THE EOCENE PERIOD. 289 



These marine strata of the " Lignitic formation " are of special 

 interest, as showing such a commingling of Cretaceous and 

 Tertiary types of life, that it is impossible to draw any rigid 

 line in this region between the Mesozoic and Kainozoic sys- 

 tems. Thus the marine beds of the Lignitic series contain 

 such characteristic Cretaceous forms as Inoceramus and Am- 

 monites, along with a great number of Univalves of a distinctly 

 Tertiary type (Cones, Cowries, &c.) Upon the whole, there- 

 fore, we must regard this series of deposits as affording a kind 

 of transition between the Cretaceous and the Eocene, holding 

 in some respects a position which may be compared with that 

 held by the Purbeck beds in Britain as regards the Jurassic 

 and Cretaceous. 



The Middle Eocene of the United States is represented 

 by the Claiborne and Jackson beds. The Claiborne series is 

 extensively developed at Claiborne, Alabama, and consists of 

 sands, clays, lignites, marls, and impure limestones, containing 

 marine fossils along with numerous plant-remains. The Jack- 

 son series is represented by lignitic clays and marls which occur 

 at Jackson, Mississippi. Amongst the more remarkable fossils 

 of this series are the teeth and bones of Cetaceans of the 

 genus Zcitglodon. 



Strata of Upper Eocene age occur in North America t,t 

 Vicksburg, Mississippi, and are known as the Vicksbtirg series. 

 They consist of lignites, clays, marls, and limestones. Fresh- 

 water deposits of Eocene age are also largely developed in 

 parts of the Rocky Mountain region. The most remarkable 

 fossils of these beds are Mammals, of which a large number of 

 species have been already determined. 



LIFE OF THE EOCENE PERIOD. 



The fossils of the Eocene deposits are so numerous that 

 nothing more can be attempted here than to give a brief and 

 general sketch of the life of the period, special attention being 

 directed to some of the more prominent and interesting types, 

 amongst which as throughout the Tertiary series the Mam- 

 mals hold the first place. It is not uncommon, indeed, to 

 speak of the Tertiary period as a whole under the name of the 

 " Age of Mammals," a title at least as well deserved as that of 

 " Age of Reptiles " applied to the Mesozoic, or " Age of Mol- 

 luscs " applied to the Palaeozoic epoch. 



As regards foe plants of the Eocene, the chief point to be 

 noticed is, that the conditions which had already set in with 

 the commencement of the Upper Cretaceous, are here con- 



