306 B. KOTO 



Extinction of the light occurs simultaneously in all crystals that have 

 taken part in the formation of the complex -individuals, and this 

 should not occur in the case of twins. Some of these remarkable 

 forms are given in the annexed plate, Figs. Ill, IV, V. 



Twins are comparatively rare, and if they are present, they 

 are of a common type whose plane of twinning and composition 

 are T (oo P^), and the extinction-direction of one individual makes 

 an angle of 6° wàth that of another ( E'ig. VI). The trace of cleavage 

 upon M of both individuals meets at an angle of about 130°, just 

 as in the case of common Epidote given by E. BeckeCO and H.H. 

 Eeusch.(^) The crystal-individuals of twins differ considerably in 

 their size, the one bearing parasitic relations to the other. The 

 colour and the behaviour of pleochroism of twins are exactly similar 

 in most cases so that the presence of twinning formation can only 

 be recognized by a slight difference in the shade of colour of both 

 crystals under crossed Niçois, and also in the direction of the traces 

 of the cleavage upon ill in both individuals. Cleavages upon the base 

 (o F), and orthopinacoid (coP^) are sometimes observed as in the 

 Fig. Ill; but in minute crystals they are, as a rule, less distinctly 

 developed than in. the larger individuals; for in the majority of cases 

 the smaller ones are perfectly free from such traces of cleavage. 



The angle of oblique extinction: c:a = 3°. The axial colour: 

 Û = deep reddish- violet ; c = brownish-red ; Ï) = light violet. The 

 degree of absorption : a > c > 1) ; while in common Epidote it 

 may be expressed by the following schemeC^); c > 1) > a. Hence the 

 clinopinacoidal sections of our mineral show most intense colours, 

 while those parallel to the orthoaxis display a lighter tinge. 



When a slide is made in the plane of schistosity of the rock, we 



(i) Tsclieimalf, Min. u. petr. Mitth. 187!), p. 837. 



(2) N. Jahrb. f. Min. u. Gcol. II, 188!, p. 87. 



(5) llüsuubuseli, ' MikioykopiiscUe riiysiographie.' I Baul, lite Auflage, p. 497. 



