ON PYROXEXIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 71 



of Plao-ioclase. Thus there seems to be much probability that they are 

 Plao-ioclase, as first sugro-ested by Penck.* Kreatz f considers them 

 as Anorthite. Doss \ also observed the gradual transition to typical 

 Plaçnoclase in the Palagonitic tuffs of Haurân. 



The 'Rhombic lamelke 7 are especially well developed in the pearlitie 

 glass of Susaki in Chichijima, and in the spheroidal glass of Ototojima. 

 In the latter, the edges of these plates are sometimes so thickly covered 

 with very delicate dark filaments of Augite, as to conceal the crys- 

 tal almost entirely. They are extremely thin, and there is usually 

 no action upon polarised light. Very sharply defined faces may 

 ofren be recognised on the sides of the rhomb. The angles of the 

 rhomb measures 52° and 128°, and may be considered to be formed of 

 the faces P (001) and x (10Ï), to which the face //(20Ï)is sometimes 

 added (fig. 22). In more perfect crystals, it has been observed that the 

 (jne side of the rhomb is formed of two faces in place of the face #(101); 

 in such cases the crystal would probably consist of faces P (001), 

 ilf(010), y (201), o(llï), p (111); the last two faces being in the 

 same zone with x. Fine fissures parallel to P. probably the cleavage- 

 fissures, are especially well developed in the specimen from Susaki. 

 The lamellae become broader in some cases, and the cross section 

 exhibits twin-lamellae of the Albite-type. The twin of the Carls- 

 bad-type has also been observed, the most typical of which is 

 copied in fig. 21. ; the zone [P : M] running practically parallel to 

 the zone [M : x~\, so that a rhombic modification sometimes results. 

 This characteristic is peculiarly analogous to the rhombic modification 

 of Anorthite-twin, e.g. that of Miyakejima described by the author. § 



* Studien üb^r lockere vulkanische Auswürfllinge. — Zeitschft. d.d. geol. Gesellft. 1878 

 p. 99. 



fUeber Vesuvlaven von 1SSI und 1883.— Tscherniak's Min. u. Petro. Mitth. 1884. p. 139. 



X Die basaltischen Laven und Tuffen der Provinz Haurân und vom I)î ret et-Tulûl in 



Sirien— Tscherniak's Min. u. Petro. Mitth. Bd. 7. 1886 p. 5^7 



§ On Anorthite from Miyakejima. This Journal, vol. II, p. 31. 



