usually tabular in habit, parallel to the plane M, as shown in the accompanying figure; sometimes, however, they are flattened parallel to P, this being a characteristic habit of the pericline variety of albite, microlitic crystals forming the ground-mass of volcanic rocks are usually elongated in the direction of the edge between P and M.
Twinning is an important character, which is almost invariably present and affords a ready means of distinguishing the plagioclases
from other felspars. Most frequent is the twinning according to the "albite law" with M as twin-plane. One half of the twin is turned through 180° about the normal to this plane and the two portions are united along the same plane (for figures of twinned crystals see Albite). The basal planes of the two portions are inclined to each other at a salient or re-entrant angle of 7° 12′ in albite and 8° 20′ in anorthite. This twinning is usually polysynthetic, being many times repeated, and giving rise to numerous thin lamellae, which are the cause of the fine striations on the cleavage planes P and parallel to the edge PM, so characteristic of the plagloclases as seen in hand specimens. Viewed in polarized light, thin sections of twinned crysta's show a very characteristic banded structure parallel to M. A second twin law is known as the " pericline-law " because of its frequent occurrence in pericline. Here the axis of rotation is the edge x P (the crvstallographic axis b) and the plane of composition is the "rhombic section": the latter is a plane which intersects the prism faces T and l in a rhomb; it is not a possible face of the crystal, and its position varies in the different species. In addition to being twinned according to these two laws, plagioclase may also be twinned on the Carlsbad-, Baveno- and Manebach-laws, as in orthoclase (q.v.).
| Composi tion. | SiO2 . | Al2O3 . | Na2O. | CaO. | Sp. gr. | Melting- point (Centigrade). | Cleavage Angle PM. | Angle of Rhombic Section. | Mean Refractive Index | Optical Extinction. | ||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| On P. * | On M. * | In sections M | ||||||||||
| Ab | 68.7 | 19.5 | 11.8 | 0 | 2.624 | - | 86° 24 ' | + 27 ° | 1.534 | + 4° 30' | +19° | - 16° |
| Ab3An1 | 62.0 | 24.0 | 8.7 | 5.3 | 2.659 | 1340° | 86° 8' | + 3° | 1.542 | + 1° 4' | + 4° 36 ' | + 7° |
| Ab1An1 | 55.6 | 28.3 | 5.7 | 10.4 | 2'694 | 1419° | 86° 1 4' | - 1 ° | 1.558 | - 5° to' | - 16° | +27° |
| Ab1Ab3 | 49.3 | 32.6 | 2.8 | 15.3 | 2.728 | 1477° | 86° 4' | - 9° | I .570 | - 17° 40' | -29° 28' | +48° |
| An | 43.2 | 36.7 | 0 | 20.1 | 2.758 | 1532° | 85° 50' | - 16° | 1.582 | -37° | -36° | +530 |
The optical characters of the plagioclases have been the subject of much study, since they are of great value in determining the constituents of rocks in thin sections under the microscope. The mean indices of refraction and the angles of extinction on the cleavages P and M are given in the accompanying table. (The meanin of the + and - directions will be seen from the figure, where the face P slopes from left to right, the the angle between the normals to the faces lettered P and M is less than 90°). The extinction angles on other faces, or in sections of known orientation in tne crvstal, also give constants of determinative value: for example, in sections perpendicular to the plane M the extinctions, which in crystals tnned according to the albite-law are symmetrical with respect to this lace, reach the maximum values given in the table. Not only do the directions of extinction (axes of light-elasticity) vary in the different species, but also the optic axial angle, so that while albite is optically positive, anorthite is negative, and a member near andesine has an axial angle of 90°. The figures seen in convergent polarized light through the P and M cleavages are characteristic of the different species. A detailed summary of the optical characters and their employment in discriminating the several members of the plagioclase series is given by H. Rosenbuch, Mikroskopische Physiographie der Minerallien und Gesteine (4th ed. Stuttgart, 1905).
The plagioclases occur as primary constituents of igneous rocks of almost every kind, and are also frequent as secondary minerals in metamorphic rocks. Albite and oligoclase are more characteristic of acidic rocks, whilst the basic members at the anorthite end of the series are characteristic of rocks containing less silica. The composition may, however, vary even in the same crystal, zoned crystals with a basic nucleus and with shells successively mole and more acid towards the exterior being common. For further particulars respecting individual species and their modes of occurrence see Albite; Andesine; Anorthite; Bytownite; Labradorite; Oligoclase. (L. J. S.)
PLAGUE (in Gr.) λοιμός, in Lat. pestis, pestilentia), in medicine, a term given to any epidemic disease causing a great mortality, and used in this sense by Galen and the ancient medical writers, but now confined to a special disease, otherwise called Griental, Levantine, or Bubonic Plague, which may be shortly defined as a specific infectious fever, one variety being characterized by buboes (glandular swellings) and carbuncles. This definition excludes many of the celebrated pestilences recorded in history-such as the plague of Athens, described by Thucydides; that not less celebrated one which occurred in the reign of Marcus Aurelius and spread over nearly the whole of the Roman world (A.D. 164–180),[1] which is referred to, though not fully described, by the contemporary pen of Galen; and that of the 3rd century (about 253), the symptoms of which are known from the allusions of St Cyprian (Sermo de mortalztate). There is a certain resemblance between all these, but they were very different from Oriental plague. " Plague " was formerly divided into two chief varieties: (1) mild plague, pestis minor, larval plague (Radcliffe), paste fruste, in which the special symptoms are accompanied by little fever or general disturbance; and (2) ordinary epidemic or severe plague, pestis major, in which the general disturbance is very severe. Cases which are rapidly fatal from the general disturbance without marked local symptoms have been distinguished as fulminant plague (pestis siderans, paste foudroyarzte).
History up to 1880.—The first historical notice of the plague is contained in a fragment of the physician Rufus of Ephesus, who lived in the time of Trajan, preserved in the Collections of Orzbasius.[2] Rufus speaks of the buboes called pestilential as being specially fatal, and as being found chiefly in Libya, Egypt and Syria. He refers to the testimony of a physician Dionysius, who lived probably in the 3rd century B.C. or earlier; and to Dioscorides and Posidonius, who fully described these buboes in a work on the plague which prevailed in Libya in their time. Whatever the precise date of these physicians may have been, this passage shows the antiquity of the plague in northern Africa, which for centuries was considered as its home. The great plague referred to by Livy (lx. Epitome) and more fully by Orosius (Histor. iv. 11) was probably the same, though the symptoms are not recorded. It is reported to have destroyed a million of persons in Africa, but is not stated to have passed into Europe.
It is not till the 6th century of our era, in the reign of Justinian, that we find bubonic plague in Europe, as a part of the great cycle of pestilence, accompanied by extraordinary natural phenomena, which lasted fifty years, and is described with a singular misunderstanding of medical terms by Gibbon in his forty-third chapter. The descriptions of the contemporary writers Procopius, Evagrius and Gregory of Tours are quite unmistakable.[3] The plague of Justinian began at Pelusium in Egypt in A.D. 542; it spread over Egypt, and in the same or the next year passed to Constantinople, where it carried off 10,000 persons in one day, with all the symptoms of bubonic plague. It appeared in Gaul in 546, where it is described by Gregory of Tours with the same symptoms as lues inguinaria (from the frequent seat of buboes in the groin). In Italy there was a great mortality in 543, but the most notable epidemic was in 565, which so depopulated the country as to leave it an easy prey to the Lombards. In 571 it is again recorded in Liguria,
