In a generalized view of the recent history of the coast-line of North America the dominant fact is that to the north of the latitude of the north shore of the Gulf of Mexico the resultant of the later movements of the continent is downward; the amount of the depression thus caused increases in a general way with increase in latitude on both the Atlantic and Pacific coasts. This downward movement has permitted the sea to encroach on the land and to flood many pre-existing valleys. On the Atlantic coast it produced such estuaries as Chesapeake and Delaware Bays, the tide-water portions of such rivers as the Hudson and the St. Lawrence, and farther north, where the submergence was greater, permitted the sea to invade the continental basin and form Hudson Bay. To this same wide-reaching cause is due also the bold ragged coast-line of the Atlantic from New England northward. On the Pacific border the downward movement is recorded by the tide-water portions of the Sacramento, Columbia, etc., and the deep picturesque fiords of the Canadian and Alaskan coasts. The most decided influence of these changes in the geography of the continent's margin on the affairs of men resulted from the production of numerous fine harbours and the extension of estuaries far inland, thus favouring commerce and fisheries in a high degree. As a result of the oscillations just referred to, the sea has gained important characteristics as well as the land. The broad submerged shelf fringing the continent furnishes conditions highly favourable to both plant and animal life, and affords some of the most valuable fishing-banks of the world. The Caribbean region is a marked exception to the broader changes that have affected the coast-line throughout the central and northern portions of the continent, and stands by itself as a conspicuous illustration of more localized earth movements which produced a remarkable submarine topography. LITERATURE In continuation of the studies outlined in this chapter the following books, most of which contain more special references, will be found of assistance: AGASSIZ, A. _Three Cruises of the Blake._ 2 vols. Houghton, Mifflin & Co., Boston, 1888. GILBERT, G. K. _The Topographic Features of Lake Shores._ In United States Geological Survey, Fifth Annual Report, 1883-'84, pp. 69-123. GULLIVER, F. P. _Shoreline Topography._ In Proceedings of the American Academy of Arts and Sciences, vol. xxxiv, 1899, pp. 151-258. Contains a valuable bibliography. HILL, R. T. _The Geological History of the Isthmus of Panama and Portions of Costa Rica._ Bulletin of the Museum of Comparative Zoology at Harvard College, vol. xxviii, 1898, pp. 151-285. HILL, R. T. _Cuba and Porto Rico with the Other Islands of the West Indies._ The Century Co., New York, 1899.