GENERAL PRINCIPLES — CONCLUDED 31 



are not indicated, and tentative because of lack of knowledge 

 concerning many areas and lack of certainty in the correlation of 

 formations in certain other areas. With progress in knowledge 

 of the strata, less generalized and more accurate maps will be made. 

 Nevertheless the series of maps used in this text will serve to give 

 the beginner a very good idea of the broader features in the geo- 

 graphic development of our continent. 



Classification of Geologic Time 



We have already shown how, by employing the law of super- 

 position of the strata together with the law of included fossils, 

 the rock formations of various parts of the earth may be corre- 

 lated and built up according to their natural order of age into a 

 standard for comparison or a geologic column. The subdivisions 

 of the geologic column represent the times when the successive 

 rock formations were deposited. Different names have, from 

 time to time, been assigned to these divisions which are in more 

 or less general use. 



For a long time the subdivisions of the geologic column were 

 made almost solely on the basis of marked differences in fossils, 

 but it is now recognized that such differences were, in no small 

 degree, caused by corresponding changes in the environment in 

 which the organisms lived, or, in other words, by changes in the 

 climate, the topography, the relations of land and sea, etc. So we 

 now try to divide the geologic record at the points where the 

 revolutionary physical changes are indicated, and to make cor- 

 responding divisions of geologic time itself. Thus there are two 

 kinds of divisions — one for the rocks themselves, and the other 

 for the time represented by the rocks. 



The following time and rock scales have been adopted by the 

 International Geological Congress. Immediately following these 

 scales is given in descending the table of main geological divisions 

 as now recognized in North America. 



Time scale Rock scale 



Era Group 



Period System 



Epoch Series 



Age Stage 



