154 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The Orogenic Revolution of Later Tertiary 953 



Epeirogenic Movements of Post-Miocene Time 95 . 



Re'sume' of the History of the Tropical American Mainland 257 



Obscurity of the Paleozoic Record 957 



Possibility of Laud Connection in Jurassic and Cretaceous 257 

 Possible Absence of Jurassic Marine Sediments on the Atlantic 



side of the North and South American Continents 257 

 Discordance of the Cretaceous fauna? of the Atlantic and Pacific 



Provinces 258 



Early Assertions of Community of Atlantic and Pacific Fossil 



Forms based upon Insufficient Data 260 



Results of Recent Researches 261 



Pacific Types in the Gatun Beds in Harmony with the Biologic 



Deductions that the Passage existed in the Eocene Tertiary . . 262 

 Evidence not conclusive that the Isthmus of Panama was the exact Locus 



of the Passage 263 



Absence of Known Tertiary Formations on the Pacific Side of Trop- 

 ical America 264 



Lack of Testimony showing that the Eocene Passage was other than 



Shallow and Restricted 264 



Absence of Off-shore Sediments indicating Deep-water Conditions . . . 264 

 Harmony of Geologic and Biologic Testimony showing the Establishment 



of the Present Land Barrier at the Close of the Miocene 265 



Both Biologic and Geologic Testimony oppose the Presumption of Marine 



Connection in Pleistocene or Later Time 266 



Fallacious Data upon which Pleistocene Connection has been assumed . 267 



Summary and Conclusion 268 



Part VI. — Appendices 271 



Report by William H. Dall on the Tertiary Fossils 271 



Report by R. M. Bagg on the Tertiary Foraminifera 275 



List of Corals identified by Mr. T. Wayland Vaughan 275 



Report by J. Elliott Wolff on the Igneous Rocks 276 



Notes by Ahe Sjogren on the Eastern Section of Costa Rica 281 



INTRODUCTION. 



The problem of an interoceanic passage, which has attracted com- 

 mercial exploration to the Isthmus of Panama since the date of its 

 discovery, is accompanied by no greater fascination than that which 

 has attended the purely scientific question of the geologic origin of 

 this narrow bond of union between the North and South American 

 continents. 



The geological history of the Isthmus has been a subject of specula- 

 tion. Zoologists and paleontologists have written upon the forms of life 

 from the opposing shores, and by comparison have made many aide and 

 astute conclusions. The Polyps, Echinoderms, Mollusks, Crustacean 



