Tertiary sedimentary rocks underlie approximately 55 per cent of the land area of the Philippines. The sedimentary formations that crop out along the coasts in many areas have been observed to pass laterally or downward in dip directions, beneath wave-cut and/or downfaulted shelves adjoining individual island, and it may be assumed that correlative strata pass into the deeps between the islands. In addition to the common interruption on land by volcanic and intrusive rocks, alluvial overlaps, upfaulting of older rocks, and removal by erosion, a great part of the geologic record is obscured by water. The geologic studies have been limited to the sedimentary cover with special emphasis on the major sedimentary basins and little will ever be known of the structurally low areas beneath the seas.