continental shelf, similar to the submerged valleys of the Hudson, have been discovered by soundings, some of

which are thought to be true stream-cut valleys; others, however, start from the coast where there are no rivers entering, and may be due to other and as yet unknown causes.

An exceptional feature in the geography of the Pacific coast to the south of the United States-Canadian boundary is furnished by the islands off the southern portion of California. These islands, of which Santa Rosa, Santa Cruz, Santa Catalina, and San Clemente are the most important, rise from water that is 300 or more fathoms deep to a height of from a few hundred to about 2,000 feet above the present sea-level. These islands are the summit portions of mountains similar to those which give a characteristic relief to southern California. One feature concerning the islands referred to which is of interest in connection with the study of the continental shelf is the presence on them of numerous terraces. These occur on a series of level, step-like areas, which sweep about the slopes of the islands, in a general way parallel to the present shore-line, and are records of an upward movement of the land. The highest well-defined terrace on San Clemente occurs at an elevation of 1,320 feet, but there are less distinct beach-lines up to 1,500 feet above the present sea-level. The evidence of movements of the land along the Pacific border of the continent might be multiplied, but enough has been stated to show that the western border of North America, like its eastern portion, is subject to fluctuation in reference to the level of the sea. The line which marks the passage of the solid earth beneath the waters of the sea is ever changing, owing to movements in the earth's crust. It is ever changing, also, owing to the action of waves and currents on the ocean's shores and other causes.

Life on the Continental Shelf.—No attempt need be made here to give an accurate description of the plants and animals which find a congenial home on the continental shelf. One needs, however, to have in mind a general idea of the wonderful abundance and variety of organic forms in the shallow waters adjacent to the continent to fully appreciate the changes in the relief of the ocean-floor, in which they

play a leading part, and also the vast economic importance of these marine harvest-fields and pastures, as they may be termed.

The continental shelf fringing North America reaches from within 500 miles of the equator to probably a less distance of the north pole. Its length following its broader curves is in the neighbourhood of 25,000 miles, and its average width is not far from 50 miles; its area is therefore something like 1,000,000 square miles. On account of the vast extent of this submarine plain and its great range in latitude, the conditions influencing the lives of the plants and animals inhabiting its surface or living in the waters covering it vary from place to place between wide extremes. The waters resting on it have a mean annual surface temperature of from 70° to 80° F. at the south, and about 32° F. at the north. In places great rivers and the turbid waters from glaciers bring in sediments and form muddy deposits; at other localities the currents, as in the path of the Gulf Stream off the Carolina coast, sweep the bottom clear of all light débris; and again bare rocks of limited extent are exposed. The depth of the water resting on the shelf varies through all gradations down to 100 fathoms. At the south the hours of light and darkness are approximately equal each day, but at the north there are six months of sunlight and six months of darkness each year. Still other variations, as of strength of currents, salinity, etc., exert an influence in this realm and lead to great diversity in its living organisms. Throughout its entire extent, however, the continental shelf abounds in both plant and animal life.

The plant life of the sea, as has been shown in recent years by the use of the dredge and net, is most abundant at the surface and is practically absent at the bottom where the water is over 100 fathoms deep. The animals of the sea, like those of the land, are dependent primarily on plants for their food. By far the most abundant supply of food plants in the sea is furnished by minute algæ, which float free in its water. Below a depth of about 100 fathoms algæ are absent because of lack of light, and all the deep-sea animals are believed to be carnivorous. For these reasons the comparatively

shallow waters adjacent to the land and mainly covering the continental shelf are the most favourably circumstanced of any portion of the sea for the support of a teeming fauna.

On the continental shelf of North America, especially to the south of Cape Cod on the Atlantic and south of the Aleutian Islands on the Pacific coast, there is a warm temperature, light penetrates to the bottom except in the unfavourable and fortunately restricted areas of muddy water, and motion of the waters produced by currents and the pulsations of waves is present. These several favourable conditions permit of an exuberance of life such as is unknown to persons who confine their attention to the study of land areas.

We may safely say, in the words of Alexander Agassiz, that the abundance of life in the many favoured localities of the ocean far surpasses that of the richest terrestrial faunal districts. The most thickly populated tropical jungle does not compare in wealth of animal or vegetable life with certain portions of the continental shelf on the western border of the Gulf Stream. In this connection we may also cite Humboldt, who before the marvellous revelations in reference to the life of the sea made by recent dredging expeditions wrote: "Upon surfaces less varied than we find on continents, the sea contains in its bosom an exuberance of life of which no other portion of the globe could give us an idea."

The distribution of life in the sea is analogous to the distribution of life on land, but in a reverse direction with reference to sea-level. A traveller passing from the tropical plains of eastern Mexico and ascending Orizaba, for example, crosses successive belts of vegetation, each with its indigenous animals, but merging one with another so as to make a gradation in the luxuriance of the flora and the abundance of animal life from the wonderfully rich plains adjacent to the Gulf coast to the snow-capped mountain top. In the sea, the tropical plains with their tangled vegetation and plentiful animal life are represented by the still more uniform plain forming the submerged continental

shelf with its strange forests of flowerless plants, the seaweeds. These submarine jungles shelter hosts of animal species, many of which swarm in countless myriads. This life embraces all grades of invertebrates, such as the microscopic protozoa, sponges, radiate animals like the coral-polyps, starfishes, sea-urchins, etc., and crustaceans in vast variety, and, among vertebrates, includes fishes, reptiles, and mammals. Even birds might be included in this category, since many of them are more at home on the sea than on the land.

The struggle for food among this multitude is intense. As with many animals on the land, adaptive coloration is here a means of escape from enemies, and many of the animals assume the brilliant hues of the surrounding vegetation. The water is less transparent than air, and in the deep sea it is always night. Counteracting to some extent this diminution or absence of sunlight, many marine animals are luminous and shine with phosphorescent light of many different tints. This property is shared also by the animals of the sunny, shallow sea as well as by those always living in the cold midnight of the great deep and in the polar