Oh. X.] LOESS FOSSILS OF THE RHINE. H9 



fossil shells, identical with species now living in the Nile, have been 

 fonnd. The probable causes of such alterations in the level of the 

 river, and the successive filling up and re-excavation of the same 

 hydrographical basin at different periods, will be presently spoken of. 

 They are changes of a kind that cannot fail to result from great conti- 

 nental movements of subsidence and upheaval, such as we may safely 

 assume that Egypt has undergone in the post-tertiary epoch, because 

 the eastern shore of the Red Sea on one side, and the great desert of 

 the Sahara on the other, have been converted from sea into land since 

 the commencement of the Post-pliocene period. 



In some parts of the valley of the Rhine the accumulation of similar 

 loam, called in Germany " loess," has taken place on an enormous scale. 

 Its color is yellowish-gray, and very homogeneous ; and Professor 

 Bischoff has ascertained, by analysis, that it agrees in composition 

 with the mud of the Nile. Although for the most part unstratified, 

 it betrays in some places marks of stratification, especially where it 

 contains calcareous concretions, or in its lower part where it rests on 

 subjacent gravel and sand which alternate with each other near the 

 junction. About a sixth part of the whole mass is composed of car- 

 bonate of liine, and there is usually an intermixture of fine quartzose 

 and micaceous sand. 



Although this loam of the Rhine is unsolidified, it usually termi- 

 nates where it has been undermined by running water in a vertical 

 cliff, from the face of which shells of terrestrial, freshwater and 

 amphibious mollusks project in relief. These shells do not imply the 

 permanent sojourn of a body of fresh water on the spot, for the most 

 aquatic of them, the Succinea, inhabits marshes and wet grassy meadows. 

 The Succinea elongata, (or S. oblonga,) fig. 107, is very characteristic 

 both of the loess of the Rhine and of some other European river- 

 loams. 



Among the land-shells of the Rhenish loess, Helix plebeia and Pupa 

 muscorum are very common. 



Fig. 107. Fig. 108. Fig. 109. 



4 



Succinea elongata. Pupa muscorum. Helix plebeia. 



Both the terrestrial and aquatic shells are of most fragile and deli- 

 cate structure, and yet they are almost invariably perfect and uninjured. 

 They must have been broken to pieces had they been swept along by 

 a violent inundation. Even the color of some of the land-shells, as 

 that of Helix nemoralis, is occasionally preserved. 



I observed the three fossils above figured in the upper fluviatile 

 loam of the Saale, near Rudolstadt, in Thuringia, a river which falls 

 into the Ilm, and belongs to the basin of the Elbe. I have also seen 



