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.
The foregoing inquiries concerning the origin of volcanoes show that at the present time they are clearly connected with some process which goes on beneath the sea. An extension of the inquiry indicates that this relation has existed in earlier geological times; for, although the living volcanoes are limited to places within three hundred miles of the sea, we find lava flows, ashes, and other volcanic accumulations far in the interior of the continents, though the energy which brought them forth to the earth's surface has ceased to operate in those parts of the land. In these cases of continental volcanoes it generally, if not always, appears that the cessation of the activity attended the removal of the shore line of the ocean or the disappearance of great inland seas. Thus the volcanoes of the Yellowstone district may have owed their activity to the immense deposits of sediment which were formed in the vast fresh-water lakes which during the later Cretaceous and early Tertiary times stretched along the eastern face of the Rocky Mountains, forming a Mediterranean Sea in North America comparable to that which borders southern Europe. It thus appears that the arrangement of volcanoes with reference to sea basins has held for a considerable period in the past. Still further, when we look backward through the successive formations of the earth's crust we find here and there evidences in old lava flows, in volcanic ashes, and sometimes in the ruins of ancient cones which have been buried in the strata, that igneous activity such as is now displayed in our volcanoes has been, since the earliest days of which we have any record, a characteristic feature of the earth. There is no reason to suppose that this action has in the past been any greater or any less than in modern days. All these facts point to the conclusion that volcanic action is due to the escape of rock water which has been heated to high temperatures, and which drives along with it as it journeys toward a crevice the rock in which it has been confined.