This group of rocks has the common property of being composed of carbonate of lime, calcite, CaCO3, or of this substance intermingled more or less with dolomite, Mg,Ca(CO3 )2 . It is also a common property that, so far as known, the carbonate of lime has primarily been separated from water, rendered insoluble and accumulated by the action of living organisms of one kind or another. Secondarily, the deposits thus made may be mechanically broken up and redeposited, or they may be taken into solution, carried away and precipitated elsewhere. There may be some possible exceptions to this rule, that the carbonate of lime is primarily precipitated by organisms, in the cases where it is concentrated in alkaline lakes by inflowing waters and finally deposited, or in the evaporation of shut-off portions of the sea, but these are of small account and negligible in comparison with the great formations produced by life agencies. Hence it is generally held that the great masses of carbonate rocks, even when they do not contain fossils, are a proof of the existence of life at the time of their original deposition. This group of rocks is soluble in hydrochloric acid; entirely so when pure carbonates, but generally leaving more or less of a residue, consisting chiefly of quartz sand, clay, silica, etc. In some cases, where dolomite is present, the acid must be heated. Their hardness is less than 4, hence they may be readily scratched or cut with the knife. The following are the important members of the group.
Limestone. This is the most common and important carbonate rock. It is fine grained to very dense in texture and its color varies from whitish, through tones of yellowish to brown, or from various shades of gray, bluish-gray, dark-gray to black. It is rarely of reddish colors. The yellow and brown colors are due to iron oxide, the gray and black to organic matter. The gray colors are most common. Compact varieties have an even to somewhat conchoidal fracture. It effervesces freely with any common acid, with vinegar (acetic acid) or lemon juice (citric acid). It is easily scratched with the knife and many of the less compact varieties are friable to the finger nail. The specific gravity varies from 2.6 to 2.8. On exposed surfaces it is apt to be cavernous and often tinted or blotched reddish or yellowish from oxidation of small amounts of ferrous carbonate it may contain. It occurs in individual beds of all thicknesses up to 100 feet or more. Some limestones consist of pure grains of calcite, others possess a fine, clay-like cement between them.
Accessory minerals, which are sometimes seen, are pyrite and quartz, the latter in minute crystals, sometimes lining cavities. Transitions to dolomite are not common; an examination of a large number of analyses shows that generally either the rock contains very little or no magnesia, or it has much and is a regular dolomite as described later. There are many varieties of limestone, depending on circumstances, especially the mode of formation. Thus in some there are abundant remains of fossils which may give the rock a distinctive character. These comprise a great variety of organisms, among which may be mentioned corals, crinoids, shells of mollusks, brachiopods, gastropods, foraminifera, remains of sponges, etc. The "encrinal limestone" of Silurian age in western New York is an example. Sometimes these fossils occur in such numbers that the entire rock is composed of masses of shells, or the hard part of one particular organism, with just enough fine carbonate of lime between them to act as a cement. Examples of this are seen in the layers composed wholly of brachiopod shells found in the Niagara formation of the Silurian in western New York. Such rocks are sometimes called "shell limestones." Certain limestones composed of corals are also examples of the same thing.
On the other hand, there are varieties which depend on the presence of some impurity which gives a particular character to the rock. Thus it may contain much clay and is termed an argillaceous limestone or it may contain much sand of siliceous character and be an arenaceous limestone. Such rocks are transitional to shales and sandstones. Others which are dark colored may yield a strong, disagreeable, bituminous odor when struck and broken and are called bituminous limestones; they contain considerable organic matter. In some, which are termed glauconitic limestone, the rock is more or less rilled with green grains of glauconite. Lithographic stone is a fine, compact, somewhat schistose limestone; the flesh colored rock from Solenhofen, Bavaria, remarkable for the well preserved fossils it occasionally contains, is especially used for this purpose. It is a very pure limestone.
Limestones are very apt to contain concretions and masses of chert, or hornstone, of the character described in a previous section; they often become so abundant as to form definite bands or layers in the rock. By the weathering of limestone the lime carbonate is removed in solution, leaving the insoluble impurities behind. These form clays or loams which are colored deeply red or yellow by the oxidation of the iron minerals originally present, and commonly contain pebbles of chert or quartzose material and masses of limonite. Such residual soils are commonly very fertile and cover large areas in the southern United States, and in other parts of the world. The use of limestone for structural purposes of all kinds is well known and needs no further comment. The same is true of its manufacture, by burning, into quicklime for mortar and cements. Large quantities are also used as a flux in smelting operations, as in the making of iron and steel. In recent years the use of certain impure limestones containing 15-40 per cent of clay, or other substances consisting of silica, alumina and iron oxide, in the manufacture of natural hydraulic cements has risen to very large proportions. Limestones and dolomites are rocks of such general distribution in all parts of the world where stratified rocks are found, that their occurrence needs no special mention.
Limestone rocks can be hosts to significant mineral deposits. They are host to many base metal replacement systems, including lead ores. They are also the chief host to the Carlin type micron gold deposits.
Dolomites. The mineral dolomite is denser than calcite and in the change above mentioned a considerable reduction of volume, amounting to 12 per cent, must occur in the limestone. This would apparently help to explain why dolomites are so frequently very porous or cavernous rocks, though if deeply buried, all such cavities would be closed by the pressure.
Oolite. Oolitic Limestone is a well-characterized variety of limestone, which consists of minute to small spherical concretions, presenting very much the aspect of a fish roe, whence the name from the Greek, meaning egg stone. The round grains vary in size from very minute up to those as large as a pea. In the larger ones it may often be observed that they have a concentric shelly structure and thus consist of successive coats.
Travertine, Calcareous Tufa. In the preceding chapter it was shown how material of the land surface is taken into solution and carried into the sea. This is especially important with regard to lime, which goes to the sea as a sulphate and carbonate, the latter being much the more momentous. This lime carbonate comes, not only from pre-existent carbonate rocks, but also from the lime silicate minerals of the igneous and metamorphic ones, which under atmospheric agencies are converted into carbonates. The lime carbonate on its way to the sea may be temporarily deposited, giving rise to rock-masses of some magnitude and importance.
Carbonate of lime has little solubility in pure water, but if the latter contains carbon dioxide, the lime carbonate is converted into the soluble bicarbonate and the amount of the latter formed and taken into solution depends on the amount of carbon dioxide present. Thus in regions where limestones or other carbonate rocks abound, the natural waters, under atmospheric pressures, attack such rocks and take the lime carbonate into solution in a relatively slow manner, but in spring waters, and especially thermal ones coming from depths, the pressure may be great, the amount of contained carbon dioxide large and the quantity of dissolved carbonate of lime proportionately so. Such waters on coming to the surface lose the greater part of the dissolved carbonic acid in the form of gas, and the lime in solution is consequently deposited rapidly and in large amount. In the waters under surface atmospheric pressure the lime is deposited by evaporation and therefore much more slowly. In warm waters the deposit of lime may be much increased by the action of low forms of vegetable life, algae, living in them, which secrete lime from the water. The rock thus formed by deposit of carbonate of lime from solution is called travertine, from the old Roman name of the town of Tivoli near Rome, where an extensive formation of the substance exists. When deposited slowly, as in the stalactites and stalagmites in caves, it is rather hard and compact, fine crystalline, sometimes white but usually tinted yellowish or brownish; it often has a fibrous or concentric structure; it breaks with a splintery fracture. When deposited more rapidly, as by springs, it is softer, not evidently crystalline, and porous to loose or earthy; when formed coating vegetation it may be open, cellular, spongy, bladed or moss-like.
These looser, less compact, varieties are commonly called calcareous tufa or calcareous sinter. Deposits of travertine, or calcareous tufa, are found in nearly all countries and especially in limestone regions. Many caves are celebrated for the number, size and beauty of the stalactite and stalagmite formations they contain. Springs depositing carbonate of lime are very common, but warm carbonated waters are chiefly found in volcanic regions or those which have recently been so, like the celebrated Mammoth Hot Springs of the Yellowstone Park, and others found in California, Mexico, Italy, New Zealand, etc. The so-called Mexican "onyx" or "onyx marble," which is extensively used as an ornamental stone, is a travertine with a banded structure, beautifully brought out by its varied tinting through metallic oxides.
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