AGE OF THE LLANO SERIES 859 



topic. Under the subheading of "Radioactivity and the Earth's Age/' 

 Becker states : 



"Now from the helium found in an analysis of fergusonite by Messrs. Ram- 

 say and Travers, Mr. Rutherford ^"^ computes an age of at least 500 million 

 years, and from an uranium mineral at Glastonbury, Connecticut, analyzed by 

 Mr. Hillebrand, a similar antiquity. The Glastonbury granite gneiss is equiva- 

 lent to the Wilbraham gneiss of Mr. Emerson,"^ who pronounces it unequivo- 

 cally early Cambrian. Messrs. Rice and Gregory"^ feel some uncertainty as to 

 its age, but do not suggest a new position for it. For the present purpose, it is 

 sufficient to regard it as at the bottom of the Cambrian. Mr. Walcott's esti- 

 mate of the lapse of time since the beginning of the Cambrian is nearly 28 

 million years, or about an eighteenth part of that suggested by Mr. Rutherford. 



"Mr, Boltwood"^ has computed the age of a large number of uranium min- 

 erals from their lead content. He shows that according to theory the age will 

 be given in years to a first approximation, if the ratio of metallic lead to me- 

 tallic uranium is multiplied by 10 million. In this way he gets for the age of 

 the Glastonbury minerals 410 million years. Now, at Barringer Hill, in Llano 

 County, Texas, there is a very remarkable deposit of rare radioactive minerals, 

 which are so abundant that they have been mined for the use of an electric 

 lighting company. It happens that the age of the granite in which the peg- 

 matite occurs is known. Mr. Walcott"® discovered In this county his Llano 

 group, which belongs to the Grand Canyon series, not far below the Cambrian. 

 The granites are intrusive in these sediments. The great masses of granite 

 which occur in western Burnet and all through Llano County belong to the 

 same age as the strata, and Mr. Walcott is careful to remark that he did not 

 observe any rocks" of undoubted Archean age in the region. A number of 

 analyses"" of the rare minerals of Barringer Hill are available, and by Mr. 

 Boltwood's rule they give the following ages for the Llano beds : 



Million years 



Yttrialite, J. B. Mackintosh 11,470 



Yttrialite, W. F. Hillebrand 5,136 



Mackintoshite, W. F. Hillebrand 3,894 



Nivenite, Mackintosh 1,671 



Fergusonite, Mackintosh 10,350 



Fergusonite, Mackintosh 2,967 



"Mr. Boltwood informs me, however, that, with the possible exception of 

 nivenite, none of these minerals is really suitable for throwing any definite 

 light on the question of the uranium-lead ratio for Llano County, since all of 

 the specimens show signs of incipient or advanced alteration; but, according 

 to theory, the state of combination is without influence on radioactivity, so 



i<e Radioactive transformations, p. 189. 



108 U. S. Geological Survey, Monograph 20, 1898. 



107 Connecticut Geological and Natural History Survey, Bulletin 6, 1906, p. 116. 



108 American .Journal of Science, vol. 23, 1907, p. 87. 



109 American Journal of Science, vol. 28, 1884, p. 431. 



110 Hidden & Mackintosh : American .Journal of Science, vol. 38, 1889, p. 474. 

 Hillebrand • American Journal of Science, vol. 46, 1893, p. 99 ; vol. 13. 1902, p. 145. 



