HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 153 



PAGE 



The Island of Cano 219 



Judas Point and the Gulf of Nicoya 219 



Evidence of former Extent of Land on Pacific Side 219 



Part IV. — A Continental Section Across Costa Rica 220 



Salient Features of the Costa Rican Section 220 



Features of the Pacific Coast 222 



The San Mateo Peneplain 222 



The so called " Boulder Clays " 223 



The Aguacate Range 225 



Central Volcanic Plateau 225 



Ancient Bolsons of the Plateau Region 226 



The Cretaceous Rocks of San Miguel 227 



Old Volcanic Rocks of the San Jose' Basin 227 



The Cartago Basin 228 



Ascent of Irazu Volcano 228 



Peculiar Loess-like Deposits of the Slopes 228 



Report of Microscopic Study thereon by H. W. Turner . . . 229 

 The Crater of Irazu and the Light it throws on the Origin of the 



Boulder Clays 229 



Descent of the Atlantic Coast . . " 231 



Upturned Tertiary Sediments of the Atlantic Coast 232 



The Probable Occurrence of Granite at Siquieres 233 



The Coastal Region, and its Resemblance to the Isthmus at 



Colon 234 



The Pliocene Localities of Gabb 234 



Comparison of the Panama and Costa Rica Sections 235 



Tabular Statement of Formations and Events 236 



Coal in the Isthmian Tertiary 237 



Part V. — The Union of the Continents, and the Problems of 



the Straits 238 



Re'surae' of Scientific Opinion concerning Periods of Continental Separa- 

 tion and Union 238 



Re'sume' of the Known Geology of the Tropical American Mainland . . 239 



Incompleteness of Exploration 239 



Older Granitic Axes of the Caribbean Region 239 



Known Paleozoic Strata 242 



The Older Mesozoic 242 



Absence of Jurassic Sedimentary Rocks 243 



The Cretaceous Strata 243 



The Marine Tertiary 243 



Formations composed of Sea De'bris 244 



Land derived Littoral Deposits 245 



Older Tertiary Littoral of the Gulf and Caribbean 245 



Guatemala, Chiapas, and Yucatan 246 



Pliocene Formations 248 



Pleistocene and Recent 249 



Diastrophism of the Tropical American Mainland 249 



Predominance of the East and West Orogenic Trends 250 



Evidence of the Existence of a Pre-Tertiary Orogenic Complex . . 250 



Epochs of Igneous Activity 251 



Evidences of Pre-Tertiary Vulcanism 251 



The Date of Post-Eocene " Syenitic " Intrusions 252 



