546 HISTORICAL PALEONTOLOGY. 



mains of this age have been with more or less probability re- 

 garded as being Ornithic. The Upper Cretaceous rocks of 

 the United States have, however, yielded the remains of un- 

 doubted birds (Laornis,Telmatornis, Scolopax, and Palceotringa). 

 More remarkable than any of the above are the recently-dis- 

 covered Hesperornis and Ichthyornis, of which the former is a 

 large Diver-like bird, whilst the latter exhibits the singular 

 peculiarity that the vertebrae are biconcave. Mammals have 

 not hitherto been detected in any Cretaceous deposit. 



CHAPTER LIV. 



EOCENE PERIOD. 



BEFORE commencing the study of the subdivisions of the 

 Kainozoic series, there are some general considerations to be 

 noted. In the first place, there is a complete and entire phy- 

 sical break between the rocks of the Mesozoic and Kainozoic 

 periods. In no instance are Tertiary strata to be found rest- 

 ing conformably upon any Secondary rock. The Chalk has 

 invariably suffered much erosion and denudation before the 

 lowest Tertiary strata were deposited upon it. This is shown 

 by the fact that the actually eroded surface of the Chalk can 

 often be seen, or, failing this, that we can point to the presence 

 of the chalk-flints in the Tertiary strata. This last, of course, 

 affords unquestionable proof that the Chalk must have been 

 subjected to enormous denudation prior to the formation of 

 the Tertiary beds, all the chalk itself having been removed, 

 and nothing left but the flints, while these are all rolled and 

 rounded. 



In the second place, there is a complete break in the life 

 of the Mesozoic and Kainozoic periods. With the excep- 

 tion of a few foraminifera, and one Brachiopod (the latter 

 doubtful), no Cretaceous species is known to have survived the 

 Cretaceous period ; while several characteristic families, such 

 as the Ammonitida and Hippuritidce, died out entirely with the 

 close of the Cretaceous rocks. In the Tertiary rocks, on the 

 other hand, not only are all the animals and plants more or 

 less like existing types, but we meet with a constantly-increas- 

 ing number of living species as we pass from the bottom of the 

 Kainozoic series to the top. Upon this last fact is founded 



