134 



SWAMP DEPOSITS 



receives the fallen leaves, twigs, and branches, and sometimes 

 even the trunks of fallen trees, preventing their complete decom- 

 position, while the dense covering of mosses, reeds, and ferns 

 which carpet the ground, add their quota to the mass of decaying 







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Fig. 51. — Great Dismal Swamp. (U. S. G. S.) 



vegetable matter. At the bottom of the bog, it is of interest to 

 observe, is a layer of fire-clay, which, by its imperviousness, tends 

 to hold the water and prevent its draining away. Peat swamps, 

 formed in a similar manner, also occur at the mouths of great 

 rivers, such as the Mississippi. 



