The Ocean Floor

THE OCEAN FLOOR. The ocean basins are vast depressions, whose surface rises and falls in gentle undulations. Throughout most of its extent the bottom lies at depths exceeding 2000 fathoms, and there are many depressions or 'deeps' lying below 3000 fathoms. The greatest depth yet re ported is 5269 fathoms, or 31,614 feet, in the Pacific near the island of Guam. Soundings of 5155 fathoms and 5147 fathoms have been ob tained in the same ocean, while the greatest known depth of the Atlantic is 4561 fathoms off the coast of Porto Rico. In the deeper or pelagic regions the floor consists of soft oozes, formed from the calcareous shells of minute animals living near the ocean surface, and from volcanic dust. The most widespread deposit is globigerina ooze, an accumulation of fossil casts of forami nifera. When the depths exceed 2500 fathoms, however, the calcareous shells are dissolved by the water and there remain only the siliceous remnants and volcanic materials which accumu late very slowly over the floor as red clay. On

the borders of the continents, the ocean some times overlaps the land in a broad belt of shal low water whose floor is commonly called the continental shelf; here the depths do not often exceed 100 fathoms for considerable dLstances off shore, and the bottom consists of sand and clays that have been derived from the adjacent land surface by the erosive and transporting ac tion of rivers. The littoral islands are mostly located an such platforms, which are prominent on the eastern coast of America, the western coast of Europe, and the southwestern coast of Asia. The deeper ocean is almost free from large islands, although by volcanic activity many small islands have been built up from great depths, as ill the South Paeific, See DEEP-SEA EXPLORATION.