746 J . BARRELL MEASUREMENTS OF GFJOLOGIO TIME 



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Part V. — The age of the LUiiio series, Texas 858 



Previous opinions 858 



Geological evidences of age of uranium minerals 860 



Upper Paleozoic age of Glastonbury uraninites *. . . . 860 



Middle Precambrian age of the IJano series 862 



The lead-uranium ratios of the Llano minerals 86o 



The original analj tical data 863 



Discussion of the value of the analyses 865 



Explanation of previous discordant results 868 



Part VI. — Convergence of evidence on geologic time and its bearings.... 871 



Methods of testing the ages given by radioactivity 871 



Age points given by uranium minerals 875 



Adjustment with geologic evidence 881 



New table of geologic time 884 



Crescendoes in diastrophism due to composite rhythms 887 



jV^ean rates of erosion and sedimentation 891 



Relations between lapse of time and organic evolution 899 



Bearings of geologic time on the problem of stellar energy 901 



Introdik'tiok ano Summary 



Nature vibrates with rhythms, climatic and diastrophic, those liuding 

 stratigraphijc expression ranging in period from the rapid oscillation of 

 surface waters^ recorded in ripple-mark, to those long-deferred stirrings 

 of the deep imprisoned titans which have divided earth history into periods 

 and eras. The iiight of time is measured by the weaving of composite 

 rhythms — da}^ and night, calm and storm, summer and winter, birth and 

 death — such as these are sensed in the brief life of man. But the career 

 of the earth recedes into a remoteness against which these lesser cycles 

 are as una ^ ailing for the measurement of that abyss of time as would he 

 for human liistory the beating of an insect's wing. We must seek out, 

 then, th(^ nature of those longer rhythms whose very existence was un- 

 known until man by the light of science sought to understand the earth. 

 The larger cf these must be measitred in terms of the smaller, and the 

 smaller must be measured in terms of years. Sedimentation is controlled 

 by them, and the stratigraphic series constitutes a record, written on 

 tablets of stone, of these lesser and greater waves of change which hav(^ 

 pulsed through geologic time. 



The doctrine of uniformitarianism has ignored the presence of age- 

 long rhythms, and where they were obtrusive has sought to smooth them 

 out; but in so doing it has minimized the differences between the present 

 and the past, and the constant variations within that past. This doctrine 



