ART. 12. SIDERITE AND ASSOCIATED MINERALS—SHANNON, 9 
angles. One face, which was taken as the:top cube face (001) and 
mounted as polar, was found to be depressed and concave and to be 
divided into four sectors by irregular sutures which yielded four 
major and numerous minor signals. The octahedral faces were bright 
enough to refle t, but each .of these was found to consist of two 
or more areas yielding separate signals. ‘The measurements were 
entirely unsatisfactory, and the vertical faces of the cube were appar- 
ently composed of numerous projecting points with shallow reentrant 
angles. A sketch of a crystal of this sort is given in figure 7. Such 
forms grade intermediately between the simple crystals and the 
spherical forms which are characteristic of this mineral at many 
localities. An excellent idea of the appearance of the cristobalite 
under a lens can be gained from the photograph, (pl. 2, left), which 
shows a small cavity in the basalt containing cristobalite associated 
with rods of feldspar and globules of first generation siderite, the 
photograph being enlarged six diameters. The most perfect crystal 
shown in this photograph was removed and measured on the two- 
circle goniometer, the results being satisfactory in that they indicated 
<r Ving 
FiGs. 6-8.—CRISTOBALITE CRYSTALS FROM BASALT. 6, SIMPLE CUBOCTAHEDRON SHOWING FORMS 
A (100) and o (111). 7, SKETCH OF IMPERFECT CUBOCTAHEDRAL CRYSTAL. 8, CUBOCTAHEDRON 
WITH EDGES BEVELED BY TRAPEZOHEDRON N (211.) 
isometric symmetry conclusively. This crystal, as shown in figure 8, 
was cuboctahedral, but with the edges beveled by the trapezohedron 
nm (211) and with no external indication of any twinning. ‘The fol- 
lowing measurements were obtained: 
Angles measured on cristobalite crystal, figure 5. 
4 
| Measured. Calculated. 
No. Form. Signal. 7 
© p eg p 
° tA ° / ° ea | ° / 
1 CAO MER tees Se Ry eye ce ose .nin:e mentees ai OROOR I sOLUS OOS GS scyerent- | 0 00 
2 ofl |PRoorits. YR HALAL eset Sa 44 17| 538 16| 45 00| 54 44 
3) roa LP's] Seth UR Os Crack ain ee 4 A oy 5 44 36) 54 28) 45 00 54 44 
4 cop WIS Sard RTE Wi ese ghee Ai ev 45 20| 54 28] 45 00 54 44 
5 oy” ly Poor. 25. S45 ite tees AA IB 01) 64.284) L451. 00 54 44 
6 CHOON RG oodke rer). Heats tes eS Ae 90 00} 90 00}; 90 00 90 00 
a m 211?| Very narrow, no signal...... 44 40/129 20] 45 00/} 35 16 
1 Measurement obviously erroneous. Indices established by zonal relations. 
