Dec, 1908. Mineralogical Notes 141 



merits were made by contact. The forms determined and measure- 

 ments obtained are as follows: 



A calcite twin bearing the Museum No. M 8692 is shown in Fig. 1, 

 PI. XLVIII. This is from the Crystal Palace mine, Central City, Mis- 

 souri. The twinning plane is c (0001). The dominant forms are the 

 rare rhombohedron ^-(0533) and e (01T2). These produce an approxi- 

 mately spheroidal twin, but only about half the spheroid is present in 

 this specimen. The halving is due to the manner of growth from the 

 attachment. The twin is complete in a polar direction, but incom- 

 plete equatorially. The length of the polar diameter is 5.5 cm. 

 The substance of the twin is wine-yellow, and transparent. All of 

 the planes, however, with the exception of those of the rhombo- 

 hedron e (01T2), are coated with a thin, firmly adhering layer of iron 

 oxide, chocolate brown in color. The planes of e are striated, as 

 represented in the drawing, and as also represented there unite with 

 those of u- by a curved edge. The common forms v and M modify 

 the dominant forms. The twin is thus made up of three rhombo- 

 hedrons, two of which are negative and one positive, and the scale- 

 nohedron v. Of the rhombohedrons, o. (0533) seems to have been 

 first noted by Thurling *.who, however, gave it no letter. Whitlock.f 

 who observed it on calcite from West Paterson, N. J., gave it the 

 letter here employed. 



The measurements which were made on the specimen were 

 by contact, but the planes were so well marked that there seems 



* Neues Jb. Beil. Bd., 1886, 4, p. 380. 

 t Am. Jour. Sci., 1007, (4), 24, p. 427. 



