Such perfect differentiation in water of fine detritus according to size can only take place in the absence (or presence of only a very small quantity) of electrolytes, such as the dissolved salts in sea, lake or river water, or of electro-positive colloids such as ferric (iron) hydroxide or decomposing organic matter (either animal or vegetable). The electronegative charge carried by colloidal clay, silica, alumina, etc., is dissipated by the action of the electrolytes, the clay particles flocculate together and settle as larger composite grains with greater rapidity. Similarly mutual discharge of the charges is effected by electro-positive colloids such as those mentioned above, with consequent floccula tion and rapid co-precipitation. Decomposing peat thus clears streams and lakes of detritus, and decomposing animal matter in lakes and confined marine areas plays a similar role. Flocculation is most effective in the sea, where the amount of dissolved salts reaches a maximum. Wherever rapid deposition of clay and silt takes place, sand and coarser silt is carried down in the flocks of clayey matter, with the result that marine "clays" are rarely, if ever, perfectly graded.
Temperature plays also a notable part in controlling the de positional sorting of muds, for the viscosity of water falls rapidly with increase of temperature ; subsidence in consequence is much slower (and therefore sorting more efficient) in cold than in warm waters. Living organisms also are less vigorous and abundant, and many chemical changes such as deposition of lime or replacement of lime by magnesia proceed at a far slower rate. In certain Canadian lakes the deposits can be "zoned" according to depth, the presence of thermal layers in the water determining different types of sediment.
While therefore the clays of the sea-bed, or the clays and their consolidation-products such as shales, mudstones, slates, etc., of past geological ages may have been in part deposited as a result of the reduced velocity of river currents and therefore their in ability to carry the load, it is probable that the constitution of the rocks undergoing denudation, (as regards their potentially clay producing character), the composition and temperature of the river and sea water, and the quantity of organic matter present play a dominant part.
Entombed in sands, clays and pebble-beds are the remains of plants and animals, which may either have drifted into the area, have lived in the water or burrowed in the deposits. Natur ally these change in character and quantity from place to place. They are abundant in the littoral or neritic zone, and in places, instead of yielding (for example) shelly sands or carbonaceous clays, may under suitable conditions give rise to molluscan com munities, coral reefs, crinoidal or polyzoan banks, saprophytic bacterial sludges (the ancestors of rocks like cannel coal) and so on. When calcareous shelly or skeletal remains are dominant, limestones are formed. Limestones often contain detrital matter (indeed they may be formed entirely of calcareous detritus them selves), and even when free from such are not necessarily deep water deposits.
In warm seas deposition of successive coats of carbonate of lime may take place on nuclei of sand-grains or fragments of organic origin. As a result of wave-rolling these develop an ovoid form varying in size from a pin-head to a pea, when they are termed oolites or pistolites respectively. (See LIMESTONES.) Bac teria are said to be effective off the Florida coast in precipitating fine calcareous mud ("drewite") thus yielding a rock comparable with Chalk or many of the fine grained Jurassic and Carbonifer ous limestones. Undoubtedly also bacteria play a great part in
the formation of deposits of iron ores in lakes, etc., and in the production of coal.
According as the melting of ice-sheets is slow or rapid, the grading of the deposits is more or less definite. River-borne (or fluvio-glacial) detritus is laid down in a manner similar to that of the torrents and streams referred to above. Chemical action however, being at a minimum, the clay consists more of unde composed "rock-flour" and less of colloidal matter, the coarser constituents are more varied in character and pebbles and bould ers often ice-scratched. Ridges of sands and gravels (eskers, kames) and moraines composed either of the same materials or of boulder clay are formed on the recession of the ice. The last named deposit (till) must have been formed under conditions which did not permit of sorting of material, i.e., the direct evapo ration of ice or a melting out in sea-water, for it usually consists of a non-bedded heterogeneous mixture of coarse boulders, pebbles, sand and fine clay.
Under the action of the wind, sand grains become well rounded and "frosted," because the lubricating action of water is absent. Sorting is usually well-marked, and silt and dust are carried far beyond the area of sand-deposition (of which the dune is a char acteristic form). By this means fine terrigenous material becomes incorporated with sediments of even deep-water type. Where, in late glacial times, the wind has played upon areas of boulder clay, it has brought about the deposit of an extra-glacial belt of a sandy clay called loess which almost encircles the earth, in about latitude N. Indurated boulder-clays or tillites occur in deposits of Cretaceous, late Carboniferous, Cambrian and Pre Cambrian age (although some are disputed) in various parts of the world. They are of ten associated with kame-like gravels and varved shales.
The term "facies" has been used to connote the sum-total of the features which characterize any particular sedimentary rock. Thus we may have a sandy facies or a coaly facies, depending on the constitution; a lagoon facies or a reef-facies, from a geo graphical or community standpoint ; or a graptolite-facies or brachiopod facies according to the dominant life.
Apart from physical changes such as consolidation under re newed deposition, and drying, sedimentary rocks may undergo chemical and other changes during their formation on the sea floor, or but slightly subsequent to it (penecontemporaneous changes). Coral reefs or other limestones may be dolomitized by partial replacement by magnesia from warm sea-water. (See DOLOMITE.) While flint and cheat may be in part deposited directly by chemical or organic action, they doubtless also repre sent in part a penecontemporaneous or subsequent segregation of silica in limestones and replacement of calcareous material. Be fore consolidation, clays and coals enclose large quantities of water, often saturated with carbonate of lime, iron, etc. By rapid segregation and crystallization of the latter substances, masses of septaria or cement-stone are formed in the clay, but where the action is slower and proceeds from many centres, beds may be formed consisting almost wholly of "spherulites" of spheroidal form and radiating structure. Where anaerobic bac teria are present, large quantities of iron (ferrous) sulphide may be formed throughout the clay, giving it a black colour, but not necessarily inhibiting other life. Subsequent segregation of this material yields nodules of pyrite or marcasite.
For additional information see ALLUVIUM, BRECCIA, CLAY, COAL, CONGLOMERATE, FLINT, LIMESTONE, MARL, OCEAN AND