AQUEOUS ROCKS. In geology, rocks which have been laid down as mechanical, chemi cal. or organic deposits from water. They be long to the sedimentary rocks, which also include rocks deposited from air (Aeolian deposits).
1. The mechanical deposits from water are derived from the destruction of prexisting rocks. Rain and rivers move considerable quan tities of disintegrated material, depositing it wherever the load is too great for the volume and velocity of the current. Waves, rolling against a shore, break from it small and large fragments, carry these fragments back with them, and deposit them in layers on the bottom of the basin. The coarse particles are left near est the shore, forming conglomerate or gravel: finer particles are carried somewhat farther out, forming sand, which by cementation becomes sandstone. quartzite. novaculite, or. when mixed with feldspar. arkose or graywacke: still finer particles are carried yet farther out and are deposited, to form mud or clay, which by cemen tation or consolidation becomes mudstone, shale, or slate.
It. Chemical deposition from water may be due to the mingling of solutions, to changes in the temperature or pressure of water containing substances in solution, or to the simple evapora tion of water. For convenience in discussion. chemical precipitates may he divided into three general classes: (a) precipitates of the alkalies and alkaline earths, giving calcareous tufa. sin ter. travertine, stalactite, onyx marbles, oiilite, gypsum, rock salt; (b) siliceous precipitates, giving ehert (flint or bornstone), geyserite, sili ceous sinter: (c) ferruginous precipitates, giv ing iron ores. These latter are largely deposited through the aid of decaying vegetable matter, and might properly be considered under class 111.. but the deposition is due rather to the chemical effect of dead organisms than to the activities of the living forms.
Ill. Organic deposits originate in the growth and decay of organisms, either in situ or after transportation. Deposits of this character are
commonly made in water which is deeper and quieter than that in which chemical deposits are made. They may be divided into—)a) cal careous accumulations, resulting in shell marl, chalk, limestone, dolomite: (b) siliceous accumu lations, such as infusorial earth, siliceous ooze, some forms of flint or cllert; (c) ferruginous ac. cumulations, resulting in certain hog ores: (d) carbonaceous accumulations. known as peat, lig nite, brown etml, or coal.
Rocks of mechanical and organic deposition form the great mass of the aqueous rocks. The common order of occurrence from the shore out wa rd—conglomera t e. sandst cam, mud. and limestone—corresponds in a general way with increase in depth of water. It follows, there fore, that if at any point. the body of water is transgressing on the land, resulting in deepen ing of the water, the vertical order I if super position of mechanical deposits will conglom erate, sandstone, shale, and limestone: and, vice versa, if the water is receding, the order will be reversed. There is thus a change of character of the sedimmits in any series both laterally and vertically. The aqueous rocks of the earth's crust are found in such recurring successions and by their study the vertical oscillations of con tinents and ocean have been determined. In a very general way, it may be said that the suc cession of aqueous deposits during geological history has been much the same the world over. The members of this succession have been grouped into divisions on the basis of their order of superposition. their structure, and their contained fossils, and these divisions correspond to the time divisions of geological history. See GEOLOGY; LIMESTONE; LITIIOGENESIS.