86 DYNAMICAL GEOLOGY. 



addition. But, if abundant water be present, then the 

 peculiar change takes place by which peat is formed, and 

 the antiseptic property of the peat prevents complete de- 

 composition, and the vegetable matter accumulates with- 

 out limit. Thus, a peat-bog represents the accumulated 

 remains of thousands of generations of plants. Every 

 year adds to the ancestral funeral-pile, and the peat- 

 ground rises higher and higher, until, although commenc- 

 ing on a low spot, it may rise above the immediately sur- 

 rounding region, and, when swollen by rains, may even 

 burst and deluge the surrounding country with black 

 mud. In the case of the great peat-swamps of southern 

 regions, the accumulation is entirely in this way i. e., 

 by growth in place. But, in small peat-bogs in hilly 

 countries, the peat accumulates also by the driftage of 

 surface-mold. In this case, the accumulation is much 

 more rapid, but the peat is less pure. 



Lastly : As a peat-swamp commenced on a low spot, it 

 was often, at first, a shallow pond or lake, and the peaty 

 matter encroached upon it from the margin. Thus, there 

 may be found in the center of the peat-swamp a small 

 remnant of the original lake. The Great Dismal Swamp 



FIG. 43. Ideal section across the Great Dismal Swamp. 



is an excellent example. This swamp, forty by thirty 

 miles in extent, is overgrown with great swamp-trees so 

 thickly that there is little or no underbrush. The peat 

 accumulates by the annual fall of leaves and branches 

 only, and the rate of thickening is, therefore, probably 

 very slow. The peat is very black, pure, and structure- 

 less, and is from twenty to thirty feet deep. The surface 

 of the swamp is decidedly higher than the immediately 



