284 



GENERAL PRINCIPLES. 



» o ,^ » 



~3I § 



Recent. 

 Qualer- 

 nary. 



Tapir, Peccary, Bison, Llan 

 Megatherium, Mylodon, Elephas. 



Pliohippus Beds. 



Pliohippus, Mastodon, Bos, etc. 



Miohippus Beds. 



Miohippus, Diceratherium, Thinohyus. 



Oreodon Beds. 



Edentates, Uycenodon, Hyracodon. 



Brontotherium Beds. 



Mesohippus, Menodus, Elotherium. 



Diplacodon Beds. 



Epihippus, Amynodon. 



Dinoceras Beds. 



Tinoceras, Uintatherium, Limnohyus, 

 Orohippus, Helaletes, Colonoceras. 



Coryphodon Beds. 



Eohippws, Monkeys, Carnivores, Ungu- 

 lates, Tillodont's, Rodents, Serpents. 



Laramie Series. Triceratops. 



Upper Cretaceous of N. J. 



Hadrosaurus, Drypto 



Pteranodon Beds. 



Birds with Teeth, ITesperornis, Ichthy- 

 ornis. Mosasaurs, Pterodac- 



tyls, Plesiosaurs. 



Dakota Group. 



Comanche Group. 



Atlantosaurus Beds. 



Dinosaurs, Apatosaurus, AUosaurus, 

 Nanosaurus. Turtles. Diplosaurus. 



Connecticut River Beds. 



First Mammals (Marsupials), (Droma- 



therium). 

 Dinosaur Footprints, Amphisaurua, 



Crocodiles (Belodon). 



Permian. 

 First Reptiles. 



Coal-Measures. 



Subcarboniferous. 



First known Amphibians (Labyrii 

 thodonts). 



Corniferous. 



Schoharie Grit. 



First Fish Fauna. 



Upper Silurian. 

 Lower Silurian. 



Primordial. 



Huronian. 



Lauren ti an. 



No Vertebrates 

 except in tite 

 Uppermost 

 Part. 



No Distinct 

 Organic 

 Remains. 



Fig. 255.— Section of the Earth's Crust, to illustrate Verte- 

 brate Life in America. (Slightly modified from Marsh.) 



the several epochs and 

 periods of the history of 

 the earth in the inverse 

 order of their occurrence. 

 Commencing with a thor- 

 ough discussion of " causes 

 now in operation" i. e., 

 geological history of the 

 present time, as that which 

 is best known, they make 

 this the basis for the study 

 of the epoch immediate- 

 ly preceding, and which, 

 therefore, is most like it. 

 Having acquired a knowl- 

 edge of this, the student 

 passes to the preceding, 

 and so on. This has the 

 great advantage of pass- 

 ing ever from the better 

 known to the less known, 

 which is the order of in- 

 duction. Other geologists 

 prefer to follow the natu- 

 ral order of events. This 

 has the great advantage of 

 bringing out the philoso-. 

 phy of the history — the 

 law of evolution. The 

 first method is the best 

 method of investigation; 

 the second method is the 

 best method of presenta- 

 tion. 



As in human history, 

 so in the geological histo- 

 ry, the recorded events of 

 the earliest times are very 

 few and meager, but be- 

 come more and more nu- 

 merous and interesting as 

 we approach the present 

 time. Our account of the 

 Archaean era will, there- 



