Many substances are known to be in solution in sea water, but in spite of this the composition is remarkably uniform. The most abundant substance by far in solution is common salt. In every 100 pounds of sea water, there are 3.5 pounds of mineral matter of various kinds dissolved. Nearly 78 per cent of the dissolved matter is common salt. The principal other constituents in solution are chloride and sulphate of magnesia, and the sulphates of lime and potash. All other dissolved mineral substances together make up less than one per cent of the total. It has been estimated that if all the dissolved mineral matter should be brought together, it would form a layer 175 feet thick over the whole sea bottom. The salts of the sea have been mostly supplied by the rivers, which in turn have derived them from the disintegration and chemical decay of the rocks.
If we make a general comparison with the surface of the land, the floor of the ocean is a vast monotonous plain. None of the sea bottom compares with the ruggedness of mountains, and even the more level portions of land surface show many sharp minor irregularities such as stream trenches. But the sea bottom is characterized by its smoothness of surface. There are under the sea, however, mountain-like ridges, plateaus, submarine volcanoes and valleys known as "deeps." But these rarely show ruggedness of relief like similar features on land.
One of the most remarkable relief features of the ocean bottom is the so-called "continental shelf." This is a relatively narrow platform covered by shallow water bordering nearly all the lands of the earth. Seaward, the depth of water is greatest, and it is seldom over 600 or 800 feet. The continental shelves of the world cover about 10,000,000 square miles or about one-fourteenth of the area of the sea.
Viewed in a broad way, there are two great classes of marine deposits; first, those laid down comparatively near the borders of the land, that is, on the continental shelf and continental slope, and second, the abysmal deposits laid down on the bottom of the deep ocean. Those found along and near the continental borders are largely land-derived materials, that is to say, they are mostly sediments carried from the land into the sea by rivers, and to a lesser extent rock material broken up by waves along many shores. Practically all such land-derived material is deposited within 100 to 300 miles of the shores. The continental border deposits are extremely variable. Near shore they are chiefly gravel and sands, while farther out they become gradually finer, and on the continental slope only very fine muds are deposited. These deposits usually contain more or less organic materials and shells or skeletons of organisms. In some cases the shells or skeletons of organisms predominate or even exist to the exclusion of nearly all other material, as is true of the coral deposits or reefs which form only in shallow water. Deposits like those just described as accumulating on the bottom of the shallow sea, comparatively near the lands, are of great significance to the geologist because just such marine deposits now consolidated into sandstone, conglomerate, shale, and limestone, are so widely exposed over the various continents. A knowledge of the conditions under which shallow sea deposits are now forming, is, therefore, of great value in interpreting events of earth history as they are recorded in similar rocks which have been accumulating through millions of years of time. One specific instance will make this matter clearer. Using the method outlined in Chapter I for the determination of earth chronology, and our knowledge of present conditions under which shallow sea deposits are formed, it has been well established that a shallow sea spread over fully four-fifths of the area of North America during the middle Ordovician period of the early Paleozoic era. Beyond this main conclusion, a careful study of these rocks has revealed many important facts regarding the physical geography, life, and climate of that time. The importance of this whole matter is still further emphasized by the statement that five-sixths of the exposed rocks of the earth are strata--mostly of shallow sea origin.