Geological Story Briefly Told 



Table of Major Divisions of Geologic 

 Time, with estimated duration of each 

 in Millions of years: 



Eras 



Caenozoic 



Mesozoic 



Year 



Palaeozoic 



540,000,000 



Pre-Cam- Algonkian 446 

 brianTime Archaean 926 1,912,000,000 



Three great erosion intervals, or "lost inter- 

 vals," occurred in Pre-Cambrian time, viz., 

 between Laurentian and Huronian; between 

 Huronian and Algonkian; and between Algon- 

 kian and Cambrian. Other erosion intervals, 

 or "lost intervals," separate the systems of 

 rocks representing the geologic periods. 



S50 MY 



1912 M.V. 



FIG. 17. The major divisions of geologic time, with thicknesses propor- 

 tionate to length. (From C. A. Reeds, modified by the substitution of Prof. 

 A. C. Lawson's classification of Pre-Cambrian.) Figures on right indicate 

 millions of years back from present time. (After W. B. Scott.) 



