Time-table for North America. 11 



nearest to the continental margins. The perfection of the column 

 will also bring about a greater harmony in the very variable 

 estimates as to the age of the earth, as given on the one hand 

 by the geologists and on the other by the physicists. 



The major breaks in the geologic record are indicated in 

 the time-table by "intervals," the marked erosion periods 

 representative in the main of wide and high continents and 

 of dominant erosion, not recorded by sediments within reach of 

 observation ; therefore in geologic chronology these are " lost 

 times " of long duration. It was not thought desirable to give 

 a new and independent name to each one of these intervals, 

 but rather to use in modified form an old and familiar one. 

 Therefore we have adopted the Greek word epi ( = upon or 

 after) as a prefix to the era terms, to indicate the subsequent 

 time, that is, the intervals, thus naming these intervals 

 Epi-Mesozoic, Epi-Paleozoic, Epi-Proterozoic, Epi-Neolau- 

 rentian, and Epi-Paleolaurentian. This method of naming 

 was first proposed by Lawson for the interval following the 

 Archean (1902, 1913). The same combination can be used, 

 when it becomes necessary, for the intervals between the 

 periods, as Epi-Silurian, etc. 



Diastrophism. — As shifting of the strand-line is the most 

 important criterion in ascertaining diastrophic action (a term 

 to include all movements of the outer parts of the earth), it is 

 well to state here briefly how these alternations are most readily 

 determined. Organically they are recorded : (1) by abrupt 

 changes in the superposed faunas, and (2) by the sudden ap- 

 pearance of newly evolved stocks ; physically (3) by more or 

 less obvious breaks in the sedimentation, due to sea withdrawal, 



(4) by changes in the character of the deposits, especially 

 when this involves abrupt transition from organically formed 

 strata (marl, chalk, limestone, dolomite) to mudstone and sand- 

 stone, or a change from continental to marine deposition, and 



(5) by marine overlaps upon rocks of earlier age, produc- 

 ing typical unconformities. 



Correlation of formations in separated regions is made in 

 part on a physical basis. This is done by finding similarities 

 in disconformities (time breaks in conformably superposed 

 or parallel strata; also called accordant unconformities), 

 unconformities (time breaks indicated by two sets of strata 

 inclined at varying angles to one another and easily seen 

 by the eye = structural unconformities), and changing petro- 

 logic characters. A physical correlation is in general, 

 however, far less reliable, and must ever remain second 

 in importance to correlation by biotas (combined faunas 

 and floras) for the discernment of diastrophic action. Of 

 course, the most easily determined crustal movements are those 



