880 historical geology. 



General Subdivisions. 



The subdivisions of the Tertiary in general use were introduced by Lyell 

 in the first edition of his Geology. They were based by him primarily on 

 his own geological 'investigations in England and Europe, and on those of 

 the French conchologist, Deshayes, who was already familiar with the fossil 

 species of the Paris Basin. The proportion of living to extinct species was 

 accepted as the distinctive character of the subdivisions. These subdivisions, 

 and the proportions now adopted for the approximate limits, are as follows : — 



1. Eocene period (from ^ws, dawn, and Katvos, recent) : no species, or less 

 than 5 per cent living. 



2. Miocene period (from /xetW, less, and Katvd?) : 20 to 40 per cent living. 



3. Pliocene period (from TrAetwv, more, and Katvos) : more than half the 

 species living. 



The Miocene and Pliocene are sometimes united under the name Neocene 

 (from veo?, new, and Katvos), especially when the divisions are not well differ- 

 entiated. The term Oligocene (from oAtyos, feio, and Katvos) is sometimes 

 used for a fourth division, consisting of the upper part of the Eocene and the 

 lower part of what had been referred to the Miocene. 



The term Oligocene was proposed by Beyrich, of Berlin, in 1855. In 1864, Homes, 

 of Vienna, proposed the term Palaeogene for the combined Eocene and Oligocene, and 

 Neogene for the Miocene and Pliocene ; Eogene has also been used in place of Palaeogene. 

 Further, the Lower Eocene has also received the separate name of Paleocene. J. W. 

 Dawson adopted, in 1889, the term Orthrocene for the Lower Eocene, Nummulitic for the 

 Middle, and Proicene for the Upper or (as he says) that of the Vicksburg Epoch. 



On the geological map published in 1884 by the U. S. Geological Survey, Eocene 

 includes the Eocene and Oligocene, and Neocene the Miocene and Pliocene. In 1887, 

 Heilprin proposed the substitution of Eogene, Metagene, and Neogene, severally, for 

 Eocene + Oligocene, Miocene, and Pliocene + Quaternary. 



The name Tertiary is a relic of early geological science. When introduced, it was 

 preceded in the system by Primary and Secondary. The first of these terms was thrown 

 out when the crystalline rocks so called were proved to belong to no particular age, — 

 though not without an ineffectual attempt to substitute for it Paleozoic ; and the second, 

 after use for a while under a restricted signification, has given way to Mesozoic. Tertiary 

 holds its place, simply because of the convenience of continuing an accepted name. Neo- 

 zoic is sometimes used in place of Tertiary, while it is also occasionally made a substitute 

 for the whole Cenozoic. It was originally proposed by Edward Forbes to comprise both 

 the Mesozoic and Cenozoic. 



NORTH AMERICA. 

 GENERAL GEOGRAPHICAL FEATURES OF THE TERTIARY ERA. 



It has been shown that the deposition of the Laramie beds and the up- 

 turning which followed left the great interior of North America emerged. 

 The Cretaceous sea, which had covered the Western Continental Interior and 

 the Eocky Summit region from Mexico to the Arctic coast, was gone, excepting 



