THE 



JOURNAL OF GEOLOGY 



OCTOBER-NOVEMBER, 1893. 



GEOLOGIC TIME, AS INDICATED BY THE SEDIMEN- 

 TARY ROCKS OF NORTH AMERICA.* 



INTRODUCTION. 



Of all subjects of speculative geology few are more attractive 

 or more uncertain in positive results than geologic time. The 

 physicists have drawn the lines closer and closer until the geolo- 

 gist is told that he must bring his estimates of the age of the 

 earth within a limit of from ten to thirty millions of years. The 

 geologist masses his observations and replies that more time is 

 required, and suggests to the physicist that there may be an 

 error somewhere in his data or the method of his treatment. 

 The geologist realizes that geologic time cannot be reduced to 

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

 tially recognized or altogether unknown factors ; but he can 

 approximate the relative position of certain formations, and by 

 comparison of their sediments, dimensions, and contained record 

 of life with estimated rates of denudation, sedimentation and 

 organic growth, form a general estimate of their relative time dura- 

 tion. It is my purpose to-day to take up the consideration of 

 the evidence afforded by the sedimentary rocks of our continen- 

 tal area, and largely of a distinct basin of sedimentation, with a 

 view of arriving, if possible, at an approximate time-period for 

 their deposition. Before so doing, I will briefly refer to a few 

 of the opinions that have been held by geologists on geologic 



* Vice-Presidential address delivered before Section E, Am. Assc. Adv. Sci., Madi- 

 son, Wis., August 17, 1893. 

 Vol. I., No. 7. 639 



