142 FIELD COLUMBIAN MusEuM—GEoLoeGy, VoL. III. 
little doubt of their accuracy. The forms and measurements are 
as follows: 
e(orl2)-4R_ iv. (0553)-3R M(4041)+4R u(2131)+R3 
Observed — Calculated 
2% te alae ws lOD Le eet ion a 46° AST inure 
Us Uh = 9 (0553) eA, (cae eee 95° 267 
M a v. = (4041) A (0553) on 5a 
UofA es (218 r) A alr) = 73° 75% 
MAM ( _ (4041) A (4041) = 29° 10! Wea eeeags 
of twin | ; ; ? 
The specimen bearing Museum No. M,8696, and shown in Fig. 2, 
Pl. XLIX, exhibits a habit resembling that described by Sterrett* 
as presented by twins from the Maybell mine, North Empire, Kan- 
sas. The Museum specimen is from the Blackberry mine, Joplin. 
It lacks the amethystine color characteristic of the Maybell mine 
twins, being colorless and transparent except for small internal 
reflections and inclusions. Neither is the Blackberry mine twin 
characterized by large size as are the majority of the Maybell 
mine twins. The greatest length of the specimen here described is 
along the twinning plane in the direction of the edge e 7, and is 8 cm. 
Normal to this in the same plane the length is 4 cm., and normal to 
the plane the length is 2 cm. 
Like the Maybell mine twins this twin exhibits a prismatic form 
produced by prominence of the planes e and?#. One end of the prism 
terminates in a re-entrant angle with modifying planes, while the 
other end was attached and exhibits the cleavage rhombohedrons of 
the two individuals, forming a salient angle. Aside from this occur- 
rence of the unit rhombohedron it does not appear on the twin although 
on the Maybell mine twins it is prominent. Two scalenohedrons 
occur on the re-entrant angle of the twin, neither of which is repre- 
sented in the Maybell mine twins. These are E (4156) and a form 
new to calcite, v:(11.4.15.3).| Tworhombohedrons, which are the com- 
mon forms, f and M, round the edge between e and E. The rhom- 
bohedron e as will be noted by the figure, is the dominant form of the 
twin and is also the twinning plane. The common scalenohedron uv 
which does not occur at all on the Maybell mine twins, occurs in this 
twin along the edge on which the individuals meet. 
The planes of the crystal are for the most part brilliant and fone 
sharp edges. The scalenohedron F however, is striated. Owing to 
* Am. Jour. Sci., 1904. 41, 8, D. 73-76. 
+ The authors are indebted to Dr. Palache for designating this letter. 
