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 

 the planes 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,f his value for c a r being 128 3' and for r a r', 

 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 an orthodome. The pyramid 

 and dome most nearly corresponding with the angles given above are 

 t (112) and / (Toi), the measured and calculated angles for these 

 forms comparing as follows: 



(I01) 



(112) 

 (Il2) 



Fig. 1, PI. 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, 1892, p. 922. 

 + Syst. Min., 1854, p. 373. 



