578 
M. DES CLOIZEAUX OX AMBLYGONITE AND MONTEBEASITE. 
occasionally observed indications of the line in which the plane of assemblage meets the 
cleavages in the form of a slight ridge (see that of fig. 2), or of striations which are only 
visible in a bright light, and which correspond to the groove drawn to the right on the 
plane on the back of the section of fig. 4 : the reentering angle mfZbvas found to be 181° 
to 182°; by calculation it should be 181° 40'. When the lamellae parallel to the plane 
of assemblage of figs. 2 and 4* are grouped in bands of a certain width, their arrangement 
is that represented in fig. 3. 
Though the determination of the crystallographic data for Amblygonite must neces- 
sarily be incomplete until at least a third face has been discovered not lying in the 
same zone with the two principal cleavages, I have given in the following Table a 
resume of the measured and of the calculated incidences, the correctness of which may 
be looked upon as fairly satisfactory. For brevity, I indicate by S the surface which 
has been worked perpendicularly to the plane of the optic axes and to the bisector of 
their acute angle, by H the twin plane lying in the obtuse angle of the two cleavages, 
and by L the plane of the laminee lying in their acute angle. At the point in which 
the three edges ^ ^ (figs. 1, 2. and 3) meet one another the plane angles are: — 
Plane angle of m = 97 17 20. 
„ ,, p = 96 55 3. 
„ „ S = 104 34 49. 
Calculated. 
m : S 
p : m 
p : H (fig. 4) 
m : H (fig. 2) 
1 P ■ J (fig- 4) 
m : w (fig. 2) 
m : d (figs. 2 and 3) 
pn : d (fig. 4) 
r S superior : H (figs. 2, 3, and 4) 
{ S inferior : 41 (figs. 2 and 4) . . 
O I 
_ f 126 16 
~ ( 53 44 
= 104 4 
= 107 28 
= 178 20 salient 
= 181 40 reentering . . . . 
= 89 36 
= 90 24 
Observed. 
O / 
99 8 mean. 
99 25 
55 
105 44 
52 2 
55 
55 
55 
104 32 
55 
55 
55 
181 to 182° 
5 3 
* The twin plane contained in the angle of 105° 44' may he conceived as being parallel to an hypothetical 
face c', which would truncate the posterior edge to the right of the parallelepiped, of which we only as yet know 
the planes p and m, with an inclination upon p of 127° 58'. According to M. Breithaupt, there should be a 
different cleavage corresponding to a similar face on the specimen of Amblygonite from Penig. In my note of 
date 1863, I had thought I could make use of the inclination of this cleavage upon m for the calculation of the 
height of the primitive form of Montehrasite (at that time supposed to be Amblygonite) from Hebron. As will 
he seen further on, Mr. Dana has established the presence of a face c for the large crystals of Montehrasite found 
at Ilehron ; but the inclinations appear very different from those which M. Breituaupt obtained on the 
Amblygonite from Penig. 
