Harbors, however, though their local quality has local value, are of primary importance to the country as a whole and may be considered today. They are naturally formed in many ways, but only the principal types need be mentioned. Wherever a river reaches the sea the continuous contour of the coast is broken, and there would be a natural harbor but for the opposition of the waves. The outflowing river endeavors to scour a channel through which ships may enter. The waves, buffeting the coast and drifting sand and gravel to and fro, endeavor to clog the riverway with submerged bars, making the water too shoal for shipping. Over small rivers the waves are victorious, and unless engineers cooperated with the rivers the entrance-ways are sealed. Large rivers overpower the waves and clear their channels faster than the waves can clog them. Only one of our rivers, the Mississippi, has proved competent to maintain its channel to the sea, but that affords a harbor of peculiar value, in that it is connected with a system of inland navigation hundreds of miles in extent. The fiord harbors associated with prehistoric ice-fields are an important group. The ice descended to the shores of both oceans, and by its remodeling of the surface left steep slopes with a tortuous contour, creating a great abundance of deep harbors. New England at the east and Washington at the west are thus endowed, and their maritime commerce requires neither piers nor dredges to maintain its natural channels. Natural harbors of a third class are connected with vertical movements of the land. When the margin of the continent is lifted the coast line, following a slope new-risen from the sea, is a simple contour on an even plain, and there are no harbors ; but when the land is deprest the sea-water enters each valley of the coastal plain, naking a bay. Then the waves, driving sand and other land waste along the coast, build a spit across the mouth of each bay, converting it into a sheltered harbor, whose entrance is scoured four times a day by the incoming and outgoing tide. Into the estuaries thus formed the streams build deltas, gradually filling and obliterating them ; but so long as subsidence continues they remain open and available for commerce. It is our good fortune that nearly the whole of our coast, both Atlantic and Pacific, is now subsiding,* so that estuaries are numerous and the maintenance of serviceable harbors requires only moderate aid from the engineer. The bays and sounds of San Francisco, Galveston, Mobile, Tampa, Savannah, Charleston, Wilmington, Pamlico, Chesapeake, and Delaware are of this type; and the Hudson estuary, which is also a fiord, carries tidewater one hundred and fifty miles from the coast.