78 



Y. KIKUCHI 



Parallel-intergrowth of the Rhombic Pijroxene with a green Augite. — 

 On examining- the microscopic crystals of the Rhombic Pyroxene 

 under crossed Niçois, it is found, as might have been expected, that 

 they extinguish the light parallel and at right angles to the direction 

 of the vertical axis. But the crystal is sometimes bordered on both 

 sides by very narrow bands, in which the direction of extinction 

 is oblique. This phenomenon is observed only when the crystal is 

 found lying horizontally upon the macropinacoidal face a. The nar- 

 row bands show a simultaneous extinction of 42°-45° with the 

 prismatic direction as represented in fig. 3. It may be observed that 

 in the crystal here figured, the terminal faces are formed by the flat 

 Pyramid e = Py, a case rarely seen in the smaller crystals. The 

 breadths of these bands are not constant ; sometimes they become so 

 narrow as to be almost inappreciable except by a most careful 

 scrutiny. 



This is a phenomenon arising from the regular intergrowth of the 

 Rhombic Pyroxene with a Monoclinic Pyroxene. We must consider that 

 the Brachypinacoid b= coPoo of the Rhombic Pyroxene is attached 

 to the Orthopinacoid a = ooPôô of the monoclinic Augite, the vertical 

 axes of both crystals being parallel. Thus the macropinacoidal face 

 a = ooPöö of the former would become parallel to or coincide with 

 the clinopinacoidal face b = ccPSc of the latter, which would ex- 

 tinguish the light obliquely. In this position, the characteristic 

 prismatic angles of the two Pyroxenes would come to similar position. 

 Such parallel-intergrowth of the Rhombic and Monoclinic Pvroxenes 

 was first observed by Trippke* in a variety of Enstatite. It has 

 also been recently observed in the Pyroxenes in some younger 

 effusive rocks. Frederick H. Hatchf describes it as occurring in 



* Ueber denEnstatit aus den Olivenknollen des Gröditzberges — Xeues Jahrb. f. Min. Geol. 

 u. Pal. 1878. p. 673. 

 1 1. c p. 327. 



