A gently sloping shelf-like border surrounds the deep central basin of the Gulf of Mexico (Fig. 3). To the west and north of Yucatan and west of Florida the shelf is from 140 to 160 miles broad, with a surface slope towards the centre of the Gulf of less than 6 feet to a mile--a slope so gentle that were the surface of the shelf exposed to view, no eye could distinguish it from a perfect plain. The deepest sounding yet obtained in the central part of the Gulf, approximately midway between Yucatan and Florida, shows a depth of 2,119 fathoms. The remarkable fact is that the slope from the 100-fathom line to the bottom of the central basin of the Gulf is precipitous. In two places on the border of the Yucatan bank a descent of about 8,500 feet occurs within a horizontal distance of 15 or 20 miles. On the east side of the southern extremity of Florida, and again on the eastern shore of Yucatan, the continental shelf is only about 5 miles broad; these are the nearest approaches of the present land to the actual border of the continent to be found on the Atlantic coast. The explanation of these exceptional conditions is that both Florida and Yucatan are portions of the continental shelf which have been raised so as to form low emerged plains. From Cape Hatteras northward to the extremity of the Newfoundland Banks the shelf increases gradually in breadth from about 15 miles in the region of the Carolinas to over 100 miles off the coast of Maine. The outer border of the shelf is an irregular curving line. Opposite the coast of Massachusetts and Maine an extension of the Atlantic basin reaches within 15 or 20 miles of the present margin of the land. The manner in which the low plain fringing the eastern border of the United States passes beneath the waters of the Atlantic and becomes a continental shelf is illustrated by Fig. 2. [Illustration: FIG. 2.--Ideal profile through a continental shelf.] Southeast of Newfoundland the continental shelf has an irregular surface, marked by shoals and depressions, and furnishes the most valuable fishing-banks in the world. The 100-fathom curve is there over 500 miles from the coast. This is the broadest portion of the continental shelf now known on the Atlantic border of the continent. Northward of Newfoundland the Atlantic basin extends far into Davis Strait and Baffin Bay, and then its border swings outward about Greenland, but its true margin is there but imperfectly known. To the north of the arctic coast of North America, as is suggested in part by the soundings made by Nansen, the submerged margin of the continent is probably broad and presents a steep escarpment to the arctic basin, but the outline of the true continent, as in the case of the present land extension in that direction, is unknown.