142 Field Columbian Museum — Geology, Vol. III. 



little doubt of their accuracy. The forms and measurements are 

 as follows: 



e(oil2)-£R v. (o5S3)-|R M(4o3i) + 4R tj( 2 i3i)+R3 



Observed Calculated 



46 



95° 



57° 



73° 



29 10' 



The specimen bearing Museum No. M 8696, and shown in Fig. 2, 

 PI. 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 f, 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 t. 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 (4136) and a form 

 new to calcite, v: (1 1.4.15.3).! Two rhombohedrons, which are the com- 

 mon forms, / 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 v 

 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 have 

 sharp edges. The scalenohedron E however, is striated. Owing to 



* Am. Jour. Sci., 1904. 41,8, p. 73-76. 



t The authors are indebted to Dr. Palache for designating this letter. 



