192 THE RIVER VALLEYS OF 



position by the waters of the Shannon, while others 

 probably are the remains of animals swamped while 

 feeding on or crossing the marshes, in the same 

 way as horses or cattle may be swamped at the 

 present day. Land and fresh-water shells ought 

 necessarily to occur more frequently in the sands 

 near the mouth of a river than elsewhere ; but they 

 may also be carried out to the terrace gravels. 

 Professor Hennessy, F.E.S., &c., has pointed out 

 the conditions under which sand and portions of 

 shells will float. 1 On a warm and dry day, the dried 

 shells and sand, when the tide rises to them, repel 

 the water, and swim thereon, and as the tide 

 moves gently past, they rise one by one, and float off. 

 We have seen shells floating also, after the manner of 

 boats ; but the sea must be perfectly calm, as the 

 moment they meet a ripple or breeze, they are over- 

 balanced, fill, and sink. In this way land shells, 

 although so frail, may be carried out and deposited 

 in salt water. 



The sea in an estuary is quite capable of levelling 

 alluvium, even although coarse shingle, and thereby 

 forming extensive flats ; this, however, is not the 

 case with a river, as the materials carried by it, 

 especially if coarse, are usually piled in banks. A 

 river-bed may be prolonged forward, by the stream 

 carrying gravel, &c., into a lake, forming a level 



1 "Floatation of Sand," Geological Magazine, vol. viii. p. 316. 



