Limestone, the generic name of all rocks which are principally composed of carbonate of calcium. It is more particularly applied to those which are not crystalline, and are not white like marble. Perfectly crystallized carbonate of lime is calc spar or Iceland spar. When it is imperfectly crystallized and has a fine grain, whether pure white or veined, it is called marble. Limestones of all kinds are found in rocks of all geologic ages, but the more crystalline varieties occur with the more distinctly metamorphic rocks. Magnesian or dolomitic limestone is noticed in the articles Cements, Dolomite, Lime, and Marble. There are limestones which are not magnesian, but whose fossils contain this substance; thus an orthoceras in the Trenton limestone of Ottawa, Canada, which is not magnesian, contained, according to T. Sterry Hunt, carbonate of lime 56.00, carbonate of magnesia 37.80, carbonate of iron 5.95 = 99.75. Whenever limestones are not metamorphic, they bear traces of organic structure, and it is generally believed that all limestones have been formed from shells and corals which have been triturated by the waves and afterward compacted together.
They usually contain more or less impurities in the form of clay, sand, talc, or other mineral substances which have been incorporated with them during the process of trituration. Hydraulic limestone is described in Cements, and oolitic and other limestones are treated under their appropriate heads.
I. A N County Of Alabama, bordering on Tennessee, bounded S. by the Tennessee river, and watered by Elk river and its branches and several creeks; area, 575 sq. m.; pop. in 1870, 15,017, of whom 7,253 were colored. The surface is hilly, and the soil very productive. The rock is limestone, from which the county derives its name. The Memphis and Charleston and the Nashville and Decatur railroads pass through it. The chief productions in 1870 were 24,010 bushels of wheat, 404,435 of Indian corn, 17,922 of Irish and 15,427 of sweet potatoes, 115,982 lbs. of butter, and 7,319 bales of cotton. There were 2,213 horses, 1,479 mules and asses, 2,188 milch cows, 3,339 other cattle, 3,960 sheep, and 13,566 swine; 1 flour mill, and 3 saw mills. Capital, Athens.
II. An E. Central County Of Texas, watered by the Navasoto and several other small streams; area, 900 sq. m.; pop. in 1870, 8,591, of whom 1,919 were colored. The surface is undulating; about two thirds of the soil is a fertile sandy loam, the rest being black sticky prairie. The Houston and Texas Central railroad passes through it. The chief productions in 1870 were 190,609 bushels of Indian corn, 13,741 of sweet potatoes, 10,608 lbs. of wool, and 3,414 bales of cotton. There were 6,000 horses, 620 mules and asses, 844 milch cows, 38,472 other cattle, 3,681 sheep, and 13,370 swine. Capital, Springfield.