While the lime deposits are by far the greatest of the chemical and organic deposits of the sea, plants and animals also secrete notable quantities of silica. Silica deposits of organic origin are relatively much more important in the deep sea than in shallow water, and will be mentioned in that connection.
Limestone.—Something concerning the origin of limestone has already been given in the preceding paragraphs, but because of the importance of this formation, it may be added by way of summary that shallow seas free, or nearly free, from terrigenous sediment, and abounding in lime-secreting life, furnish the conditions for nearly pure deposits of limestone, and that most of the limestone within the areas of the present continents appears to have originated under such conditions. The common notion that limestone is normally a deep-water formation is a serious error. Although limestones are formed in deep as well as in shallow waters, by far the more important classes of lime-secreting organisms are photobathic, i.e. are limited to the depths to which light penetrates. In the shallow waters, these plants and animals are in part free and in part attached. Within the areas of deep water they are free and at the surface, and their remains drop to the bottom, if not sooner dissolved. But few forms live on the deep, dark, cold bottoms of abysmal depths. Clear waters, free from abundant terrigenous sediments and abounding in lime-secreting life, rather than deep waters, are, therefore, the most favorable conditions for the origin of limestone.
The purely chemical deposits of limestone are probably all of shallow-water origin. Once made, they are subject to solution, redeposition, and other mutations like other deposits. As a result, they often lose many of their original characteristics, but enough usually remain to tell the story of their origin.
Deep-sea Deposits.
Contrasted with shallow-water deposits.—The deep-sea deposits cover the ocean-bottom below the 100-fathom line. Their area is considerably more than half the earth’s surface. The characteristic deposits are muds, organic oozes, and clays, which in their physical characteristics are remarkably uniform. In regions of floating ice, greater diversity is introduced from the varied nature of the materials which the ice transports, but gravels and sands, comparable to those of shallow water, are rarely found. “Tides, currents, and waves produce some mechanical effects at the upper limits of the deep-sea region, but on the whole there is an absence of the phenomena of erosion, and mechanical action would appear to be absent except in the case of submarine eruptions. The depth is too great for sunlight to penetrate, and vegetable life is limited to the upper zone. Animal life is present in the same zone and on the bottom, but absent or nearly so in the middle depths. The temperature (at the bottom) is below 40° Fahr. throughout the larger part of the area, and if subject to variation with latitude or change of season, these changes affect only the depths immediately beyond the 100-fathom line. Throughout the whole region there is a very uniform set of conditions. In the shallow-water and littoral zones, owing to the rapid accumulation and the mechanical effects of transportation and erosion, the effects of chemical modification are not very apparent in the deposits; but in deep-sea deposits, in consequence of the less rapid rate of accumulation, absence of transport, the nature and small size of the particles, many evident chemical reactions have taken place, resulting in the formation in situ of glauconite, phosphatic and manganese nodules, zeolites, and other secondary products.”[176] With increasing depth and distance from the shore, the character of the deposits undergoes a change. There is less and less material derived directly from the land, and more “amorphous matter arising from the ultimate decomposition of minerals and rocks, and accompanied, in all moderate depths, by an increase [relative] of the remains of pelagic organisms. We thus pass insensibly from those deep-sea deposits of a terrestrial origin, which we call ‘terrigenous,’ to those deep-sea deposits denominated ‘pelagic,’ in which the remains of calcareous and siliceous organisms, clays and other substances of secondary origin play the principal rôle.”[176]
The following table[177] shows the relations of the various groups of marine deposits.
| 1. Deep-sea deposits beyond 100 fathoms |  | Red clay |  | I. Pelagic deposits formed in deep water removed from land. |
| Radiolarian ooze | ||||
| Diatom ooze | ||||
| Globigerina ooze | ||||
| Pteropod ooze | ||||
| Blue mud | | II. Terrigenous deposits formed in deep and shallow water, mostly close to land. | ||
| Red mud | ||||
| Green mud | ||||
| Volcanic mud | ||||
| Coral mud | ||||
| 2. Shallow-water deposits between low-water mark and 100 fathoms | | Sands, gravels, muds, etc. | ||
| 3. Littoral deposits between high- and low-water marks | | Sands, gravels, muds, etc. |
Sources.—The pelagic deposits are made up in part of materials of organic origin, and in part of materials of inorganic origin. The inorganic materials may be of mechanical or chemical origin. Mechanical pelagic deposits originate in various ways. They may come (1) from the land by the ordinary processes of gradation, (2) from volcanic vents, or (3) from extra-terrestrial sources. Chemical deposits may be formed (1) in situ by the chemical interaction of substances in the sea-water on materials of organic and inorganic origin, and (2) by direct precipitation from the sea-water.
Mechanical inorganic deposits.—The terrigenous materials which reach the deep sea are, as a rule, only the finest products of land decay, and are carried out by movements of water or by the winds. They are not commonly recognized in the dredgings more than 200 miles from the shore, but opposite the mouths of great rivers they extend much farther,—1000 miles in the case of the Amazon. They are especially abundant on the slopes of the continental shelves. Here occur the blue, green, and red muds, with which are associated volcanic and coral muds. The color of these various muds is dependent in part on the changes which they have undergone since their deposition. The green muds usually contain enough glauconite to give them their color, and are most commonly found off bold coasts where sedimentation is not rapid. The blue muds indicate lack of oxidation, or perhaps deoxidation. Red muds are not common, though they have been found in some situations. In general, these deposits are analogous to certain shales, marls, etc., found within the continents.
Though coarse materials derived from the land are occasionally found in the deep-sea deposits, their presence must be looked upon as in some sense accidental. Occasional pebbles, or even bowlders, are carried out into the ocean entangled in the roots of floating trees. Within limits, too, icebergs have carried out land débris, though it is probable that transportation by this means has been exaggerated. The amount which icebergs might carry, if fully loaded, is far greater than the amount which they do carry.