Beneath the seas, except in their very greatest depths, and perhaps even there, the process of forming strata is continually going on. Next the shores, sometimes for a hundred or two miles away to seaward, the principal contribution may be the sediment worn from the lands by the waves and the rivers. Farther away it is to a large extent made up of the remains of animals and plants, which when dying give their skeletons to form the strata. Much of the materials laid down--perhaps in all more than half--consist of volcanic dust, ashes, and pumice, which drifts very long times before it finds its way to the bottom. We have as yet no data of a precise kind for determining the average rate of accumulation of sediments upon the sea floor, but from what is known of the wearing of the lands, and the amount of volcanic waste which finds its way to the seas, it is probably not less than about a foot in ten thousand years; it is most likely, indeed, much to exceed this amount. From data afforded by the eruptions in Java and in other fields where the quantity of volcanic dust contributed to the seas can be estimated, the writer is disposed to believe that the average rate of sedimentation on the sea floors is twice as great as the estimate above given. Accumulating at the average rate of one foot in ten thousand years, it would require a million years to produce a hundred feet of sediments; a hundred million to form ten thousand feet, and five hundred million to create the thickness of about ten miles of bed. At the rate of two feet in ten thousand years, the thickness accumulated would be about twenty miles. When we come to consider the duration of the earth's geologic history, we shall find reasons for believing that the formation of sediment may have continued for as much as five hundred million years.