HARPER: CLIMAX VEGETATION 525 
other inflammable herbs, is not usually sufficiently protected from 
fire to allow hammock vegetation to obtain a foothold there, while 
the sandhills, whose vegetation is so sparse that fires on them are 
very rare, furnish an excellent protection on that side. The trees 
in the swamp at the base of nearly every stream sandhill probably 
furnish the humus to start with, and the birds and squirrels bring 
the berries and nuts of the hammock plants, which gradually spread 
up the bare sandy slopes, making their own humus as they go. 
The same theory also seems to explain why pine barrens ap- 
proach nearer to salt marshes and salt water in Mississippi than 
in Georgia; a fact which I commented upon a few years ago* but 
could not account for at the time. The country within a few 
dozen miles of the coast in Georgia is considerably lower and 
flatter than that similarly situated in Mississippi, and tidal 
estuaries have encroached on it to a much greater extent, making 
many islands and narrow-necked peninsulas, as pointed out on 
a preceding page. In Mississippi the pine flatwoods are sparingly 
represented if at all, and undulating pine barrens like those of 
the Altamaha Grit region of Georgia come right down to the 
coast, the typical branch-swamp vegetation occurring within a 
few hundred yards of Mississippi Sound at Gulfport and elsewhere. 
The shell hammocks on the coast of Alabama and Mississippi, 
described by Mohr ft and Hilgard,{ must owe the peculiarities of 
their vegetation at least in part to the protection from fire afforded 
by the marshes or waterways by which they are invariably 
bordered. 
GEOLOGICAL SURVEY OF ALABAMA. 
Dee ee 
* Torreya 6: 201, 203. 06 
+ Contr. U. S. Nat. Herb. 6: 133. I901. 
t Geol. & Agric. Miss. 373, 374, 380. 1860; Tenth Census U. S. 6: 270, 271. 
1884; Soils 495, 497. 1906. 
