634 HISTORICAL GEOLOGY 



among which walnuts and hickory nuts have been identified, al- 

 though most of them are of unknown or doubtful affinity. In Colo- 

 rado (Florissant) the deposits of a Miocene lake (p. 580) have afforded 

 large numbers of plants, such as alders, oaks, narrow-leafed cotton- 

 woods, pines, roses, thistles, asters, and Virginia creepers. Mixed 

 with these are others of a more southern type, such as the holly, 

 smoke tree, sweet gum, and persimmon. The vegetation of the 

 Pliocene was probably almost identical with that in the adjoining 

 regions to-day, but so few plant fossils have been found that no 

 definite statements can be made. 



Climate 



Difficulty in Determining Tertiary Climates. — The determina- 

 tion of the climates of the Tertiary is complicated by a mingling of 

 plants whose relatives are no longer found associated, some being at 

 present restricted to tropical or subtropical regions and others to 

 temperate zones. Since closely related modern species sometimes 

 live under very different climatic conditions, it will readily be seen 

 that the presence of plants in the Tertiary related to species now living 

 only in subtropical regions, for example, does not necessarily prove 

 that these early species lived under similar conditions, but is certainly 

 strong evidence in favor of such a supposition ; especially when it 

 can be shown that the plant associations were then the same as now; 

 e.g., breadfruit trees, cycads, and many ferns grew in association in 

 Greenland (72 N.) as they now do in the tropics. This mingling of 

 what are now tropical and temperate region types has led investi- 

 gators to very different conclusions. For example, the Miocene 

 climate of Colorado, because of the presence of genera now living in 

 Colorado, is believed by one investigator (Cockerell) to have been in 

 no sense tropical, while another (Knowlton), because of the presence 

 of West Indian genera, believes that the climate was not unlike that 

 of certain parts of the West Indies of to-day. Certain trees, such as 

 palms, are generally agreed to indicate warm, tropical, or subtropical 

 conditions, even when they grew in forests with the maple, elm, and 

 other temperate region plants. When, however, the remains of 

 insects, reptiles, or mammals occur in deposits with plants, the total 

 evidence often becomes conclusive. 



Eocene. - I he vegetation suffered so little change between the 

 Upper Cretaceous and the Eocene that it is evident the climates of the 



