TER TIAR Y SYSTEM. 83 



been identified from the Fort Union Group, thus specifically uniting the Cretaceous 

 era with the present time. It is possible, too much confidence in this identification 

 may lead to error, and better specimens may show specific distinctions ; but it is an 

 important fact, they so closely resemble the living forms as to be mistaken for 

 them, and show how closely the living are connected with the ancient dead. 

 Among the Cretaceous genera of invertebrates, about one-third survive ; three 

 genera of reptiles, Grocodilus, Trionyx, and Emys survive ; but no genus of birds or 

 mammals has come down from that age to the present. There is no great break 

 or chasm discoverable in vegetable or animal life in passing back to the Cretaceous 

 era. No sudden physical change has taken place over which some deposit may 

 not furnish a connecting bridge. No evidence of any great climatic change is 

 furnished, either in the animal or vegetable world, but on every hand we are en- 

 couraged to look at uniformity in the organisms, subject only to a constant, almost 

 imperceptible evolution. Seams of productive coal occur at different places in this 

 Group. 



CHAPTER XXXVIII. 



TERTIARY SYSTEM. 



§ 184. The organic remains of the Tertiary are so completely blended with 

 the living, that no Quaternary age or period can be distinguished. The words 

 Primary and Secondary have become quite obsolete in Geology, while Tertiary is 

 so interwoven with the science as to be permanently fastened to the nomenclature, 

 notwithstanding its definition, as the third age, has no application to the period to 

 which it relates. The subdivision of the Tertiary, with reference to the survival of 

 conchological species into Eocene, Miocene, Pliocene, and Post-pliocene, brings us to 

 the living species as gradually as the species change within any of the subdivisions 

 of geological time, or within any division of the strata into Groups. The Tertiary 

 rocks generally consist of marls, clays, sands, or other friable material, filling de- 

 pressions in the underlying rocks, and, though widely distributed, seldom form hard, 

 continuous strata. This condition of the rocks made it difficult to determine the 

 order of superposition, until a comparison of the shells had been made with living 

 species. This comparison led to the naming of the rocks containing about 3 or 4 

 per cent of living species, the Eocene, which signifies the dawn of the present state of 

 things; those containing 15 to 20 per cent of living species, the Miocene, which im- 

 plies less recent ; and those containing 90 to 95 per cent of living species, the 

 Pliocene, which means more recent; and those having all the imbedded fossil shells 

 identical with living species, though containing extinct mammalian remains, Post- 

 pliocene. Instead of determining the rocks by the per cent of living species, the con- 

 trary course is now adopted, and the age is determined by the extinct species. 

 Certain species are regarded as types of Eocene age, or Miocene, as the case may 

 be, and from the presence of these the rocks are referred to the proper Group. 

 This subdivision of the Tertiary, with reference to the survival of conchological 

 species and the division into geographical Groups, have made a double system of 

 nomenclature. 



