Dctenniiiation of tlie Feldspars — WuicJicll. 
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Fig. 26.— Extiuctions on 010 referred to the trace of the edge 001 : 010, positive, in the 
obtuse angle OOLvlOO, (Michel Levy). 
the face oio. It ought then to disappear in sections parallel 
to 010, or, rather, it should only be visible by reason of the 
superposition of two individuals of different optical orienta- 
tion; for the ellipse of the indices is returned about the edge 
100:010, and that of the twin takes a position in oio which is 
symmetrical with respect to the direction of lOO. In reality, 
in the case of a Carlsbad twinning, contrary to that of albite 
twinning, the junction of the twins is not a plane; there is a 
reciprocal penetration which is often very irregular, and the 
sections cut, although rigorously parallel to oio, very often 
show two individuals adjoining and partly superposed — in a 
word, penetrating each other along a line more or less sinuous 
and irregular. When the traces of the easy cleavages (ooi) 
