

MODERN GEOLOGY 



"centuries" and the "weeks" of every-day computa- 

 tion. They are convenient, and they may even stand 

 for local divisions in the strata, but they are bounded 

 by no actual gaps in the sweep of terrestrial events. 



Moreover, it must be understood that the "ages" of 

 different continents, though described under the same 

 name, are not necessarily of exact contemporaneity. 

 There is no sure test available by which it could be 

 shown that the Devonian age, for instance, as outlined 

 in the strata of Europe, did not begin millions of years 

 earlier or later than the period whose records are said 

 to represent the Devonian age in America. In at- 

 tempting to decide such details as this, mineralogical 

 data fail us utterly. Even in rocks of adjoining regions 

 identity of structure is no proof of contemporaneous 

 origin; for the veritable substance of the rock of one 

 age is ground up to build the rocks of subsequent ages. 

 Furthermore, in seas where conditions change but little 

 the same form of rock may be made age after age. It 

 is believed that chalk-beds still forming in some of our 

 present seas may form one continuous mass dating back 

 to earliest geologic ages. On the other hand, rocks dif- 

 ferent in character may be formed at the same time in 

 regions not far apart say a sandstone along shore, a 

 coral limestone farther seaward, and a chalk-bed be- 

 yond. This continuous stratum, broken in the process 

 of upheaval, might seem the record of three different 

 epochs. 



Paleontology, of course, supplies far better chrono- 

 logical tests, but even these have their limitations. 

 There has been no time since rocks now in existence 

 were formed, if ever, when the earth had a uniform 



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