232 Part III. Chapter 3. 
The results of this survey as to the southern extremity of Florida may be 
arranged below: 3 
Ancient Strand Flora Ancient Sea Island Flora 
I 
= Northern Element 
B 
| 3 bs 
2 Plain Flora ci Swamp Flora 
og 
= Savanna Flora es Grassland Flora. 
3 X as 
Present Strand Flora Pineland Flora Present Hammock Flora 
leaving the southern part of the Mississippi valley still submerged. In the 
Tertiary, the water had still further receded, only a narrow embayment 
presenting the once more extensive northward projecting lobe of the oce 
(See Fig. 3, 4, page 179.) A plain was formed with the recession of the se 
and in all probability it stood then much at its present elevation. This f 
was invaded at once by the grass formation which formed dense mats of 
with deeply penetrating roots. Thus we have the vegetal covering of N 
America at the close of the Tertiary period divided into two great kinds, fo | 
and plain. The plain flora consisted of turf-builders which characterize 3 
prairie and plains of the present day. During the middle Tertiary period down 
to the Pleistocene, the grassy plains were much less extensive than now, 
more extended area, especially during Miocene times. The recession of 
inland waters was followed by a much drier steppe climate and the west: 
trees receded to about their present position. The arid plains east of the 
Rockies thus had their origin, and their floral covering was derived from 
several sources of supply. 
ı. An invasion took place from the Great Basin and Mexico. This ma} 
be called the southwestern arid contingent. 
2. The prairie contingent derived from the prairies lying immediately | 
the east. f | 
trees. However, east‘of these dry plains existed a grass covered territory = 
prairie on both sides of the Mississippi River. The invasion of trees into this 
area was not possible because the turf and mat formation was so dense y 
because a steppe climate with strong winds prevented the encroachment be: 
