The sea is the great theatre of sedimentary accumulation, and rocks of marine origin form the larger part of the present land surfaces. Important as other classes of deposits may be, they are less so than those laid down in the ocean and the waters immediately connected with it. There is great variety in the sedimentary deposits made in the sea, which change in accordance with climate, the depth of water, the nature of the coast rocks, the force of winds and tides, and the nearness or remoteness of the mouths of rivers and, in a very important degree, with the elevation and relief of the adjoining land masses; very different materials are supplied by bold and rocky shores from those derived from flat, sandy coasts. Large land-locked seas, like the Gulf of Mexico and the Mediterranean, again, have deposits more or less different from those of the open ocean, a difference which is largely due to the absence or insignificance of the tide, and the reduced force of the waves.

It is important to remember that the actual line of meeting of sea and land is not the structural margin of the continent, for the water may cover a broad submerged shelf of the latter. For 1oo miles east from the coast of New Jersey the water deepens very gradually to the Ioo-fathom line, whence it shelves very steeply to the profound oceanic abyss. Shoal water, less than Ioo fathoms deep, surrounds all coasts, sometimes as a narrow belt, again as a very broad zone. The transition from shoal water to the deep sea is by steep slopes, with well-defined upper margin or edge; as a rule, these steep slopes of the continental mass begin at the Ioo-fathom line, but there are frequent departures from this rule. For example, in the Gulf of Guinea the steep descent begins at the 40-fathom line, while off the west coast of Ireland the descent is gradual almost to a depth of 200 fathoms and then becomes steep. The bed of the Atlantic off the Carolinas displays no well-marked edge of the continental shelf.

The ocean thus fills its own basin and overflows the margins of the continental platforms to a greater or less extent; this submerged shelf constitutes the shallow sea (Fig. 124).

Basin of the Gulf of Mexico, showing the submerged margin of the continental platform.

Fig. 124. - Basin of the Gulf of Mexico, showing the submerged margin of the continental platform and the steep descent of the botton. at the 100-fathom line. Vertical scale much exaggerated. (From a model by the U. S. Coast Survey).

Marine deposits may be classified primarily in accordance with the depth of water in which they were laid down, one of the most valuable guides to the history of ancient rocks, and secondarily in accordance with the nature of the material of which they are composed, and the processes by which they were accumulated. The following table gives, in a somewhat modified form, the classification of Murray and Renard, founded upon the great collections of modern marine deposits made by the "Challenger" expedition.

Marine Deposits

1. Littoral Deposits, between high and low water marks.

Sands, Gravels, Muds, etc.

I. Terrigenous Deposits, material derived from the land in suspension (except the calcareous masses).

2. Shoal-water Deposits, between low-water mark and 100 fathoms.

Sands, Gravels, Muds, Calcareous Accumulations, etc.

3. Aktian Deposits, laid down on the conti-nental slope.

Coral Mud. Volcanic Mud. Green Mud and Sand. Blue Mud. Red Mud. Calcareous Deposits.

4. Abysmal Deposits, laid down on the ocean floor.

Foraminiferal Ooze. Pteropod Ooze. Diatom Ooze. Radiolarian Ooze. Oceanic Red Clay.

II. Pelagic Deposits, formed in deep water far removed from land.

The material brought into the sea by rivers, or washed from the shore by waves, is partly mechanically suspended and partly in a state of solution; the former is deposited when the water is no longer able to transport it, while some of the latter is extracted by animals and plants, and some remains permanently dissolved. The sorting power of the water arranges the mechanically borne sediments according to the coarseness and fineness of their constituent particles, at the same time separating them according to their mineralogical composition, a separation which is usually imperfect, but sometimes very complete. Marine deposits are thus typically stratified, though when deposition continues long and uninterruptedly, thick masses, not obviously divided into layers, may be accumulated, but this is exceptional in those parts of the sea where deposition is most rapid.