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Fes. 1900, CrystaL Forms or CALCITE-——-F ARRINGTON. 237 
FORMS OF SMALLER CRYSTALS. 
None of the following crystals have the large size of those of the 
‘types above described. They show, however, characteristic combi- 
mations which warrant their description as types. 
Type 3.—Crystals of this type are illustrated in Fig. 3, Pl. xxix. 
‘As is usual with the Joplin crystals they are not highly modified, 
being made up solely of the scalenohedrons v, + R3 and w, 3 Ra. 
‘The specimens of these crystals in the Museum collection come 
from the Pelican mine in Joplin, occurring with sphalerite on the 
characteristic chert breccia of the region. A large group is shown 
in the frontispiece. The largest single crystal is not over 4 inches 
(zo cm.) in length, and from this the crystals grade to a very small 
‘size. Measurements ofthe planes made with the reflecting goniome- 
iter are as follows: 
Measured, Calculated. 
OS SEEN | emery ABS ra a aa ae re Ne yA 
eeoeeeee PERT ePRENE NES a IE. os ka a5° 36) ane ah 
SIRE TREC Se. ao ll eo ea 46° 53' 47° 1%" 
SiN ETEE 6S GSAS SCD Wn Ds. Od a a 50° 58’ 49° 23’ 
Ga ea Ai AG AT 95 00 tise ale ASE gene te ee 152500 16° 
The most striking characteristic of these crystals is the fact, repre- 
‘sented in the figure, that the planes of scalenohedron v are smooth 
and brilliant, while those of w are rough. This differentiation of 
planes is constant for all the crystals which have come under the 
writer’s observation. The rough planes do not show definite etch- 
ing figures, but rather under a lens series of troughs crossing one 
another and following the lines of the cleavage rhombohedron. In 
the writer’s opinion the roughness is due to irregularities of growth 
of the crystals rather than to etching. The reasons for this opinion 
are: First, it is difficult to conceive of any etching agent which 
could leave the planes of one scalenohedron so entirely untouched 
while those of the other were so deeply corroded. Second, on sub- 
mitting the crystal to the action of weak vinegar as an etching agent 
the planes of the scalenohedron 7 were affected much more rapidly 
and noticeably than those of w. The etching produced by the vine- 
gar on the planes of w was in the forms of troughs or striz follow- 
ing the lines of cleavage, on w there occurred only a slight additional 
roughening of the planes. It is, therefore, in the writer’s opinion, 
not always correct to speak, as is usually done, of the rough planes 
as etched planes. They will often be more correctly described by 
the term zmperfect or incomplete planes. 
