Aqueous Rocks. —The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a larger part of the earth’s surface than any others. They consist chiefly of mechanical deposits (pebbles, sand, and mud), but are partly of chemical and some of them of organic origin, especially the limestones. These rocks are stratified , or divided into distinct layers, or strata. The term stratum means simply a bed, or any thing spread out or strewed over a given surface ; and we infer that these strata have been generally spread out by the action of water, from what we daily see taking place near the mouths of rivers, or on the land during temporary inundations. For, whenever a running stream charged with mud or sand, has its velocity checked, as when it enters a lake or sea, or over¬ flows a plain, the sediment, previously held in suspension by * the motion of the water, sinks, by its own gravity to the bottom. In this manner layers of mud and sand are thrown down one upon another.
If we drain a lake which has been fed by a small stream, we frequently find at the bottom a series of deposits, dis¬ posed with considerable regularity, one above the other; the uppermost, perhaps, may be a stratum of peat, next be¬ low a more dense and solid variety of the same material; still lower a bed of shell-marl, alternating with peat or sand, and then other beds of marl, divided by layers of clay. Now, if a second pit be sunk through the same continuous lacustrine formation at some distance from the first, nearly the same series of beds is commonly met with, yet with slight variations; some, for example, of the layers of sand, clay, or marl, may be wanting, one or more of them having thinned out and given place to others, or sometimes one of the masses first examined is observed to increase in thick¬ ness to the exclusion of other beds.
In the estuaries of large rivers, such as the Ganges and the Mississippi, we may observe, at low water, phenomena analogous to those of the drained lakes above mentioned, but on a grander scale, and extending over areas several hundred miles in length and breadth. When the periodical inundations subside, the river hollows out a channel to the depth of many yards through horizontal beds of clay and sand, the ends of which are seen exposed in perpendicular cliffs. These beds vary in their mineral composition, or col¬ or, or in the fineness or coarseness of their particles, and some of them are occasionally characterized by containing drift-wood. At the junction of the river and the sea, espe¬ cially in lagoons nearly separated by sand-bars from the ocean, deposits are often formed in which brackish and salt¬ water shells are included. In Egypt, where the Nile is always adding to its delta by filling up part of the Mediterranean with mud, the newly de¬ posited sediment is stratified , the thin layer thrown down in one season differing slightly in color from that of a previous year, and being separable from it, as has been observed in excavations at Cairo and other places.*
Fossil shells, of forms such as now abound in the sea, are met with far inland, both near the surface, and at great depths below it. They occur at all heights above the level of the ocean, having been ob¬ served at elevations of more than 8000 feet in the Pyrenees, 10,000 in the Alps, 13,000 in the Andes, and above 18,000 feet in the Himalaya.* These shells belong mostly to marine testacea, but in some places exclusively to forms characteristic of lakes and rivers. Hence it is concluded that some ancient strata were deposit¬ ed at the bottom of the sea, and others in lakes and estuaries. We have now pointed out one great class of rocks, which, however they may vary in mineral composition, color, grain, or other characters, external and internal, may nevertheless be grouped together as having a common origin. They have all been formed under water, in the same manner as modern accumulations of sand, mud, shingle, banks of shells, reefs of coral,and the like, and are all characterized by strati¬ fication or fossils, or by both.