Abstract Hyaloclastite and pillow lavas were dredged a short distance northwest of St. Paul’s Rocks. These rocks are more alkaline than the basalts erupted from spreading ridges. In addition to olivine and plagioclase phenocrysts, the rocks contain complex sector-zoned pyroxene phe-nocrysts. Fe 2 O 3 /FeO of about 0.20 was cal-culated for the glass from FeO-MgO partitioning between olivine and liquid. Clay minerals, mainly potassian smectite, are,abundant in both rock types. These are divided into three groups on the basis of their composition, optical properties, and mode of occurrence. All these groups have structural formulas that are intermediate between dioctahedral and trioctahedral smectite. Potassium, sodium, and calcium, in order of abundance, are the dorninant interlayer cations. These smectites occur with small amounts of analcite and even less abundant phillipsite. The abundance and composition of these clays suggest 4 that they are important traps lor potassium and magnesium from sea waiter.