374

FOSSIL SHELLS OE THE LOESS.

CHAP.. XYI.

rock and the stones and sand frozen into the ice are exposedin the manner before described (p. 357). We may thereforereadily conceive that a much greater volume of fine sedimentwas swept along by rivers swollen by melting ice at the timeof the retreat of the gigantic glaciers of the olden time. Thefact that a large proportion of this mud, instead of beingcarried to the ocean, where it might have formed a delta onthe coast, or have been dispersed far and wide by the tidesand currents, has accumulated in inland valleys, will be foundto be an additional proof of the former occurrence of thosegrand oscillations in the level of the Alps and parts of theadjoining continent which were required to explain thealternate advance and retreat of the glaciers, and the super-position of more than one boulder-clay and stratified alluviumas before mentioned (p. 366).

The position of the loess between Basle and Bonn is suchas to imply that the great valley of the Rhine had alreadyacquired its present shape, and in some places perhaps morethan its actual depth and width, previously to the time whenit was gradually filled up to a great extent with fine loam.The greater part of this loam has been since removed, so thata fringe only of the deposit is now left on the flanks of theboundary hills, or occasionally some outliers in the middle ofthe great plain of the Rhine where it expands in width.

These outliers are sometimes on such a scale as to. admit ofminor hills and valleys having been shaped out of them bythe action of rain and small streamlets, as near Freiburg inthe Brisgau and other districts.

Fossil Shells of the Loess.

The loess is generally devoid of fossils, although in manyplaces they are abundant, consisting of land-shells, all ofliving species, and comprising no small part of the entire