The most important of the swamp and bog deposits are the vegetable accumulations, for the preservation of which a certain 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.

The processes of organic decomposition depend upon the activities of bacteria, but, for the sake of simplicity, we may treat the subject as if the processes were chemical only. 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 tissues are completely oxidized, and form such simple and stable compounds as C02, H20, NH3, 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 complete. Some C02, H20, and CH3 (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 gradually fill up the basin and convert the pond into a bog.

The Great Dismal Swamp of Virginia and North Carolina probably more nearly reproduces than do most existing peat-bogs the conditions of the ancient coal swamps. The swamp, which measures 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 receives the fallen leaves, twigs, and branches, and sometimes even the trunks of fallen trees, preventing their complete decomposition, while the dense covering of mosses, reeds, and ferns which carpet the ground, add their quota to the mass of decaying 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.

Great Dismal Swamp, Virginia. (U. S. G. S).

Fig. 90. - Great Dismal Swamp, Virginia. (U. S. G. S).

The bogs of northern latitudes are due principally to the bog moss Sphagnum, which forms dense and tangled masses of vegetation, dead and decaying below, green and flourishing above. As these mosses hold water like a sponge, they will develop bogs in any shallow depression, or even on a flat surface, where they may get a foothold. The depth of peat is sometimes as much as fifty feet, and its density and fineness of grain increase with the depth and the length of time it has been macerating in water.

Fire-clay is frequently found at the bottom of peat-bogs, and is directly connected with the processes of vegetable decomposition, though not itself of organic origin. Fire-clay contains a large admixture of siliceous sand, but is free from lime, magnesia, the alkalies, and any high percentage of iron; it is thus a mixture of nearly pure clay and sand, which may be heated very highly without melting or crumbling. The iron, alkalies, and alkaline earths are gradually leached out of the clay by the action of the peaty water, which is charged with organic acids, and thus an ordinary clay is converted into a fire-clay. Fire-clay occurs frequently beneath coal seams; as the percentage of silica becomes very high, fire-clay passes over into gannister, which is largely used for the lining of iron furnaces.

Bog Iron-ore is another substance which is indirectly due to the decay of plants; it is found at the bottom of bogs, or lakes, in deposits which are sometimes many feet thick. Iron is a very widely disseminated substance, occurring in almost all rocks and soils, though usually in very small quantities; by the action of the bog water the oxide is converted into the soluble carbonate (FeC03). Solutions of FeC03 accumulate under peat-bogs and deposit their mineral by concentration; but when the iron-bearing waters evaporate in contact with the air, the carbonate is reconverted into the red oxide, by the loss of C02 and absorption of O.