142 ORGANIC AGENCIES. 



sents so much matter withdrawn from the atmosphere and added to 

 the soil. In some cases, besides the material deposited from the 

 growth of vegetation in situ, the accumulation may be partly also the 

 result of organic matter drifted from the surrounding surface-soil. 



Rate of Growth. — The rate of peat-growth must be very variable, 

 since it depends upon the vigor of the vegetation and upon the manner 

 of accumulation, whether entirely by growth of plants in situ, or partly 

 by driftage. Many of the European bogs are evidently the growth of 

 not more than eighteen hundred years, for they were forests in the 

 time of the Romans, or even later. The felling of these forests, as a 

 military measure to complete the subjugation of the country, and the 

 consequent impediments to drainage thus produced, have changed 

 them into bogs. At their bottoms, and covered with eight to ten feet 

 of peat, are found the trunks and the stumps of the original forests, 

 the axes and coins of the Roman soldiers, and the roads of the Roman 

 army. The rate of accumulation has been variously estimated, from 

 one or two inches to several feet per century. In all cases of simple 

 growth in situ, however, and therefore always in great peat-swamps, 

 the increase is very slow. 



Conditions of Growth. — The conditions usually considered necessary 

 for the formation of peat are cold and moisture; and of these the 

 former is considered the more important, as without cold it is supposed 

 vegetable matter would be destroyed by decay. In proof of this it is 

 stated that peat-bogs are more numerous in cold climates. But it is 

 more probable that excess of moisture is the only important condition. 

 This condition may be rarer in warm climates on account of the greater 

 capacity of the air for moisture in these climates ; but when it is pres- 

 ent, immense accumulations of peat occur in extensive swamps. The 

 Great Dismal Swamp is a good illustration. This swamp, situated 

 partly in North Carolina and partly in Virginia, is forty miles long by 

 twenty-five miles wide. It is covered with a dense forest of cypress 

 and other swamp trees, by the annual fall of whose leaves the peat is 

 formed. These trees, by means of their long tap-roots and their wide- 

 spreading lateral roots, maintain a footing in the insecure soil, but are 

 often overthrown, and add their trunks and branches to the vegetable 

 accumulation. The original soil, upon which the accumulation was 

 formed, must have been lower in the center, but the surface of the peat 

 rises very gently toward the center, which is twelve feet higher than 

 the circumference (Fig. 114). Near the center there is a lake of clear, 

 wine-colored water, seven miles across and fifteen feet deep, the banks 

 and bottom of which are composed of pure peat. 



In the Mississippi River swamps there are also large areas where pure 

 peat has been accumulating for ages, and is still accumulating, by 

 growth of trees in situ, though subject to the annual floods of the river. 



