266 ^ TERTIARY FORMATION. 



Its organic remains distinguish it from the sec- 

 ondary formations, and its layers mark its differ- 

 ence from the alluvial 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 whicif 

 they contain. In the pliocene formations, more 

 than one half the shells belong to existing species; 

 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 tons are con- 

 fined almost exclusively to the Atlantic plain of the 

 United States, and to the southern part of the Valley 

 of the Mississippi. I'he 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 tertiar}^ of Massachusetts under 

 two divisions. The first, which he calls the most 

 r'ecent, 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 tipper 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 stat«. 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 



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