins to assume great force, notwithstanding considerable in- 

 definiteness in the geological evidence regarding the exact stratigraphic 

 position of many of the minerals. 



This test could be, and was, applied to the first paper on the subject, 

 published by Boltwood in 1907. It showed that his work bore internal 

 evidence of the essential correctness and importance of the hypothesis of 

 the lead-uranium ratio as a means of determining age, though a number 

 of subordinate and modifying questions remained to be settled, especially 

 in regard to the presence of various kinds of lead. This internal evidence 

 was stated by Boltwood, in 1907, as follows : 



"The actual value of the ratio varies considerably for the primary minerals 

 from different localities, the maximum value being about six times the mini- 

 mum. It is beyond the writer's province to discuss the data bearing on the 

 geological ages of the different deposits ; but he is indebted to Professor Joseph 

 Barrel!, of Yale University, for the statement that, so far as the knowledge of 

 the latter extends, the relative values of the ratios are not contradictory to 

 the order of the ages attributed by geologists to the formations in which the 

 different minerals occur. 



"From the data which have been presented in the preceding tables, it is ap- 

 parent that the requirements for a disintegration product of uranium are ful- 

 filled by lead within the limits of probably experimental error. On the basis 



