266 
TERTIARY FORMATION. 
Its organic remains distinguish it from the sec- 
ondary formations, and its layers mark its differ- 
ence from the alhivial and diluvial deposites. 
We have already stated that this formation is di- 
vided into three groups, called by Mr. Lyell plio- 
cene, miocene, and eocene, distinguished by the char- 
acter of the shells of molluscous animals vi^hicl/ 
they contain. In the pliocene formations, more 
than one half the shells belong to existing specie^ ; 
in the miocene, less than half; and in the eocene, 
the proportion is still less. In Europe this group 
contains at least fifty species of terrestrial animals, 
none of which belong to existing species, 100 spe- 
cies of fish, and several of extinct birds and rep- 
tiles. 
The tertiary formations yet known to us are con- 
fined almost exclusively to the Atlantic plain of the 
United States, and to the southern part of the Valley 
of the Mississippi. The northern limit of this for- 
mation has generally been supposed to be in the 
southeastern corner of New-Jersey, adjacent to the 
Delaware Bay. But it is now known that they ex- 
ist in every state of the Union. Prof. Hitchcock 
has described the tertiary of Massachusetts under 
two divisions. The first, which he calls the most 
recent, consists of horizontal alternating layers of 
white silicious sand and blue plastic clay, the sand 
occupying the highest place in the series, and cov- 
ering most of the surface. The sand and gravel 
are often several feet thick, their upper portion be- 
ing disturbed and piled up irregularly by diluvial 
action. 
The most extensive deposites of this class are 
in the Valley of the Connecticut, though they ex- 
tend in patches all over the state. These clay beds 
are often of great thickness. In Deerfield they are 
over seventy feet thick, and covered by sand fifteen 
or twenty feet deep ; while in the vicinity of Bos- 
ton the same clay is from 70 to 120 feet thick 
