No. 5. — TJie Geological History of the Isthmus of Panama and 

 Portions of Costa Pica. Based %L r pon a Peconnoissance made 

 for Alexander Agassiz. By Eobert T. Hill. 



TABLE OF CONTENTS. 



PAGE 



Introduction 154 



Part I. — The Geographic Position of the Isthmian Region . . . 158 

 Independence of the North, Central, and South American Orogenic Sys- 

 tems 159 



The Later Volcanic Mountains, — their Distribution and Relation to the 



Folded Regions 160 



Physical Differences between the Lands surrounding the Caribbean Sea 



and the Gulf of Mexico 161 



The Greater Isthmian Region includes the whole of Central America . 161 



The Recent Volcanic Eminences mostly on the Pacific Side . . . 162 



The Caribbean Side a Region of East and West Mountain Folds . . 162 



The Isthmus of Panama defined 162 



The Mountainous Topography of the Isthmus . . . 163 



Absence of a Dividing Continental Ridge 163 



The Coastal Topography void of a well defined Coastal Plain .... 164 



The Drainage of the Isthmus 165 



The Isthmian Topography contrasted with that of the adjacent Andean 



and Costa Rican Heights 1G6 



Topographic Evidence of the Antiquity of the Isthmus 167 



Part II. — The Geology of the Continental Section, Colon to 



Panama 172 



The Caribbean Coast 172 



Colon Harbor 172 



Manzanilla Island 173 



Manzanilla Bay 173 



The Isthmian Swamps 173 



Colon to Bujio 175 



The Monkey Hill Base Level 175 



The Inland Swamps 176 



The Foraminiferal Marls of Bujio 177 



Section from Bujio to Colon along the Line of the Panama Canal ... 179 



Bujio to the Pacific 182 



The Barbacoas Subsection 183 



The Bujio Formation 183 



The Pena Negra Hill 183 



vol. xxviii. — no. 5. 1 



