UNIVERSITY OF CALIFORNIA 



Bulletin of the Department of Geology 



Vol. 2, No. 8, pp. 231-267 ANDREW C. LAWSON, Editor 



THE GEOLOGY 



OF THE 



CENTRAL PORTION OF THE ISTHMUS OF PANAMA. 



BY 



Oscar H. Hershey. 

 CONTENTS. 



Page. 



Introduction 232 



Area Studied 233 



Topography 234 



Formations Earlier Than the Pleistocene 237 



The Azuero Formation 237 



The Torio Limestone 239 



The Montijo Conglomerate 241 



The Santiago Formation 241 



The Tertiary Basal Conglomerate 243 



The Tertiary Red Shale 243 



The Panama Formation , 244 



The Canazas Formation 247 



The Veraguas Crystalline Series 247 



Resume of the Pre-Pliocene Isthmian History 249 



Development of Present Topography 251 



The Tertiary Dissected Peneplain 252 



The Pleistocene Peneplain 255 



Pleistocene Formations , 257 



The Aguadulce Formation 258 



The San Carlos Formation 259 



The Mariato Formation 261 



The Middle Pleistocene Uplift 261 



A Recent Depression of the Coastal Lands 262 



The Parita Formation 263 



Modern Stream Gravels 264 



Summary of Post-Eocene Earth Movements 264 



Origin of Isthmian Stream Courses 265 



Pleistocene Oscillations of the Southwest Coast of North America 265 



