324 J". D. Dana on American Geological History, 



Two or three times in the course of the Tertiary Period, the 

 life of the seas was exterminated, so that the fossils of the later 

 Tertiary are not identical with any in the earliest beds, — ex- 

 cluding some fish remains, species not confined to the coast 

 waters. The crust of the earth was still oscillating; for the 

 close of the first Tertiary epoch was a time of subsidence ; but 

 the oscillation or change of level was slight, and by the end of 

 the Tertiary, the continent on the east stood within a few feet of 

 its present elevation, while the Gulf of Mexico was reduced 

 nearly to its present limits.* 



I have thus brought this rapid sketch to the close of the Ter- 

 tiary, having omitted much of great interest, in order to direct 

 attention to the one grand fact, — that the continent from the 

 Potsdam sandstone, or before, to the Upper Tertiary, was one in 

 its progress, — a single consecutive series of events according to a 

 common law. It is seen, that the great system of oscillations, 

 due to force pressing or acting from the southeast, which reached 

 its climax in the rise of the Appalachians, then commenced a 

 decline. We mark these oscillations still producing great results 

 in the Jurassic Period along the whole eastern border from Nova 

 Scotia to the Carolinas. Less effect appears in the Cretaceous 

 Period ; and gradually they almost die out as the Tertiary closes, 

 leaving the Mississippi Valley and the eastern shores near their 

 present level. 



Thus were the great features of Middle and Eastern North 

 America evolved ; nearly all its grand physical events, includ- 

 ing its devastations and the alternations in its rocks, were con- 

 sequent upon this system of development. Moreover, as I have 

 observed, this system was some way connected with the relative 

 position of the continent and the oceanic basin. 



We need yet more definite knowledge of the Pacific border 

 of North America to complete this subject. It is in accordance 

 with the fact that the highest mountains are there, that volca- 

 noes have been there in action ; and also that, in the Tertiary 

 Period, elevations of one to two thousand feet took place ; and 

 that immediately before the Tertiary, a still greater elevation of 

 the Rocky Mountains across from east to west occurred. The 

 system of changes between the Eocky Mountains and the Pacific 

 has been on a grander scale than on the Atlantic border, and 

 also from a different direction, — and this last is an element for 



numerous freshwater shells, species of Melania, Physa, Paludina, Cyrena, and all 

 are such kinds as inhabit fresh and brackish waters. The tertiary deposits of the 

 Bad Lands, or that part where the bones occur, have afforded no evidence of salt 

 water origin ; and the same is true of the Lignite beds of the far north. "While there- 

 fore the tertiary beds are extensive, the marine tertiary, indicating the presence of 

 the sea, as far as present knowledge goes, is quite limited. 



* Naming the North American Tertiary Epochs from prominent localities, as in 

 the Paheozoic, they are : — 1. The Claiborne, or Older Eocene; 2. The Vicksburg, 

 or Newer Eocene ; 3. The Yorktown, or Pliocene and Miocene in one. 



