Page:A Treatise on Geology, volume 2.djvu/41

 CHAP. VI. and more regular deposits than those produced by merely mechanical agency.

Shallow lakes, subject to fluctuation, produce on the deposits of coarse gravel and sand, which are brought into them by rivers, an effect intermediate between that of deep water and mere fluviatile currents. The conoidal lamination due to the former is complicated with variation of the point of influx arising from the latter; and thus the upper ends of such lakes become irregular in outline, and are filled by insulated subaqueous banks.

The deposition of sediments from a river happens in all parts of a valley, even very near to the sources of the stream, if the slopes of the ground permit; but as towards the sea, generally, the inclination becomes the most gentle, it is there that the finer sediments drop most abundantly.

The cross section of the 'straths' or narrow meadows which are produced in the upper parts of valleys are usually level, or rather a little highest near the edge of the river, and a little lowest where the new surface touches the old (technically 'hard') land. The sediment is rather coarser near the river edge, rather finer at a distance from it, but every where laminated according to the frequency and continuity of the inundations.

Inland seas, which by their position are exempt from strong tides and currents, become filled with river sediments, under the same conditions as large lakes. Their area is contracted, by the addition of new land on the margin, and their depth is lessened by the diffusion of fine sediment over the bed, to various distances, according to circumstances already pointed out while treating of lakes.

Some of the most considerable deltas at the mouths of rivers have been accumulated in seas of this quiet character; as the delta of the Nile, which is a continuation of the long valley of Egypt; the wide sediments at