IOWA ACADEMY OF SCIENCE 
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lative criteria. The series is all-important and not its time equivalent, which 
begins to have very vague definition even when different parts of the same 
geologic province are compared; and necessarily so in provinces with dissimilar 
histories. This plan gives prominence to the structural unit of geologic map- 
ping, the lithologic formation, or terrane. Minor subdivisions of the latter 
may be delimited by the specific mineral peculiarities of the fossils. 
By clearly distinguishing between geologic history and biotic history geologic 
correlation is placed upon a rational, genetic and philosophic foundation. 
Stratigraphy is immeasureably advanced. 
