238 Field Columbian Museum — Geology, Vol. i. 



Type 4. — The only crystals of this type in the Museum collec- 

 tion come from Leddy's Lease, Central City, about 5 miles from 

 Joplin. They are, however, a characteristic type for the Joplin dis- 

 trict. They are scalenohedral in character, often doubly termi- 

 nated, colorless, and frequently reach a length of from 2 to 3 inches 

 (5-7.5 cm.). The forms characterizing the type are v, +R3 domi- 

 nant with e,-}4K, /,-2R and 2,-nR, subordinate. A characteristic 

 feature is the habitually rounded and cross striated character of the 

 planes of /. In fact this form often grades into a scalenohedron of 

 large indices, the symbols of which it is impossible to determine. 

 The planes of / likewise usually widen toward the apices of the 

 crystal, so as to crowd out the planes of v, and themselves define 

 the termination of the crystal. Fig. 1, PL xxx, sufficiently illustrates 

 the average development of planes on crystals of this type, although 

 the one figured is a twin crystal and larger than the average of the 

 type. 



Type 5. — Crystals of this type are of rhombohedral development. 

 Type specimens in the Museum collection come from the Meadow- 

 croft mine, Joplin. They are colorless, and not over 1 inch (2.5 cm.), 

 in length. They are made up of the negative rhombohedrons 7/,-4R 

 and/,-2R dominant, with the latter truncated by e, -J^R. A typical 

 crystal is shown in Fig. 4, PI. xxix. The planes of /and r t are con- 

 siderably curved, and admit of only approximate measurement. The 

 symbol of/ was, however, sufficiently determined by its relation to 

 the cleavage rhombohedron, and that of r t was determined from the 

 mean of measurements of ten crystals, which gave e : ^ — 0221 : 0441 

 = 49°. The calculated value is 49° 32'. The planes of e are usually 

 smooth and bright, those of/ are deeply striated parallel to the edge 

 fy, and those of r t parallel to the longest diagonal of the rhomb. 

 Rounding of the planes of the three rhombohedrons e, /and r t into 

 one another is common. One crystal showed an intermediate rhombo- 

 hedron between e and/ but it was impossible to obtain a measure- 

 ment sufficiently satisfactory to determine its symbol. In some 

 crystals again the rhombohedron /is absent and the crystal is made 

 up of only the forms r h -4R and e, -^R. 



FORMS OF TWINNED CRYSTALS. 



Only a few twinned crystals were noted among those examined. 

 The larger number of those noted are twinned upon the basal plane. 

 These occur of both Types 3 and 4. The crystal chosen for special 

 study was of Type 4, and is from the collection of the University of. 



