140 FIELD COLUMBIAN MusEuM—GEo .oeGy, VoL. III. 
measurement of ¢ (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: 
Measured Calculated 
PALS). +=" QOL) SEtOT) ein a Oa noe 29° 57" 
Si WO SS.) t= SOT) aeAD Cr) ak 2 eee 62° 46’ 
Moh MT = (LTO A 1G) ee ee 70s 
mr 0 = (110) A (010) =  §2° 20! bade 
GAL GUILE 
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, Pl. XLVIII, is from the Cuban mine, Joplin. The 
twinning plane is e (0112) and the crystal shows its greatest elongation 
in the direction of this plane. "The length in this direction is rr cm. 
(414 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 (1011) and the 
scalenohedron $8: (5161).* 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 5mm. 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. 
