Limestone of Scotland are sometimes converted into dolomite over
wide areas. There has been an introduction of magnesia, with
sometimes a little silica and iron; the rock recrystallizes owing to
the formation of small rhombohedra of dolomite; it frequently
becomes porous and full of drusy cavities, owing to the contraction
in volume which takes place, and the fossils and other organic
structures of the original rock disappear. The change proceeds
outwards from fissures and bedding planes and spreads gradually
through the mass of the limestone; often the transformation is
complete and no unaltered rock remains. Silicification or the
replacement of limestone by chalcedony, chert or quartz, is often
exhibited on a large scale. The formation of flint nodules and chert
bands is of this nature; the silica is not really an introduction from
without, but is merely the material of the fine siliceous skeletons of
sponges, radiolaria and other organisms, which at first were widely
scattered through the limestone and at a later time were dissolved by
percolating waters, percolated through the rock and were deposited
in certain situations as bands, nodules and tabular masses of cryptocrystalline
silica. In limestones extensive deposits of zinc ore may
occur, usually calamine. These are important sources of the metal
and there is little room for doubt that they have formed by a process
of metasomatic replacement, e.g. Carthagena, Raibl (in Carinthia)
and Belgium. In many parts of western North America (Nevada,
Arizona, &c.) great deposits of copper, lead and silver ores are worked
in crystalline limestones. They are often highly silicified, and associated
with them are intrusive igneous rocks such as granite,
dacite, porphyry and diabase. The ores occur not only in veins and
shoots, but also in great masses replacing the limestone, and the
geologists who have examined these mining districts are nearly
unanimous as to the metasomatic nature of a large part of these
deposits. Other rocks such as tuff, volcanic breccia, shale, porphyry
and granite may also be impregnated with metalliferous ores, but
the largest ore bodies are found in the limestones. Secondary
enrichment has often taken place on a considerable scale. The
constant presence of igneous rocks indicates that they are connected
with the introduction of the metals, and the deposits are often of
such a kind as to show that post-volcanic discharges of magmatic
gases and water have been the actual mineralizing agents. Bisbee,
Clifton and the Globe district in Arizona, Flagstaff in Utah, and the
Eureka district in Nevada are good examples of the deposits in
question.
As indicated above, shales, sandstones and igneous rocks may be silicified and mineralized under suitable conditions. Rhyolites and rhyolitic tuffs are often impregnated with silica to such an extent that they become almost massive quartz, and the fluidal, porphyritic, spheroidal and other igneous structures of the original rock may be retained in the siliceous pseudomorph. There are many examples of this in North Wales and the Pentland Hills. In andesites, serpentines and trachytes silicification is frequently found in circumstances indicating that the changes are not due to weathering but are the effect of post-volcanic emanations of heated waters. Silicified shales may accompany mineral deposits, e.g. in the Cornish tin mines the killas or grey slate may be converted into quartz and brown tourmaline and contains small quantities of tin stone. In the copper mines of Parys Mountain, Anglesey; formerly of great importance as producers of this metal, there are large areas of silicified slate and silicified porphyry. White mica, kaolin, gilbertite, chlorite and epidote are frequently present in silicified igneous rocks. As a further instance of mineral deposition in metasomatized igneous rocks we may quote the Cripple Creek goldfield in Colorado, where syenites, latites, phonolites, breccias, &c., have been filled with pyrite, dolomite, fluorite, calaverite and other new minerals together with quartz.
Another type of metasomatic alteration is phosphatization. This is most common in limestones, and many of the most important bedded phosphate deposits are of this origin. Trachytes and other igneous rocks are occasionally phosphatized. The source of the phosphate is for the most part the skeletons of animals, vertebrate bones and teeth, shells of certain brachiopods, trilobites and other organisms. Guano, the excreta of birds, is rich in phosphates and these are washed downwards by rain producing metasomatic changes in the underlying rocks. Phosphatized limestones are obtained in great quantities in Christmas Island, Sombrero, Curaçoa and other uninhabited limestone islands. (J. S. F.)
METASTASIO (1698–1782). Pietro Trapassi, the Italian
poet who is better known by his assumed name of Metastasio,
was born in Rome on the 13th of January 1698. His father,
Felice Trapassi, a native of Assisi, came to Rome and took service
in the Corsican regiment of the papal forces. He subsequently
married a Bolognese woman, called Francesca Galasti, and
established himself in business as a grocer in the Via dei Cappellari.
Two sons and two daughters were the fruit of this
marriage. The eldest son, Leopoldo, must be mentioned, since
he played a part of some importance in the poet’s life. Pietro,
while quite a child, often held a crowd attentive in the streets
while he recited impromptu verses on a given subject. It so
happened that, while he was thus engaged one evening in the
year 1709, two men of distinction in Roman society stopped to
listen to his declamation. These were Gian Vincenzo Gravina,
famous for legal and literary erudition, famous no less for his
dictatorship of the Arcadian Academy, and Lorenzini, a critic of
some note. Gravina was at once attracted by the boy’s poetical
talent and charm of person, interested himself in the genius he
had accidentally discovered, made Pietro his protégé, and in the
course of a few weeks adopted him. Felice Trapassi was glad
enough to give his son the chance of a good education and introduction
into the world under auspices so favourable. Gravina
hellenized the boy’s name Trapassi into Metastasio; and
intended his adopted son to be a jurist like himself. He therefore
made the boy learn Latin and begin the study of law. At
the same time he cultivated his literary gifts, and displayed the
youthful prodigy both at his own house and in the Roman
coteries. Metastasio soon found himself competing with the
most celebrated improvisatori of his time in Italy. Days spent
in severe studies, evenings devoted to the task of improvising
eighty stanzas at a single session, were fast ruining Pietro’s
health and overstraining his poetic faculty. At this juncture
Gravina had to journey into Calabria on business. He took
Metastasio with him, exhibited him in the literary circles of
Naples, and then placed him under the care of his kinsman
Gregorio Caroprese at a little place called Scaléa. In country
air and the quiet of the southern seashore Metastasio’s health
revived. It was decreed by Gravina that he should never
improvise again, but should be reserved for nobler efforts, when,
having completed his education, he might enter into competition
with the greatest poets.
Metastasio responded to his patron’s wishes. At the age of twelve he translated the Iliad into octave stanzas; and two years later he composed a tragedy in the manner of Seneca upon a subject chosen from Trissino’s Italia liberata—Gravina’s favourite epic. It was called Giustino. Gravina had it printed in 1713; but the play is lifeless; and forty-two years afterwards we find Metastasio writing to his publisher, Calsabigi, that he would willingly suppress it. Caroprese died in 1714, leaving Gravina his heir; and in 1718 Gravina also died. Metastasio inherited house, plate, furniture and money, which amounted to 15,000 scudi, or about £4000. At a meeting of the Arcadian Academy, he recited an elegy on his patron, and then settled down, not it seems without real sorrow for his loss, to enjoy what was no inconsiderable fortune at that period. Metastasio was now twenty. During the last four years he had worn the costume of abbé, having taken the minor orders without which it was then useless to expect advancement in Rome. His romantic history, personal beauty, charming manners and distinguished talents made him fashionable. That before two years were out he had spent his money and increased his reputation for wit will surprise no one. He now very sensibly determined to quit a mode of life for which he was not born, and to apply himself seriously to the work of his profession. Accordingly he went to Naples, and entered the office of an eminent lawyer named Castagnola. It would appear that he articled himself as clerk, for Castagnola exercised severe control over his time and energies. While slaving at the law, Metastasio in 1721 composed an epithalamium, and probably also his first musical serenade, Endimione, on the occasion of the marriage of his patroness the Princess Pinelli di Sangro to the Marchese Belmonte Pignatelli. But the event which fixed his destiny was the following. In 1722 the birthday of the empress had to be celebrated with more than ordinary honours, and the Viceroy applied to Metastasio to compose a serenata for the occasion. He accepted this invitation, but it was arranged that his authorship should be kept secret. Under these conditions Metastasio produced Gli orti esperidi. Set to music by Porpora, it won the most extraordinary applause. The great Roman prima donna, Marianna Bulgarelli, called La Romanina from her birthplace, who had played the part of Venus in this drama, spared no pains until she had discovered its author. La Romanina forthwith took possession of him, induced him to quit his
