The shoal-water deposits of cold and temperate seas are much alike, both having a preponderance of sand and gravel, with occasional limestones which are more frequent in lower latitudes, while in the deeper waters of the continental slopes, blue mud is laid down. On the floor of the oceanic abysses, however, is a difference in that the terrigenous material is more widely distributed, while foraminiferal ooze and oceanic red clay are absent from the polar seas, and in the Antarctic Ocean is a complete zone of diatom ooze, encircling the earth. The tropical seas are characterized by red muds, and by great development of limestones, especially of the coral reefs, which are almost entirely confined to the tropical waters. The deposits of the truly deep sea are essentially the same in the tropical and temperate zones, and are determined almost entirely by the depth of water; foraminiferal and radiolarian oozes and red clay cover almost the whole floor of the oceanic basins in these regions. As conditions are more uniform in the sea than on the land, climatic differences are less clearlv marked in marine than in continental deposits.