DEc., 1908. MINERALOGICAL NOTES 147 
ish-green, translucent and of resinous luster, but the remaining sur- 
faces are coated with a white, opaque mineral evidently derived from 
alteration of the underlying substance. This coating is about 1 milli- 
meter in thickness. The blowpipe characters of the principal mineral 
agree with those of leadhillite, and those of the coating mineral with 
cerussite. Alteration of leadhillite to cerussite was also observed by 
Penfield on crystals from this locality.* While the large crystal under 
consideration had an apparently cubical form, measurements of the 
angles with the reflecting goniometer, secured by attaching cover 
glasses to the planes, gave 87° 5’,87° 30’ and 87°48’. These results sug- 
gested the presence of a rhombohedron, especially as an apparently 
rhombohedral form of leadhillite, known as susannite, has been 
observed. Moreover, a well-marked cleavage truncating the solid 
angle in a manner corresponding to the basal cleavage of the rhom- 
bohedron occurs on the crystal. The angles of this cleavage upon 
thesplanes of the crystal are as follows: 51° 20’, 51° 57’, 52° 45’. 
These angles closely resemble those given for the rhombohedron of 
susannite: by Dana,t his value forc A 7 being 128° 3’ and forr A 7’, 
94°. It is thus apparent that the interfacial angles of the crystal 
would admit of its interpretation as a rhombohedron. On examin- 
ing the cleavage plates with the polarizing microscope, however, well- 
marked biaxial characters appear. Interference figures perpendicular 
to the acute bisectrix are exhibited, with dispersion p< v and a nega- 
tive optical character. It is thus obvious that the crystal should be 
interpreted as monoclinic and must therefore probably be regarded 
as made up of a negative pyramid and anorthodome. The pyramid 
and dome most nearly corresponding with the angles given above are 
t (112) and f (lor), the measured and calculated angles for these 
forms comparing as follows: 
Observed. Calculated 
pen(cleawace)- A.ij =- (oor) A. (lor) = aed Tye Towa Tay ad 
C i Pai — 92. (O0T)e-/A2 (1725 = SoCs e eee Lie is 
bees, 18 een. (£12)A..(112) = lee eae se ee 
Fig. 1, Pl. L, shows the form: thus produced. The habit seems 
not to have been hitherto observed in leadhillite except in so far as it 
may resemble the rhombohedral habit of susannite. The specific 
gravity of the mineral was found to be 6.42. 
* Dana, System of Mineralogy, 18092, p. 922. 
HoyStMin.. £654, Pp. 37.3- 
