ON PYROXENIC COMPONENTS IN CERTATN VOLCANIC ROCKS. 77 



examination has revealed the fact that innumerable crystals of the 

 Rhombic Pyroxene are also developed in the glass in smaller but 

 sharply defined forms which, though on the whole closely allied to 

 those already described, exhibit certain differences in habitus. They 

 are more slender in form, being elongated along the vertical axis. The 

 pleochroism is very feeble ; in still smaller crystals being hardly 

 noticeable, transmitting only a very faint greenish colour. 



Most of the crystals are formed of the faces a = coPôc (100), 

 b = ooPao (010), m = coP (110), and i = 2 Pt (211) as repre- 

 sented in fig, 2. The flatter pyramid e = P2 (212) appears rather 

 seldom so that the crystal appears sharper at the terminations. 

 Sometimes a pseudo-monoclinic form results by the shifting of the 

 pyramidal faces i as in fig. 7. The plane-angle formed by the 

 edges of the pyramid i over the vertical axis, when measured 

 along the brachypinacoidal face b, is 80 1 /°, while the corresponding 

 angle when measured along the macropinacoidal face a, is always near 



120 v 2 °* 



These crystals are found in all sizes usually '05 — 1 mm. in 

 length, gradually passing to the larger forms. 



Twin. — Numerous stellate aggregates of crystals are often observed 

 under the microscope. This is probably due to the union of the 

 crystals according to several twin-laws. One of these is a cross-shaped 

 penetration-twin (fig. 6), in which the two individuals have the faces 

 of the Brachypinacoid parallel to or coinciding with each other, and 

 the directions of the vertical axes make an angle of 60 1 / 2 °. This 

 twin is therefore analogous to that described by F. Becke,f the 

 twinning-plane being Macrodome P 5c (101). 



* Rosenbusch, gives the two values as 80° 52', and 120° 38' respectively— Mikroskop. Phy- 

 siogr. 2te Aufl. Bd. I, p. 392. 



f Uebar Zvnllingsverwachsungen gesteinbildender Pyroxene u. Amphibole — Tschermak's 

 Min. u. Petro. Mitth. Bd. 7. 1887. p. 95. 



