Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . / I ^ \ /a \VT- Cassiterite.—Axes « : c = i : In this the ratio of « to c is closely as in zircon but the commonassociation is now the unit pyramid / with the second order pyra-mid d zs shown in Fig. 102. In Fig. T03 these forms occur with a ditetragonal pyramid z =(a : ^ a : ^c) and the unit prism 7n. CLASS OF THE THIRD ORDER PYRAMID. 13. No. 21—Pyramidal Hemihedry, Liebisch. No. 8—Pyramidal Group, Dana. Certain crystals of the minerals scheelite and wernerite are sym-metrical to one horizon
Elements of mineralogy, crystallography and blowpipe analysis from a practical standpoint .. . / I ^ \ /a \VT- Cassiterite.—Axes « : c = i : In this the ratio of « to c is closely as in zircon but the commonassociation is now the unit pyramid / with the second order pyra-mid d zs shown in Fig. 102. In Fig. T03 these forms occur with a ditetragonal pyramid z =(a : ^ a : ^c) and the unit prism 7n. CLASS OF THE THIRD ORDER PYRAMID. 13. No. 21—Pyramidal Hemihedry, Liebisch. No. 8—Pyramidal Group, Dana. Certain crystals of the minerals scheelite and wernerite are sym-metrical to one horizontal plane and one vertical tetragonal axis only. Fig. 104. Fig. 105. Fig. 30 CR YSTALLOGRAPHY. The General Form or Third Order Pyramid.—Weiss,a??mc\ Dana, m — n; Miller, \kkl\. Composed of eight faces, Fig. 105, each cutting the two basalaxes at unequal but simply proportionate distances, and the ver-tical axis at a distance not simply proportionate to the other dis-tances. In the ideal forms the faces are isosceles triangles. That eight such faces satisfy the symmetry is shown by Fig. 104. Any occurringface must, because of the vertical axis of tetragonal symmetry, occur with three otherfaces above the horizontal axes, and because of the horizontal plane of symmetry eachof these must be accompanied by a face below the plane. The Limit Forms. Five of the limit forms are geometrically like forms alreadydescribed, namely : 1. Basal Pinacoid. Fig. 85. 2. Prism of Second Order. Fig. 86. 3. Prism of First Order. Fig. 87. 5. Pyramid of Second Order. Fig. 89. 6. Pyramid of the First Order. Fig. geometrically new form is : 4. Prism of Third Order.—a:
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