BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 28, pp. 745-904, PLS. 43-46 DECEMBER 4, 1917 



RHYTHMS AND THE MEASUEEMENTS OF GEOLOGIC TIME ' 



BY JOSEPH BARRELL 



(Presented before the Society. Decemher 29, 1916) 



CONTENTS 



.:: . , Page 



Introduction and summary- • • • • • -v- • 746 



Part I. — Rhythms in denudation. . 753 



Present rates of denudation ,. , 753 



Relations of rate to the cycle of erosion , 755 



Significance of present A'alley forms 761 



Contrast of Cenozoit; and Paleozoic continental reliefs 767 



Contrast of present and past rates of denudation 773 



Part II. — Rhythms in sedimentation 776 



Generality of accumulation in shallow water 776 



Rate of sedimentation determined by subsidence 785 



Progressive tilting combined with rhythmic oscillation of baselevel. . . 789 



Nature of stratigraphic record resulting from composite rh.\ thms .... 795 



Illustrations of rhythms in sedimentation 798 



Part III. — Estimates of time based on geologic processes 809 



Magnitude of the Cenozoic era on evidence chiefly from erosion 809 



Magnitude of the Paleozoic era on evidence chiefly from sedimentation 815 



Estimates of time based on rhythms in sedimentation 824 



Estimates of total time based on oceanic salts 834 



Estimates of time based on loss of primal heat 834 



Part IV. — Measurements of time based on radioactivity 842 



Outline of the theory 842 



Accumulation of helium. 845 



Pleochroic halos 847 



Accumulation of lead 849 



Discussion of the evidence 849 



Conclusions on accumulation of lead 857 



1 The second of a series of papers composing a "Symposium on the Interpretation of 

 Sedimentary Rocks." It was presented at the Albany meeting, December 29, 1916, under 

 the title "The Significance of Sedimentary Rhythms." It formed the first and second 

 parts of a paper entitled "The Measurements of Geologic Time," delivered as two ad- 

 dresses in the University of Illinois Public Science Lectures on January 16 and 18, 1912, 

 at Urbana, Illinois. The author feeling that condensation would diminish greatly the 

 value of the paper, the Publication Committee of the Society consented to publish it in 

 full under the title given above. 



Manuscript received by the Secretary of the Society April 16. 1917. 



(745) 



