130 Tin-; XAUTILUS. 



the ocean-currents, affected the cliaracter of the Floridian fauna even more pro- 

 foundly than did those clianges whicli terminated the Eocene. 



" The period hetween the inception of the Miocene and tlie modification of its 

 original fauna covered the deposition of the beds comprising the Chattahooche 

 group of Langdon and tlie Tampa group of Dall, and, from the fact that its warm- 

 water fauna is best displayed in ihe Chijjola beds of Northwest Florida, along the 

 river of the same name, may be called the Chipola epoch. During this epoch sub- 

 tropical mollusks, such as Cymia and Voluta, flourished as far north as New 

 Jersey. The temperature-indications of the fauna do not differ essentially as far 

 as our knowledge goes, from those of ihe previous later Eocene fauna. At no 

 succeeding epoch do we find subtropical or tropical mollusks extending northward 

 to such a distance from their present range. If any of the leaf-beds of Greenland 

 are really Miocene, these facts authorize the suspicion that the period when wal- 

 nuts ripened on the shores of the Arctic Sea may have been synchronous with the 

 warm Chipola epoch of the early Miocene. 



" Wiiether an eastward deflection of the Gulf Stream, connected with elevation of 

 the Great Carolinian Ridge, or some other undetermined cause, offered the oppor- 

 tunity, a colder inshore current seems to have crept southward along the conti- 

 nent, penetrated the strait between Georgia and Florida, and washed the northern 

 shores of the Gulf of Mexico. With it came the cold-water fauna appropriate to 

 its temperature. This fauna be^an early in the north, nearly the whole mass of 

 the New Jersey, Maryland and Virginia Miocene being of this character. South- 

 ward the mass relative to that of the Chipola epoch gradually diminishes, being 

 less in the Carolinas and least in the Floridian region. With this fauna were 

 introduced the conspicuous forms which are known as characteristic of the Mio- 

 cene of Maryland and Virginia, the large Pectens and Areas, Venus and Ecphora. 

 Profusely developed about Chesapeake Bay, where it is found in those beds to 

 which Darton and the writer, independently, came to apply the name of Chesa- 

 |ieake, the period in which it flourished may appropriately be designated as the 

 Chesapeake epoch. The fauna introduced at this time has left lasting traces on 

 the fauna of the Gulf of Mexico even to the present moment, but never reached as 

 far south as the Florida Keys or the southern portion of the peninsula. The 

 faunal change was decidedly the most important mutation which is traceable in 

 the fossil vertebrate faunas of the Gulf and Floridian region during the whole of 

 Post-Eocene time. 



" The Chipola epoch here, in general, was a period of very slow and gentle ele- 

 vation, followed at or near its close by a slight depression equally gentle. 



" The Chesapeake epoch in the South was in the main a period of quiescent 

 deposition, and was closed by a very important movement in elevation. In the 

 Central American region (notably Costa Rica), the Miocene rocks were elevated 

 to a height of 12,f)00 feet above the sea. The Panamic connections between tlie 

 Caribbean Sea and the Pacific Ocean were definitely terminated, and the connec- 

 tion b.;tween the continents of North and South America finally brought about. 

 On the northern shores of the Gulf of Mexico the elevation was more moderate, 

 but considerable, and by it the island of Florida was united to the Georgian 

 mainland and the previously existing strait permanently clo^ed. This event, in 

 the classification proposed i)y the writer, terminates the Miocene. 



