FRESH-WATER AND MARINE FOSSILS.
Strata, whether deposited in salt or fresh water, have the same forms; but the
imbedded fossils are very different in the two cases, because the aquatic
animals which frequent lakes and rivers are distinct from those inhabiting the
sea. In the northern part of the Isle of Wight formations of marl and limestone,
more than 50 feet thick occur, in which the shells are of extinct species. Yet
we recognise 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-- strata 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 fossils except snail-shells (helices); as, for instance, the
limestone on the left bank of the Rhine, between Mayence and Worms, at
Oppenheim, Findheim, Budenheim, and other places.
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