GEOLOGIC TIME, AS INDICATED BY THE SEDIMENTARY 

 ROCKS OF NORTH AMERICA.* 



By Charles D. Walcott. 



TNTRODUrTION. 



Of all subjects of speculative iieology few are more attractive or more 

 uucertaiu in positive results thau geologic time. The physicists have 

 drawn the lines closer and closer until the geologist is told that he 

 must bring his estimates of the age of the earth within the limit of 

 from 10,000,000 to 30,000,000 years. The geologist masses his observa- 

 tions and replies that more time is required, and suggests to the physi- 

 cist that there maj^be an error somewhere in his data or the method of 

 his treatment. The geologist realizes that geologic time can not be 

 reduced to actual time in decades or centuries ; there are too many par- 

 tially recognized or altogether unknown factors; but he can approxi- 

 mate the relative position of certain formations and, by comparison of 

 their sediments, dimensions, and contained record of life with the esti- 

 mated rates of denudation, sedimentation, and organic growth, form a 

 general estimate of their relative time duration. It is my purpose to-day 

 to take up the consideration of the evidence afforded by the sedimentary 

 rocks of our continental area, and largely of a distinct basin of sedimen- 

 tation, with a view of arriving, if possible, at an approximate time period 

 for their deposition. Before proceeding to examine the conditions of 

 denudation, sedimentation, etc., that enter as factors into the calcula- 

 tion of the age of the earth on the basis of sedimentary geology, we will 

 refer to some of the opinions that have been held by geologists on geo- 

 logic time and the age of the earth. 



Soon after geology emerged from its pre-systematic stage and assumed 

 an independent position among the inductive sciences speculations on 

 the age of the earth were made by both geologists and jihysicists. Hut- 

 ton, Werner, Smith, and Cuvier, among the former, arranged and pub- 

 lished their observations and those of their predecessors during the 

 closing years of the eighteenth century, and in the three succeeding 

 decades rapid progress was made in many lines of investigation by 



* Vice-presidential address before section E, American Association for the 

 Advancement of Science, Madison, Wis., August 17, 1893. 



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