476 



PROF. E. KOTO ON OCCURRENCES OF 



crystal individuals of twins differ considerably in their size, and the 

 one bears a parasitic relation to the individual to which it is attached. 

 The colour and the behaviour of pleochroism in twins are exactly 



Fig. 3. 



similar in most cases, so that the existence of twinning can only be 

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

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

 of cleavage npon 



Cleavages upon the base (oP), and orthopinacoid ( coPoo ) are 

 sometimes observed ; 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 colours : 

 A = deep reddish- violet ; C=brownish red; B = light violet. The 

 degree of absorption: A>C>B ; while in common epidote it may 

 be expressed in the following scheme *, C >B > A. Therefore the 

 clinopinacoidal sections of this mineral show the most intense colours, 

 while those parallel to the ortho-axis display a lighter tint. When 

 a slide is prepared in the direction of the planes of schistosity in 

 the rock, we usually see sections approximately parallel to the 

 6-axis ; but we also perceive a marked difference in the colours 

 of various sections suggestive of entirely different minerals ; the one 

 is a deep violet, the other a brownish yellow. As there are great 

 differences in the axial colours already stated, it may be naturally 

 expected that a section parallel to the basal pinacoid II is of a 

 brownish yellow ; for we see the combination of the axial colours of 

 C and B ; that which is taken nearly parallel to the orthopinacoid 

 T is of a deep violet (a combination of A and B). The clinopina- 

 coidal section shows the deepest shade of colour, the face-colour 

 being a combination of C and A. 



* Eosenbuseh, ' Mikroskopische Physiographic,' i. p. 497, 2te Auflage. 



