202 J. P. Cooke, Jr., on some American Chlorites. 
the small differences between these values and those in column 
A are real variations in the angle, such as are quite common m 
foliated minerals. All the crystals measured were macled as in 
g. 2, and we have distinguished the faces of the two members 
of the macle by the symbol O, R, etc., for the first, and O, and 
R, for the second. It is a fact worthy of notice that the faces 
of the same subordinate rhombohedrons are not necessarily found 
on both of the twinned crystals, while those of the fundamental 
rhombohedron are never wanting. On the first crystal we were 
able to measure every angle of the zone of planes between the 
opposite basal planes of the macle, and the observations were 
verified by repeated measurements. It will be noticed that the 
corresponding angles on the two crystals composing the m 
agree absolutely. From these I have calculated the value of 
the half vertical axis, and from the last I have deduced t 
values of the angles in the column headed calculated. 
a= 3'4951 
Calculated. Observed. c 
A B 
OonR 103°55’ 103°55’ | ~—S«0g"41’ 108° 
Oon AR 128° 514 128° 59’ 129° 6’ 129° 8’ 
O on 2K 121° 47! 121° 54! 122° (nearly) 
ys on 2R 172° 56/ 172° 55! 
2R on 162° 8’ 161° 59! 
RonR, 152° 10’ 152° 10’ 
R, on O, 108° 55’ -103° 55’ 
AJ, on O, 180° 
Ron R (vertical edge) 65° 36’ 
Des Cloizeaux obtained from the same mineral O on R 108° 
45’, O on 2R, 122° (about), and from the Pennine of Zermatt 
for O on R 108° 40’ to 50’. Ee 
Optical characters of Kiémmererite——The crystals are uniaaral, 
and the double refraction though feeble is distinctly positive. + 
As stibed above as No. 1, showed distinctly the first 
two rings with the familiar black cross. In white light the® 
rings are alternately blue and pink surrounding a pink-colore 
center. In homogeneous light the rings formed by the red tJ* 
are much narrower than those bined by the blue, indicating 
the somewhat unusual fact that the crystal exerts the feeble 
persive wer, on the most refracted, in this case the extt® 
stals of Kimmererite are strongly dr 
