CRETACIC PERIOD 597 



The fresh-water fauna is composed of existing genera, which in them- 

 selves are not of much stratigraphic value. Taken in connection with the 

 local stratigraphic sections in which they occur, however, it is seen that 

 the formations are intimately bound with those of unmistakable Cretacic 

 age. Again, the Unios are unlike those of the Eocene in that the umbos 

 of the shell are often sculptured. The brackish-water faunas are certainly 

 those of the marine Cretacic, and the succession shows that these finally 

 vanish from the interior region, the Cretacic series going. over completely 

 into fresh-water beds of a continental character. Nowhere has the marine 

 Tertiary entered this region, and the nearest Eocene sea did not advance 

 beyond Tennessee or coastal Texas. 



Stanton concludes : 



"The 'Ceratops beds' are of Cretaceous age as decided by stratigraphic rela- 

 tions, by the pronounced Mesozoic character of the vertebrate fauna with ab- 

 sence of all Tertiary types, and by the close relations of its invertebrate fauna 

 with the Cretaceous. The relations of the flora with Eocene floras is believed 

 to be less important than this faunal and stratigraphic evidence. Taken in 

 their whole areal extent they probably include equivalents of the Laramie, 

 Arapahoe, and Denver formations of the Denver Basin." 



"The Fort Union formation, properly restricted, is of early Eocene age, the 

 determination resting chiefly on its stratigraphic position and its primitive 

 mammalian fauna which is related to the earliest Eocene fauna of Europe. 

 The very modern character of the flora tends to confirm the correlation" (293). 



TERTIARY OR NEOZOIC (CENOZOIC) ERA 



See plates 96 to 100 



Familiarity at first hand with the Tertiary formations requires a vast 

 amount of detailed knowledge of the plants and land animals, and espe- 

 cially of the marine invertebrates, which the writer does not possess. In 

 order to make the paleogeography of North America as complete as possi- 

 ble, however, he has devoted his eiforts toward the compilation of the maps 

 and the table of formations more for the benefit of teachers of historical 

 geology than for stratigraphers. With the maps he has had the assist- 

 ance of Dall, Arnold, and Vaughan. 



The more important literature pertaining to this era is as follows : 



Dall and Harris : Correlation papers — Neocene. Bulletin 84, U. S. Geological 

 Survey, 1892. 



Dall : A table of the North American Tertiary horizons, etc. Eighteenth 

 Annual Report of the U. S. Geological Survey, 1898, pages 323-348. Contribu- 

 tions to the Tertiary fauna of Florida, parts I-VI, 1886-1903. Transactions of 

 the Wagner Free Institution of Science, Philadelphia. 



