[Limestone] is a [sedimentary] rock made up chiefly of [calcite], a calcium-carbonate mineral. This rock also commonly contains grains of [quartz], [clay] minerals, the mineral [dolomite], or other materials. If a large amount of dolomite is present, the rock is called dolomitic limestone. In some limestones, the mineral grains are too small to be distinguished from each other without a magnifying glass or a microscope, but in other limestones, the individual mineral grains are easily seen.
Pure [limestone] is white, but if it contains [clay] or plant or animal matter it is light gray, dark gray, or black. Limestone also may be some shade of yellow, brown, or red. It is fairly soft and can be scratched with a knife. Because this rock contains [calcite], an easy chemical test will help identify it: a drop or two of dilute hydrochloric acid will quickly fizz and bubble when placed on the limestone.
Limestones form in fresh water, such as in lakes, but most of them form in the seas. As some earlier-formed rocks are weathered, the calcium minerals that they contain are dissolved. Creeks and rivers carry this dissolved material to the sea. There, small animals, such as corals, crinoids, sponges, and foraminifers, take the dissolved material out of the water to build their calcium carbonate shells. Plants, such as algae, can take calcium carbonate out of solution too, and it collects on them. Shells, shell fragments, and plant remains accumulate on the sea floor, forming limy deposits that later become [limestone].
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.
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.
[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.