ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 



255 



I 



came wider and extended farther inland, forming ravines. 

 The ravine, often assisted by the settling of the earth crust 

 through the breaking down of the roofs of caverns, gradually 

 became wider and acquired gentler slopes. This action 

 continues till base level is reached. 



It is probable that at first the entire upland was covered 



by innumerable 



pools of 



of time became so shallow that, at least durin 



situated 



they were dry or nearly 



The higher 



d islands, soon occupied by grasses and sed 

 short time became associated with other ol: 



These 



chiefly annuals 



biennials, such as Artemisia. Xanthiu 



and Oenothera and which, if not the same species as those 



occurnn 



to-day, were at least closely related. Willows 



hackberries. coffee trees, birches and poplars followed 



here and there an ash. All of 

 drier were replaced by elms, hi 



became 



ckories and finally by 

 greater, submerged p< 



of the plateau did not take place as rapidly. The conditions 

 existing at that time are, perhaps,to be compared with those 

 we meet with to-day in the lowlands of the Mississippi. The 



limited 



typical lake flora, at first entirely 

 probably consisted chiefly of Polygonum, 



ed 



I ^ 



. These, encroaching upon 

 raising the level of the lake 



occupied by rushes and sedge: 



the water, and by their decay 



bottom, gave the willows an opportunity to gain a foothold. 



Or circumstances may have arisen by which an entire lake 



was suddenly drained. 



probably 



up an extensive growth of Forestiera. Both willows and 



be 



the oak stage was reached. It is the stage reached at the 

 present day and represents a condition which probably has 

 existed for thousands of years. Yet evidence is not wanting 

 that the oaks in their turn will be replaced by other trees 

 such as sassafras and maple. 



described 



the islands of 



Mississippi 



of the 



