M. DES CLOIZEAIJX OX AMBLYGOXITE AXD MONTEBRASITE. 
577 
The presence of the numerous lamella; of which I have spoken ought, one would 
suppose, to be evidenced on the cleavages p and m by fine strife parallel to their mutual 
intersection ; of these, however, I have never been able to find the smallest trace in the 
midst of the irregularities which the surfaces of these cleavage-planes present. This 
is no doubt due to the lamellee, which are so thin as not even to produce the phenomena 
of the colours of thin plates, not permitting of our distinguishing from p and m their 
faces in the turned position g and the inclination of which on the former planes has 
the calculated angle p g =179° 5G' and 5^ = 180° 4', a difference alike unappreciable 
to the eye and to direct measurements*. 
Figures 2 and 4 present plates compounded of two great divisions twinned round 
an axis perpendicular to a surface of assemblage which is contained in the obtuse angle 
p m=105° 44', and gave to direct measurement a mean angle of 52° 2' with p, and con- 
sequently an angle of 53° 42' with m. The two figures only differ in other respects 
from one another in the relative dimensions of their constituent portions ; and these 
dimensions may vary to any extent, according to the spot where the plane of assemblage 
may meet the cleavages p and to. Each of these portions is furrowed by fibrous bands, 
which are sometimes reduced to mere threads and present an orientation like that of 
fig. 1, and by lam el he, generally very thin, which are parallel to the plane of assem- 
blage. 
The tessellated pattern, often very complex, which is produced by the two systems of 
lamelke is perfectly visible in polarized light with parallel rays ; and one can even 
obtain by photography f magnified images in which the threads situated in the obtuse 
angle may be recognized as being in general more blended and less undulated than are 
those situated in the acute angle. • The mutual inclination of the lamellae which belong 
to the two systems has been approximately estimated at 89° 12' and 90° 48' upon 
plates cut nearly normal to the plane of the optic axes. By combining with the 
angle of 58° 22' found on the twins of the type No. 1 this number 89° 12', an inclina- 
tion of 123° 14' may be deduced for that between the plane of the optic axes 
belonging to one region of the plate and that belonging to the other region of it. 
(The measures that were made with plates similar to those of figs. 2 and 4 vary 
between 122° 42' and 123° 31'.) As for the adjustment of the two regions of the 
section with regard to the plane of the optic axes and to their bisector, when this 
has been made as accurate as possible with regard to the one it is slightly inac- 
curate with regard to the other, exactly as is the case with the twins similar to 
those in fig. 1. 
On the laminar masses from which I obtained the plates of the types 2 and 4 I have 
* The same fact may be observed in crystals of calcite and of aragonite traversed by twin lamellae, where the 
trace of these ceases to be visible upon the faces at which they come out, when they present more than a 
certain degree of thinness. 
4 I am indebted to M. Cornu for images of these obtained by means of a small solar microscope in the 
laboratory of the Ecole Poly technique. 
4 h 2 
