It is obvious that there are mountains in the sea of as great height above the lowest valleys as the mountains of continents are above the level of the sea. If a new continent should hereafter be formed by the elevation of a large area of the bed of the sea, the existing mountains, now appearing in the form of islands, would partake of the general movement, and the new continent would have the same general diversities of surface as existing continents. The mountains would have existed long before the continent. It is therefore to be supposed that the mountains of the present continents were elevated before the continents, and that they stood for long periods as islands, exposed to the action of waves, tides, and marine currents. 2. _The Elevation of Continents._--Continents have been elevated by so slow a movement that it has not generally been perceived, even when they have been peopled by nations advanced in civilization. And yet satisfactory evidence is always left of former sea-levels. Almost every seaboard furnishes examples of beaches, evidently once washed by the sea, but now elevated more or less above high water. At Lubec, near the northern extremity of the coast of Maine, barnacles[B] are found attached to the rocks eighteen feet above high water. The pilots at that place, and for a hundred miles north and south of it, speak of the ship-channels as diminishing in depth, though it is certain that they are not filling up. Such facts are to be explained only by supposing that the coast is rising. [B] The barnacle is a marine animal, permanently fixed to the rocks, and live but a short time without being surrounded by sea-water. Lakes are numerous throughout the northern portions of North America, which are receiving annually large quantities of sediment, and must ultimately become alluvial plains. Those of moderate depth, as Lake Erie, cannot require periods very protracted to fill them. Their continuance in such abundance indicates that the elevation of the continent to its present height is comparatively recent. This conclusion is confirmed by evidence of another kind. Throughout this region of lakes, beds of clay containing the remains of existing species of marine animals, are found at all elevations from the sea-coast, to the height of about four hundred feet, but not higher. These clay beds are very recent, and were deposited when the surface was four hundred or five hundred feet lower than it now is; and this amount of elevation has left the existing lakes scattered over the surface.[C]