858 J. BAilRELL MEASUREMENTS OF GEOLOGIC TIME 



he finds that radioactivity maintains only one-seventh of the present 

 gradient, and that the age of the cooling earth is 68,000,000 years. With 

 the data he uses it is calculated that if two-thirds of the gradient is main- 

 tained by radioactivity, then the age becomes 1,314,000,000 years, and the 

 depth of easiest fusion becomes 300 km. In a series of papers running 

 through the Journal of Geology, 1914 and 1915, J. Barrell has shown 

 that the zone of easiest fusion must be below the level of compensation, 

 and from considerations based on the strength of the earth's crust and 

 on tidal phenomena, he places the source of igneous activity at about the 

 middle of the Asthenosphere — that is, at a depth of 350-500 km. Com- 

 bining this conclusion with Becker^s analysis, it is easily seen that the age 

 of the coaling earth is not 68,000,000 years, but considerably greater than 

 1,314,000,000 years.— A. H. 



Part V. — ^The Age of the Llano Series, Texas 



- PREVIOUS OPINIONS 



A discussion of the age of the pegmatites which intrude the Llano series 

 of Precambrian sediments would be out of place in an article of this 

 general character if it were merely to settle a disputed point in regard to 

 local geology, but it happens that Becker has used the uranium minerals 

 of the pegmatites in this series in an argument which completely discounts 

 the value of the uranium-lead ratio as a means of determining tlie age 

 of a uraniutn mineral.^^* Becker's conclusions are so adverse and ap- 

 parently so final to one not otherwise familiar with the subject that they 

 may in part account for the lack of further papers treating of the sub- 

 ject in American geological journals since Boltwood's publication in 1907. 

 In England, on the contrary, considerable work has been done. In regard 

 to the radioactive minerals of the Llano series, however, Holmes, relying 

 in part on Becker's statements regarding their age and the variations in 

 the lead -uranium ratios, discusses them as a warning example of the 

 effects of alteration in minerals subsequent to their origin. Holmes takes 

 this position, since, as seen, he holds that for many localities such min- 

 erals do give valuable indications of age. The present re-examination of 

 the data goes to show, however, that these minerals of the Llano pegma- 

 tites do give definite and accordant measures of age when they are prop- 

 erly interpreted. So many differences arise between the views of Becker 

 and the present writer in the analysis of this subject that, as a basis for 

 discussion, Becker's statements must be quoted as an introduction to the 



1"* Relations of radioactivity to cosmogonj^ and geology. Bull. Geol. Soc. Am,, vol. 19, 

 1008, pp. 11.3-146. 



