Percent. More than 6000 feet above sea-level 2.3 Between sea-level and 6000 feet above 25.5 Between sea-level and 6000 feet below 14.8 Between 6000 and 12,000 feet below sea-level 14.8 Between 12,000 and 18,000 feet below sea-level 39.4 Between 18,000 feet and 24,000 feet 3.1 =Epicontinental seas.=—Those shallow portions of the sea which lie upon the continental shelf, and those portions which extend into the interior of the continent with like shallow depths, such as the Baltic Sea and the Hudson Bay, may be called epicontinental seas, for they really lie _upon_ the continent, or at least upon the continental platform; while those other detached bodies of water which occupy deep depressions in the surface are to be regarded as true abysmal seas, as, for example, the Mediterranean and Caribbean seas and the Gulf of Mexico, whose bottoms are as profound as many parts of the true ocean basin itself. =Diversities of surface.=—The bottoms of the oceanic basins are diversified by broad undulations which range through many thousands of feet, but they are not carved into the diversified forms that give variety to land surfaces. The ocean bottoms are also diversified by volcanic peaks, many of which rise to the surface and constitute isolated islands. Some of them have notable platforms at or near the surface, cut by the waves or built up by the accumulation of sediment and of coralline and other growths about them. Aside from these encircling platforms, the solid surface usually shelves rapidly down to abysmal depths, so that the islands constitute peaks whose heights and slopes would seem extraordinary if the ocean were removed. The surface of the land is diversified in a similar way by broad undulations and volcanic peaks, and also by narrower wrinklings and foldings of the crust; but all of these irregularities have been carved into diversified and picturesque forms by subaërial erosion. In this respect the surface of the land differs radically from the bed of the sea. The agencies which have produced the continental platforms and abysmal basins, and the great undulations and foldings, as well as the volcanic extrusions that mark them, are yet subjects of debate. Here lie some of the most difficult problems of geology, but these cannot be stated with sufficient brevity to find a place here.