148 Tertiary. 



Pliocene he subdivided iuto the Older Pliocene and Newer Pliocene. 

 In the latter, out of 226 fossil species of shells, he found 216 to be 

 living. He afterward proposed the name Post-pliocene for rocks hav- 

 ing all the imbedded fossil shells identical with living- species, though 

 they may contain extinct mammalian remains. We now include in this 

 group strata which belong to more modern time, and which are fre- 

 quently called "Eecent." 



This subdivision of the Tertiarv, with reference to the survival of 

 conchological species, and the subdivision of the strata, or rocks, into 

 groups, have made a double system of nomenclature, which does not 

 prevail in the older geological periods. The determination of the North 

 American equivalents of the European strata, by the per cent, of living 

 species, was soon ascertained to be impracticable, and, instead of that 

 method, the age is determined b^^ the extinct species. Certain species 

 have come to be regarded as types of Eocene age, or Miocene, as 

 the case ma}^ be, and, from the presence of these, the rocks are referred 

 to the proper subdivision of the Tertiary. 



I have not found tiine to separate the consideration of the Tertiary, 

 into the groups into which it has been subdivided, and preserve the 

 chronological order, or history of our knowledge of it. For this reason? 

 I will follow the order of discovery in matters relating to the Tertiary, 

 separating only that part relating to the fresh water drift of the central 

 part of the continent, which will form the conclusion of this essaj'; 

 nor will I dwell upon the few vertebrate fossils mentioned prior to 1820. 



In 1824, Prof. Silliman* noticed the Tertiary exposed at Martha's 

 Vineyard and the Elizabeth Islands. Prof. 01mstead,f in the first re- 

 port ever made, as it is said, in any country, upon geology, with State 

 or Government funds, described the country through which the Beau- 

 port canal was excavated, and separated the strata into: 1st. A black 

 mould; 2d. Potters' cla}', of a yellowish brown color; 3d. A thin layer of 

 sand, full of sea shells and the remains of land animals, particularly 

 of the mammoth, from three to eight feet deep; and, 4th. A soft 

 blue clay. 



Thomas SayJ described, from strata now referred to the Miocene of 

 Maryland, Turritella plebeia, Natica interna, Buccinum porcinum^ 

 now Ptychosalpinx porcina, B. aratum, Fusus cinereus, now Urosal- 

 pinx cinereus, F. 4-costatus, now Ecpthora quadricostata, L'alyptraea 



* Am. Jour. Sci. and Arts, vol. vii. 

 t Rep. on the Geo. of North Carolina. 

 \ Jour. Acad. Nat. Sci. vol. iv., pt- 1. 



