210 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
into the peat deposit in the form of dust. Peat containing over 25%o of mineral matter (which is-commonly spoken of as ash) is not worth much for fuel and certain other purposes.
In texture, peat is more or less fibrous, being least so in the oldest and most thoroughly decomposed samples, which are rather plastic and look much like mud. The color is always some shade of brown, varying from light brown (this usually in fresh or imperfectly decomposed peat) to nearly black.
The principal use of peat, like that of coal, is for fuel. This and some of its numerous other uses will be discussed in some detail in a subsequent chapter.
The term muck is often used more or less interchangeably with peat. When a distinction is made between them the former is usually applied to peat which is dark colored and throughly decomposed, especially if it contains a large proportion of ash, or is used as a soil to grow crops in. When regarded as synonymous, peat is the more classical term, and muck the one more used by the masses.
Humus is another substance, sometimes confused with peat. But there is one essential difference between them. Humus is formed by the slow decay of vegetable matter in places where it is exposed to the air most of the time, which allows a part of the carbon to oxidize,* and thus increases the concentration of some of the other constituents of the plant, such as lime, potash, nitrogen, etc., correspondingly. Typical humuus is rarely more than a few inches deep, while peat often has a depth of many feet. Another difference between peat and humus is that the former always cohtains certain organic acids which prevent the growth of bacteria and fungi.t (These, however, disappear' or become considerably changed when peat is subjected to extensive aeration, as described below).
It should not be inferred from all this however that a sharp line can always be drawn between peat and humus. Although the extremes are distinct enough, they often grade imperceptibly into each other, either vertically or horizontally. Vertical intergradation occurs where the upper layers of a peat deposit become exposed to the air by being built up above the ground-water level, by the climate becoming drier, or by any one of several other causes; and hor*A certain amount of shade and moisture is necessary for the formation of humus, however, for the carbon would he completely oxidized in very dry or svnny places, or where fires are frequent.
tThe reader wxho is sufficiently interested can find a pretty full discussion of humus formation in Chapter 8 of Hilgard's "Soils," 19o6.