140 Field Columbian Museum — Geology, Vol. III. 



measurement of t (301) on s (101) having been secured on one crystal. 

 The striations in the prismatic zone make measurements there unsatis- 

 factory although results of sufficient accuracy for the identification 

 of the planes were secured. The following are some of the measure- 

 ments obtained : 



t A S = (301) A (ioi) = 



5 A S' = (ioi) A (Toi) = 



m a tn™ = (no) a (ilo) = 



m a b = (no) a (010) = 



CALCITE 

 JOPLIN DISTRICT, MISSOURI 



PLATE XLVIII AND FIG. 2, PLATE XLIX 



A number of crystallized specimens of calcite from the Joplin Dis- 

 trict, Missouri, received for the most part from Maynard Bixby, 

 present features hitherto undescribed. ' Most of the specimens are 

 twin crystals. The specimen bearing the Museum No. M 8695 and 

 shown in Fig. 2, PI. XLVIII, is from the Cuban mine, Joplin. The 

 twinning plane is e (01I2) and the crystal shows its greatest elongation 

 in the direction of this plane. The length in this direction is 11 cm. 

 (4X inches), while at right angles to this plane it is only about half 

 as long (5 cm.). The form of the twin is roughly prismatic, the sides 

 of the prism being planes of the unit rhombohedron r (ioli) and the 

 scalenohedron S3! (5 161).* At one end the crystal was attached, and 

 here it shows only the cleavage rhombohedrons, but at the other end 

 occur a re-entrant angle and a number of modifying forms. The latter 

 forms are the rare scalenohedrons enumerated below. All are about 

 equally developed. The substance of the twin is white and opaque 

 in the interior and yellowish and nearly transparent on the exterior. 

 The boundary between these two portions is rather distinctly marked, 

 the thickness of the exterior portion being about 5 mm. The planes 

 of the crystal have brilliant, flat surfaces as a rule, but the larger ones 

 are more or less undulating both as to surfaces and edges. Measure- 



♦Goldschmidt's letter. One of the authors has elsewhere (Pub. Field Col. Mus. Geol. Ser. Vol. 

 I., p. 239) given reasons for combining the use of Dana's and Goldschmidt's letters. The two 

 kinds of letters can be distinguished by remembering that Goldschmidt's letters are followed by 

 dots. 



