Peat bogs consist of decomposed mosses, grasses, aquatic plants, and mud. In mountainous districts when they are superimposed upon hard or non-weathering rock, such as quartz, they have little thickness; if upon clay rock, or any that decomposes under weather influences, they are usually of considerable depth. As a rule, the greater the specific gravity of peat, the firmer it is and also the darker. The top layers are generally the most fibrous and the driest, although spongy and containing vegetable matter, and are of a light brown colour, and of less specific gravity than the next deposit, which is of a darker brown tint, and is denser and more decomposed; the peat then becomes brown black or black in hue and approaches a coaly condition. Compact turf usually contains little water, but should it be of a mossy nature it is generally saturated. Wet bog, peat, or moorland may contain as much as 80 per cent. of water, and any soil having so large a proportion must necessarily subside and change upon being thoroughly and regularly drained, and have its bearing power increased in a ratio approximate to the percentage of water in it before draining and that after such operation has been effected, other conditions being similar. The depth and character of the top layers will indicate the best method of procedure, and whether it is advisable to excavate them and deposit an embankment upon the lower and firmer bed. In Holland, where the peat is superimposed upon sandy soil, in order to prevent slips in embankments it is found advisable to excavate a trench about 15 to 20 feet in width and as wide as the formation width of a railway or road, until it reaches the subsoil, and to fill it with sand; and when an embankment has to be formed of very porous earth, to make a trench at the toe of each slope, and to fill it with sand so as to act as a counterfort. Peat and sand have been found to laterally spread the most, clay and sand less, and therefore the latter soil stands at a steeper slope. On the South Austrian system of railways marsh-land has been made firm, and malaria-fever almost annihilated by depositing ashes over the earth. When they are mixed with the bog or swamp deposit, it is found that they cause it to become fertile.