38 T. KIKUCHl 



ment. Fia'. 7 shows sucli a crystal. Tlie o'enernl outline of this 

 modified tvi)o reminds us of a tabidnte Anorthite-crystîd (Cvclopite 

 of V. AValtershaasen) described by A. v. Lasaiilx* from Cyclopean 

 Island, near Etna, where the crystals are found in a druse within the 

 doleritic rock. In the crystal sliown in üo-. 7, severnl subindividuals 

 are found makin_i^' ]xu-alle] intero-rowth. Tn another modification of 

 this twin, one of the individuals becomes much smaller than the other, 

 and the smaller individunl is attached on one side or at the middle of 

 the laro-er one. Xerx rarely the halves of the two individuals nre 

 attached bv 3/, without making penetration (üg. <S). Of the two 

 modifications arising in this twin, according as the orie individual 

 is attached to the riii'ht or the left side of the brach vpinacoidal face 

 of the other, it is almost impossible to sav anything certain. 



Another kind of twin, found only as twin-lamelke, is that known 

 as the Pericline-type ; the twin-axis being the Macroaxis h. The 

 trace <->f the "Rhombische Schnitt" <^n the cleavage-face of J/, is 

 indicated by very fine stria^ making an angle of -15° to -17° with 

 the edo-e of P: M (fig. 1). These twin-lamellae are extremely fine, 

 but they become comparatively much ))roader in the crystals im- 

 bedded in the lava, sometimes attaining a breadth of 7, mm. Fig. 11, 

 12, show these broader twin-lamella^ as observed in sections of por- 

 phyritic crystals, cut nearly perpendicular to the edge 1' : 3/. 



Optical characters — The direction of extinction has been ex- 

 amined on the cleavage-faces of 7', and of 3/ under parallel polarised 

 lio-lit. As the mean of several observations, tlie direction of maxi- 

 mum extinction on the fa'C P was found to be-38° to— 10°. Avhile 

 that of the face 31 was -40° to-4r; tlie latter does not completely 

 extinçruish the lioht, owini»- to the interposition i-f the Pericline twin- 

 lamellai. 



• Sartorius-Lasaulx, Der Aetua. Bd. II. p. 505. also Zaitsclift, f. Krystallographie, Bd. 5. 

 1881 p. 32G. 



