THE PENNSYLVANIAN PERIOD. 



567 



essentially free from sediment, and composed of the trunks, branches, 

 leaves, and fruits of the trees, shrubs, and herbs which grow there, 

 has been long accumulating, and its mass is now great. In various 

 cypress swamps, too, great thicknesses of vegetable matter are found, 

 essentially free from foreign matter. The same is true of mangrove 



Fig. 253. — Map of the Cape May peninsula, showing coastal marshes. The unshaded 

 areas inside the coast line are dry land. 



swamps, which differ from the cypress swamps in that they spread 

 into salt water. The multitude of marshes and peat-bogs in the United 

 States and Canada are further illustrations of the accumulation of 

 vegetable matter, sometimes mixed with abundant sediment and some- 

 times nearly free from it. 



The vegetation in such situations need not be more luxuriant than 



