The light and porous kinds of peat appear in general to be formed in shallow swamps or on the surface of bogs, where there is considerable access of air to the decaying matters, while the compacter, older, riper peats are found at a depth, and seem to have been formed beneath the low water mark, in more complete exclusion of the atmosphere, and under a considerable degree of pressure.
The nature of the vegetation that flourishes in a bog, has much effect on the character of the peat. The peats chiefly derived from mosses that have grown in the full sunlight, have a yellowish-red color in their upper layers, which usually becomes darker as we go down, running through all shades of brown until at a considerable depth it is black. Peats produced principally from grasses are grayish in appearance at the surface, being full of silvery fibres—the skeletons of the blades of grasses and sedges, while below they are commonly black.
Moss peat is more often fibrous in structure, and when dried forms somewhat elastic masses. Grass peat, when taken a little below the surface, is commonly destitute of fibres; when wet, is earthy in its look, and dries to dense hard lumps.
Where mosses and grasses have grown together simultaneously in the same swamp, the peat is modified in its characters accordingly. Where, as may happen, grass succeeds moss, or moss succeeds grass, the different layers reveal their origin by their color and texture. At considerable depths, however, where the peat is very old, these differences nearly or entirely disappear.
The geological character of a country is not without influence on the kind of peat. It is only in regions where the rocks are granitic or silicious, where, at least, the surface waters are free or nearly free from lime, that mosses make the bulk of the peat.
In limestone districts, peat is chiefly formed from grasses and sedges.
This is due to the fact that mosses (sphagnums) need little lime for their growth, while the grasses require much; aquatic grasses cannot, therefore, thrive in pure waters, and in waters containing the requisite proportion of lime, grasses and sedges choke out the moss.
The accidental admixtures of soil often greatly affect the appearance and value of a peat, but on the whole it would appear that its quality is most influenced by the degree of decomposition it has been subjected to.
In meadows and marshes, overflowed by the ocean tides, we have salt-peat, formed from Sea-weeds (Algæ,) Salt-wort (Salicornia,) and a great variety of marine or strand-plants. In its upper portions, salt-peat is coarsely fibrous from the grass roots, and dark-brown in color. At sufficient depth it is black and destitute of fibres.
The fact that peat is fibrous in texture shows that it is of comparatively recent formation, or that the decomposition has been arrested before reaching its later stages. Fibrous peat is found near the surface, and as we dig down into a very deep bed we find almost invariably that the fibrous structure becomes less and less evident until at a certain depth it entirely disappears.