Ch. IE.] FRESHWATER AND MARINE FOSSILS. 27 



sands of well-preserved organic bodies, which abound in every minute 

 grain of chalk, and are especially apparent in the white coating of 

 flints, often accompanied by innumerable needle-shaped spiculae of 

 sponges. After reflecting on these discoveries, we are naturally led on 

 to conjecture that, as the formless cement in the semi-opal of Bilin 

 has been derived from the decomposition of animal and vegetable re- 

 mains, so also many chalk flints in which no organic structure can be 

 recognized may nevertheless have constituted a part of microscopic 

 animalcules. 



" The dust we tread upon was once alive !"— Byron. 



How faint an idea does this exclamation of the poet convey of the 

 real wonders of nature ! for here we discover proofs that the calcareous 

 and siliceous dust of which hills are composed has not only been once 

 alive, but almost every particle, albeit invisible to the naked eye, still 

 retains the organic structure which, at periods of time incalculably re- 

 mote, was impressed upon it by the powers of life. 



Freshwater 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 fre- 

 quent lakes and rivers are distinct from those inhabiting the sea. In 

 the northern part of the Isle of Wight formations of marl and lime- 

 stone, more than 50 feet thick, occur, in which the shells are prin- 

 cipally, if not all, of extinct species. Yet we recognize their freshwater 

 origin, because they are of the same genera as those now abounding 

 in ponds and lakes, either in our own country or in warmer latitudes. 



In many parts of France, as in Auvergne, for example, strata of lime- 

 stone, marl, and sandstone are found, hundreds of feet thick, which con- 

 tain exclusively freshwater and land shells, together with the remains of 

 terrestrial quadrupeds. The number of land shells scattered through 

 some of these freshwater 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, Find- 

 heim, Budenheim, and other places. In order to account for this phe- 

 nomenon, the geologist 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 strewed over with innumerable dead land shells, which 

 have been brought down from valleys in the Alps in the preceding springy 

 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 individuals have been washed away from 

 the alluvial plains of the great river and its tributaries, some from 

 mountainous regions, others from the low country. 



