Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 4. — Idiomorphic augite crystal in camptonite. The section is about at rightangles to the vertical axis c, and shows the intersecting cleavages parallel to the prismof 87° Ob. Keene Valley, N. Y. of several sections of the same mineral and by a measurement ofthe angles, it is possible to determine the common crystallo-graphic forms of the mineral. Very misleading outlines may,however, be observed ; as for exam
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 4. — Idiomorphic augite crystal in camptonite. The section is about at rightangles to the vertical axis c, and shows the intersecting cleavages parallel to the prismof 87° Ob. Keene Valley, N. Y. of several sections of the same mineral and by a measurement ofthe angles, it is possible to determine the common crystallo-graphic forms of the mineral. Very misleading outlines may,however, be observed ; as for example the triangular outline of a. Fig. 5. — Allotriomorphic quartz (/, showing no relief, plagioclase / and decom-posed feldspar o in granite. As seen with crossed nicols. FORM. 15 section of a cube, with the corner truncated. For measuring theangles have the stage accurately centered, then bring the vertexof the angle to be measured to the intersection of the cross-wiresin the eye-piece. Bring one of the sides in coicidence with oneof the cross-wires. Note the reading of the graduated circle androtate the section until the other side is in coincidence with thesame wire. Take the reading again and the difference will be theangle required. Incomplete Crystals, without definite crystalline outline, whosebounding surfaces are more or less determined by those of ad-jacent crystals. Such crj-stals are frequently called Allotrio-niorphic,^ see Fig. 5. This allotriomorphic form must not be
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