138 Field Columbian Museum — Geology, Vol. III. 



There is little variation in the development of the crystals, the 

 principal differences being in the development of the macropinacoid 

 a (100). When this is extended, as shown in Fig. 4, PL XLVII, 

 the crystals have a generally rectangular outline ; when it is developed 

 about equally with the prisms, the crystals have an apparently hex- 

 agonal outline if, as is usually the case, only half of the crystal is pres- 

 ent. Again the unit prism m (no) may be wanting entirely. If so, 

 the crystal is usually elongated in the direction of the macro-axis 

 and attached by the brachypinacoid b (010) so that the appearance 

 illustrated in Fig. 5, PI. XLVII is obtained. This drawing is made 

 with b (010) in front in order to show the characteristic appearance. 

 The form / ( 2 °3)> as illustrated in the figures, occurs at only one end of 

 the vertical axis. The absence of a corresponding plane indicates 

 hemimorphism in the direction of the vertical axis such as was noted 

 by Penfield.* The edge opposite to / (203) produced by the junction 

 of c (00T) and a (100) and that upon which a plane corresponding to 

 / would normally appear if the crystal were holomorphic, is never 

 sharp, but grades irregularly toward the center of the crystal by suc- 

 cessive overlying lamellae, all of which have irregular edges. Such 

 indications of lamellar structure suggest twinning similar to that 

 noted by Penfield on crystals from Mt. Antero,f but study of cross- 

 sections of the crystals in polarized light gives no evidence to support 

 such a view. Extinction in polarized light occurs parallel to the pina- 

 coidal cleavage of the crystals, thus affording additional proof of the 

 orthorhombic crystallization of the mineral. On slight heating the 

 crystals become strongly electric so that they pick up pieces of paper. 

 Before the blowpipe they exfoliate slightly and when heated in the 

 closed tube decrepitate. The other blowpipe characters observed 

 were similar to those which have been mentioned by previous ob- 

 servers. 



CALAMINE 



LEADV1LLE, COLORADO 



Among specimens received by the Museum from the World's 

 Columbian Exposition, a series of ores from the Maid of Erin mine, 

 Leadville, Colorado, contained an ocherous substance thickly coated 

 with long, slender crystals. These crystals proved on examination 

 by means of a blow-pipe to be calamine. The occurrence seems 

 not to have been hitherto described, although Pratt has given an 



* Loc. cit. t Loc. cit. 



