PLAGIOCLASE  , an important

group of rock-forming minerals, constituting an isomorphous series between albite, or soda-felspar and anorthite, or lime-felspar . Inter- mediate members are thus soda-lime-felspars, which in their crystallographical, optical and other physical characters vary progressively with the chemical composition between the two extremes albite (Na.AlSisOs) and anorthite (CaAl2Si2Os) . This variation is continuous in the series, but specific names are applied to members falling between showing the relations of the foetal villi to the placental sinuses, the Oligoclase> AbsAni to AbsAni'> Andesine, AbsAns to fusion of the amnion with the inner surface of the chorion, and the thinning of the fused deciduae (capsularis and vera) . AbsAni; Labradorite, AbsAni to AbsAni; Bytownite, phylum as Saipa, a placenta is formed, and the embryo is nourished Ab1Ana to AbiAns; Anorthite, An (= CaAl2Si2Os) . within the body of its parent . In some of the viviparous sharks, All the members of the series crystallize in the anorthic e.g. the blue shark (Carcharias), the yolk-sac has ridges which fit (triclinic) system . They possess a perfect cleavage parallel into grooves in the wall of the oviduct and allow an interchange of to the basal pinacoid P (oor) and a somewhat less pronounced materials between the maternal and foetal blood . This is an example of an " umbilical placenta." In the viviparous blennies (Zoarces cleavage parallel to the pinacoid M (Or)) . The angle between viviparus), among the teleostean fishes, two or three hundred young these two cleavages varies from 86° 24' in albite to 85° 50' in are nourished in the hollow ovary, which develops villi secreting anorthite . It was on account of the oblique angle between nutritive material . Among the Amphibia the alpine salamander the cleavages that A . Breithaupt in 1847 gave the name plagio-(Sa ra atra) nourishes its young in its oviducts until the gilled a stage e o of f development is past, while in the Reptilia the young o. clase (Gr .

7rXa ytoc, oblique, and tcXav, to cleave) to these felspars, viviparous

lizard (Seps chalcides) establish a communication between to distinguish them from the orthoclase felspar in which the the yolk-sac anteriorly and the allantois posteriorly, on the one corresponding cleavage angle is a right angle . It should be hand, and the walls of the oviduct on the other . In this way both noted that the potash--and potash-soda-felspars, microcline an umbilical and an allantoic placenta are formed . The mammals are divided into Placentalia and Aplacentalia; (q.v.) an anorthoclase, though also anorthic, are not included in the latter group, to which the monotremes and most marsupials in the plagioclase series of soda-lime-felspars . Crystals are Fused decidua capsularis and decidua vera . Fused mesoderm of amnion and chorion . From A . H . Young and A . Robinson, in Cunningham's Text-Book of Anatomy . 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 feispars .

Most frequent is the twinning according to the "albite

law " with M as twin-plane . One half of the twin is turned through 18o° 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 plagioclases as seen in hand specimens . Viewed in polarized light, thin sections of twinned crystals 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 crystallographic 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.) . 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 2S3), the symptoms of which are known from the allusions of St Cyprian (Sermo de mortalitate) . There is a certain resemblance between all these, but they were very different from Oriental plague .

" Plague " was formerly divided into two

chief varieties: (I) mild plague, pestis minor, larval plague (Radcliffe), peste 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, peste foudroyante) . History up to 880.—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 Oribasius.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; Constants of Plagioclase Felspars . Composi- SiO2 . Al20, . Na2O . CaO . Sp . Melting- Cleavage Angle of Mean Optical Extinction . Rhombic Refractive On P.* On M.* In sections non . gr. point Angle Section .

*

Index (Centigrade) . PM . Ab 68.7 19.5 11.8 0 2.624 — 86° 24' + 27° 1.534 + 4° 30' +19° -16° Ab3An, 62•o 24.0 8-7 5.3 2.659 1340° 86° 8' + 3° 1.542 + 1° 4' + 4° 36' + 7° Ab,Anl 55.6 28.3 5.7 10.4 2.694 1419° 86° 14' — 1° 1.558 — 5° 10' -16° +27° Ab,Ab, 49.3 32.6 2.8 15.3 2.728 1477° 86° 4' — 9° 1.570 -17° 40' -29° 28' +48° An 43.2 36-7 0 20.1 2.758 1532° 85°50' — 16° 1.582 -37° -36° +53° * Angles measured to the edge PM . 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 meaning of the + and — directions will be seen from the figure, where the face P slopes from left to right, i.e. 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 the crystal, also give constants of determinative value: for example, in sections perpendicular to the plane M the extinctions, which in crystals twinned according to the albite-law are symmetrical with respect to this place, 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 Mineralien and 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 more and more acid towards the exterior being common . For further particulars respecting individual species and their modes of occurrence see ALBITE ; ANDESINE ; ANORTIIITE ; BYTOWNITE LABRADORITE; OLIGOCLASE . (L . J .