A series of scuba dives were made during the summer of 1971 to examine the development and morphology of pillow lavas. A prehistoric and a fourteenth-century lava flow were sampled, and the subaerial to submarine transition observed and photographed. The structure of the pillows formed is believed to be dependent on rate of flow and general topography. The lack of hyaloclastite is discussed and related to the pillow formation. The development of palagonite rims to the pillows by the contact of sea water on sideromelane is described. Comparative analyses obtained by electron microprobe are presented. These results indicate a leaching of Si, Ca, Na and K in the altered portions of the pillows. The possible role of this chemical exchange to the bulk chemical composition of the oceans is discussed.