DEc., 1908. MINERALOGICAL NOTES 155 
been hitherto described. The largest of the crystals measure 20 
millimeters in length by 17 millimeters in width, and from this 
they diminish to about one-half this size. They are arranged 
upon the matrix in a nearly parallel position though not exactly 
so. The mode of attachment may be in general stated to be that 
of the lower end of the vertical axis, though this attachment varies 
somewhat. The crystal planes do not present brilliant surfaces, 
but though dull are not rounded. They do not afford sharp signals 
with the reflecting goniometer, but give tokens sufficiently well de- 
fined so that very close estimates of the angular values can be ob- 
tained. The cleavage parallel to the clinopinacoid is, as usual, very 
strongly marked. This cleavage affords reflections which are sharp 
but vicinal. The color of the crystals is a dark orange-red, on 
cleavage surfaces bright golden-yellow. The crystals are opaque. 
In development the crystals exhibit monoclinic symmetry throughout 
and leave little doubt that orpiment should be considered as crystal- 
lizing in this system. They are all of the same habit and one which 
seems to be new for this mineral. Itis especially characterized by the 
large development of the positive pyramid v (343). This occurs in 
broad planes, sometimes 1.5 cm. in length by 1 cm. in width. 
Grouped with this pyramid occurs in greater or less development 
the pyramid v(i2z1). Accompanying this occur several prisms and 
in less prominent development several other pyramids. The 
habit generally exhibited is illustrated in Fig. 1, Pl. LIII, the crystal 
being drawn in the normal position. As this position is not, however, 
favorable to exhibiting the positive pyramid, Fig. 2, Pl. LIII, shows 
the crystal drawn in reverse position. Three forms new to orpiment 
were detected upon the crystals. These were the 3/2 clinodome, 023, 
designated as /, the 1/3 negative pyramid 133 designated as n, and 
the 1/3 positive orthodome 103 designated asd. In addition two 
forms were noted which had been observed by Stevanovic,* but to 
which he had assigned no letters, apparently because he did not 
regard his results as conclusive. These forms were the 1/3 negative 
orthodome 103 to which the letter e has been assigned, and the posi- 
tive pyramid 123 to which the letter k has been assigned. The 
habit of the Mercur crystals, it may be noted, somewhat resembles 
that of one figured by Stevanovicy from Allchar except that in the 
crystal figured by him, the prisms are the prominent forms instead 
of the pyramids. A basal projection showing the usual development 
of the different forms found upon the Mercur specimen is given 
ES. Kr. F904. 30, DP. 14. TOC, Cit. Migs 3. 
