32G FOSSIL SHELLS OF THE LOESS. chap. xvr. 



conceive ithiit a much greater volume of fine sediment was 

 swept along by rivers swollen by melting ice at the time of 

 the retreat of the gigantic glaciers of the olden time. The 

 fact that a large proi^oi'tion of this mud, instead of being 

 carried to the ocean, where it might have foi'med a delta on 

 the coast, or have been dispersed far and w^ide by the tides 

 and currents, has accumulated in inland valleys, will be found 

 to be an additional proof of the former occurrence of those 

 grand oscillations in the level of the Alps and parts of the 

 adjoining continent which were required to explain the 

 alternate advance and retreat of the glaciers, and the super- 

 position of more than one boulder-clay and stratified alluvium 

 before mentioned, p. 321. 



The position of the loess between Basle and Bonn is such 

 as to imply that the great valley of the Ehine had ah'eady 

 acquired its present shape, and, in some places, perhaps more 

 than its actual depth and width, previously to the time when 

 it was gradually filled up to a great extent with fine loam. 

 The greater part of this loam has been since removed, so that 

 a fringe only of the deposit is now left on the flanks of the 

 boundary hills, or occasionally some outliers in the middle of 

 the great plain of the Rhine where it expands in width. 



These outliers are sometimes on such a scale as to admit 

 of minor hills and valleys having been shaped out of them 

 by the action of rain and small streamlets, as near Freiburg 

 in the Bi'isffau and other districts. 



Fossil Shells of the Loess. 

 The loess is generally devoid of fossils, although in many 

 l^laces they are abundant, consisting of land-shells, all of 

 living species, and comprising no small part of the entire 

 molluscous fauna now' inhabiting the same region. The 

 three shells most frequently met with are those represented in 



