RISE OP CHRONOLOGY 497 



new periods of TJlrich and Schuchert that the average stratigrapher is 

 chary in accepting them (see figure 2). 



BREAKS 



The breaks in the geologic record are known to be many, and yet few 

 stratigraphers appreciate their great number. The easily seen, marked 

 unconformities, as, for instance, the one between the Cambrian and the 

 Archeozoic in the Grand Canyon of Arizona, are of course accepted at 

 full face value; but the many more apparently conformable and yet 

 broken contacts, the disconformities, are generally overlooked, or when 

 seen are generally undervalued (see plate 19, figures 1 and 2). It is 

 probable that as much time is represented by the breaks as by the entire 

 sedimentary record. This statement may appear to many as overdrawn, 

 and yet the age of the earth estimated from geologic data is greatly at 

 variance with the results attained by physicists on the basis of radium 

 emanations. The differences are about as one is to eight, or even ten, and 

 as the physicists have more reliable data on which to base their calcula- 

 tions, it follows that stratigraphers must either considerably elongate the 

 geologic time-table or show that the rate of sedimentation varies greatly 

 during the opening and closing epochs of the period when compared with 

 the peneplained middle epoch. To emphasize the importance of breaks 

 and their very unequal duration, it is sufficient to recall to mind the 

 vastly long record that is absent on the Canadian Shield, where the Pleis- 

 tocene drift generally reposes on the Laurentian granites of Archeozoic 

 or Proterozoic time, or the case of the horizontal Pleistocene loess that 

 rests conformably on Silurian limestone at Grafton, Illinois. The latter 

 is a disconformable contact, and yet the record that is absent is equal to 

 about one-half of the entire stratigraphic chronology. 



In regard to the breaks, the statement can be made that there are at 

 least ten disconformities for every known angular unconformity, and in 

 the Ohio and Mississippi valleys, where the Paleozoic strata are nearly 

 everywhere practically horizontal, there may be a hundred disconformi- 

 ties, and yet hardly anywhere is there to be seen a marked unconformity. 



Methods underlying a determined Chronology 



general discussion of methods 



The methods now in use in determining the stratigraphic sequence and 

 in correlating the formations from place to place are of four categories. 

 These in the order of immediate use are (1) the sedimentary method, 

 (2) the paleontologic method, (3) the paleogeographic method, and (4) 



