THE EOCENE OF EUROPE AND NORTH AMERICA 93 



by salt water was worn down to base level, antl widening expanses of flats 

 were covered with marshes which eventually formed coal, preserving a similar 

 flora from the outlyers of the Mississippi valley almost to the Pacific coast. 

 These coal-forming conditions reached far uj) into Alaska, where almost under 

 the arctic circle t.^q^es of plants flourished that to-day could not live in the 

 open, north of Mexico. 



In Eocene times the climatic and geographic conditions remained the 

 same, but the sea had encroached still fartluu- on the land, and the base level- 

 ing of the backbone of the continent was more complete. Tropical conditions 

 still prevailed up as far as Alaska, and coal was still formed al)undantly 

 where vegetation is now scanty. This is truly an "Eocene carboniferous," 

 for most of the coal on the west coast belongs to this epoch. At this time 

 also a temporary connection was established between the Pacific and Atlantic 

 oceans, for the peculiarly Atlantic tjq^e of mollusc, Venericardia planicosta, 

 is found along with Pacific types in California and Oregon. 



In his recent discussion of this region, Scharff/ after a complete review 

 of the existing knowledge of the present and former distribution of animal 

 and plant life in the two continents, comes to the following somewhat extreme 

 conclusion, namely : that there was a great lobe of land connecting western 

 North America with the southern portion of the South American continent 

 during the beginning of Csenozoic times, while Central America and the 

 northern portion of the South American continent were still largely submerged. 



Late Cretaceous and Early Eocene Flora 



The plant life is naturally one of the keys to the evolution of the herbiv- 

 orous mammals. We have to consider the food afforded by bark, leaves, 

 twigs, and buds, by shrubs, and by the plants of softer tissue, such as mam- 

 mals browse upon to-day, as well as by the grasses, plants of harder tissue, 

 upon which they graze. The diminution of browsing mammals and increase 

 of grazing mammals is one of the great features of Csenozoic evolution. 



Grasses. — If we observe the conditions of the preservation of plant 

 remains along existing ponds, river borders, or swamps we see at once that they 

 are as favorable for the preservation of deciduous leaves as they are unfavor- 

 able for the preservation of grasses. Grasses are firmly attached to their 

 roots and are not swept away either by water or wind. Leaf deposits, there- 

 fore, abound ever>^vhere and give us sure indications of the forest flora, 

 while we know but little of the field and meadow flora, which is of great im- 

 portance in connection with the evolution of the grazing herbivorous Ungu- 

 lates especially. In fact, the evidence as to grasses is very limited throughout 

 the entire Age of Mammals. The number of kinds of grasses (Gramincir) 

 found in the whole Ca?nozoic of Europe is comparatively small, and it is diffi- 



' Scharff, R. F., On an Early Tertiary Land-Connoction between North and South Amer- 

 ica. Amer. Natural., Vol. XLIII, Sept., 1909, pp. 513-531. 



