576 M. DES CLOIZEAUX ON AMBLYGONITE AND MONTEBRASITE. 
The result of these tentative trials proved that the most perfect plates were those 
the surfaces of which made nearly equal angles with the two cleavages (affording a 
mean result of 99° S' with m and 99° 25' with p). The optic axes are situated in a 
plane which divides into two very unequal parts the acute angle = 74° 16' of the two 
cleavages. This direction is entirely different from that which I have found for Mon- 
tebrasite (formerly Amblygonite) of Hebron and of Montebras, in which the plane of 
the axes lies in the obtuse angle of 105° formed by the two principal cleavages*. 
With regard to the exact orientation of this plane, I have deduced it from that which 
it presents in the natural macles of which all the plates of Amblygonite of Montebras 
which I have examined are constituted, and which result from the interposition in the 
midst of their mass of twin lamellae that bisect almost exactly, the one set the obtuse 
angle of 105° 44', the other the acute angle supplementary to it. These lamellae, which 
are often met with on the same specimen, in which they cross one another at an angle 
of very nearly 90°, are sometimes so thin and so numerous that they convert the whole 
mass into a network with close meshes, to all appearance rectangular, in which the 
coloured rings are only visible in the polarizing microscope at a few isolated points. 
Fortunately this is not always the case, and plates are pretty often found which are 
referable to one of the four subjoined types, and which offer more or less facility for 
the study of the optical doubly refracting properties of the substance. 
Fig 1 (p. 579) represents those plates which are divided into slices, triangular or tra- 
pezial in section (1, 3, 5, 7, 9, &c.), which are more or less homogeneous, by a series of 
bands which are generally narrow (2, 4, G, 8, &c.), parallel to one another, and very nearly 
parallel to the bisector-plane of the acute angle of 74° 16'; in these bands we may 
recognize, by the fibrous aspect which they exhibit even under illumination by ordinary 
light, a number of excessively thin lamellae. Thus, while in the sectional figures indi- 
cated by an odd number the disposition and dispersion of the coloured rings can be 
studied in the polarizing microscope, it is only in some narrow regions of the bands 
indicated by the even numbers that one can examine them. This examination suffices, 
however, to show that wdienever the section has been so successfully made as that the 
plane of the optic axes is as nearly normal as possible to the worked surface in the case 
of the slices with the odd numbers, it still remains in a very slight degree oblique to 
the plane in the case of those with the even numbers. In the latter case the orientation 
of this plane is only to be explained by the existence of lamellae parallel to the edgep m 
which have undergone a revolution of 180° round an axis within a few minutes of being 
perpendicular to that edge. A great number of observations were made with an apparatus 
in which the two Nicols were crossed, on the maximum extinction of the light in the bands 
successively even and odd ; and these gave the result, that the plane of the optic axes in 
the bands of the former order formed, with that in the latter case, a mean angle of 
58° 22'. 
* It- will be seen further on that the Montebrasite which I have lately found among the specimens from 
Montebras presents the same optical characters as that from Hebron in the State of Maine. 
