PIEDMOXTITE-SCHIST US' JAPA^". 



475 



rhomboidal outline, caused by the predominance of the traces of T 

 and M as in the figure, and if the faces r or i be also developed 

 the section will then be six-sided ; but the latter case is less frequently 



seen. In common epidote the face r (^do ) is said to be a predominating 



element, and, as a rule, it is more perfect than the face T ( ooPob ). 

 In these piedmontites the face r is very poorly developed, and it 

 is usually not visible even in the clinopinacoidal section, the outlines 



Fig. 1. 



of which are never regular, owing to the fact that there are an 

 infinite number of prominences and indentations, and they are some- 

 times even knee-shaped, just like twins of rutile. All these facts 

 are due to the parallel growth and intergrowth of two or more in- 

 dividuals of different sizes ; the striations commonly observed on 

 the faces parallel to the ortho-axis arise mainly from these causes, 

 and proportionally few of the striations can be attributed to the 

 twinning. 



Extinction takes places simultaneously in all the individuals that 

 enter into the formation of the complex crystals, and this would not 

 occur in the case of twins. Some of these remarkable forms are 

 given in figs. 2, a, 6, c. 



Via- 9 



ah c 



Twins are comparatively rare, and, if present, they are of a common 



. . ... 1 



type whose plane of twinning and composition is T ( ooPoo ), the ex- 

 tinction-direction of the one individual making an angle of 6° with 

 that of another (fig. 3). The traces of cleavage upon M, in both 

 individuals, meet at an angle of about 130°, just as in the case of 

 common epidote described by E. Becke * and H. Eeusch t. The 



* Tschermat, Min. u. petr. Mitth. 1879, p. 837. 

 t Neues Jahrb. fur Min. etc. ii. 1883, p. 87. 



