M. DES CLOIZEAUX OX AMBLT GONITE AKD MOXTEBEASITE. 
575 
cleavages, presenting nearly the same degree of facility, but yielding surfaces so uneven 
and rough that, for measuring their inclination, one has to isolate small portions, 
as little uneven as possible. But, even with this precaution, one can never obtain 
sufficiently perfect reflections to afford exact measurements ; and it is only by repeating 
these measurements on a great number of fragments, and by taking the mean of these, 
that I have succeeded in determining with tolerable exactness the incidence of the two 
cleavages to be 105° 44'. Close observation shows, furthermore, that the sharpness of 
the reflected images is generally a little greater on one of the surfaces of cleavage than on 
the other, which would induce one to suppose that they do not both belong to equivalent 
crystallographic planes ; the difference, however, is so small that one does not notice it 
at first, still less can it be compared to that which I have pointed out as distinguishing 
the face p (with pearly cleavage) from the face m (the vitreous cleavage) of the Monte- 
brasite of Hebron in the State of Maine*. 
The insufficiency of the geometrical characters naturally led to the inquiry whether 
the study of some of its optical properties might not enable me to determine the 
crystalline type of the mineral ; and this investigation, which presents certain special 
difficulties arising from the small extent of the transparent portions and the presence 
of numerous twin plates even in the specimens that to all appearance are the most 
homogeneous, has proved that, without doubt, the laminar masses of Montebras must be 
referred to the triclinic system. 
In order better to compare the points of analogy or of difference which distinguish 
Amblygonite and Montebrasite (minerals composed of the same constituent elements but 
in different proportions), I have assumed, in the first of these minerals, that the less 
perfectly reflecting cleavage took place parallel to the base jj, and the more per- 
fectly reflecting cleavage parallel to the face to the left, on, of an oblique-angled 
parallelepiped, of which the face to the right t, and the relative dimensions are still 
unknown. The angle p m of Amblygonite equals 105° 44', which differs by only 44' 
from the corresponding angle in Montebrasite. 
The optical investigation was commenced by the examination of thin plates cut per- 
pendicularly to the two cleavages ; and it served to establish that these plates were 
obviously oblique to the plane of the optic axes. In order to obtain plates as perpen- 
dicular as was possible to this plane and to the bisector of the acute angle of the axes, 
it became necessary to arrange a sort of tentative method by the aid of the polarizing 
microscope f. 
* “ Sur les proprietes optiques birefringentes et sur la forme crystalline de l’amblygonite,” Comptes Eendus 
des Seances de l’Acad. tom lvii. p. 357. 
t All these tentative sections have been effected by M. H. Soleil with bis well-known patience and skill. 
The working of these plates of Amblygonite (formerly Montebrasite) is all the more delicate since the best 
specimens that I obtained of M. Moissenet would not allow of plates being cut of a thickness greater than J of 
a millimetre, in order to present the transparency necessary for the subsequent execution of the slight modifi- 
cations which the polarizing microscope might prove their worked surfaces still to require. 
MDCCCLXXIII. 4 H 
