This name has been applied to the remains of plants that have accumulated in the course of centuries on the margins of shallow lakes and in marshy land. The lakes or marshes gradually disappear with the encroachment of the vegetation and the latter becomes pressed down into more or less solid or spongy fibrous layers of organic material often several feet deep. Wherever natural peat beds exist, they are found on soil or rock that has been hollowed out like a bowl or saucer into which the water from the surrounding land drains and keeps in wet condition for a great portion of the year.

Peat when dry burns readily, and is used in the same way as coal in parts of Ireland and Great Britain. It absorbs water freely and is therefore valuable when mixed with sandy soil. Some plants, like Rhododendrons, Azaleas, Kalmias, Heaths, Andromedas, and many other Ericaceous plants like to have a good deal of peat in their compost; but very few plants would thrive in peat alone.