Page:EB1911 - Volume 18.djvu/275

 METAPONTUM (Gr. , mod. Metaponto), an ancient city of Magna Graecia situated on the Gulf of Tarentum, near the mouth of the river Bradanus, and distant about 24 m. from Tarentum and 14 m. from Heraclea. It was founded by an Achaean colony from Sybaris and Crotona about 700 Metapontum was one of the cities that played a conspicuous part in the troubles arising from the introduction of the Pythagorism into Magna Graecia, and it was there that Pythagoras died in 497 His tomb was still shown in the time of Cicero.

At the time of the Athenian expedition to Sicily (415 ) Metapontum appears to have been an opulent and powerful city, whose alliance was courted by the Athenians; but it contented itself with a very trifling support. IN 332, at the time of the expedition of Alexander, king of Epirus, into Italy, it was one of the first cities to enter into an alliance with him. The Second Punic War gave a fatal blow to its prosperity. After the battle of Cannae in 216 it was among the first cities in the south of Italy to declare in favour of Hannibal, and became for some years the headquarters of Hannibal. Hence, when the defeat of Hasdrubal at the Metaurus (207 ) compelled him to abandon this part of Italy, the inhabitants of Metapontum abandoned their city, and followed him in his retreat.

From this time Metapontum sank; though it was still existing in the days of Cicero, Pausanias tells us that in his time nothing remained of it but a theatre and the circuit of the walls.

Metapontum has the remains of two temples, both of which seem to belong to the period 510–480 (Koldewey and Puchstein, Die griechischen Tempel in Unteritalien und Sicilien, Berlin, 1899, pp. 35–41). The so-called Chiesa di Sansone, which lay within the ancient town, and was probably dedicated to Apollo Lycius, was a peripteros measuring 186 by 91 ft., of which only the foundations are left. The capitals were 3 ft. in diameter. The temple was decorated with finely painted terra-cottas. Of the other temple, the so-called Tavole Paladine, which lay outside the area of the ancient city, and was a peripteros with 6 columns, 3 ft. in diameter, in front and 12 ft. at the sides, 15 columns are standing, with the lower portion of the epistyle. It measured 105 ft. by 49 ft. without the steps. There are also traces of the town walls, which have served for the construction of farmhouses, of tombs, and of a harbour by the shore. Pliny speaks of a temple of Juno at Metapontum supported by columns of vinewood (Hist. nat. xiv. 9). An archaic treasure-house dedicated at Olympia by the people of Metapontum has been discovered there. The railway station is the junction of the line from Battipaglia (and Naples) with that from Taranto to Reggio.

METASOMATISM (Gr. , change,  , body), in petrology, a process of alteration of rocks by which their chemical composition is modified, new substances being introduced while those originally present are partly or wholly removed in solution. For example a limestone may be converted into a siliceous chert, a dolomite, an ironstone, or a mass of metalliferous ores by metasomatic alteration. The process is usually incomplete, greater or smaller portions of the original rock remaining. The agencies of metasomatism are in nearly all cases aqueous solutions; probably they were often at a high temperature, as metasomatic changes are especially liable to occur in the vicinity of igneous intrusions (laccolites, dikes and necks) where large quantities of water were given off by the volcanic magma at a time when it had solidified but was not yet cold. Metasomatism also usually goes on at some depth, so that we may readily believe that it is favoured by increase of pressure. On the other hand, there are many instances in which these processes cannot be shown to have taken place at temperatures or pressures above those which normally exist in the upper part of the earth’s crust (e.g. dolomitization and silicification of many limestones). There are also cases of metasomatism in which steam and other vapours are supposed to have been operative; the temperatures were probably above the critical temperature of water. Changes of this sort are described as pneumatolytic, being induced by gases (see ).

By metasomatism new minerals replace the primitive ones; at the same time the original rock-structures may be completely obliterated. An igneous rock for example may be entirely replaced by crystalline massive quartz, a fossiliferous limestone by granular crystalline dolomite. It is equally common, however, to find that the structure of the original rock is preserved though its substance has been entirely altered. An oolitic limestone may become an oolitic ironstone or chert (see, Pl. IV. fig. 5.) and casts of the fossils which the limestone contained may be formed of siderite or of chalcedony. In this case the rock resembles a pseudomorph, which is the term applied to a mineral which has been entirely replaced by another mineral without losing its original crystalline form. As a result of metasomatism rocks usually become more crystalline, especially those which have been in large part built up of fossil organic remains; this is a consequence of the new substances having been deposited by purely inorganic processes from solution in water.

The chemical change is often complete, as when a limestone is replaced by chert or otherwise silicified, but it is probably more usually incomplete, part of the substance of the original rock having been retained though possibly in new mineral combinations. When a limestone is replaced by ironstone (e.g. carbonate of iron or siderite) part at least of the carbonic acid may be that of the limestone. A dolomite, formed from a limestone, contains more than one-half of its weight of carbonate of lime presumably derived from the limestone itself; yet in this case the mineral transformation may be perfect, as the dolomite retains none of the calcite of which the limestone was formed; it is all present as the double carbonate of lime and magnesia (or dolomite). When a granite is converted by emanations containing fluorine and boron into a quartz-tourmaline rock (, q.v.) or a quartz mica rock (, q.v.) it can be proved by analysis that there has been very little modification of the chemical composition of the original mass. This resembles paramorphism in minerals, in which one mineral is substituted for another having the same chemical composition (e.g. kyanite for andalusite).

The relations between metamorphism and metasomatism are very close; in fact some authors regard metasomatism as a variety of metamorphism. It is generally true, however, that in metamorphic changes there is little chemical alteration; sandstones pass into quartzites, clays into mica-schists and gneisses, limestones into marbles without any essential modification in chemical composition, for the original minerals new ones being substituted and new structures being produced at the same time. In metasomatism, on the other hand, chemical alteration is supposed by most geologists to be an essential feature; new minerals appear, but the original structures are sometimes retained.

The facility with which a rock undergoes metasomatism depends partly on its nature, and partly on the circumstances in which it is placed. Limestones, being readily soluble under natural conditions, are especially liable. The Cleveland iron ores of Yorkshire are limestones replaced by siderite and limonite; the Whitehaven iron ores are metasomatic replacements of limestone by haematite. The former are of Mesozoic, the latter of Palaeozoic age, but both have been changed in very much the same way by percolating solutions containing salts of iron. In some cases limonite and magnetite are the principal ores. Often the changes have taken place very irregularly, along bedding planes, faults and fractures. An ironstone may in many places be traced laterally into a limestone, the amount of iron in the rock gradually diminishing. Some ironstones (Carboniferous, Jurassic, &c.) retain the oolitic structures of the original limestone; others show corals, shells and other calcareous fossils replaced by iron ores. When beds of shale or sandstone are intercalated among the limestones they usually show little change, a fact which indicates that the ready solubility of the calcareous rocks was a dominating factor in determining the metasomatic deposits. It is believed that the Whitehaven iron ores may be derived from the ironstones of the Coal-Measures which once covered the limestone districts.

Dolomitization of limestones is even more common than replacement by iron ores. That it is going on at the present day is evident from the fact that cores obtained by boring in recent coral reefs have shown that these may be extensively dolomitized in their deeper parts, and the older limestones such as the Triassic of the Alps, the Carboniferous Limestone of England and the Cambrian