20S Costa Rica Miocene — Olsson 33 



Anomia simplex d'Orb St. Mar3''s to recent 

 Cardium spifioszcm var. Tur- 



toni Dall Miirfreesboro to recent 



Cardium serratum Linne Murfreesboro to recent 



Cardizim medium Linne St. Mary's to recent 



Labiosa lineata Say Duplin to recent 



Tellina umbra Dall Duplin to recent 



Chione grus Holmes Duplin to recent 



Phacoides radians Oonrad Duplin to recent 



Phacoides trisulcatus Conrad Chesapeake to recent 



DivaricellaguadristdcatadiOrh Murfreesboro to recent 

 Oliva say ana va.x .i^nynortua 



Pilsbry and Brown typical shell, upper 

 Chesapeake to recent 



Fasciolaria tulipa Linne Pliocene 



The Costa Rican Geosynci^ine 



The wide distribution of Miocene beds around the borders 

 of the Gulf of Mexico and Caribbean sea, indicates a condition 

 of general deposition during Miocene time. Costa Rica was 

 largely under water, at least during the Lower Miocene, and 

 was the locus of a strait or geosyncline that then connected the 

 Atlantic and Pacific Oceans; while Panama, parts of Nicaragua, 

 and other central American areas were mainly above sea level. 

 This condition is shown in the general igneous character of the 

 backbone of Panama, while in Costa Rica, sedimentary rocks of 

 Miocene age occur even in the high interior and at elevations of 

 several thousands of feet. This geosj^ncline covered the greater 

 part of Costa Rica, a portion of western Panama, and western 

 Nicaragua. It should be noted that this area, at the present 

 time, is the locus of a large number of active volcanoes. 



Certain changes of importance took place at the close of 

 Uscari time, resulting in a brief withdrawal of the Miocene sea. 

 The effects of these changes were widespread. These great land 

 movements, which probably reached a culmination in the Plio- 



