158 Field Columbian Museum — Geology, Vol. III. 



This is also usually truncated by the positive % rhombohedron d 

 (01T2). The unit prism m, ( 1 0T0) also appears as small planes. The 

 third specimen (Mus. No. M 10276) consists of a large crystal of ortho- 

 clase 3x5 inches in size more or less intergrown with albite, and show- 

 ing also three crystals of topaz 2 to 3 inches in length. Scattered 

 about upon the albite and orthoclase about 50 crystals of phenacite 

 occur. These vary in diameter from 1 cm. to 1 mm. All are 

 whitish to colorless, the larger crystals tending to be semitrans- 

 parent and the small ones perfectly transparent. In habit all show 

 the lenticular shape previously described, which is produced by 

 the forms already mentioned. Many of the smaller crystals, how- 

 ever, are more highly modified than the large ones and the planes 

 being brilliant and giving good reflections afford easy identification 

 of the forms. The following is a list of the forms observed and some 

 of the measurements obtained : 



The appearance of one of the crystals is shown in the accompany- 

 ing figure, Fig. 4, it having been drawn as is usual with phenacite, 

 with the negative rhombohedrons in front. 



REALGAR 

 MERCUR, UTAH 



PLATES XLIV AND LIV 



Among other specimens from Mercur, Utah, obtained from May- 

 nard Bixby, two exhibiting realgar deserve especial mention. In 

 one of these specimens (Mus. No. M 8204), the realgar occurs as small 

 crystals partially filling a narrow fissure in limestone: in the other 

 (Mus. No. M 8205) it occurs as elongated prisms intergrown with 



