AND FRESH-WATER FORMATIONS. 329 



fresh water, might not have been marine, or the re- 

 verse. 



This same difficulty exists with regard to the fishes ; 

 by the remains of which naturalists have so often pre- 

 tended to decide on the character of a deposit. As 

 the facts which prove this are important, and as they 

 are new to geology at least, I shall quote them from 

 my former writings. They will show that while many 

 fishes change their residence voluntarily, others can 

 be compelled to do so ; and that marine species will 

 not only thrive, but breed in fresh waters, under com- 

 pulsory change. 



That this fact was known to the Romans, is proved 

 from the writings of Columella ; though the species 

 of sea fish thus transferred to fresh water can not be 

 fully ascertained. This practice descended to Sicily, 

 where it is now in use as to the Mullet and Lobster. 

 In nature, a certain number of fishes are either migra- 

 tory between fresh and salt water, or else permanently 

 indifferent to either. Such are the Sturgeon, the com- 

 mon and the grey salmon, the gwiniad, smelt, salmon 

 trout, and Salmo migratorius, together with the Lam- 

 prey, stickleback, Eel, and others. The Mullet, con- 

 ger, torsk, sprat, shad, cottus quadricornis, rockling, 

 whiting pout, mackerel, herring, Delphinus leucas, and 

 a few more, also quit the sea for fresh water, either 

 for the purpose of spawning or for other reasons ; 

 while the Cod, in Shetland, is the voluntary inhabitant 

 of a fresh water lake. Two or three species of flounder 

 also abandon the sea to live entirely in rivers. In a 

 reverse manner, the pike, and eight species of Cypri- 

 nns, which elsewhere are inhabitants of fresh water, 

 abide permanently in the Caspian. Further, the 

 salmon trout, supposed necessarily migratory, and the 

 herring, purely a sea fish, reside permanently in fresh 



