AGE OF THE LLANO SERIES 871 



equal eliminatiou of both lead and uranium will make a large difference in the 

 ratio. From the nature of the mineral, apart from the chemical analysis, it is 

 clearly unsuited for the present purpose. 



Eliminating nine of the twelve analyses from Holmes^ list for reasons 

 given above, leaves numbers 14, lo, 16 of Boltwood^s list and gives for 

 this remainder a highly accordant ratio. These appear to serve, therefore, 

 as a reliable means of measuring the age of the Llano series and add their 

 weight to the value of the method. 



Part VI. — Cojstvergenoe oe Evidence on geologic Time and 



ITS Bearings 



METHODS OF TESTING THE AGES GIVEN BY RADIOACTIVITY 



In the last third of the nineteenth century physics, in the embodiment 

 of its leaders, Kelvin, Helmholtz_, Tait, and others, spoke with assurance 

 on the limits of geologic time. Geologists sought to meet their demands, 

 in so far as they could, but such men as Huxley, Geikie, Goodchild, and 

 others, giving greater weight to the geologic evidence, refused to accept 

 the restrictions which were set. We have lived to see unsuspected sources 

 of energy discovered, stupendous in amount, which wholly remove the 

 former limitations on the age of the earth and set new boundaries far 

 beyond what, to most geologists, has seemed the testimony of the evidence. 



After the one experience in the fallibility of physical argument not- 

 withstanding its mathematical character, it would certainly be unwise for 

 geologists to accept unreservedly the new and larger measurements of 

 time given by radioactivity. There may be here, also, factors undetected 

 and unsuspected which vitiate the results. The radioactive measurements, 

 however, can and should be tested by the degree of concordance -or dis- 

 cordance of the several results when compared with each other, and also 

 with independent lines of evidence, especially geological. 



In the preceding parts of this paper a reexamination has been made of 

 these various lines of evidence. This concluding part will bring these 

 results together and point out what seem promising fields for future 

 investigation. The first conclusion of importance is the essential lack of 

 uniform rate of erosion and sedimentation through geologic time. The 

 introduction of the idea of rhythm as ever-present in geological processes 

 in here made fundamental. It is seen on applying this to the present 

 condition of the earth, as compared to past geologic time, that there exists 

 at present a ^^ery unusual rate of aggregate erosion. Joly, overlooking 

 the significa^ice of composite rhythms, holding to the essential uniformity 

 of geiogic pi'ocesses, and, I'lirthermore, regarding the present rates as 



