AQUEOUS ROCKS. In geology, every layer which forms a portion of the solid crust of the earth is called a rock, it matters not whether its particles are incoherent, like soil or sand, or compacted together, like limestone and sandstone—to all alike, irrespective of popular usage, the geologist applies the term rock. In this wide sense, the rocks of the earth's crust are either igneous (q.v.) or sedimentary. These sedimentary rocks have an aqueous origin, with the exception of a very limited number, like drift-sand, which are brought into their present position by the action of the wind. Unlike the igneous rocks, whose particles have assumed their present form in the position they occupy, the materials of the A. R. have evidently been brought from a distance. They owe their origin to some older rock, whose decomposition or destruction has afforded the materials. The parent rock can often be identified. Its distance is indicated by the condition, of the materials, whether they are rounded and water-worn, or angular and shingly.
The agents now at work, and which have been active in past geological ages, rubbing down and transporting the materials from which these rocks are formed, are the follow ing : 1. The sea, destroying the rocks and cliffs, and beaches which form its boundary, and carrying off the eroded materials to form new rocks below the level of the sea. 2. _Rivers, including the action of their smallest tributary rills, and even of the drops of rain, for these abrade and carry off the almost imperceptible particles from the surface where they fall; and when united they form the rill with its suspended sediment, and these again unite to form the river, which in its course not only retains what it has got, but scoops up more from its own bed, and carries all to the sea or lake, to deposit it there as a new stratum. It is difficult to estimate the influence of this agency. Sir Charles Lyell calculates that the Nile annually deposits in the Mediterranean 3,702,758,400 cubic feet of solid matter. 3. Glaciers and icebergs. These enormous moving masses of ice
are not only loaded with rock-fragments, which are deposited as the ice melts, but are ever abrading the rocks over which they pass, and thus supply materials to new layers. 4. Several stratified rocks have an evidently organic origin, such as chalk, and some limestones chiefly composed of animal remains, and coal consisting of vegetable carbon; but even these lave been influenced in their formation by water so much as to justify us in classifying them with A. R. 5. The same remark applies to rocks which have been precipitated from a fluid with which the materials existed in chemical combina tion, as has been the case with beds of salt, gypsum, and calcareous tufa.
As the result of these various actions, we have a series of rocks which, from their composition, may be classed as arenaceous, argillaceous, calcareous, carbonaceous, saline, and officious. We must refer to these terms for the descriptions of the various rocks included under them.
The arrangement of the A. R. depending on their different ages, is of more impor tance in modern geology than that depending on their internal constitution. When a section of the earth's crust is examined, it is found to be composed of a series of layers which have been produced in succession, Comparing this with sections in other dis tricts, it is noticed that there is a regularity in the several parts; for beds of the same structure are found in different localities, and these occupy the same relative position to the adjacent beds. A number of observations have shown that the crust of the earth is composed of a regular series of earthy deposits formed one after another, during suc cessive periods of time. This general induction forms the basis of the following classi fication. For the description of the included strata we must again refer to the names of the different divisions: •