Many areas of both California and the Great Basin contain diatom-bearing sediments which range in age from Late Cretaceous to Recent. The diatom assemblages in these rocks contain both short-ranging species that are useful for stratigraphic correlation and others, still represented in living assemblages elsewhere, that are useful for paleoecologic interpretations. Although thousands of square miles of diatom-bearing sediments have been mapped by petroleum geologists and have been penetrated and cored during drilling operations, the diatoms have been neglected as a stratigraphic and paleoecological tool. Some of the reasons most often presented for this lack of attention are claims that Foraminifera are easier to work with, are better known, and that laboratory manipulatio of them is easier and more suited to assembly-line methods. This is true in part only, as assembly-line methods have been developed for handling large numbers of samples of diatomaceous sediments. Furthermore, diatoms are commonly found in sediments that are completely barren of Foraminifera or other fossils.

The present interest in palynology by the oil companies indicates that new techniques are no longer viewed with disfavor. As far as the necessary laboratory preparation and study are concerned, diatom samples can be prepared and significant species identified at least as expeditiously as samples containing pollen and spores.

Distinctive diatom assemblages are known from the Moreno Shale of Late Cretaceous and Paleocene (?) age and from many sedimentary formations in Eocene, Oligocene, Miocene, Pliocene, and Pleistocene rocks in California from the San Francisco Bay area southward. These assemblages from rocks of Cretaceous through Miocene age are virtually all marine. Pliocene rocks in different localities contain either marine or non-marine

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diatom assemblages. Pleistocene assemblages are dominantly non-marine.

Extensive areas of Miocene, Pliocene, and Pleistocene sediments in Nevada also contain distinctive non-marine diatom assemblages. In that region, diatoms, more commonly than otherwise, are the only fossils present. Here also the diatoms can provide much needed paleoecological information, as the Cenozoic lake basins varied greatly in depth, temperature, salinity, pH, and other factors of paleoecological importance.

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