PART I. CHAPTER III. 



43 



Distinction of Freshwater from Marine Formations. 



extends over the northern part of the Isle of Wight, composed 

 of marl and limestone more than fifty feet thick. The shells are 

 principally, if not all, of extinct species ; but they are of the 

 same genera as those now abounding in ponds and lakes, either 

 in our own country or warmer latitudes. 



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

 of limestone, marl, and sandstone occur, hundreds of feet thick, 

 which contain 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 even districts where the rocks 

 scarcely contain any other fossils except snail-shells {helices ;) 

 as, for instance, the limestone on the left bank of the Rhine, 

 ]t)etween Mayence and Worms, at Oppenheim, Findheim, Buden- 

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

 non, the geologist has only to examine the small deltas of tor- 

 rents 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 innume- 

 rable dead land shells, which have been brought down from val- 

 leys 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 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. 



Although freshwater formations are often of great thickness, 

 yet they are usually very limited in area when compared to 

 marine deposits, just as lakes and estuaries are of small dimen- 

 sions in comparision with seas. 



We may distinguish a freshwater formation, first, by the 

 absence of many fossils almost invariably met with in marine 

 strata. For example, there are no corals, no sea-urchins, and 

 scarcely any other zoophytes; iio chambered shells, such as the 

 nautilus, nor microscropic Foraminifera. But it is chiefly by 

 attending to the forms of the mollusca that we are guided in ^ 

 determining the point in question. In a freshwater deposit, the 

 number of individual shells is often as great, if not greater, than 

 in a marine stratum ; but there are fewer species and genera. 

 This might be anticipated from the fact that the genera and spe- 

 cies of recent freshwater and land shells are few when contrasted 

 with the marine. Thus, the genera of true mollusca according 

 to Blainville's system, excluding those of extinct species and 

 those without shells, amount to about 200 in number, of which 



