200 E. 8. Dana—Crystalline form of 
The crystals examined were quite small, averaging from one 
to two millimeters in length. The are generally rhombic 
(fig. 1) at one extremity, and hexagonal in outline on the other; 
occasionally, however, from the extension of the clinodomes 
they take the form of hexagonal tables (fig. 2). The hemihedral: 
character of the crystals, shown in the figures, is a marke 
feature which they all possess. No trace of either of the miss- 
ing pyramids or clinodomes was in any case discovered ; a few 
isolated crystals of apparently holohedral development were 
without exception found by optical means to be twins, though 
showing no reéntrant angles. Other twins had the form of 
fig. 2. e twinning-axis is here a normal to the basal plane. 
The cleavage is highly perfect parallel to the base (c), and the 
_ erystals have a pearly luster on this face. 
The examination of the optical properties of the crystals 
proves that, though the angle of obliquity is small (89° 4+’), 
they unquestionably belong to the monoclinic system, and, on 
the other hand, that they are not triclinic. : 
natural crystal, or still better a cleavage section, viewed 
in the polariscope in a direction normal to the basal plane (¢), 
The apparent optic-axial angle in air was obtained with con- 
siderable exactness : f 
2E=68° 23’ for red rays, =67° 30’ for blue rays. The ordi- 
nary dispersion is consequently p>v; the dispersion of the 
bisectrices, on the other hand, v>p. The character of the 
double refraction is negative. 
The twins (see fig. 2) gave interference figures of great beauty, 
both sets of axes being visible, situated symmetrically on either 
