Limestones also originate in a slightly different way. When the temperature and chemical composition of the water permit, calcium carbonate precipitates as millions of tiny grains of calcite and forms a limy mud that is converted to limestone. Many limestones contain shell or plant fragments in addition to these tiny grains of calcite. [Illustration: Polished section of Lower Cretaceous Edwards Limestone from Travis County, Texas, containing fossil gastropods.] There are several special kinds of limestone. If the rock is made up of many little rounded calcite grains that resemble fish eggs, it is called _öolitic limestone_. Another limestone, _chalk_, is soft, white, and fine grained. It consists mostly of tiny shell fragments and fine-grained calcite. _Coquina_ is a porous limestone made up of loosely cemented shells and shell fragments. Another special kind of limestone, known as _lithographic limestone_, because it can be used in printing, is smooth, firm, and hard. Its mineral grains are too small to be recognized without a microscope. This kind of limestone breaks with a smooth, sometimes curved, fracture. Still another variety, _pulverulent limestone_, is loose, soft, powdery, and white. It occurs in the Lower Cretaceous Edwards Limestone in Williamson and Bell counties of central Texas. Some of this limestone is used to polish rice grains, and it is added to livestock feeds to provide calcium for the animals. [Illustration: Limestone quarry in Lower Cretaceous Edwards Limestone at Georgetown, Williamson County, Texas.] Much limestone is found at the surface in Texas in Cambrian, Ordovician, Mississippian, Pennsylvanian, Permian, and Cretaceous formations. If you will look at numbers 5, 6, 9, 10, and 11 on the Texas geologic map (pp. 4-5), you will see that these strata appear at the surface in central, north-central, and Trans-Pecos Texas. Limestone has many important uses. Much Texas limestone is crushed and used as a road-building material and as an aggregate that is mixed with cement to make concrete. Farmers in some areas improve their crops by adding limestone to the soil. Limestone also is sent to the iron furnaces in east Texas to be used in the production of pig iron and steel. Some of the Texas limestones are heated to a fairly high temperature in order to change them into _lime_ (calcium oxide). Industry uses a large amount of lime in making chemicals, steel, glass, paper, and other products. Builders use it to make plasters, mortars, and stuccos. At plants in Comal, Johnson, Travis, and Williamson counties, lime is made from Cretaceous limestones.