TRANSACTIONS OF WAGNER 

 1550 



TERTIARY FAUNA OF FLORIDA 



an inshore cold current to penetrate the Gulf, depositing on the floor of the 

 shallow Suwannee Strait, separating the island of Florida from the continental 

 shore, a thin series of Miocene sediments, which were also carried as far south 

 as Lake Worth on the east coast of Florida and Tampa on the west coast, as 

 shown by artesian borings. 



The movement in elevation which ushered in the Miocene continued, prob- 

 ably, during its entire term. It amounted in Costa Rica, according to Gabb, 

 to several thousand feet, and permanently united the two continents. 



I concur with Hill in the belief that, whatever changes of level may have 

 taken place since, no discontinuity of the link between North and South 

 America from the Miocene to the present time is probable, and certainly none 

 amounting to a free communication between the two oceans. 



The Miocene of the Gulf coast is essentially the older Miocene of Maryland 

 and Virginia. No trace of it appears anywhere in the Antilles or on the Gulf 

 coast west of the Mississippi embayment. The Miocene fauna of the coast of 

 Texas, revealed by the Galveston artesian borings, is of a different stamp, more 

 nearly allied to that of the Pacific coast. It is probable that the wide stretch 

 of the Mississippi water pouring into the Gulf served as a barrier to the west- 

 ward migration of species of marine invertebrates. 



As the elevation culminated, leading to the termination of the Miocene 

 epoch, Florida became united to the continent, the Suwannee Strait was ob- 

 literated, and the influx of cold water into the Gulf ceased. . Gradually the 

 temperature rose and the exiled subtropical species began to return. The cold 

 current must have been diverted off shore or elsewhere, for a migration north- 

 ward, during the latter part of the Miocene, of many species and genera be- 

 longing in warmer water succeeded in reaching as far as North Carolina along 

 the coast, and some of them even as far as southern Virginia. This is quite 

 a marked element in the Duplin fauna. Some of the northern invaders kept 

 their foothold in the Gulf, became acclimated to the warmer temperature, and 

 survived. It is always easier for a cold-water invertebrate to survive in water 

 warmer than it is accustomed to than for one belonging in warm waters to 

 persist when there is a change to a lower temperature. Brooks' experiments in 

 Chesapeake Bay showed that a fall of two degrees Fahrenheit in the tempera- 

 ture of the water killed all the swimming larvae of Ostrea virginica, but a rise 

 of twice as many degrees would probably only have hastened their develop- 

 ment. 



After the culmination of the upward movement terminating the Miocene, a 

 slight depression of the continental border and a change in the fauna indi- 



