src="@public@vhost@g@gutenberg@html@files@36024@36024-h@images@i012.jpg" alt="Terrain profile at edge of continent." title="" tag="{http://www.w3.org/1999/xhtml}img"/> 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.

Soundings to the north of Cape Lisburne, on the northwest coast of Alaska, show that the 100-fathom curve is there over 200 miles from land. The exceptionally shallow sea covering this portion of the shelf continues westward to the coast of Asia, and southward through Bering Strait, so as to embrace the eastern portion of Bering Sea. The continental mass of North America is thus directly connected with the continental mass of Asia. A rise of the bottom of less than 200 feet in Bering Strait would bring about a land connection between the Old and the New World. This, as will appear later, is a most significant fact to students of geography and geology.

On the Pacific coast of North America the continental shelf is throughout much narrower than its average breadth on the Atlantic side of the continent, and is also more deeply submerged on its seaward border. The broad platform beneath the northern and eastern portions of Bering Sea—from which rise the low islands, St. Lawrence, St. Matthew, Nunivak, and the Pribilof group, now separated by water from 25 to 35 fathoms deep—extends to the south of the more easterly of the Aleutian Islands, and is prolonged eastward along the south border of Alaska, where the 100-fathom curve is from 10 to 20 miles from the coast-line, and approaches still nearer the land in the neighbourhood of the islands of southeastern Alaska and British Columbia. The shelf is narrow but well defined along the coasts of Washington and Oregon. Adjacent to California, Mexico, and Central America, its outer margin is barely 10 miles from land. Throughout the entire distance from the Aleutian Islands to Panama the outer border of the shelf is in general well defined, and its seaward escarpment descends abruptly to the floor of the vast Pacific

basin, where the sounding-line shows depths of from 2,000 to 3,000 fathoms.

Could the waters of the sea be removed and North America viewed from a distance, in the manner we are enabled to examine the surface features of the moon through a powerful telescope, an observer would behold a great plateau, having the present well-known triangular shape of the continent, rising boldly between the Atlantic and the Pacific basins. The surface of the plateau would be rough, in comparison with the generally smooth contours of the adjacent troughs, but even the highest mountains would be less in elevation above its general surface than the crests of its bordering escarpments above the adjacent depressions. The mountain-peaks when illuminated by the sun would appear as points of light with long, tapering morning and evening shadows, and the east and west plateau-borders would be strongly drawn bands of light or shadow, according to the time of day, 6,000 or 8,000 miles in length. The Bermuda, Hawaiian, and other islands now rising above the surface of the deep sea would stand on its desiccated floor as isolated, gigantic mountains—"Bermuda mountain" with an elevation of 15,000 feet, and the Hawaiian group of peaks with a culminating point of light 30,000 feet above the surrounding plain. The bordering slopes of the "North American plateau" and its slightly bevelled margin forming the present continental shelf would be lacking in details, and appear as a vast, smooth, curving belt of light or shadow, in striking contrast to the roughened surface now above water.

The North American continent is not exceptional in being partially submerged at the present time. Similar conditions occur about the margins of other continents which, as is well known, are fringed with broad submarine terraces built in part of their own débris. In fact, every large land mass on the earth under existing climatic conditions and present distribution of life, if it remained moderately stable for a sufficient length of time, would have a submarine shelf built about its borders.

Of what is the Continental Shelf Composed?—The rocks forming the present land surface of North America extend seaward from the existing shores and constitute the basal portions of the continental shelf, thus suggesting that the submerged platform is due, in part at least, to shore erosion—the waves having eaten into the land so as to make a terrace. That this is not the true explanation, however, may be shown in several ways.

The superficial covering which gives the continental shelf its smooth contours is composed largely of sediments such as rivers bring from the land. This material is coarsest and in greatest abundance near shore and decreases both in the size of the particles composing it and in abundance towards the seaward borders of the shelf. The wash from the land is mostly deposited within a few miles of the coast-line and, as has been shown by dredging, is seldom carried, even under the most favourable conditions, more than about 100 miles seaward. Supplementing the fragmental material derived from the land, and increasing in thickness towards the seaward margin of the continental shelf—coincident with the increase in depth of the water—is a deposit of light-coloured calcareous mud or ooze, formed of the hard parts of animals and plants which live in the waters of the sea. The organisms which supply this material are in the main microscopic and live especially in the warmer seas in countless myriads. Their dead shells or cases fall to the sea-floor in a constant shower, much as the snow falls from the air, but continuously year after year and century after century. This descent of the hard parts of organisms, both calcareous and siliceous, from the waters of the sea has led to the accumulation of a sheet of slimy sediment over almost the entire sea-bottom. How thick this layer is we have no means of knowing, but it is probably many hundreds of feet.

The organic débris falling on the continental shelf descends through only a few hundred feet of water and is but little affected by its solvent action. The great number of organisms, such as the Foraminifera which secrete calcareous tests or "shells" causes the slime on the continental

shelves to be calcareous and in the condition to form limestone if cemented or subjected to sufficient pressure. In the deep sea, where the hard parts of dead organisms fall through many thousands of feet of water, their more soluble portions are removed and the bottom is covered throughout vast areas with a pinkish clay composed of the more insoluble residue of the calcareous shells and the cases of silica-secreting animals and plants.

The continental shelves are, in general, within the influences of ocean currents, and fine débris, as we seem justified in concluding, is removed from their surfaces, carried beyond