MARBLE is a term applied to any limestone which is sufficiently close in texture to admit of being polished. Many other ornamental stonessuch as serpentine, alabaster, and even graniteare sometimes loosely desig-nated as marbles, but by accurate writers the term is invariably restricted to those crystalline and compact varieties of carbonate of lime which, when polished, are applicable to purposes of decoration. The crystalline structure is typically shown in statuary marble. A fractured surface of this stone displays a multitude of sparkling facets, which are the rhombohedral cleavage-planes of the component grains. On placing a thin section of Carrara marble under the microscope, it is seen that each grain is an imperfect crystal, or crystalloid, of calc-spar, having an irregular boundary, and being itself made up of a number of crystalline plates twinned together (see fig. 5, article GEOLOGY, vol. x. p. 231). It is said that a somewhat similar polysynthetic structure may be artificially induced in calc-spar by means of pressure. As marble appears to be, in many cases, a metamorphic rock, it is probable that pressure and heat have been the principal natural agents concerned in the alteration of compact into crystalline limestones. It was shown many years ago by Sir James Hall that even an earthy limestone, like chalk, when strongly heated in a closed vessel, might assume a saccharoidal texture ; and it is a fact familiar to the field-geologist that a crystalline structure is often locally developed in limestone where it happens to have been invaded by an eruptive rock. Prof. Geikie proposes to distinguish this kind of metamorphism by the term mar-marosis (Text-Book of Geology, 1882).
Among statuary marbles the first place may be assigned to the famous Pentelic marble, the material in which Phidias, Praxiteles, and other Greek sculptors executed their principal works. The characteristics of this stone are well seen in the Elgin marbles, which were removed from the Parthenon at Athens, and are now in the British Museum. The marble was derived from the quarries of Mount Pentelicus in Attica. The neighbouring mountain of Hymettus likewise yielded marbles, but these were neither so pure in colour nor so fine in texture as those of Pentelicus. Parian marble, another stone much used by Greek sculptors and architects, was quarried in the isle of Paros, chiefly at Mount Marpessa. It is called by ancient writers lychnites, in allusion to the fact that the quarries were worked by the light of lamps. The Venus de' Medici is a notable example of work in this material. Carrara marble is better known than any of the Greek marbles, inasmuch as it constitutes the stone invariably employed by the best sculptors of the present day. This marble occurs abundantly in the Apuan Alps, an offshoot of the Apennines, and is largely worked in the neighbourhood of Carrara, Massa, and Serravezza. Stone from this district was employed in Rome for architectural purposes in the time of Augustus, but the finer varieties, adapted to the needs of the sculptor, were not discovered until some time later. It is in Carrara marble that the finest works of Michelangelo and of Canova are executed. The purest varieties of this stone are of snow-white colour and of fine saccharoidal texture. Silica is disseminated through some of the marble, becoming a source of annoyance to the workman ; while occasionally it separates as beautifully pellucid crystals of quartz known as Carrara diamonds. The geological age of the marbles of the Apuan Alps has been a subject of much dispute, some geologists regarding them as metamorphosed Triassic or even Liassic rocks, while others are disposed to refer them to the Carboni-ferous system. Much of the common marble is of a bluish colour, and therefore unfit for statuary purposes ; when streaked with blue and grey veins, the stone is known as barcliglio. Curiously enough, the common white marble of Tuscany comes to England as Sicilian marblea name probably due to its having been formerly re-shipped from some port in Sicily.
Although crystalline marbles fit for statuary work are not found to any extent in Great Britain, the limestones of the Palaeozoic formations yet yield a great variety of marbles well suited for architectural purposes. The Devonian rocks of South Devon are rich in handsome marbles, presenting great diversity of tint and pattern. Plymouth, Torquay, Ipplepen, Babbacombe, and Chudleigh may be named as the principal localities. Many of these limestones owe their beauty to the fossil corals which they contain, and are hence known as madrepore marbles.
Of far greater importance than the marbles of the Devonian system are those of Carboniferous age. It is from the Carboniferous or Mountain Limestone that British marbles are mainly derived. Marbles of this age are worked in Derbyshire and Yorkshire, in the neighbour-hood of Bristol, in North Wales, in the Isle of Man, and in various parts of Ireland. One of the most beautiful of these stones is the encrinital marble, a material which owes its peculiarities to the presence of numerous encrinites, or stone-lilies. These fossils, when cut in various directions, give a characteristic pattern to the stone. The joints of the stems and arms are known from their shape as " wheel-stones," and the rock itself is sometimes called entrochal marble. The most beautiful varieties are those in which the calcareous fossils appear as white markings on a ground of grey limestone. On the Continent a black marble with small sections of crinoid stems is known as petit granit, while in Derbyshire a similar rock, crowded with fragments of minute encrinites, is termed bird's-eye marble.
Perhaps the most generally useful marbles yielded by the Carboniferous system are the black varieties, which are largely employed for chimney-pieces, vases, and other ornamental objects. The colour of most black limestone is due to the presence of bituminous matter, whence the mineralogical name anthraconite. Such limestone com-monly emits a fetid odour when struck ; and the colour, being of organic origin, is discharged on calcination. Black marbles, more or less dense in colour, are quarried in various parts of Ireland, especially at Kilkenny and near Galway ; but the finest kind is obtained from near Ashford in Derbyshire. From Ashford is also derived a very beautiful stone known as rosewood marble. This is a dense brown laminated limestone, displaying when polished a handsome pattern somewhat resembling the grain of rosewood; it occurs in very limited quantity, and is used chiefly for inlaid work.
With the rosewood marble may be compared the well-known landscape marble or Gotham stone, an argillaceous limestone with peculiar dendritic markings, due probably to the infiltration of water containing oxide of manganese. This limestone occurs in irregular masses near the base of the White Lias, or uppermost division of the Rhaetic series. It is found principally in the neighbourhood of Bristol. The arborescent forms depicted in bluish-grey upon this landscape marble form a marked contrast to the angular markings of warm brown colour which are seen on slabs of ruin marble from Florencea stone occasionally known also as landscape stone, or pietra paesina.
British limestones of Secondary and Tertiary age are not generally compact enough to be used as marbles, but some of the shelly beds are employed to a limited extent for decorative purposes. Ammonite marble is a dark brown limestone from the Lower Lias of Somersetshire, crowded with ammonites, principally A. planicostata. Under the name of Forest marble, geologists recognize a local division of the Lower Oolitic series, so named by W. Smith from Wychwood Forest in Oxfordshire, where shelly limestones occur; and these, though of little economic value, are capable of being used as rough marbles. But the most important marbles of the Secondary series are the shelly limestones of the Purbeck formation. Purbeck marble was a favourite material with mediaeval architects, who used it freely for slender clustered columns and for sepulchral monuments. It consists of a mass of the shells of a fresh-water snail, Paludina carinifera, embedded in a blue or grey limestone, and is found in the Upper Purbeck beds of Swanage in Dorsetshire. Excellent examples of its use may be seen in Westminster Abbey and in the Temple Church, as well as in the cathedrals of Salisbury, Winchester, Worcester, and Lincoln. Sussex marble is a very similar stone, occurring in thin beds in the Weald clay, and consisting largely of the shells of Paludina, principally P. sussexiensis and P. fluviorum. The altar stones and the episcopal chair in Canterbury cathedral are of this material.
Mixtures of limestone and serpentine frequently form rocks which are sufficiently beautiful to be used as orna-mental stones, and are generally classed as marbles. Such serpentinous limestones are included by petrologists under the term ophicalcite. The famous verde antico is a rock of this character. Mona marble is an ophicalcite from the metamorphic series of the Isle of Anglesey, while the " Irish green " of architects is a similar rock from Connemara in western Galway. It is notable that some of the " white marble" of Connemara has been found by Messrs King and Bowney to consist almost wholly of malacolite, a silicate of calcium and magnesium.
A beautiful marble has been worked to a limited extent in the island of Tiree, one of the Hebrides, but the quarry appears to be now exhausted. This Tiree marble is a limestone having a delicate carnelian colour diffused through it in irregular patches, and containing rounded crystalloids of sahlite, a green augitic mineral resembling malacolite in composition. When dissolved in acid the marble leaves a brick-red powder, which has been studied by Dr Heddle, who has also analysed the sahlite.
Many marbles which are prized for the variegated patterns they display owe these patterns to their formation in concentric zones,-such marbles being in fact stalagmitic deposits of carbonate of lime, and probably consisting in many cases of aragonite. One of the most beautiful stalagmitic rocks is the so-called onyx marble of Algeria. This stone was largely used in the buildings of Carthage and Borne, but the quarries which yielded it were not known to modern sculptors until 1849, when M. Delamonte rediscovered the marble near Oued-Abdallah. The stone is a beautifully translucent material, delicately clouded with yellow and brown, and is greatly prized by French workmen. Large deposits of a very fine onyx-like marble, similar to the Algerian stone, have been worked of late years at Tecali, about 35 miles from the city of Mexico. Among other stfdagmitic marbles, mention may be made of the well-known Gibraltar stone, which is often worked into models of cannon and other ornamental objects. This stalagmite is much deeper in colour and less translucent than the onyx marbles of Algeria and Mexico. A richly tinted stalagmitic stone worked in California is known as Californian marble. It is worth noting that the " ala-baster " of the ancients was stalagmitic carbonate of lime, and that this stone is therefore called by mineralogists " Oriental alabaster " in order to distinguish it from our modern " alabaster," which is a sulphate, and not a car-bonate, of lime.
The brown and yellow colours which stalagmitic marbles usually present are due to the presence of oxide of iron. This colouring matter gives special characters to certain stones, such as the giallo antico, or antique yellow marble of the Italian antiquaries. Siena marble is a reddish mottled stone obtained from the neighbourhood of Siena in Tuscany; and a somewhat similar stone is found in King's County, Ireland. True red marble is by no means common, but it does occur,'of bright and uniform colour, though in very small quantity, in the Carboniferous lime-stone of Derbyshire and north-east Staffordshire. It may be noted that the red marble called rosso antico is often confounded with the porfido rosso antico, which is really a red porphyritic felstone.
Fire marble is the name given to a brown shelly lime-stone containing ammonites and other fossil shells, which present a brilliant display of iridescent colours, like those of precious opal. It occurs in rocks of Liassic age at the lead-mines of Bleiberg in Carinthia, and is worked into snuff-boxes and other small objects. By mineralogists it is often termed lumachella, an Italian name which may, however, be appropriately applied to any marble which contains small shells.
It would unnecessarily extend this article to enumerate the local names by which marble-workers in different countries distinguish the various stones which pass under their hands. The quarries of France, Belgium, and Italy, not to mention less important localities, yield a great diversity of marbles, and almost each stone bears a distinc-tive name, often of trivial meaning.
America possesses some valuable deposits of marble, which in the eastern States have been extensively worked. The crystalline limestones of western New England furnish an abundance of white and grey marble, while a beautiful material fit for statuary work has been quarried near Rutland in Vermont. A grey bird's-eye marble is obtained from central New York, and the greyish clouded limestones of Thomaston in Maine have been extensively quarried. Of the variegated and coloured marbles, perhaps the most beautiful are those from the northern part of Vermont, in the neighbourhood of Lake Champlain. A fine brecciated marble is found on the Maryland side of the Potomac, below Point of Rocks. Among the principal localities for black marble may be mentioned Shoreham in Vermont and Glen Falls in New York. In Canada the crystalline limestones of the Laurentian series yield beautiful marbles.
Turning to India, we find important quarries at Makrana in Rajputanaa locality which is said to have yielded the marble for the famous Taj Mahal at Agra. In the valley of the JSTerbudda, near Jabalpur, there is a large development of marble. The white marble which is used for the delicately-pierced screens called jalee work is obtained from near Raialo, in Ulwar. See Ball's Economic Geology of India, 1882.
For descriptions of ancient marbles see F. Corsi's treatise Belle Pietre antiche ; and for marbles in general consult Professor Hull's Building and Ornamental Stones, 1872. (F. W. R.*)
The above article was written by: F. W, Rudler, Curator of the Museum of Practical Geology, London.