568 GEOLOGY. 



on moist lands which are not swampy. On fertile prairies and in 

 great forests the annual growth of vegetation is great; but as the 

 leaves, fruits, twigs, and trunks fall, they decay, and the larger part 

 of their substance is returned to the atmosphere. If the decay of vegeta- 

 tion does not keep pace with its growth and death, an undecayed 

 or partially decayed residuum of organic matter remains, giving the 

 soil a black color. It is a matter of common knowledge that black 

 soils are more common in moist regions than in dry ones, in cool regions 

 than in warm ones, and on flat surfaces than on sloping ones. In a 

 moist region there is more growth (and therefore more death) of vege- 

 tation than in a dry one, and a better chance that decay will not keep 

 pace with death. Decay is less rapid in a cool climate than in a hot 

 one, so the former is more likely to' have a residuum of partially decayed 

 organic matter. From a flat surface the undecayed organic matter 

 is less likely to be removed by erosion than from a sloping one, and 

 the poorer drainage has the effect of greater moisture. Forests, by 

 shading the ground and checking the drainage, have the effect of addi- 

 tional moisture. 



An increasing blackness of the soil may often be observed as the 

 borders of a marsh are approached. The increase in blackness means 

 an increase in the amount of undecayed or but partially decayed organic 

 matter, and this in turn appears to be immediately connected with 

 the increasing moisture of such situations. In the marshes them- 

 selves, where the vegetation falls into water, it usually undergoes 

 a slow decay only, and not that type of decay suffered by the vegeta- 

 tion which falls on drier lands. The preserving influence of water 

 is seen in many ways. Posts and piles set partly in water, and 

 partly above, decay just above the water-level, while the portions 

 below remain sound. It is the preservation of organic matter in the 

 water of marshes and very shallow lakes which converts them into 

 peat-bogs, for the peat is nothing more than accumulated vegetable 

 matter undergoing those changes to which vegetable matter in water 

 is subject. 



If the surroundings of a marsh be low, little sediment is washed 

 into it, and there is every gradation from the earthy soil about it, 

 merely colored by organic matter, to the deposit in the marsh itself, 

 made chiefly of organic matter. Given continued favorable condi- 

 tions, and the organic matter of a bog may become very deep, as it 



