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 
ters 
close of the first Tertiary epoch was a time of subsidence; but 
the oscillation or change of level was slight, and by the e nd of 
e 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. 
T have thus brought this ca sketch to the close of the Ter- 
iary, 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 reache 
its climax in the rise of the Appalachians, then nope a 
decline. We mark these oscillations still producing grea 
in the Jurassic Period along the whole eastern border pon Nova 
Scotia to the Carolinas. Less effect appears in the Cretaceous 
eriod; and gradually they almost die out as the Tertiary closes, 
tobving g ihe = ssippi Valley and the eastern shores near 
eve 
sequent upon this system of aivileedeany Moreover, 
observed, this system was some way connecte ected with the ealasive 
position of the continent and the oceanic 
We need yet more definite Sippel is of the Pacific _—. 7 
of North America to complete this subject. It is 
with the fact that the highest mountains are there, that ser 
noes have been there in action; and also that, in -_ Tertiary 
Period, elevations of one to t two thousand feet’ took lace; and seas 
that immediately before the Tertiary, a still gi on che of 
the Rocky Mountains across from east to west o 
system of changes between the Rocky Mountains eer ‘tha » Pacifi 
nm on a grander scale than on the Atlantic border, and 
also from a different direction,—and this last is an element for 
ghia fr pipes: shells, species of Melania, Physa, Paludina, and all 
as inhabit fresh and brackish waters The ery op of the 
Ait 
the sea, as far as ponies knowledge goes, is quite limited. 
* Naming the North American Tertiary Epochs from prominent localities, as in 
the Palsozoic, they are :—1. The Oxaro’ con or Older ee ne ; 2. The VIcKsPURG, 
or Newer Eocene ; 3. The Yorxtown, or Peat and Miocene 
