DEPOSITS ON THE OCEAN-BED.
Something has already been said concerning the sediments which accumulate in the shallow waters along shores; but the area of marine sedimentation is as extensive as the ocean itself, and the deposits must now be reviewed from another point of view.
Oceanic deposits may be conveniently divided into two chief groups, dependent on the depth of the water in which they are made.[170] These groups are (1) shallow-water deposits, made in water less than some such depth as 100 fathoms, and (2) deep-sea deposits, laid down in water of greater depth. The selection of the 100-fathom line as the dividing depth is less arbitrary than it seems, for passing outward from the shore, it is at about this depth that the bottom ceases to be commonly disturbed by the action of currents and waves; that sunlight and vegetable life cease to be important at the bottom; and that the coarser sediments which predominate along shore give place, as a rule, to muds and oozes. Furthermore, the 100-fathom line (or some line very near it) is an important one in the physical relief of the globe, for it appears to mark, approximately, the junction of continental plateaus and ocean-basins. Only because the latter are a little over-full does the water run over their rims, covering about 10,000,000 square miles of the borders of the continents, converting them from land into epicontinental seas.
Aside from the deposits made by organisms, shallow-water deposits are divisible into two groups—(a) those immediately along the shore, the littoral deposits, and (b) those made between the littoral zone and the 100-fathom line. Both are terrigenous. The deep-sea deposits likewise are divisible into two groups, (a) terrigenous deposits formed close to land, and made up chiefly of materials derived immediately from the disintegration of land formations; and (b) the pelagic deposits, made up chiefly of the remains of pelagic organisms and the ultimate products arising from the decomposition of rocks and minerals. The former predominate in the less deep waters relatively near shore; the latter in the deeper water far from land. The shallow- and deep-water deposits grade into each other in a belt along the 100-fathom line.
Shallow-water Deposits.
Littoral deposits.—The littoral zone is the zone between high- and low-water marks. It is the zone in which bowlders, gravels, sands, and all coarser materials accumulate, though muds are occasionally met with in sheltered estuaries. Generally speaking, the nature of these deposits is determined by the character of the adjoining lands and the nature of the local organisms. “The heavier materials brought by rivers from high terrestrial regions, or thrown up by the tides and waves of the sea, are here arranged with great diversity of stratification through the alternate play of the winds and waves. Twice in the twenty-four hours the littoral zone is covered by water and exposed to the direct rays of the sun or the cooling effects of the night. There is a great range of temperature; mechanical agencies produce their maximum effects,”[171] and physical conditions in general are most varied. Still greater diversity is introduced by the fact that the zone is inhabited by both marine and terrestrial organisms, while the evaporation of the sea-water which flows over tidal marshes and lagoons leads to the formation of saline deposits. If the length of the coast-lines of the world be taken at 125,000 miles (about 200,000 kilometers), and the average width of this zone at half a mile, these deposits are now forming over an area of 62,500 square miles (about 160,000 square kilometers) of the earth’s surface.
Non-littoral, mechanical deposits in shallow water.—These deposits are laid down in the zone of the ocean between low-water mark and the 100-fathom line. They cover about 10,000,000 square miles.[172] Their composition is much the same as that of the littoral deposits, with which they are continuous, though on the whole they are finer. At their lower limit they pass insensibly into the fine deposits of the deep sea. Coarse material, such as gravel and sand, prevails, though in special situations, such as depressions and inclosed basins, muddy deposits are found. While some of the deposits are wholly composed of inorganic débris, organic remains are freely mingled with others. The mechanical effects of tides, currents, and waves are everywhere present, but become less and less well marked as the 100-fathom line is approached. The forms of vegetable and animal life are numerous, though the former decrease as depths which exclude the sunlight are approached.
Both littoral deposits and deposits in shallow water outside the littoral zone have already been referred to in connection with the work of waves and currents (pp. 355–66). A few additional points only need here be added.
Figs. 321 and 322.—Diagrams showing how shallow-water deposits may attain considerable depth by the shifting of the zone of deposition seaward.
Fig. 323.—Diagram showing the interwedging of gravel-, sand-, and mud-beds.
In general the coarser sediments are lodged near shore and those farther from the land become progressively finer. Even the coarser part of the material carried in suspension by the undertow is partly left in the shallow water. On the other hand, waves of exceptional strength may carry coarse material into water of some depth. Thus coarse shingle (gravel) and even bowlders have been found at depths of 10 fathoms.[173] Coarse deposits may extend far out from land if the waves are strong, and especially if the water is shallow, and since the zone of shallow water may be extended seaward by the aggradation of the bottom, shallow-water deposits may cover extensive areas. They may become deep at the same time, for as the outer border of the shallow-water zone is shifted seaward by aggradation, the vertical space to be filled becomes greater (compare Figs. [321] and [322]). Again, if the coast be sinking, new deposits of coarse material may be made on older ones. In this way also great thicknesses of sediment may be accumulated, all parts of which were deposited in shallow water. The great thickness of some of the conglomerate beds of the past shows how far this process may go.
Fig. 324.—Ripple-marks.
Fig. 325.—Rill-marks resembling impressions of seaweeds. Beach at Noyes Point, R. I. (Walcott, U. S. Geol. Surv.)
Fig. 326.—Rill-marks. Same locality as 325. (Walcott.)
As a rule, no definite line marks the seaward terminus of the coarse detritus, since coarse material is carried farther out when the waves run high (and the undertow is strong) than when they are feeble. In calm weather, therefore, fine sediment may be deposited where coarse had been laid down in the preceding storm, only to be covered in turn by other deposits of a different character. Thus gravel grades off into sand, with more or less overlapping or interwedging, and sand grades off into silt in the same way. This is diagrammatically illustrated by [Fig. 323].
Characteristics of shallow-water deposits.—Clastic sediments laid down in shallow water have several distinctive characteristics. While they are, in the aggregate, coarse, they are characterized by frequent variations in coarseness. The surfaces of successive beds are likely to be ripple- and rill-marked (Figs. [324], [325], [326]), and cross-bedding ([Fig. 327]) is of common occurrence. Clayey sediments accumulated between high and low water are often sun-cracked ([Fig. 328]), and the tracks of land animals are sometimes preserved on their surfaces. Shallow-water deposits often contain fossils of organisms which live in waters of slight depth. These characteristics are sufficient to differentiate sedimentary formations made in shallow water from those made in deep water, even after they have been converted into solid rock and after the rock has emerged from the sea. Many of these characteristics are, however, shared by deposits made by streams on the land. Subaërial and lacustrine sediments are usually distinguishable from those made in the sea by their fossils, and sometimes by their distribution.
Fig. 327.—Cross-bedding. (Gilbert.)
Fig. 328.—Sun-cracks. These cracks were on the mud-flats of the Missouri a few miles above Kansas City, but the sun-cracks on shore-deposits are not essentially different. (Calvin.)
Topography of shallow-water deposits.—The shallow-water deposits have, on the whole, a rather plane surface, though there are some notable departures from flatness. The steep slopes of the delta fronts and of wave-built terraces have already been spoken of. Barriers often shut in depressions, and the disposition of the material deposited is sometimes uneven, owing to shore and tidal currents. The result is that the surface of the shallow-water deposits is often affected by low elevations and by shallow depressions. The elevations and depressions may be elongate, circular, or irregular in form. These general facts are shown in Figs. [319], [320], and [329]. This topography is sometimes preserved on newly emerged lands, as at various points on the Coastal Plain of the United States.
Fig. 329.—Irregularities of topography of shallow-water deposits. The depths of the water are shown in fathoms. (Chart of C. and G. Surv.)
Chemical and organic deposits.—There is no sharp line of distinction between the deposits usually classed as chemical and those regarded as organic. The latter are chemical in the broader sense of the term, but as they are immediately associated with life and are dependent upon it, it is a matter of practical convenience to separate them. Aside from the organic deposits, the chemical deposits made in shallow sea-water embrace (1) those due to reactions between constituents so brought together that new and insoluble compounds are formed and precipitated, and (2) those due to evaporation. The points of saturation for the various substances dissolved in sea-water are reached at different stages, and hence they are deposited more or less in succession.
The chemical deposits made in the shallow water of the sea, or in shallow bodies of water isolated from the sea, are chiefly simple precipitates resulting from evaporation; but new combinations are sometimes made in the process of concentration and precipitation. All substances in solution are necessarily precipitated on complete evaporation, but since the sea-water is in general far from saturation, so far as all its leading salts are concerned, only a few are thrown down in quantity sufficient to have geological importance where evaporation is incomplete. The leading deposits are lime carbonate (CaCO3), lime sulphate (gypsum, CaSO4,2H2O), common salt (rock-salt, NaCl), and the magnesium salts, usually the chlorides and sulphates, which are later changed to carbonates. In investigations on Mediterranean water[174] which had an initial density of 1.02, no deposit took place until concentration by evaporation had brought the water to a specific gravity of 1.05. Between this density and that of 1.13, lime carbonate and some iron oxide were deposited. Between 1.13 and 1.22, lime sulphate was the most abundant precipitate, while between 1.22 and 1.31, 95% of the deposit was common salt. With still further concentration, the remaining substances in solution, especially the magnesium salts, were thrown down.
While there is somewhat more than ten times as much lime sulphate as lime carbonate in the ocean ([p. 324]), the deposits of the carbonate (including the organic) have been very much greater than those of the sulphate. This is due partly to the fact that the sulphate is much more soluble in natural waters than the carbonate. Rivers bring much more carbonate than sulphate to the sea, so that the point of saturation for the sulphate would normally be reached much later than that of the carbonate. The more important fact, however, is that marine plants and animals use lime carbonate freely for skeletal and housing purposes. It is held by some that they get their lime from the sulphate, but if so they convert it into carbonate before it takes the form of shells, coral, etc., the sulphuric acid set free in the process reproducing, directly or indirectly, more sulphate. The secretion of lime carbonate by organisms is not dependent on the saturation of the water, but may be carried on when the amount in solution is very small.
There can be little doubt that the chief deposits of lime carbonate have been and are being made through the agency of plants and animals in the form of shells, coral, bones, teeth, and other devices for supporting, stiffening, housing, protecting, and arming themselves; but while it is agreed that the larger part of the lime carbonate deposited in the open sea is of organic origin, it is equally clear that in closed seas subject to concentration from evaporation, simple precipitation takes place freely. There is some difference of opinion as to the importance of these two classes of deposits, past and present. The debated point is whether simple precipitation takes place in any appreciable degree under the usual oceanic conditions. There is much more evidence of solution by sea-water than of precipitation from it. The ocean appears to be under-saturated with lime carbonate on the whole, though it is still possible that deposition may take place in favorable situations, as, for example, where the very calcareous waters of rivers are spread out in thin sheets on the surface of the heavier salt water, and thus exposed to exceptional evaporation, or where there is very exceptional agitation and aëration.[175]
Gypsum appears to be deposited in quantity only in the closed basins of arid regions where concentration reaches an advanced state.
Since normal sea-water is far from saturation with common salt, the latter is precipitated only in lagoons, closed seas, or other situations favorable to great concentration. This is usually achieved only in notably arid regions, and in basins that receive little or no drainage from the land.
Deposits of salt usually, therefore, signify highly arid conditions, and where they occur over wide ranges in latitude and longitude, as in certain periods of the past, unusual aridity is inferred. Where confined to limited areas, their climatic significance is less, for topographic conditions may determine local aridity. The total area where salt is now being precipitated is small, though on the whole the present is probably to be regarded as a rather arid period of the earth’s history. On the other hand, ancient deposits of salt preserved in the sedimentary strata show that the area of salt deposition has been much more considerable than now at one time and another in the earth’s history. The salt and gypsum deposits of the past seem, therefore, to tell an interesting tale of the climates of the past.
The magnesium salts are among the last to be thrown down as the sea-water is evaporated, and they most commonly take the form of sulphates and chlorides. They often form double salts with potassium, a relatively small and soluble constituent of sea-water. In the artificial evaporation of salt water to obtain common salt, the process is usually stopped before the saturation-point for the magnesium salts is reached, and the residue, the “mother-liquor,” or “bittern,” is drawn off to prevent these “bitter” salts from mixing with the common salt. The magnesium salts are among the last to be precipitated, not only because they are readily soluble, but because their quantity is small; yet in the original rock from which all the sea-salts came, there is at least as much magnesium as sodium, while in the sea there is about five times as much sodium as magnesium. Just what becomes of the remaining magnesium is not yet well understood. It has a notable disposition to form double salts with some other constituent, as noted above. In the earlier marine strata, dolomite, that is, limestone composed partly or wholly of the double carbonate of lime and magnesia, (CaMg)CO3, abounds. This appears to have been formed by a gradual substitution of molecules of magnesium for those of calcium, but just how and when and why it was done has not been fully worked out. It appears to be a case where the saline matter of the sea made its contribution to the sedimentary deposits by chemical reaction upon them, rather than by precipitation because of saturation.
The relatively small amount of potash in the sea-water is probably due to its disposition to remain united with the clays and earths of the mantle rock and of the shaley deposits.
To some extent the salts in solution act directly on the earthy matter brought down into the sea by rivers, but where sedimentation is rapid, as it often is in shallow water, this action is limited and obscure. In the main, the ocean-waters protect the sediments from weathering and similar changes, except as organic matter buried with them induces change.
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.
Of the identifiable inorganic materials in the deep sea, the most abundant are of volcanic origin, and among these the most common is pumice, which is frequently so light that it floats readily until it becomes water-logged. Pieces of pumice brought up by the Challenger and thoroughly dried were found to float for months in sea-water before settling even through the depth of water contained in the vessel in which the experiment was performed.[178] The next most abundant substance of volcanic origin in pelagic deposits is volcanic glass. This ranges from pieces of the size of a walnut down to the smallest fragments, which often serve as centers for concretions. Lapilli (cinders) and volcanic ash also are abundant in parts of the deep sea. The distribution of these volcanic products is essentially universal, though by no means uniform. Some of them are probably from submarine volcanoes.
The study of the deep sea deposits has revealed the presence of many nodules and grains which are believed to be of extra-terrestrial origin. Many of them are magnetic.[179] The dust of countless meteors which enter the atmosphere daily settles on land and sea alike, and enters into the sediment of the bottom of the latter. It is probably no more abundant in deep water than in shallow, but it is relatively more important, since other sedimentation is more meager. The number of meteorites which enter the atmosphere daily has been estimated at from 15,000,000 to 20,000,000.[180] If on the average the meteorites weigh ten grains each, probably a rather high estimate, the total amount of extra-terrestrial matter reaching the earth yearly would be 5,000 to 7,000 tons, and something like three-fourths of this must, on the average, fall into the sea. But even at this rate it would take some fifty billion years to cover the sea-bottom with a layer one foot in thickness.
Organic constituents of pelagic deposits.—With increasing distance from shores, and especially with increasing depth of water, terrigenous deposits become less and less abundant, and sediments derived from pelagic life increase in relative importance. Beyond the upper part of the outer slopes of the continental shelves, the pelagic deposits are largely made up of shells and skeletons of marine organisms which live in the surface-waters. Pelagic molluscs, foraminifera, and algæ secrete shells of lime carbonate, while diatoms and radiolarians secrete shells of silica. When the organisms die, they sink to the bottom with their shells, and these mineral matters of organic origin are mingled with the volcanic products which are universal over the sea-floor. Pelagic deposits of organic origin are named according to their characteristic constituents. Thus there are pteropod oozes, globigerina oozes, diatom oozes, radiolarian oozes, etc.[181] It is not to be understood that these oozes are made up exclusively of the shells which give them their names. Diatom ooze is an ooze in which diatom shells are abundant, not an ooze made up wholly of diatom shells; and globigerina ooze is an ooze in which globigerina shells are abundant, though in many cases they do not make up even the bulk of the matter. While samples of these various oozes might be selected which are thoroughly distinct from one another, there are all gradations between them, since pelagic life does not recognize boundary-lines.
It is a significant fact that with increasing depth the proportion of lime carbonate in the ooze decreases. Thus in tropical regions remote from land where the depths are less than 600 fathoms, the carbonate of lime of the shells of pelagic organisms may constitute 80% or 90% of the deposit. With the same surface conditions, but with increasing depth, the percentage of lime carbonate decreases, until at 2000 fathoms it is less than 60%; at 2400 fathoms, 30%, and at 2600 fathoms, 10%. Beyond this depth there are usually no more than traces of carbonate of lime. The data at hand show that the percentage of lime carbonate falls off below 2200 fathoms more rapidly than at lesser depths.
When the percentage of lime carbonate becomes very low, the calcareous oozes grade off into the red clay with which the sea-floor below 2400 to 2600 fathoms is covered.
Chemical deposits.—The chemical deposits of the deep sea are chiefly the alteration products of sediments which reach the sea-bottom by mechanical means. All sediment deposited in the sea undergoes more or less chemical change, but it is only when the change is very considerable that the product is referred to this class. Where sedimentation is rapid and the sediment coarse, the chemical change is relatively slight; but where the sedimentation is slow and the sediment fine, the chemical change is relatively great; for the longer exposure to the sea-water and the greater proportion of surface exposed to attack, both favor change. Both the area and the mass of sea-bottom sediment radically changed in this way are large, but most of the deposit does not correspond to any formation known on the land.
The red clay already referred to belongs to this class of deposits. Its origin has been the subject of much discussion. It contains much volcanic débris, various concretions, bones of mammals, zeolitic crystals, and extra-terrestrial spherules, and doubtless the insoluble products of the shells of pelagic life; but it is still a mooted question how far the clay itself is the product of decomposed shells, and how far the altered product of pulverized pumice, volcanic ash, dust, etc. Pelagic life does not seem to be less abundant at the surface where the water is deep than where it is shallow, and it would appear that the shells must sink in such situations as elsewhere. If the lime carbonate of globigerina ooze be removed by dilute acid, the inorganic residue is similar to the red clay in the ocean-bottom. This suggests that owing to the more complete solution in the very deep water, the lime carbonate of the shells has been dissolved, leaving the red clay as a residuum. The more complete solution at the bottom might be the result either of the greater pressure, or of a greater percentage of CO2 in the water due to emanations from the sea-floor, or to both; but the suddenness of the transition from oozes to red clay, with increasing depth, does not seem to be fully explained by these assumptions. The study of the dredgings has inclined the students of these materials to the conclusion that volcanic materials, rather than shells, are the principal source of the red clay.[182] The volcanic materials are thought to have accumulated slowly and to have been long exposed to the action of sea-water. The various nodules and crystals in the clay are believed to be secondary products, the materials for which were derived from the decomposition of the same materials. Eolian dust may be a notable constituent of the red clay.
Various specific products of chemical change may be briefly referred to. The decomposition of certain mineral particles, such as feldspar, gives rise to kaolin, and kaolin is a very considerable constituent of most of the clayey deposits of the ocean-bottom. The kaolinization of feldspar may take place both on land and in the sea. Manganiferous deposits are widespread in the ocean-bottom, occurring both as coatings on grains of mechanical sediments, shells, etc., and as concretions ranging in sizes from minute particles to nodules an inch or more in diameter. The concretions are sometimes approximately spheroidal, but often botryoidal. These manganiferous nodules are believed to have arisen from the decay of fragments of volcanic rocks. In their decay, the manganese and iron are believed to have been first changed to carbonates, and subsequently to oxides. After manganese oxide, iron oxide and silica are by far the most abundant constituents, but many other substances enter into their composition in minor quantities.
Another substance somewhat widely distributed in the sea-bed, though by no means universal, is glauconite, a complex silicate of alumina, iron, potassium, etc. Glauconite is, on the whole, most abundant along the edges of the continental shelves, though it is by no means universal in this position. It is not commonly found in deep water, nor very near the shore, but approximately at the “mud-line.” The glauconite grains begin to form, as a rule, in tiny shells, chiefly the shells of foraminifera. After filling the shell, the shell itself may disappear, while the glauconite goes on accumulating around the core already formed, until the grain attains considerable size. Glauconite is believed to be an alteration product of certain sorts of mechanical sediment, the change being effected under the influence of the decaying organic matter in the shells.[183] It does not occur where sedimentation is rapid, and its formation appears to be favored by considerable changes of temperature. Glauconite deposits occur on the land and are commonly known as green sand marl. Glauconite also occurs sparingly in many other sedimentary rocks.
Fig. 330.—Distribution of various sorts of deep-sea deposits. (Murray. Challenger Reports.)
Another substance which is somewhat widespread in the ocean-bottom is phosphate of lime, which occurs in various sorts of oozes, in the manganiferous nodules, in glauconite, and in independent nodules. Like the grains of glauconite, the grains of phosphate of lime appear to have started as concretions in shells, and to be the result of the reaction of organic matter on the contents of sea-water. The immediate source of the lime phosphate in the water appears to have been the shells or bones of the numerous animals living in the sea.
Secondary minerals made from the constituents of volcanic matter which has been decomposed occur not uncommonly in the bottom of the sea. These minerals belong to the general class of zeolites, phillipsite being the most abundant. Their distribution is somewhat wide, but their quantity is slight.
Unfortunately, knowledge of the deep-sea deposits is limited to their superficial layers. Soundings do not usually penetrate more than a few inches, or at most a foot or two.
Unlike shallow-water deposits, those of the really deep sea seem to find no correlatives in the known rock formations of the land.