212 Part II. Chapter 3. 
comprehensively in the ratio of rainfall to evaporation, expressed in percent- 
ages which, if plotted on the map, says TRANSEAU, exhibit climatic centers 
which correspond in general with the centers of plant distribution. 
To return to the bog plant associations previously mentioned, we must 
describe the envelopment of these by the species of the broad-leaved trees 
which, as we have just seen, were derived from a southeastern center. In the 
Ohio Valley, the northern fringe of this deciduous forest encompassed the bog 
societies which here existed along the most southern lobe of the great ice 
sheet. The oaks, hickories, maples, ashes andelms following the line of their 
specific habitat, the stream valleys, or the uplands, the sandy stretches left by 
glacial drainage, or the eskers, kames and drumlins surrounded the bog asso- 
ciations in their northward progression. 
2. Evolution of Coastal and Southern Mountain Flora. 
Different Areas. With the final elevation of the land along the Atlantic 
and Gulf shores of the North American continent, we have the evolution of 
the coastal plain which has passed through an interesting history since the be- 
ginning of Cretaceous times. A zone extending from the “fall line” representing 
the eastern edge of rocky desposits of Archean age has at times been dry- 
land, as it is now, and at times sea bottom. Along the advancing and reced- 
ing coast, gravels have been piled by streams coming from the landward side 
and with them sands and clays have been deposited ‘). Now it is a great plain 
marked by few hills and slightly terraced with bluffs on the margins of food 
plains. Near the coast and along the flood plains, extensive marshes ‚are found. 
This irregular zone of marsh is clearly distinguished from the higher plain. 
A lower plain extends from the coastal marshes out to the sea for many miles, 
until at last shallow waters change into deep waters, and the bottom plunges 
down with steep declivity into the depths of the Atlantic. 
This region, which may be divided into scveral well marked areas, Viz., 
the northern Atlantic coastal plain, the southern Atlantic coastal plain and the 
Gulf coastal plain, shows some marked differences in the flora of the different 
areas. These differences will be emphasized in succeeding chapters of this 
work. Suffice it to say here that the flora of the coastal plain is comparatively 
recent, more recent than the latest submergence when the Columbia gravels 
a ee 
were deposited, perhaps as late as Pleistocene times and the endemic species 
have been evolved by mutation or otherwise since the Pleistocene, while tbe 
other plants have been derived from other sources of supply, one of which, 
not mentioned previously, is the West Indian group of islands. Many plant 
associations are met with on the coastal plain and their development has foll- 
owed the elevation of the land above sea-level. For example in New Jersey) 
where the most northern extension of the plain occurs, we have the pine barren 
1) MACGEE, W. J.: The Lafayette Formation. Twelfth Annual Report U. S. Geological 
Survey 1890—1891, Part I, Geology, pp. 219—515. 
