THE 



AMERICAN JOURNAL OF SCIENCE 



[FO URTH SERIES.] 



Art. I. — A Revised Geologic Time-table for North America ; 

 by Charles Schtjchert and Joseph Barrell. 



Preface. 



The writers, having to teach historical geology to elementary 

 classes in the two undergraduate schools of Yale University, 

 have felt the need of a table of geologic chronology which 

 shall assemble in brief space and in proper order and propor- 

 tion the more significant facts of earth history. Some kind of 

 a time-table is indeed very necessary for the elementary 

 student in order that he may coordinate the mass of data 

 which accumulates day by day, but not all textbooks contain 

 such tables, and those which are given are either brief or may 

 not express the latest views. The ideas of geologists as to what 

 such a table should indicate grow with succeeding years. 

 This perhaps has been especially true of the past decade. 



A table of geologic ages and events is needed also as a wall 

 chart for teaching other branches of the earth sciences, such 

 as physical geology and organic evolution, where references 

 must be made to the periods and eras, and it was for the pur- 

 pose of such a wall chart that the table accompanying this 

 paper was first drawn up. It is thought", however, that the 

 results may be of use beyond this University. 



The senior author is chiefly responsible for the general dis- 

 cussion of principles and for the portion of the chart embracing 

 post-Proterozoic time. The junior author is responsible for 

 the pre-Cambrian discussion and classification. 



Part I. Post-Proterozoic Time (C. S.). 



The well-known and beloved Californian geologist, LeConte, 

 in reviewing the progress of geology during the Nineteenth 

 Century, said: Through this century a gradual movement of 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 223.— July, 1914. 

 1 



