Limestone

a well known and widely distributed rock. It consists chiefly of carbonate of lime. Magnesium, iron oxide, alumina, and silica are usually present. Most limestones are formed in comparatively shallow, clear seas by the accumulation of shells. Some of these shells are so small that they require a microscope to make them out. Coral and shells of mollusks contribute largely. Sometimes limy materials are ground to a fine ooze before they consolidate. Sometimes they remain almost entire. A close examination of a limestone will reveal usually a large number of fossils, ranging in size from the head of a pin to shells several feet in length or diameter. Some of the later limestones are a mere mass of shells. The varieties of limestone are endless. Some are pure white, others yellow, others are blue, and others again, when stained with bitumen, are black. Under great heat and pressure limestone is converted frequently into marble of various colors, sometimes beautifully mottled. Chalk is a soft, powdery limestone. Some limestones are so full of sand that it is difficult to distinguish them from sandstones. At the other end of the series, they pass into marls and shales. Disintegrating limestone makes rich soil. The famous blue grass region of Kentucky and other sections owe their reputation to an underlying layer of limestone that disintegrates just fast enough to feed the soil. Crops grown on a limestone soil withstand early frosts and drought well. Our limestone quarries lead those of all other countries in money value. For some of the purposes to which limestone is put, see MARBLE; LIME; CEMENT.