xiv CONTENTS. 



PAGE 



CHAPTER III. 

 Primary or Paleozoic System of Rocks and Paleozoic Era. 289-413 



General Description, 289 ; Rocks — Thickness, etc, 290 ; Area in the United States, 

 291 ; Geological Map of the United States, 291 ; Physical Geography of the Ameri- 

 can Continent, 292 ; Subdivisions, 293 ; The Interval, 294 ; Importance, 295. 



Section 1. Silurian System: Age of Invertebrates. — The Rock System, 296; 

 Subdivisions, 296 ; Character of the Rocks, 296 ; Area in America, 296 ; Physi- 

 cal Geography, 297 ; Primordial Beach and its Fossils, 298 ; General Remarks 

 on First Distinct Fauna, 301. General Life- System of the Silurian Age, 302; 

 Plants, 304 ; Animals, 304 ; Protozoans, 304 ; Radiates, Corals, 304 ; Hydrozoa, 

 30S; Polyzoa, 309; Echinoderms, 309; Mollusks, 313; General Description of 

 a Brachiopcd, 314 ; Lamellibranchs, 316 ; Gasteropods, 316 ; Cephalopods, 317 ; 

 Articulates, 320 ; Crustacea, 320 ; General Description of a Trilobitc, 320 ; Af- 

 finities of Trilobites, 325 ; Eurypterids, 326 ; Anticipations of the Next Age, 

 326 ; Land-Plants, Insects, Fishes, 326. 



Section 2. Devonian System and Age or Fishes, 327 ; Area in United States, 327 ; 

 Physical Geography, 327; Subdivision into Periods, 328. Life-System of Devo- 

 nian Age — Plants, 328 ; General Remarks on Devonian Land- Plants, 329 ; Ani- 

 mals, 332 ; Radiates, 332 ; Brachiopods,332 ; Cephalopods, 333 ; Crustacea, 333 ; 

 Insects, 334 ; Fishes, 334; Examples of Devonian Fishes, 335 ; Classification of 

 Devonian Ganoids, 338 ; Nearest Living Allies, 339 ; General Characteristics of 

 Devonian Fishes, 340 ; Devonian Fishes were Generalized Types, 342 ; Bearing 

 of these Facts on the Question of Evolution, 343 ; Suddenness of Appearance, 344. 



Section 3. Carboniferous System : Age of Acrogens and Amphibians. — Retro- 

 . spect, 345 ; Subdivisions of the Carboniferous System and Age, 345. Carbonif- 

 erous Proper — Rock-System or Coai-Mcasv.res, 346 ; The Name,' 346 ; Thickness 

 of Strata, 346; Mode of Occurrence of Coal, 346; Plication and Denudation, 

 343 ; Faults, 349 ; Thickness of Seams, 349 ; Number and Aggregate Thickness, 

 349 ; Coal Areas of the United States, 350 ; Extra-Carboniferous Coal, 351 ; Coal- 

 Areas of Different Countries compared, 351 ; Relative Production of Coal, 351. 

 Origin of Coal and of its Varieties, 352 ; Varieties of Coal, 353 ; Varieties de- 

 pending upon Purity, 353; upon Degree of Bituminization, 353; upon Propor- 

 tion of Fixed and Volatile Matter, 353 ; Origin of these Varieties, 354; Modes 

 of Decomposition — 1. In Contact with Air, 355 ; 2. Out of Contact with Air, 

 355 ; Mctamorphic Coal, 356. Plants of the Coal, their Structure and Affinities, 

 358 ; Where found, 358 : Principal Orders, 358 ; 1. Conifers, 358 ; Affinities of 

 Carboniferous Conifers, 361 ; 2. Ferns, 362 ; 3. Lepidodendrids, 365 ; 4. Sigil- 

 larids, 368 ; 5. Calamites, 370 ; Conclusions as to the Affinities of the Last Three 

 Orders, 372. Theory of the Accumulation of Coal, 373 ; Presence of Water, 

 373; Growth in situ, 373; At the Mouths of Rivers, 374; Application of the 

 Theory to the American Coal-Fields : (a) Appalachian Coal-Field, 375 ; (6) West- 

 ern Coal-Fields, 376 ; Appalachian Revolution, 377. Estimate of Time, 377 : 1. 

 From Aggregate Amount of Coal, 377 ; 2. From Amount of Sediment, 377. Physi- 

 cal Geography and Climate of the Coal Period, 378 ; Physical Geography, 378; 

 Climate, 379 ; Cause of this Climate, 380. Iron-Ore of the Coal- Measures, 383 ; 

 Mode of Occurrence, 383 ; Kinds of Ore, 383 ; Theory of the Accumulation of 

 the Iron-Ore of the Coal-Measures, 3S4. Bitumen, Petroleum, and Natural Gas, 

 386 ; Geological Relations, 386 ; Oil-Formations, 387 ; Principal Oil-Horizons of 

 the United States, 387 ; Laws of Interior Distribution, 387 ; Kinds of Rocks 

 which bear Petroleum, 388. Origin of Petroleum and Bitumen, 389; Theories 

 of, 389 ; Chemical Theory, 389 ; Organic Theory, 38^ ; Origin of Varieties, 390 ; 



