FEB. 1900, CRYSTAL ForMS OF CALCITE—FARRINGTON. 235 
of these forms together with the rhombohedron /, — 4 R, as seen ona 
crystal in the Museum collection. There is also present one plane 
of the negative rhombohedron /,-2R. 
Some measurements made on this crystal with the contact goni- 
ometer are as follows : 
Measured. Calculated. 
Mp eee TAD O4AG fi ere ici hae eee 43°-44' aa? 6! 
ss AE” Ea teeter» | Ge rire nc area ae oa ee 35 °-38' Ly oboe i 
Pema NAN 2a PO4AR SRT th ee ca hte wach F1°=32° T2. 2" 
a Stal hy Hie Ct ae ea pee Prete he 50° 36’ 
As has often been observed on calcites from other localities, the 
planes of the rhombohedron ¢,-% R are, in this crystal, striated 
parallel tothe edge r/7’. They are also somewhat rounded. The 
curious sporadic occurrence of the plane f and the accompanying 
imperfect growth of planes seems to be quite common for Joplin 
crystals showing the above combinations. At least the writer has 
noticed the same appearance on several other similar but smaller 
crystals. It would be interesting to know the nature of the molecular 
forces which would uniformly produce:degradation of this sort. The 
planes of the scalenohedron v on the crystal are in general flat and 
bright, but show peculiar markings suggesting stages of growth. 
These markings are quite irregular in form and seem to have no 
angular values whatever. They follow, however, so far as they have 
any arrangement, the lines of the cleavage rhombohedron. Partings 
along planes of this rhombohedron are to be seen through the crystal 
and the surface of attachment is usually characterized by cleavage 
planes. 
Other crystals of Type 2 are more highly modified than the one 
just described. Fig. 1, Pl. xxix, represents one such crystal now in 
the Museum collection. Here the primary scalenohedron v, + R3 is 
modified by the scalenohedrons 2, + R$ and w, + 2 R2, also by 
the positive rhombohedronsyz, + Rand /, +:4R, and by the negative 
rhombohedron /,-4R. Only about half the complete form is present 
in the specimen. In Fig. 2, Pl. xxix, is shown the completed ideal 
form which would be produced by these combinations. Some of the 
measurements made on this crystal with the contact goniometer are 
as follows: 
Measured. Calculated. 
cae Pe OSA) papain (0) eB ark oN Sree ee 30° 7a nd 
fa BO eile go alae? Co = Ser ar ie BAe Orc ee I 18° 19° 24' 
PMN OLE AOA 1 Reh or AON echt Oe weet ko he ai 31° 10%’ 
v WEBEEESL SE o C4AS Said, a HA helin Midale hk oooh s 44° 44° 6! 
