Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/21

8 by a tideless or a tidal sea, and to its relative amount of exposure to prevalent winds and breakers. In a tideless sea the effects produced would be similar to those in a lake. We may, indeed, consider such seas, as those of the Caspian, as mere saline lakes (whether portions of the ancient sea, detached by alterations in the relative levels of land and sea, or bodies of water rendered saline by reposing on saliferous rocks), in which not only the matters mechanically suspended and brought down by rivers into them are accumulated, but those in chemical solution also, inasmuch as such waters are only retained as they are by the evaporation of those so brought into them. Hence, in such isolated seas the river-borne saline additions should have an influence on the deposits formed in them.

The Mediterranean affords a good example of the varied accumulations which take place in tideless seas, for the Mediterranean may be so termed as regards the subject under consideration, as its coasts afford examples of both shallow and deep water immediately adjoining them, into which the matter brought by rivers, mechanically and chemically, is discharged.

When we regard the mouths of the great rivers which have flowed over much level ground, due in a great measure to the mud, sand, and gravel brought down by themselves, such as the Nile, the Po, and the Rhone, little else is discharged into the sea except finely comminuted matter. This, especially during floods, is carried bodily outwards; no small portion moving with the fresh waters over those of the sea, from the less specific gravity of the whole. The fine detritus eventually settles, and charts show us the form in which this is effected. The action of breakers prevents the accumulation of the muddy matter within their range, so that the accumulations in the immediate coast line become complicated. The river action, and the piling action of the breakers, combine to produce lines of sand, behind which accumulations of mud are effected, and these continue to be intermingled as the mouth of the river advances. Such is not the case with the detrital matter carried into the sea itself, beyond the action of the breakers. The sand forced forward on the bottom of the river will be more in flattened sheets, and nearest the mouth of the river, while the mud accumulates at a greater distance.

We may thus have three kinds of accumulations, one above the other, as the embouchures advance seaward. First and lowest, clay or mud, deposited at the greatest distance from the land, and inhabited by marine animals suited to such conditions, the thickness of the deposit depending on the original depth of bottom. Secondly; as the detrital accumulations of the river advanced, the clay or mud would become covered by the sand (mingled, perhaps, with some clay) forced over the