Mesozoic and Caenozoic Geology and Palceontology. 147 



TBJE C^NOZOIC AGE, OR TERTIARY PERIOD. 



When tlie woi'ds Primaiy, Secoudaiy and Tertiary are used to dis- 

 tinguish geological subdivisions, the rocks are so comprehended as to 

 leave none to which the word Quaternar}^ can be properly applied. 

 The organic remains of the Tertiary are likewise so completel}' blended 

 with the living organisms, that we can not distinguish a Quaternary 

 age or period. The subdivision of the Tertiary, with reference to the 

 sur\-ival of conchological species, into Eocene, Miocene, Pliocene and 

 Post-pliocene, brings us to the living species as graduall}' as the spe- 

 cies are found to change within any of the subdivisions of geological 

 time, or within any of the minor subdivisions of the strata into groups. 

 It is, therefore, evidently a mistake to use the word Quaternary, in a 

 geological subdivision, with reference either to the rocks or their organic 

 contents. 



The Tertiary rocks, genei'ally, consist of marls, clays, sands, or other 

 friable material, filling depressions in the underlying rocks, and, though 

 widel3' distributed, seldom form hard continuous strata. This condi- 

 tion of the rocks in Europe made it very difficult to determine the 

 order of superposition, and led Deshayes to suggest, after having ex- 

 amined 1,122 species of fossil shells from the Paris basin, and having 

 identified onlj- thirty-eight with the living, that a subdivision of the 

 Tertiary might be based upon the relative proportion of the extinct 

 and living species of shells. He drew up, in tabular form, lists of all 

 the living shells known to him as occurring in Tertiary rocks, and sub- 

 mitted the same to Mr. Lj^ell. The number of species of fossil shells 

 examined by Deshayes was about three thousand, and the living spe- 

 cies with which they were compared about five thousand. With this 

 assistance, and that furnished by the works of Basterot and some 

 Italian authors, .Mr. Lyell, in 1833, estimated that, in the lower Ter- 

 tiar3' strata of London and Paris, 3|^ per cent, of the species are iden- 

 tical with the living; that, in the middle Tertiarj' of the Loire and 

 Gironde, about 17 per cent, are living; that in the upper Tertiary, or 

 Subappenine beds, from 35 to 50 percent.; and that, in strata still 

 more recent, in Sicily, from 90 to 95 per cent. He proposed to call the 

 lower Tertiary " Eocene," which signifies the dawn of the present state 

 of things; the middle Tertiary " Miocene," which implies less recent; 

 and the upper Tertiar}- *' Pliocene," which means more recent. The 



