116 Brush and Dana—Fairfield County Minerals. 
Calculated, Measured. 
cad, 0014100, = *61° 30’ 
caa, 001,301, = *42° 30’ 
cap, 0014111, = 61° 87 61°—62° 
¢xs, OO1K9%1, = ~82°° 9” 82°—82° 30’ 
ana, 100,301, = 19° 0” 
a’ xp, 1004711, = 81° 7 
a’~8, 100,221, = 68° 22’ 68° 
bap, 010,111, — 40° 40’ 
Das, 010 . 321, = 30° 56’ 
pap’, 111,111, = 98° 40’ 
eae, 2214921, = 118° 9 
It will be seen from the above table that pe : angle between 
the base and one of the two pyramids (eA p=61° 8’) differs but 
little from the angle between the base and the orthopinacoid 
(cAa=61° 30’); there are thus three planes which have nearly 
equal inclinations to the base. This fa et, which is analogous to 
that true of the Vesuvian biotite (meroxen) as pointed out by 
Tschermak,* gives to the crystals a marked rhombohedral aspect 
especially as the planes a (801) and s (221) have usually a minor 
development. As exact measurements were not possible the 
true relations could hardly be established beyond doubt until 
recourse was had to an optical examination. This showed that 
the cleavage planes are not isotrope as they must be if rhom- 
bohedral; on the contrary one plane of piconet is exactly 
parallel rs the edge c/a, and the other normal to i 
he rhombohedral pseudo-symmetry is also aces in the 
fact that the plane angle of the base differs very little if at all 
from 120°. th he most careful measurements practicable failed 
to establish any variation. That the angle really is 120° 
seems, moreover, to be indicated by the fact that on many 
cleavage lamine triangular markings are visible, which are 
apparently equilateral the angles measuring 60°; other analo- 
gous markings have four or ore! ares mi always me Pe 
of 60° or 120° as near as the can be 
The above facts show that ¢ er ryetallographically ickingonibe 
[ related to the micas and chlorites, although most unlike 
chemi 
ee Groth, Zeitschrift fir eevmaliaenphis, ii, p. 19, 1877. 
