408 J.D. Dana on the Feldspar group of minerals. 
135° the angle between a face of a cube and a plane trunca- 
ting its edges, 
The plane 2-2, at the top of the figure, the only remaining 
one, is a face of the octahedron,—a plane which on the dode- 
cahedron truncates its trihedral solid angles,—and it is there- 
fore lettered @. Theangle J (D) : 2-=145° 47’, while the an- 
gle between a dodecahedral and octahedral plane, is 144° 44. 
The angle 2-i (#@): 2-i=124° 51’, while the angle between a 
cubic and octahedral plane is 125°16’. That the plane is oc- 
tahedric is further apparent from the parallel intersections of 
§-t, 2-1, 1-3, and of O, 2-7, 2-i ; for (1) a cubic face makes par- 
ed, are 
The dodecahedric, I, I,i-1,0,1,1, with their opposites, or 
the whole twelve. : 
_ The trapezohedric, 1-7, i-3', i-3', with their opposites, or si# 
The octahedric, 2-i, with its opposite, or two, ; 
The cubic, 2-1, 2-1, 2-7, with their opposites, or the whole six. 
The angles of this oblique or clinohedrized cube are 2-i:2-i 
over O —90° 6’, 2-7 : 2-7=96° 48’. ; ‘ 
The true or normal apex of the orthoclase dodecahedron is 
that part of the crystal occupied by the octahedric plane 
§-i ; and the true obliquity of the crystal (or angle C,) instead 
of being 63° 53’ and 116° 7’ (or the angle O : 1-7) would be the 
€ {-i:1-i, which equals 81° 54’ and 98° 06’. Hence 8° 6' 18 
the extent to which the octahedron has been clinohedrized in 
its conversion into the form of orthoclase. 
For the triclinic feldspars, the only peculiarity is that already 
mentioned,—the fact of lateral obliquity of the principle se¢- 
tion, so that the two cleavage planes QO, i-) (dodecahedric planes 
on opposite sides of a tetrahedral solid angle) make with one 
another an angle of 93°15’ to 94° 15’ instead of 90°. The 
form of the crystals are so similar to those of orthoclase, with 
this exception, that special explanations are unnecessary. 
G. Rose, ina recent article on Albite (Pogg., cxxv, P- 45 
observes (p. 466), that it isa remarkable fact that the planes 
2-i situated either side of 0, between it and 7-i, (planes 
e and m of Rose,) make with one another in albite very neatly # 
Tight angle, 90° 35’ according to Neumann, and 90° 4’ accord- 
ing to DesCloizeaux’s measurements, It is not so surprisitg 
