250 ELEMENTARY GEOLOGY 
overflowing, spreads out as a shallow sheet of water, 
covering the entire marsh. The salt marsh is always 
covered with marsh grass, and it owes its existence 
partly to the action of this vegetation. Sometimes 
entire bays are converted into salt marsh plains, and 
on the eastern coast of the United States there are 
many thousand acres of these. 
Fie. 140. 
A salt marsh on the shores of the Bay of Fundy. 
Mangrove Swamps. Salt marshes are treeless, and 
the plants that help to build them are all of a low type; 
but in tropical or semi-tropical regions, a form of tree 
known as the mangrove, is able to grow with its roots 
im the salt water (Fig. 141). On the coast of Florida 
there are extensive mangrove swamps, and little by 
little these encroach upon the sea in a manner similar 
to that of the treeless salt marshes. 
In earlier geological ages it seems probable that other 
trees had this habit, and perhaps some of the coal beds 
