Text 10. Limestone

Limestone is a common type of sedimentary rock composed principally of calcite (calcium carbonate, CaCO3). When "burned" or calcined (raised to a high temperature), it yields lime. Crystalline metamorphosed limestone is known as marble. Many varieties of limestone are formed by the consolidation of seashells, which are formed by the largely CaCO3 secretions of various marine animals. Chalk is a variety of porous, fine-grained limestone composed mostly of foraminifera shells; coquina is a soft limestone made up of shell fragments. A variety of the rock, known as oolitic limestone, is composed of small spherical concretions, each containing a nucleus of a sand grain or other foreign particle around which deposition has taken place.

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NOTES:

• coquina - ракушечник;

• oolitic limestone - оолитовый известняк.

Text 11. Marble

Marble is a crystalline, compact variety of metamorphosed limestone, consisting primarily of calcite (CaCO3), dolomite (CaMg (CO3)2), or a combination of both minerals. Pure calcite is white, but mineral impurities add color in variegated patterns. Hematite, for example, adds red; limonite, yellow; serpentine, green; and diopside, blue. Marble is capable of taking a high polish and is used principally for statuary and for building purposes. Extensive deposits are located in Italy, Great Britain, and in the United States. Commercially the term marble is extended to include any rock composed of calcium carbonate that takes a polish, and it includes some ordinary limestones. The term is further extended in the loose designation of stones such as alabaster, serpentine, and sometimes granite.

The surface of marble crumbles readily when exposed to a moist, acid atmosphere, but marble is durable in a dry atmosphere and when protected from rain. The purest form of marble is statuary marble, which is white with visible crystalline structure. The distinctive luster of statuary marble is caused by light penetrating a short distance into the stone and then being reflected from the surfaces of inner crystals. The most famous variety of statuary marble is the Pentelic marble of Greece, found in the quarries of Mount Pentelikon in Attica. Pentelic marble was used by the great sculptors of ancient Greece, including Phidias and Praxiteles, and it was used to make the Elgin marbles. Parian marble, also used by ancient Greek sculptors and architects, was quarried chiefly at Mount Parpessa on the Grecian island of Paros. Carrara marble - which occurs abundantly in the Apuan Alps of Italy and is quarried in the region of Carrara, Massa, and Serravezza - was used in Rome for architectural purposes in the time of Augustus, the first emperor, but the finer varieties of sculptural marble were discovered later. The greatest works of Michelangelo are made of this marble, and it is used extensively by present-day sculptors.

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Text 12. Chalk

Chalk is a soft white or whitish form of limestone, composed of the remains of small marine organisms such as foraminifera and coccolithophores; it formed after the shells and skeletons of these organisms were deposited in a thick layer on the sea bottom. Chemically, chalk is almost pure calcium carbonate with traces of other minerals. It ranges in hardness and texture from very soft porous varieties to harder close-grained types. Chalk is particularly common in strata of the Cretaceous period (Latin creta, "chalk"). Large deposits are found in the United States, and in the British Isles. Cretaceous chalk is exposed in the White Cliffs of Dover on either side of the English Channel.

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