490 University Geological Survey of Kansas. 
crystals have their vertical axis inclined at almost right angles 
(89° 647). 
The fourth twinning law, in which the twinning plane and 
composition face is the negative rhombohedron, ¢g.,—2R, was 
observed on two large, amber-colored crystals from Joplin, in 
the collection of Mr. W. G. Kane, of Kansas City, Mo., whose 
collection of Joplin minerals, both for show and for scientific 
interest, is one of the finest in the world. The dominant 
form on these crystals is K:,+R3, but as the ends are broken 
off, the other forms, if any occurred, cannot be given. An 
ideal unmodified twin of this kind is represented by figure 66. 
The color of calcite varies from colorless or white to a deep 
amber, though when inclusions are present, it often assumes 
darker colors. A pale amethystine tint is common in the in- 
terior of the crystals, and, sometimes, this tint is limited to 
an interior phantom. 
Pseudomorphs of smithsonite and calamine after calcite are 
frequently found, being especially abundant at Granby. 
Calcite is one of the latest minerals formed. Of the primary 
ones, barite alone follows it. 
Dolomite; ‘‘SPaAr.’’ 
Composition, calcilum-magnesium carbonate, (CaMg)COs: Ca, 32-26 %; 
Mg, 19-35%; C', 9-67%; O, 38-70%. Hardness, 3-5-4; gravity, 2-8-2-9; 
color, white, varying to pink, green, brown, gray, yellow; crystallization, 
rhombohedral; cleavage, rhombohedral. 
Observed forms: p-(1011)+R. 
The mineral dolomite almost invariably exists as crystals 
with curved faces of the rhombohedron. The curvature of 
the faces, as in many other minerals, is due to the fact that 
the sub-individuals, which make up the crystal, are only in 
approximately parallel positions, as may be seen on close ex- 
amination of the crystals. The color of the crystallized dolo- 
mite varies from white to various shades of a pale, delicate 
pink, the latter color being the more frequent one. 
A gray, granular, crystalline dolomite is of common occur- 
rence. Frequently well-formed crystals of a light gray color 
are found embedded in a dark, siliceous matrix, as at the 
Tennessee Prairie mines, south of Galena. 
