36 Y. KIKUGHI 



The most prödominating faces are P, M", ?/, 6, m, to which T, I, 

 are somatimas combined. The most primitive form i.s a parallelopiped 

 formed by the faces P, M, y. Usually the hemidom^ e or n trun- 

 cates the edge of P : J/, or P : il/', to which prisms 7', /, and quarter- 

 pyramids 0, p, are added, (fig. 1, 2). The différent modes of facial 

 development give rise to different forms, which may conveniently be 

 distinguished into two types. 



First Type — The crystal assumes a tabular shapj by the [»re- 

 dominance of the Base 1\ while the Brachypinacoid M appears as a 

 narrow band extending in the direction of the Brachyaxis a (fig. 1, 2). 

 Fig. 3. represents in projection this type of crystal, in which the 

 development of tlie faces is most perfect. Some of these crystals 

 show an irregularity arising from parallel intergrowtli. (tig. -A). 



Second Type — In this type, the form is also tabular, but the pre- 

 dominating lace is Jl/, while P appears as a narrow band extending 

 along the Brachyaxis. The faces forming this type are the same as 

 those of the first type (tig. 5). 



Simple crystals oî the first type are more numerous than those 

 of the second ; the latter however appear more frequently as twins. 

 In both types, the form becomes sometimes irregular by the unequal 

 development of the faces on the back and front sides, (renerally o, p, 

 appear on one side as narrow bands, while their opposite faces o', //, 

 become much broader and intersect in a line, which would be parallel 

 to the face x, did this face occur, as it generally does not. But owing to 

 the imperfection of many specimens it is often difficult to say whether 

 the faces o,]>, have greater extension on the back or on the front side. 



The predominance of the faces 1\ M, y, o, p, and tlie absence 

 or rarity of prisms T, /, resemble to some extent the case of the 

 Anorthite-crystals first described by G. vom Rath,* found as a second- 



• Beitraege zur Pétrographie— Zeitschft. d. deutsch, geol. Gesellft. 13d. 27. 1875 p. 302. 



