203 



Costa Rica Miocene — Olsson 



31 



tory of our southeastern Tertiary, and indicates physical changes 

 in the surrounding region, if not in Florida itself, sufficient to 

 alter the course of ocean currents and wholly change the temper- 

 ature of the waters of our southern coasts." 



The fauna of the Lower Miocene or Alum Bluff series is of 

 warm- water character and largel}^ composed of Antillean types. 

 In the Chesapeake series, the fauna is of cold or temperate-wat- 

 er character and it finds its relations with the present Atlantic 

 fauna north of Cape Hatteras. These faunal differences must be 

 take into account in an}^ correlation between the Miocene of the 

 United States with that of the West Indies and Central 

 America. 



In the Correlation Charts,! have placed the Gatun formation 

 as equivalent to the Chesapeake series, the Uscari formation 

 with the Alum Bluff. The evidence upon which this correla- 

 tion is based, may be summarized as follows: 



1. The two-fold division of the Costa Rican and Panama 

 Miocene, by an unconformity at the base of the Gatun forma- 

 tion. 



2. Wherever the Gatun beds have been found overlain by 

 more recent formations, the age of these overlying beds is Plio- 

 cene or younger. 



3. The high percentage of recent species in the Gatun 

 fauna, vv^hich agrees with the average as shown for the Chea- 

 peake Miocene. 



4. The Gatun fauna, contains a few species which appear 

 identical with strictly Chesapeake species, such as the follow- 

 ing: 



Dosonia acctabuliuti Conrad 

 Petaloconchus sc2ilpturat2ts H. C. Lea 

 Chajna congrcgata Conrad 

 Plicahila marginata Say ? 



