Detcr))ii nation of the Fthhpars. — ]]'i/icJic/l. 47 
])arallel to 001, and it has not appeared to me to depart sen- 
sibly from it in labradorite (see Fig. 16). On the other hand, 
the abundance of lamellae of the pericline type impairs the 
diagnostic which might be drawn from the disappearance of 
the albite lamellae, especially in anorthite in which the per- 
icline made sometimes predominates over that of the albite. 
In anorthite from St. Clement the face of association is about 
— 15^ from 001, in the zone 001:100, and in the acute angle 
001 Aoio. 
/. It remains to revert to the images seen in convergent 
light. The bisectrix n„ is visible in the face 010 in albite, 
olfgoclase and andesine. The oligoclase of the second class 
(Abi Ani) gives a figure almost at the center of the field of the 
microscope, with an extinction at + 6° in 010. 
As the concentric zones of growth of the plagioclases very 
often attain an acidity in a narrow belt at the periphery which 
involves the centring of the figure seen in convergent light, it 
is useful to proceed to this method of verification. 
(2). Once the suitable section is chosen, it is necessary 
to orient it; that is to say, to discover the trace of 001, and 
that of the edge of the prism. It is to be remembered that the 
extinction will be positive, according to the rule of Schuster 
(p. 23, Fig. 19), when it is in the obtuse angle 001 Aoio. and 
negative when it is in the acute angle. 
(7. The occurrence of the Carlsbad made, combined with 
the trace of the easy cleavages, gives the complete solution of 
the problem; it is enough then to measure the angle {co) com- 
prised between the two extinctions of the same sign. For 
the purpose of avoiding all error it will be well to select from 
the two supplementary angles that which has the same bisec- 
trix as the angle 128'' of the basal cleavages (Fig. 27). 
For albite Oi? =168° .extinction at + 20° in 010. 
For oligoclase &?=i28°, extinction at 0° in 010. 
For andesine 00=112°, extinction at — 8° in 010. 
For labradorite co^^=g6° , extinction at — 16° in 010. 
For anorthite &5=54°. extinction at — 37° in 010. 
When one of the profiles 001, loi, 201, can be found, or 
simply the cleavage of 001 and the coarse "cracks parallel to 
no or no, the acute angleooi A 100 can be recognized; the 
