CHAPTER XIII

Summary

We have hitherto considered the destroying agency of running Water, as exhibited in the disintegration of rocks and transportation of matter from higher to lower levels. It remains for us to examine the reproductive effects of the same cause. The aggregate amount of matter accumulated in a given time at the mouths of rivers, where they enter lakes or seas, affords clearer data for estimating the energy of the excavating power of running water on the land, than the separate study of the operations of the same cause in the countless ramifications into which every great system of valleys is divided. We shall proceed to select some of the leading facts at present ascertained respecting the growth of deltas, and shall then offer some general observations on the quantity of subaqueous sediment transported by rivers, and on the manner of its distribution. Deltas may be divided into, first, those which are formed in lakes ; secondly, those formed in inland seas ; and thirdly, those formed on the borders of the ocean. The most characteristic distinction between the lacustrine and marine deltas consists in the nature of the organic remains, which become imbedded in their deposits; for, in the case of a lake, it is obvious that these must consist exclusively of such genera of animals as inhabit the land or the waters of a river or lake; whereas, in the other case, there will be an admixture and most frequently a predominance of animals which inhabit salt water.