FRESH-WATER AND MARINE ' FOSSILS. 
53 
50 feet thick occur, in which the shells are of extinct species. 
Yet we recognize their fresh-water origin, because they are 
of the same genera as those now abounding in ponds, lakes, 
and rivers, either in our own country or in warmer latitudes. 
In many parts of France^—in Auvergne, for example—stra¬ 
ta occur of limestone, marl, and sandstone hundreds of feet 
thick, which contain exclusively fresh-water and land shells, 
together with the remains of terrestrial quadrupeds. The 
number of land-shells scattered through some of these fresh¬ 
water deposits is exceedingly great; and there are districts 
in Germany where the rocks scarcely contain any other fos¬ 
sils except snail-shells [helices ); as, for instance, the lime¬ 
stone on the left bank of the Rhine, between Mayence and 
Worms, at Oppenheirn, Findheim, Budenheim, and other 
places. In order to account for this phenomenon, the geol¬ 
ogist has only to examine the small deltas of torrents which 
enter the Swiss lakes when the waters are low, such as the 
newly-formed plain where the Kander enters the Lake of 
Thun. He there sees sand and mud strewn over with innu¬ 
merable dead land-shells, which have been brought down 
from the valleys in the Alps in the preceding spring, during 
the melting of the snows. Again, if we search the sands 
on the borders of the Rhine, in the lower part of its course, 
we find countless land-shells mixed with others of species 
belonging to lakes, stagnant pools, and marshes. These in¬ 
dividuals have been washed away from the alluvial plains 
of the great river and its tributaries, some from mountain¬ 
ous regions, others from the low country. 
Although fresh-water formations are often of great thick¬ 
ness, yet they are usually very limited in area when com¬ 
pared to marine deposits, just as lakes and estuaries are of 
small dimensions in comparison with seas. 
The absence of many fossil forms usually met with in ma¬ 
rine strata, aflTords a useful negative indication of the fresh¬ 
water origin of a formation. For example, there are no sea- 
urchins, no corals, no chambered shells, such as the nautilus, 
nor microscopic Foraminifera in lacustrine or fluviatile de¬ 
posits. In distinguishing the latter from formations accu¬ 
mulated in the sea, we are chiefly guided by the forms of 
the mollusca. In a fresh-water deposit, the number of indi¬ 
vidual shells is often as great as in a marine stratum, if not 
greater; but there is a smaller variety of species and gen¬ 
era. This might be anticipated from the fact that the gen¬ 
era and species of recent fresh-water and land shells are few 
when contrasted with the marine. Thus, the genera of true 
mollusca according to Woodward’s system, excluding those 
