238 FieELD 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 
(5-7-5cm.). The forms characterizing the type are v, +R3 domi- 
nant with ¢,-%R, f-2R and 2,-11R, subordinate. A characteristic 
feature is the habitually rounded and cross striated character of the 
planes of f In facet 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 f,-2R dominant, with the latter truncated by ¢, -4%R. A typical 
crystal is shown in Fig. 4, Pl. xxix. The planes of fand 7 are con- 
siderably curved, and admit of only approximate measurement. The 
symbol of f was, however, sufficiently determined by its relation to 
the cleavage rhombohedron, and that of 7 was determined from the 
mean of measurements of ten crystals, which gave ¢ : 7=0221 : 0441 
=49°. The calculated value is 49° 32’. The planes of eare usually 
smooth and bright, those of fare deeply striated parallel to the edge 
yn, and those of 7 parallel to the longest diagonal of the rhomb. 
Rounding of the planes of the three rhombohedrons e, f and 7 into 
one another iscommon. Onecrystal showed an intermediate rhombo- 
hedron between ¢ and f, 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 7, —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 
