150 The American Geologist. March. 1904. 
show, from the Jacksonian upward, a yellowish tinge due to oxidation. 
In the Oak Grove sands come the first indications of a change towards 
the gray of the Miocene marls. With the incursion of the colder water 
the change becomes complete. Not only do northern animals compose 
the fauna, but the southern ones are driven out, some of them surviving 
in the Antilles to return later. Some change along the northern coast 
permitted 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 Flor- 
ida and Tampa on the west coast, as shown by artesian borings. 
"The movement in elevation which ushered in the Miocene continued, 
probably, during its entire term. It amounted in Costa Rica, according 
to Gabb, to several thousand feet, and permanently united the two con- 
tinents. 
"I concur with Hill in the belief that, whatever changes of level may 
have taken place since, no discontinuity of the Hnk 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 Mio- 
cene epoch, Florida became united to the continent, the Suwannee Strait 
was obliterated, and the influx of cold water into the Gulf ceased. Grad- 
ually the temperature rose and the exiled subtropical species began to 
return. The cold current must have been diverted off shore or else- 
where, for a migration northward, during the latter part of the Miocene, 
of many species and genera belonging 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 per- 
sist when there is a change to a lower temperature. Brooks' experiments 
in Chesapeake Bay showed that a fall of two degrees Fahrenheit in the 
temperature of the water killed all the swimming larvae of Ostrca vir- 
ginica, but a rise of twice as many degrees would probably only have 
hastened their development. 
"After the culmination of the upward movement terminating the 
Miocene, a slight depression of the continental border and a change in 
