Accurate correlations and zonal sequences have been used in the Tertiary for many years, but in general they have been only of regional application. One paleontologic method is to compare the proportion of species which occur in both of 2 formations with the proportion of species which is restricted to each one alone. This resemblance of 2 assemblages indicates similar environments, but not necessarily contemporaneity. More accurate are correlations based on a good sequence of index fossils which are found to appear always in the same stratigraphic order, with the fossils chosen so as to be related to one another in an evolutionary sequence. The most successfully used groups are characteristically planktonic. To useful stratigraphically, new forms should succeed each other in quick succession; each new form should be dispersed widely and rapidly; it should be numerous; and it should be relatively indifferent to facies changes. In addition, a good index fossil should be easily recognizable and should not demand elaborate preparation techniques for study. Correlations of Tertiary rocks have been based generally on pelecypods, gastropods, and large Foraminifera; these benthonic organisms are a partial explanation of the difficulties in trying to establish intraregional correlations of the Tertiary strata. Work on the planktonic globigerine Foraminifera promises to produce a framework of long-distance correlation for the Tertiary. In North America, the Gulf Coast will provide a standard of reference for the Tertiary of the Western Hemisphere which must ultimately be correlated with the European type localities.

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