In an outline sketch of the present coastal topography of the continent we can generalize, and say that the whole continent during the late Tertiary, glacial, and recent times has swayed up and down about a hinge-line situated in the region of the Gulf of Mexico, and the movements, although not uniform, have increased in amount from the south northward. Let us glance at the evidence on which this broad statement, involving the up and down surging of a vast continent, is based. The Hudson, as stated above, is a drowned river as far as Troy, a distance from the present land margin of 160 miles. In the next great river to the northward, the St. Lawrence, the tide rises and falls nearly up to Montreal, a distance of about 800 miles from the general shore-line. Still farther north are Hudson Strait and Hudson Bay, which, although but imperfectly explored, seem to be an example not only of the drowning of a river-valley, but of the largest part of a river-basin. The geography of the arctic archipelago fringing the north shore of the continent also suggests that a strongly stream-cut plateau has there been deeply submerged. In addition to the drowned river-valleys and ragged coasts which record a subsidence of the land, there are raised terraces and beaches which begin at the south near New York and increase in elevation above the present sea-level, when followed northward, all the way to the arctic region, and have in the far north an altitude of about 1,200 feet. These old beaches and terraces show that the land was formerly depressed and has since risen; but, as shown above, has not regained the elevation it had previous to the glacial epoch. The marked differences in the geography of the coast from New York northward to the Arctic Ocean, and from the same locality southward to Central America, are due primarily to the fact that the oscillations of the land have been such that at the north the continental shelf is entirely submerged and the sea has encroached on a rough land; while at the south the recent oscillations have been less and a broad margin of the continental shelf is exposed and forms the coastal plain. At the north, we find innumerable islands, bold, rocky shores with many capes and headlands, separated by deep inlets, sounds, straits, bays, etc., or, in brief, a ragged coast such as finds typical illustration on the shores of Maine (Fig. 10), while at the south (Fig. 6) the shores are low, sandy, remarkably uniform in trend, and without islands, excepting such as are built by the waves and currents. The West India Islands will, no doubt, be recalled by the reader, but their history is again different. Intermediate between the land that has experienced great oscillation at the north and the region of less energetic movements at the south is the series of large estuaries mentioned above, in the narrower portions of the coastal plain. [Illustration: FIG. 10.--A portion of the coast of Maine.]