INTRODUCTION AND SUMMARY 751 



publication in England in 1915 ol an article by Professor Arthur 

 Kolnies,'' an investigator who has done much work on this subject. 



Becker in 1908 published an article on the "Eelations of Eadioactivity 

 to Cosmogony and Geology/" ^ On pages 127 to 135 of this article is 

 discussed the subject of '^Radioactivity and the earth's age." Becker^s 

 argument and conclusions are strongly adverse to the value of the radio- 

 active methods for determining age. This result is based in considerable 

 part on the conflicting testimony derived from the radioactive minerals 

 of the Llano district in Texas. In order to weigh these criticisms, the 

 writer has examined the original data and finds that, instead of being 

 opposed, they strongly support the hypothesis that the ratio of lead to 

 uranium in uranium minerals, serves, when properly used, as an index of 

 age. A hypothesis gains greatly in sti^ength when what appeared to be 

 conflicting evidence is resolved into supporting evidence. For this reason 

 the facts for the Llano region are given in Part V in some detail. 



Thus this article seeks to bind the geological and physical argument^ 

 into a unity — the geological data giving evidence of highly variable rate 

 and imperfection of record, the physical evidence supporting the assump- 

 tion of constant rate for radioactive processes and giving the magnitude 

 of the framework into which the geological picture must be set. That 

 framework is given in Part VI, the concluding part, as a new table of 

 geologic time, and is of more generous proportions than geologists would 

 have dared to assume from the data of their field alone ; but it appears 

 on testing from various points of vieA\^ to have been logically constructed 

 and gives room for a bolder and larger treatment of earth history than 

 has been imagined to be held within the past. This table of geologic 

 time is constructed by the dovetailing of two lines of evidence, as follows : 



First, the thicknesses and character of the sediments give stratigraphic 

 ratios of the lengths of the several periods. These, for reasons discussed, 

 are subject to considerable uncertainty; yet, when derived with a knowl- 

 edge of the variables involved, the ratios give some measure of the relative 

 durations. Second, the quantities of helium and lead in radioactive min- 

 erals give minimum and maximum measurements of age. These ages are 

 open to some uncertainty because of loss of helium or the presence of 

 original lead, and the stratigraphic positions of the rocks holding the 

 minerals are also in most cases not closely known. Nevertheless, the radio- 

 active minerals give measures of absolute age wliich are of the right order 

 of magnitude and in proper sequence, as shown by the geologic data. 



" Radioactivity and the measurement of geological time. Proc. of the Geologists' Asso- 

 ciation, vol. xxvi, part 5, lOl.''), pp. 280-309. 

 ■A Bull. Geol. Soc. Am., a^oI. 10, pp. 11.3-14(;. 



