5. Sedimentary Rocks of Organic Origin
Limestone.—A generally white or gray rock composed of carbonate of lime with varying proportions of clay, silica, and other impurities. The lime carbonate is usually derived from the hard parts of marine organisms, and the argillaceous and siliceous impurities from the finer land-derived sediments which descend with them to the bottom.
Dolomite (dolomitic or magnesium limestone).—Differs from limestone in containing varying proportions of the mineral dolomite (ante, [p. 455]), which is made up of equal parts of calcium and magnesium carbonates. Difficult to distinguish from limestone unless a chemical test is made for magnesium, though it may be said in general that dolomite is less soluble in cold mineral acids.
Peat.—An accumulation of decomposed vegetable matter within small lakes and in lagoons separated from larger ones (ante, [p. 429]). Peat represents the first stage in the formation of coal from vegetable matter, and differs from the coals by its larger proportion of contained water. Because of this water its fuel value is correspondingly small. It is usually dark brown or black and reveals something of the structure of the plants out of which it was formed.