LARAMIE, OR TRANSITION EPOCH. 



497 



Fig. 832. 

 Figs. 832, 833.-832. Viburnum Newberrianum. 



Fig. 833. 

 3. Alnus Grewiopsis. 



it was taken possession of by new types of Plants, probably migrated 

 from the north : and thus Cretaceous land-animals and Tertiary land- 

 plants existed side by side. Meanwhile, the marine shells by changing 

 conditions were most of them destroyed, but some changed through 

 brackish- water forms into fresh-water forms of the Tertiary. Some 

 of the steps of this change of molluscan types have been traced (White). 



Such transition strata are of especial interest, and deserve separate 

 treatment in order to emphasize their transitional character. 



Area. — From what is said above it is evident that the Laramie ex- 

 ists over very wide areas in the region of the interior Cretaceous sea, 

 but it is largely covered by Tertiary lake deposits. It is, however, 

 exposed along the eastern base of the Colorado mountains, from Mexico 

 northward far into British America-; also in the Laramie plains, 

 where it is traversed by the Union Pacific Eailroad. This is the typical 

 locality from which it takes its name. On the Pacific coast, a part at 

 least of the Tejon group, and also the principal coal-fields of California, 

 Washington, and British Columbia, probably belong to this horizon. 

 In the typical locality, on the Laramie plains, the strata are several 

 thousand feet thick. It represents, therefore, a long period of time. 



Life-System — Plants. — The vegetation was very abundant, and the 

 plants had already assumed a Tertiary aspect. About 323 species of 

 plants are known, of which 226 species are Dicotyls. We give a few 

 illustrations of these from Ward (Figs. 829-833). 



32 



