Vegetation south of the Land Ice. 201 
City Colorado two fossil species of Platanus, three of Juglans, one of Ulmus, 
one of Zindera (Benzoin), one of Sapindus and one of Fagus. None of these 
genera exist in Colorado at the present day. 
Prairies. With the gradual disappearance of the sea which extended over 
the central prairie region during the Miocene period, the soil at first was 
largely impregnated with the salts of sea water, which were afterward leached 
out by the rain which with the drying up of the land fell with less constancy, 
producing more arid conditions on the eastern slopes of the Rocky Mountains. 
HARVEY ') suggests that on account of the low precipitation this region bounded 
on the west by the crest of the Rocky Mountains was denied at first to tree 
invasion and came to be occupied by a prairie formation, which was displaced 
after the glacial period by the invasion of trees, while the plains became grass- 
covered. The formation of the prairies introduced a drier climate in the interior 
of North America and new elements of plant life were introduced which de- 
veloped into the numerous and peculiar forms, characteristic of the prairie 
region of today. Numerous Cactaceae, many Chenopodiaceae, the Chlorideae, 
the peculiar Polemoniaceae of the prairies appeared during Miocene and Plio- 
cene times. 
Coast Plains. We have thus far traced the developments of the main types 
of the North American flora during the Glacial period. Before treating of the 
post glacial and present distribution of North American plants, let us briefly 
allude to the conditions which probably existed on the Atlantic and Gulf 
coast plains. 
During the Tertiary period that portion of North America which now com- 
prises the Atlantic and Gulf coast plains was beneath the Atlantic Ocean. 
Later during the upper Tertiary, it was elevated excepting the portion com- 
prising the Florida peninsula. With its appearance above the sea, it was 
tenanted from two main sources of plant supply, viz., the flora which covered 
the elevated mountain and table lands of the present eastern states as far 
north as New York and the coastal flora which must have been differentiated 
and fringed the tertiary sea coast composed of typic sea coast plants. The 
sandy plains of the coast were tenanted chiefly by pines which if they did not 
arise as coastal plain species probably were derived from the near, by tertiary 
forest so frequently referred to. The assortment of species in the newly formed 
land depended largely upon edaphic conditions. The sandy soil of the coastal 
plain would only support those species which had previously existed in the 
near by land areas under similar edaphic conditions, which may have been a 
rocky ledge of some mountain side, or the sandy bottom of some silt filled 
ravine, or which were sufficiently plastic, as species, to adapt themselves to 
the new surroundings. 
Colony of northern plants in Florida. The topography of Florida in the 
neighborhood of the Appalachicola River is in striking contrast to the level 
1) Botanical Gazette XLVI: 84. 
