GREAT DISMAL SWAMP 1 33 



amount of water is necessary. The vast quantities of coal which 

 occur in so many parts of the world, testify to the significance of 

 the part which bog and swamp accumulations of vegetable matter 

 have played in the earth's history. The nearest approach to coal 

 that we have in process of formation at the present day, we find 

 in the peat bogs, which are especially abundant and extensive in 

 cool, damp climates, as in Ireland, Scandinavia, and the northern 

 parts of North America. In northern regions the peat is formed 

 principally by mosses, and especially by the bog moss, Sphagnum ; 

 elsewhere, as in the Great Dismal Swamp of Virginia, the leaves 

 of trees and various aquatic plants are the sources of supply. 



Vegetable matter consists of carbon, hydrogen, oxygen, and 

 nitrogen, with a certain proportion of mineral matter, or ash. 

 When decaying on the ground, exposed to the air, the plant tis- 

 sues are completely oxidized, and form such simple and stable 

 compounds as C0 2 , H 2 0, NH 3 , and the more complex humous 

 acids, and thus hardly any solid residue is left. In forests the 

 accumulation of leaves for many centuries results only in a shallow 

 layer of vegetable mould. Under water, where the supply of 

 oxygen is very limited, vegetable decomposition is much less com- 

 plete. Some COo, H 2 0, and CH 4 (marsh gas) are formed, but 

 much of the hydrogen and nearly all of the carbon remain ; the 

 farther decomposition proceeds, the higher does the percentage 

 of carbon rise, and the darker does the colour of the mass become. 

 Peat frequently forms in small lakes and ponds, aquatic plants 

 growing out from the edges and on the surface, until they gradu- 

 ally fill up the basin and convert the pond into a bog. 



The Great Dismal Swamp 6f Virginia and North Carolina prob- 

 ably more nearly reproduces than do most existing peat bogs the 

 conditions of the ancient coal swamps. The swamp, which meas- 

 ures thirty miles by ten, is a dense growth of vegetation upon a 

 water-covered soil of pure peat about fifteen feet deep and with no 

 admixture of sediment. The swamp cypress grows abundantly in 

 the bog, and prevents, by its dense shade, the evaporation which 

 would take place in summer, could the sun's rays penetrate to the 

 wet soil. The shallow layer of water which covers the ground 



