124 LOESS OF THE EHINE [Ch. X. 



and depth to allow of the simultaneous accumulation of the loess, at various 

 heights, throughout the whole area where it now occurs, I formerly suggest- 

 ed that, subsequently to the period when the countries now drained by the 

 Rhine and its tributaries had nearly acquired their actual form and geo- 

 graphical features, they were again depressed gradually by a movement 

 like that now in progress on the west coast of Greenland.* In propor- 

 tion as the whole district was lov/ered, the general fall of the waters 

 between the Alps and the ocean w^as lessened ; and both the main and 

 lateral valleys, becoming more subject to river inundations, were partially 

 filled up with fluviatile silt, containing land and freshwater shells. When 

 a thickness of many hundred feet of loess had been thrown down slowly 

 by this operation, the whole region was once more upheaved gradually. 

 During this upward movement most of the fine loam would be carried 

 off by the denuding power of rains and rivers ; and thus the original 

 valleys might have been re-excavated, and the country almost restored to 

 its pristine state, with the exception of some masses and patches of loess 

 such as still remain, and which, by their frequency £.nd remarkable ho- 

 mogeneousness of composition and fossils, attest the ancient continuity 

 and common origin of the whole. By imagining these oscillations of 

 level, we dispense with the necessity of erecting and afterwards removing 

 a mountain barrier sufficiently high to exclude the ocean from the valley 

 of the Rhine during the period of the accumulation of the loess. 



The proportion of land shells of the genera Helix^ Pupa^ and Buli- 

 mus, is very large in the loess ; but in many j^laces aquatic species of 

 the genera Lyinnea, Paludina, and Planorbis are also found. Thesf; 

 may have been carried away during floods from shallow pools and 

 marshes bordering the river ; and the great extent of marshy ground 

 caused by the wide overflowings of rivers above supposed would favor 

 the multiplication of amphibious mollusks, such as the Succinea (fig. 

 106), which is almost everywhere characteristic of this formation, and is 

 sometimes accompanied, as near Bonn, by another species, S. amphibia 

 (fig. 34, p. 29). Among other abundant fossils are Helix pleheia and 

 Pupa TYi'iscGrum. (See Figures.) Both the terrestnal and aquatic shells 

 preserved in the loess are of most fragile and delicate structure, and yet, 



Fig. 106. Fig. 107. Fig. 108. 



Succinea elongata. Pupa muscorum. Eelixplebeia. 



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 ncnioralis, is 

 occasionally preserved. 



* Princ. of Geol. 3d edition, 1834, vol. iii, p. 414. 



