The primary components of biogenic silica in deep-sea sediments are the tests of diatoms and radiolarians. Their distribution and concentration in open ocean deep-sea sediments are strongly influenced by the productivity of the overlying water masses. In general, in deep-sea sediments far from land where masking by continental detritus is not an important factor, sediments rich in opaline silica underlie regions of strong

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vertical mixing and consequent high productivity. Therefore, the presence and fluctuation of the concentration of opaline silica in deep-sea sediments can be an important indicator of paleo-oceanographic conditions. Upward changes from sediments low in opaline silica to sediments high in opaline silica about 2 m. y. ago in the Antarctic region and about 4.5 m. y. ago in the North Pacific suggest an increase of vertical mixing at these times in these areas.

The widespread occurrence of middle Eocene cherts in the Atlantic indicates a strikingly different circulation pattern for Eocene time than exists today. A modified Eocene circulation pattern is suggested on the basis of the probable shape of the Eocene Atlantic.

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