Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Fig. 352.

Fig. 351.—Globigerina ooze. Magnified 20 times. (Murray and Renard.)

Fig. 352.—Pteropod ooze. Magnified 4 times. (Murray and Renard.)

Limestones and dolomites.—Of the lime, magnesia, soda, and potash leached out of the surface-rocks and carried to the ocean in solution, the lime is largely extracted to form the shells, skeletons, teeth, armor, and other hard parts of sea-animals and sea-plants. These limy parts are at length left on the floor of the ocean and become more or less disintegrated and help to form beds of lime-mud and lime-sand which in time are cemented into limestone (Figs. [349], [350], [351], and [352]). A larger proportion of the magnesia remains in solution in the sea-water, but in ways not yet well understood, the magnesia sometimes unites with the lime to form dolomite, a double carbonate of lime and magnesia (Ca,Mg)CO₃. This change is sometimes local, and sometimes affects great series of beds, more commonly the ancient ones than the modern. Sometimes the dolomization appears to have taken place long after the original limestone was formed and probably sometimes after it was lifted out of the sea, while in other cases it seems to have taken place while the sediment was accumulating, or at least before the next overlying beds were laid down. The potash in solution is to some large extent taken up by the land- and sea-plants or is retained in the clays, and through them becomes again incorporated in the sediments. The soda largely remains in solution in the sea-water.

Precipitates.—When a portion of the ocean-water is isolated in a region where evaporation from the surface of the water is greater than the rainfall on it, and the inflow from the tributary basin, the lime, magnesia, soda, potash, and other dissolved substances (solutes) are concentrated until the water becomes saturated. The solutes are then precipitated in the order in which they reach the point of saturation. This order, when taken in strict and full detail, gives a very complex series, but the leading deposits are calcium carbonate (limestone), calcium sulphate (gypsum), and sodium chloride (halite or rock salt) (see [p. 375]). Isolated lakes in arid regions may give rise to similar deposits. It has sometimes been thought that the ancient limestones were produced largely by precipitation from concentrated sea-water. While this is probably the case in some instances and to some degree, it has not been demonstrated that the great limestone formations were made to any large extent in this way. The more accepted view is that the limestones in the main were made from organic remains. The lime in solution in the ocean is chiefly in the form of the sulphate.

Fig. 353.—Diatom ooze. Magnified 150 times. (Murray.)

Iron Ore-beds.—In a somewhat different way iron ore-deposits are formed by the precipitation of iron oxide or iron carbonate from solutions of ferrous compounds. The ferrous compounds in solution were leached from iron-bearing rocks by percolating waters. The most familiar case is that of iron-bearing springs. On exposure to the air, the iron compounds in solution undergo change, and ferric oxide is thrown down, usually forming limonite (Fe₂O₃,3H₂O), but sometimes hematite (Fe₂O₃). This change is common in marshes and gives origin to “bog-ore.” Similar deposits take place in certain shallow lakes, and hence are known as “lake ore.” Iron ore sometimes also forms at the bottom of a peaty bed or in muddy soil. In connection with the great coal formations, beds of iron carbonate (siderite) occur. Organic matter seems to play a great part both in the original solution and the later deposition of these ores. From certain soils and clay-beds on which the ancient coal-producing forests grew, the iron has been almost completely removed, either by the action of the roots, or more probably by organic acids arising from their decay and from the decaying vegetation on the surface. On flowing into shallow bodies of water or into marshes, the waters containing such dissolved iron compounds usually throw down their iron content either as a carbonate (siderite), or as a hydrous ferric oxide (limonite). The siderite is formed where decaying vegetation is present to furnish abundant carbon dioxide and to partially protect the iron solution from oxidation, and the limonite where free oxidation takes place. Sand, silt, clay, or calcium carbonate often accumulates with the iron precipitate, and the result is an impure deposit which becomes an ironstone. Such deposits often become segregated into nodules, as will be explained later. It is thought that diatoms sometimes aid in the deposit of iron ore in shallow waters.

Silicious deposits.—In the decomposition of igneous rocks, a certain portion of the silica, as well as of the bases, is dissolved and carried away in solution. Certain organisms extract this from solution for their skeletons, just as others extract calcium carbonate. The accumulation of these silicious skeletons often forms silicious rocks. The diatom, radiolarian, and other oozes ([Fig. 353]) of the deep sea are the great examples. Sometimes layers of infusorial earth, tripolite, arise from the shells of diatoms and other aquatic organisms secreting silica. The waters in which such earths accumulate are rather shallow, and either fresh or salt. The most familiar examples of indurated rocks formed in this general method are the flints and cherts (impure flints) that occur in limestone and chalk, chiefly as nodules, but sometimes in distinct beds.

Organic rocks.—While most limestones, chalks, flints, cherts, and the silicious and calcareous oozes are formed through the agency of organisms, they are not themselves strictly organic. There is, however, a small but important group of rocks formed directly from organic matter. In favorable situations the woody parts of plants, falling into water, are so far preserved from decay that they accumulate in beds, and by slow changes pass into peat, lignite, bituminous coal, anthracite, and graphite. The first of these is composed essentially of carbohydrates and hydrocarbons much as plants are, while the last two are mainly carbon, and the intermediate members represent stages of passage from the first to the last. They are all derived from the strictly organic part of the plants, and spring essentially from the atmosphere and hydrosphere. They are only indirectly associated with the evolutions of the inorganic series.