Abstract
CORRELATIONS of marine strata using the convenient numerical zones of the Neogene1,2, based on planktonic foraminiferans, represent an important advance in refining and developing greater precision in biostratigraphic correlations. This zona-tion is most effective in tropical and subtropical regions where the nominate planktonic taxa are best developed; progressive difficulty is encountered in applying it away from equatorial areas as these taxa disappear and are replaced by temperate and then by cold-water populations in polar areas. The experimental Mohole (Fig. 1), drilled near Guadalupe Island off Baja California, is significant as far as tropical-temperature planktonic foraminiferal populations are concerned because it is located in a subtropical area in which there is a commingling of tropical and temperate planktonic species3.
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