ON ANORTHITE FROM MIYAKE JIMA. 37 



ary minernl in the contact-rock of Pesmeda-Alps, in Monzoni, Tyrol. 

 Bat the difference between the two lie3 in the fact tliat the Pesmeda- 

 Anorthite is elongated along the Mncroaxis h, Avliile the Miyake- 

 crystal has o-reater elong^ation alono- the Brachyaxis ci. 



T^vins — Simple crystals are rare ; most of the crystals being 

 found as penetration-twins. The twin law is analogous to that of 

 the Carlsbad type in Orthoclase. The twinning-axis is the A^ertical 

 axis c, the plane of composition the Brachypinacoid M ( r^P^). Bnt 

 as the crystals have often no prismatic edge, the two individuals have 

 apparently no common direction. An exact measurement of the 

 angles made l)y the faces of the two individuals is impossible. The 

 angular divergence of the Brachyaxis, as measured along the edo-es of 

 P : /)/ and V : M is approximately 128°. Fig. 6 rejiresents a typical 

 form of the twin, the faces consisting of P, i)/, ?/, <?, n, o, jt ; the last 

 two faces being usually developed unequally on the two sides of 

 the Brachyaxis. It is well-known that in this twin, the face 

 X (^1\<^) of one individual comes very near the Base /" of the other 

 individual. The faces o, p, M, lying in one zone with x, their inter- 

 section-edges become very nearly parallel to the zone of /\ n, M. 

 According to vom Bath* the angular difference of these two directions 

 is calculated to be 0° 2-1' 4" from the axial elements of the Vesuvian 

 Anorthite, and this value is found to be more constant than in the 

 case of the Orthoclase of Elba, in which the two faces P', x tend to 

 coincide. In the crystals of Miyakeiima, this anovular difference can 

 not be detected at least by the eye, and the intersection-edges of 

 7-*, i)/, e, n, become apparently parallel to those of il/, o, p. On 

 account of this fact, this kmd of twin crystal very often assumes a 

 rhombic outline, when the faces o, p, attain their maximum develop- 



Ein Beitrag zur Kenntniss des Anorthits— Poggendf. Annalen der Physik n. Chemie. 

 Bd. 147. 1872. p. 55. 



