LOESS OR mUNDATION-MUD. 
155 
Fig. 88. Fig. 89. Fig. 90. 
Succinea elongata. Pupamuscorum (Linn.). Helix hispida {JAmi.) {plebeia). 
nea^ inhabits marshes and wet grassy meadows. The 8uc- 
einea elongata (or & oblongata)^ Fig. 88, is very characteris¬ 
tic both of the loess of the Rhine and of some other Europe¬ 
an river-loams. 
Among the land-shells of the Rhenish loess, Helix hispida^ 
Fig. 90, and Pupa muscorum^ Fig. 89, are very common. 
Both the terrestrial and aquatic shells are of most fragile 
and delicate 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. 
In parts of the valley of the Rhine, between Bingen and 
Basle, the fluviatile loam or loess now undSr consideration 
is several hundred feet thick, and contains here and there 
throughout that thickness land and amphibious shells. As 
it is seen in masses fringing both sides of the great plain, 
and as occasionally remnants of it occur in the centre of the 
valley, forming hills several hundred feet in height, it seems 
necessary to suppose, first, a time when it slowly accumu¬ 
lated ; and secondly, a "later period, when large portions of 
it were removed, or when the original valley, which had 
been partially filled up with it, was re-excavated. 
Such changes may have been brought about by a great 
movement of oscillation, consisting first of a general depres¬ 
sion of the land, and then of a gradual re-elevation of the 
same. The amount of continental depression which first 
took place in the interior, must be imagined to have exceed¬ 
ed that of the region near the sea, in which case the higher 
part of the great valley would have its alluvial plain grad¬ 
ually raised by an accumulation of sediment, which would 
only cease when the subsidence* of the land was at an end. 
If the direction of the movement was then reversed, and, 
during the re-elevation of the continent, the inland region 
nearest the mountains should rise more rapidly than that 
near the coast, the river would acquire a denuding power 
sufficient to enable it to sweep away gradually nearly all 
the loam and gravel with which parts of its basin had been 
filled up. Terraces and hillocks of mud and sand would 
then alone remain to attest the various levels at which the 
