46 
The Ajucrican Gcolooisf. 
January, 189N 
are distinctly visible it is necessary that the angle between 
them be about 128°, but that is a condition which is not suf- 
ficient, and which only acquires a true value if the extinctions 
of the same sign in the two twinned individuals occur sym- 
metrically with respect to the bisectrix of the angle 128°. 
There is then only one other plane in the zone 100:010 which 
enjoys the same property. (Fig. 27.) 
c. The faces which bound the supposed section parallel 
to 010 can often be distinguished, sometimes by means of the 
zones of increment, or growth, and sometimes by evident ex- 
ternal contours. These faces are, in order of frequence, 001. 
201, loi, no, 1 10, 201 and 203. The profile of their angles con- 
stitutes a very good verification. Compare Fig. 28 for albite. 
Fig. 27.— Carlsbad made visible in 010, 
Fig. 28.— Face 010 in Albito, 
d. Quite often the cleavage ooi is represented by fine 
straight cracks, best visible in high powers and by lowering 
the polarizer. Sometimes there is an excellent verification by 
cracks more coarse and more visible which are parallel to the 
cleavages of the prism no or no. 
The discovery of the fine cracks (ooi) is facilitated by two 
circumstances: They are always near the negative direction 
//p of extinction, since this last oscillates between -|-20° and 
— 36°. They are often indicated by fine hemitropic lamellze 
of the pericline twinning. 
c. We have not mentioned, up to the present, this man- 
ner of twinning, because, from an optical point of view, it 
blends closely with the individual (i') (p. 37) of the albite law 
of twinning; but it relieves all uncertainty as to the face of 
association which is peculiar to it, for it may be said that, at 
least for the oligoclases and the andesines, this face is almost 
