The alluvial matters, which terminate this latest col lection of independent strata, appear also to be a deposit from fresh water. The whole bed consists of marl, clay, and sand of different characters, intermixed with fragments of rock add of vegetable matters. But its most remarkable feature is, that it contains organic remains, consisting of the bones of oxen, deer, elephants, and other large quad rupeds, intermixed with trunks of trees. It is doubtful if any one of the species of these is identical with those belonging to similar genera now existing ; but analogous remains occur in many parts of the world in similar alluvia, where the same inferior strata are not present, as in England and Ireland, in the upper strata that border the Appenines, and in many well-known situations which it is unnecessary to enumerate. It must not therefore be considered as having any necessary relation to the basin over which it lies, or as increasing the difficulty of explaining that which, with out it, is sufficiently difficult of explanation.
It is by no means easy to explain these alternations of fresh and salt water deposits, although the general principles of stratification will account for the deposition of the strata, with that of all the bodies which they con tain. It has been argued, that there is no fixed distinction between a salt water and a fresh water shell, so as to enable an observer to pronounce on their origin. This remark is unquestionably just. The texture or thickness of a shell, as we already observed, is no test ; and we might add to the instance of Argonauta, that many marine species of Pinna, Patella, and Pecten, are as delicate as the shells of fresh waters. It has lately been remarked, by M. Freminville, that different species of Anodon, Cyclas, Unio, Tellina, Cardium, and Venus, some belonging to fresh and others to salt water, live promiscuously together in the gulf of Livonia. The habits, indeed, of many shell fish, in this respect, are convertible. Our own muscles and oysters thrive even better in fresh water than in salt. Some of our turbines and trochi seem equally indifferent to the quality of the water in which they live; and it is not improbable that a great many of these insensible tribes of animals are exceedingly versatile in this respect, and very indifferent to the quality of the element in which they reside.
As this question of the versatility of aquatic animals is of great importance in all our reasonings respecting fresh water deposits, it will not be here out of place to prolong these remarks, for the purpose of stating a few facts yet unattended to, respecting the facility with which many existing species of fishes change their residence, not only for temporary purposos, but permanently. The plaise, (Pleuronectes platessa,) has been established in the ponds of East Friesland, and the grey mullet, (Mugil cephalus„` has, in the same manner, been naturalized to fresh water in Guernsey. The Pleuronectes flessus, or flounder, which is truly a salt water fish, is permanent in the freshest waters of the Thames. The sea trout is equally so in
fresh water lakes in Lismore. These are examples of the complete transfer of marine fish to fresh waters.
In a great many species, it is found, that part of their time is passed in fresh water, and part in the sea ; the migrations from the latter to the former being, in many instances, for the purpose of depositing their spawn ; and, in others, it would appear either the result of accident or caprice, or intended for the pursuit of their prey. In one instance, that of the common eel, (Mut xna anguilla,) the migration appears intended for the purpose of depositing the spawn in the salt water. It is probable that those who can thus alter their residence for temporary purposes, may, under compulsion, or change of views, alter it per manently ; and this, in fact, appears to be the case with many species which will be enumerated presently.
If the essential function of respiration can go on for a time in both qualities of water, it may go on for ever. The migrating fish find food under all their migrations ; and there is no reason why they should not find it per manently; while there is as little reason to apprehend that numerous species may not propagate their kind under changes of habitation, as those already named are known to do. There is no reason, a priori, against the trans ference of the residence of fishes, and there are many proofs of its existence.
It is almost superfluous to point out the change of re sidence which is sought for by the salmon, the sturgeon, the smelt, the eel, the lamprey, the stickel back, the salmon trout, the grey salmon, the gwiniad, (Salmon lavaretus,) and the Salmo migratorius. The pike lives in the Caspian Sea, and in the fresh waters of our own country. The conger enters rivers, as does the torsk ; and Liancourt found the herring in the Potowmack, Hudson, Elk, and Delaware rivers. The Gadus barbatus, and Tricirratus, frequent the fresh-water extremities of the Highland lochs, as does the mackarel; and the cod, in Shetland, resides, for a time at least, in a fresh-water lake that communicates with the sea by a short and narrow stream. The Cottus quadricornis, like the sprat and shad, also frequents rivers to spawn. In the same man ner, the Delphinus leucas, or white whale, is known to ascend the rivers of North America, even to a great dis tance from the sea, misleading Hearne and Mackenzie in their geography, as its bones may hereafter mislead geologists, when the alluvia of the Hudson shall have buried them in its banks, a hundred miles from the ocean. We may terminate this enumeration, by remarking, that eight species of Cyprinus inhabit the Caspian Sea, like the pike; although with us they are all the inhabitants of fresh water, showing a perfect indifference to the nature and quality of the element in which they reside. These are the Cyprinus Idus, Nasus, Aspius, and Ballerus, to gether with the Carassius, Rutilus, Alburnus, and Brama, our crucian, roach, bleak, and bream.