I have endeavoured to show that the deposits forming the delta and alluvial plain of the Mississippi consist of sedimentary matter, extending over an area of 30,000 square miles, and known in some parts to be several hundred feet deep. Although we cannot estimate correctly how many years it may have required for the river to bring down from the upper country so large a quantity of earthy matter--the data for such a computation being as yet incomplete--we may still approximate to a minimum of the time which such an operation must have taken, by ascertaining experimentally the annual discharge of water by the Mississippi, and the mean annual amount of solid matter contained in its waters. The lowest estimate of the time required would lead us to assign a high antiquity, amounting to many tens of thousands of years to the existing delta, the origin of which is nevertheless an event of yesterday when contrasted with those terraces, _c_, and _d e_, fig. 106., formed of the loam No. 2. above mentioned. These materials of the bluffs _a_ and _d_ were produced, the reader will observe, during the first part of that great oscillation of level which depressed to a depth of 200 feet a larger area than the modern delta and plain of the Mississippi, and then restored the region to its former position.[117-A] _Loess of the Valley of the Rhine._--A similar succession of geographical changes, attended by the production of a fluviatile formation, singularly resembling that which bounds the great plain of the Mississippi, seems to have occurred in the hydrographical basin of the Rhine, since the time when that basin had already acquired its present outline of hill and valley. I allude to the deposit provincially termed _loess_ in part of Germany, or _lehm_ in Alsace, filled with land and freshwater shells of existing species. It is a finely comminuted sand or pulverulent loam of a yellowish grey colour, consisting chiefly of argillaceous matter combined with a sixth part of carbonate of lime, and a sixth of quartzose and micaceous sand. It often contains calcareous sandy concretions or nodules, rarely exceeding the size of a man's head. Its entire thickness amounts, in some places, to between 200 and 300 feet; yet there are often no signs of stratification in the mass, except here and there at the bottom, where there is occasionally a slight intermixture of drifted materials derived from subjacent rocks. Unsolidified as it is, and of so perishable a nature, that every streamlet flowing over it cuts out for itself a deep gully, it usually terminates in a vertical cliff, from the surface of which land shells are seen here and there to project in relief. In all these features it presents a precise counterpart to the loess of the Mississippi. It is so homogeneous as generally to exhibit no signs of stratification, owing, probably, to its materials having been derived from a common source, and having been accumulated by a uniform action. Yet it displays in some few