P. 33, line 23, for beat read be at.
P. 66, line 10, after appearance add (sometimes called “twinkling”).
Corrections for mean index of refraction n′: p. 60, line 1, 1.551 to 1.584; p. 62, line 29, 1.766; p. 66, line 3, 1.601; p. 67, line 24, 1.622; p. 71, line 10, 1.633 to 1.674.
Corrections for Indices of Refraction (mean) in Table on p. 142: Corundum 1.766; Tourmaline 1.674; Tourmaline (precious) 1.633; Dolomite 1.622; Scapolite (Meionite) 1.584; Scapolite (Marialite) 1.551; Calcite 1.601.
Corrections for mean indices of refraction in Scheme (insert folder): Scapolite 1.551 to 1.584; Calcite 1.601 and transfer to Dolomite rectangle; Dolomite 1.622; Corundum 1.766; Tourmaline 1.633 to 1.674.
1. Often called a direction of maximum elasticity, c′ being a direction of minimum elasticity.
2. Rosenbusch’s Microskopische Physiographie, p. 156.
3. For a more complete discussion of optics, in connection with Optical Mineralogy, the student is referred to A. J. Moses’ Characters of Crystals, p. 85, et seq.; Moses’ & Parsons’ Mineralogy, Crystallography and Blowpipe Analysis, Chap. XVI., 4th Ed., 1909; Miers’ Mineralogy, 1902; L. Fletcher’s Optical Indicatrix, etc., 1892; Groth’s Physikalische Krystallographie, 3d Ed.; Rosenbusch’s Mikroskopische Physiographie, 4th Ed. and Iddings’ Rock Minerals, 1911.
4. The electromagnetic theory of light is now very generally held, but, whatever may be the recurrent change of state to which light is really due, the principles of wave motion furnish a satisfactory geometric description of optical phenomena.
5. These sections are supposed to have plane parallel faces, such being the case in ordinary practice, and to be examined with parallel perpendicularly incident light.
6. It is interesting to remember in this connection that in the isometric system there is also the greatest possible symmetry of “form.”
7. A. J. Moses, Characters of Crystals, pp. 85–97.
8. For this branch of optical physics, see A. J. Moses, Characters of Crystals, pp. 97–100.
9. This can be demonstrated by using a nicol and a plate of calcite which shows a double image. If the nicol is held between the calcite plate and the observer’s eye it can be so adjusted that only one image is seen. If now the nicol is revolved 90° the first image will disappear and the other image alone will be seen.
10. In some cases a peculiar form of double refraction does take place parallel to this direction, as in the circular polarization of quartz and cinnabar; but in very thin sections these results are not noticed and can be disregarded.
11. A. J. Moses, Characters of Crystals, pp. 98, 99.
12. The terms axes of elasticity are commonly used for these principal vibration directions in text-books on petrography.
13. Instead of ω and ε, for convenience in tables, etc., α and γ are used, denoting the indices of refraction of the rays traversing the crystal with greatest and least velocity respectively, without regard as to which is the O or E ray. A good reason for this convention is that the symbol (γ − α) is used to express in decimals the relative strength of the double refraction of a crystal, whether uniaxial or biaxial. γ is always greater than α.
14. For most cases in observations with white light the “optics axes” may be regarded as approximately fixed in position.
15. Often spoken of as: a, the axis of maximum; b, the axis of intermediate, and c, the axis of minimum elasticity.
In the more recent American text-books on Optical Mineralogy by Iddings, Winchell and Phillips a is denoted by X, b by Y and c by Z.
16. For a short historical sketch of the use of the microscope in connection with Petrology, see G. H. William’s pamphlet, Modern Petrography (Monographs of Education), Boston, 1886. For detail description and adjustments, see Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, pp. 11, 61.
17. For description of the ordinary microscope, eye-pieces, objectives, magnification, etc., see Manipulation of the Microscope, by Ed. Bausch.
18. Text Book of Mineralogy, by E. S. Dana, 1898 Ed., p. 176.
19. It is convenient to assume that the vibrations of the polarized light are taking place in this plane, called the “plane of vibration,” but all the phenomena caused by polarized light could be also explained on the assumption that the vibrations were taking place at right angles to this plane.
20. This condensing lens must be removed when very low power objectives are used.
21. Some microscopes are provided with adjusting screws bearing on the frame holding the objective, which can then be accurately centered to the axis of rotation of the stage.
22. In the Seibert microscope use objective No. 00 for the first general study of a rock section, No. II for general use and No. V for observations with convergent light. In the Fuess microscope use objective No. 4 for general use and No. 7 for convergent light tests. In the case of an English microscope a 1″ to ¾″ objective is used for general purposes and a ¼″ to ⅕″ for observations with convergent light.
24. In some microscopes the analyzer is in the form of a “cap” nicol, arranged to be fitted over the top of the eye-piece, and not introduced in the microscope tube as shown here. This form is not so convenient, as the “cap” nicol must be set by hand every time it is desired to make observations with crossed nicols. But at the same time it avoids any possible refocusing which may be necessary when the other type of analyzer is introduced in the tube.
25. When accurate adjustments are possible of the vibration planes of the nicols and the cross-wires parallel to these vibration planes, reference should be made to Methods of Petrographic-Microscopic Research, p. 61, 1911, Fred. E. Wright.
26. In the Seibert microscope eye-piece No. 0 is used for most purposes. Other eye-pieces, Nos. 1 and 2, with cross-wires, are used for different degrees of magnification, and one eye-piece, No. 3, without cross-wires, is provided to be used in connection with an eye-piece micrometer.
27. With the Seibert microscope, Fig. 2, the No. 0 eye-piece and the No. II objective will prove most satisfactory for the following tests. With the Fuess microscope use No. 4 objective. With an English microscope use an ordinary eye-piece and a 1″ or ¾″ objective.
28. Twins may be recognized just as in macroscopic specimens, and zonal structure noticed if the zones differ in color. When a colorless mineral is surrounded by other colorless minerals, of about the same index of refraction, its outline is often best brought out by observation between crossed nicols.
29. The term “Anhedron,” meaning without planes, has been suggested by L. V. Pirsson to describe in rocks the crystal fragments which have no plane faces, as, for example, the augites of augitic rocks. Science, Jan. 10, 1896, p. 49.
30. Partial resorption and recrystallization may produce a border of secondary minerals, surrounding the original crystal.
31. The surfaces of all minerals in sections are more or less rough, but this roughness is only made visible when there is a marked difference between the indices of refraction of the minerals, and the index of refraction of the balsam in which the minerals are embedded. The index of refraction of balsam is about 1.54, so it is only when the mineral has a higher or lower index of refraction that its surface appears rough. When internal structure is to be studied, the crystal should be surrounded by a fluid of nearly the same index of refraction as that of the crystal, and when the exterior of the crystal is to be studied, then a fluid should be used with a very different index of refraction.
32. The Seibert microscope has a very convenient and quick lowering adjustment, by means of the lever d, for making this test. For convenience the lower nicol and condensing lens are generally left in place below the stage of the microscope, as polarized light serves as well for these investigations as ordinary light. An additional advantage in this arrangement is that the condensing lens is always ready for the “relief” test and the lower nicol for the pleochroism test; but it must be remembered that the polarizer or lower nicol cuts out one half of the light, which comes to it from the reflector, and this loss is important when high power objectives are to be used. When very low power objectives are used, the condensing lens must be removed.
33. Mem. de l’Acad., Paris, 1767–68.
34. Min. Mag., Vol. I., p. 193; Vol. II., p. 1.
35. Sitzungsberichte der k. k. Akad. der Wiss., Wien, 1893, I Abt., p. 358. Translation by L. McI. Luquer in School of Mines Quarterly, Vol. XXIII., Jan., 1902, No. 2, p. 127. Review by Viola in Min. Pet. Mitt., Vol. 14, p. 554. Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 95.
36. Most of the petrographical microscopes carry over the polarizer a convex lens the effect of which is to widen the illuminating cone and hence make less visible this phenomenon.
37. In Seibert student microscope, No. II a, use next to smallest light-stop. Some of the Fuess microscopes are supplied with an iris-blende for limiting the cone of light.
38. In Seibert microscope use No. V, not sufficiently marked results being obtained with No. II.
39. See Viola’s diagram, Minn. Pet. Mitt., Vol. XIV., p. 556.
40. Seibert, No. II. Fuess, No. 4.
41. Kurze Anleitung zur mikrosk. Krystallbestimmung, Wiesbaden, 1898, and Tabellen zur mikrosk. Bestimmung, etc., Wiesbaden, 1900. Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 93.
This method is particularly favorable for the accurate determination of the refractive indices of small isolated fragments or grains, by using liquids of known indices. In this case a bright line appears on the “near” edge and a dark line on the “far” edge if the grain has a higher index than the liquid. The reverse occurs when the index is lower than the liquid. When the index is the same (using white light) the “far” edge is blue and the “near” edge red and also the contours about disappear.
F. Krantz, of Bonn, furnishes a series of 21 liquids in small bottles, with indices from 1.447 to 1.83.
The indices of a few convenient liquids are: water, 1.34; alcohol, 1.36; glycerine, 1.41; olive oil, 1.47; nut oil, 1.50; clove oil, 1.54; aniseed oil, 1.58; almond oil, 1.60; cassia oil, 1.63; monobromnapthalene, 1.65; methylene iodide, 1.75.
42. Crystals that have two good cleavages often develop so that the direction of elongation is parallel to the intersection of the two cleavages, while in the case of crystals with one good cleavage the tendency seems to be towards a tabular habit parallel to the cleavage.
43. Harker’s Petrology for Students, p. 306.
44. The lower nicol is generally so adjusted that its plane of vibration is parallel to the north and south cross-wire in the eye-piece. This adjustment can be tested by means of a section of biotite, showing cleavage cracks. When the plane of vibration of the polarizer is parallel to the N. and S. cross-wire in the eye-piece, the biotite section becomes almost dark when its cleavage cracks are parallel to the same cross-wire. The upper nicol, or analyzer, must, of course, be removed during this test. This method is more convenient than taking the nicol out of its frame, in order to ascertain its plane of vibration (the direction of its shorter diagonal).
45. Although the “absorption directions” may not necessarily coincide with the principal vibration directions in Monoclinic and Triclinic crystals; still for convenience the absorption colors are usually given for the light rays vibrating parallel to these principal vibration directions.
46. In the Fuess and Seibert microscopes the analyzer or upper nicol is so fitted that it slides in and out of the tube of the microscope with its plane of vibration always at right angles to the plane of vibration of the polarizer or lower nicol.
47. Moses’ Characters of Crystals, p. 106. Moses and Parsons’ Min. Cryst. and B. P. Analysis, p. 163.
48. Iddings’ Rock Minerals, 1911, p. 172.
49. These sections always contain the principal vibration directions a and c.
50. Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 101.
51. A chart of interference colors can be obtained from Baudry et Cie, Paris, and is also published in Les Minéraux des Roches, by Lévy and Lacroix, Rock Minerals (1911), by Iddings and in Rosenbusch’s Mikroskopische Physiographie.
52. Iddings’ Rock Minerals, 1911, pp. 141, 183.
53. Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 132.
54. The ¼ undulation mica plate consists of a thin cleavage of mica on which is marked c, the vibration direction of the slower ray, which in mica is the line joining the “optic axes.” The thickness is such that the slower ray is ¼ wave-length behind the faster and the interference color is a bluish-gray. The gypsum plate is a thin cleavage of gypsum, on which is usually marked a, the vibration direction of the faster ray. The chosen thickness is such as to produce the red interference color of the 1° order.
55. The test-plates are generally introduced in the slot k, in a microscope of the Seibert type, or if a cap-nicol is used in a slot below this. In case no provision is made by the instrument maker for these test-plates, the regular analyzer is left out of the tube, and a simple nicol prism is used as an analyzer and is held by the observer over the eye-piece. Care must be taken to have the plane of vibration of this nicol at right angles to that of the polarizer, and to leave sufficient room for the introduction, by hand, of the test-plate between the eye-piece and the nicol. With care the plates can be introduced with sufficient accuracy to make the test practical.
56. A scale or chart of interference colors, or the interference color diagram, should be before the observer in order to avoid any mistakes as to whether the new color is higher or lower in the scale.
57. The quartz wedge is cut so that one of its faces is exactly parallel to the ć axis (hence also parallel to the c vibration direction) while the other face makes a very small angle with it. The direction c is marked on the wedge.
58. Described under next test.
59. In applying this rule count the 1° order white as green and the 1° order gray as blue.
60. A. J. Moses, Trans. N. Y. Acad. Sci., Vol. XVI., p. 55, Jan., 1897.
61. E. von Federow, Zeit. f. Kryst., etc., Vol. XXXV., p. 340, 1895.
62. After the first rough determination of the phase difference by the mica wedge, the more exact phase difference can be obtained by the aid of a good color chart or diagram, see end of book.
63. It is not safe to use minerals near the edge of the section, as the thicknesses are apt to be unequal.
64. See at end of appendix.
65. In this way eliminate, so far as possible, the effect of the orientation of the mineral section.
66. The different mineral sections are all supposed to have the same thickness throughout the rock section.
67. Harker’s Petrology for Students, 1895, p. 14.
68. Iddings’ Rock Minerals, pp. 153, 173. Moses’ Characters of Crystals, p. 115.
69. In the Seibert microscope use No. V objective, in Fuess microscope No. 7 objective, and in English microscopes a ¼″ or ⅕″ objective.
70. Each convergent ray will have its vibration direction either in or at 90° to the plane through the ray and the optic axis. Hence all rays vibrating parallel to the vibration planes of both nicols will be completely cut out. As the section is rotated new rays successively come into these positions, so the same effect is maintained.
71. In the Seibert microscope there is a little slot k for this purpose just above the objective.
72. The optical character may also be determined in parallel light by proving ć = c(+), ć = a(−). The optical character of the principal zone or the sign of the elongation is often given in tables. This optical character or sign is (+) when the principal zone axis or the direction of elongation is parallel to c and (−) when parallel to a.
73. Iddings’ Rock Minerals, 1911, p. 172.
74. This assumes the optic axes for different colors to emerge about at the same points. If there is marked “dispersion” the black bands and hyperbolas may be rainbow-hued, as with titanite.
75. The interference figure, perpendicular to the obtuse bisectrix, would be of the same type with a larger axial angle. Ordinarily this figure would not come within the limits of the field of view of the microscope. Confusion may arise, however, but in a section perpendicular to the acute bisectrix the cross dissolves more slowly into the hyperbolas than in the case of a section perpendicular to the obtuse bisectrix. At times it may be necessary to measure the axial angle to be sure. When, however, the mineral is known, the section perpendicular to the acute bisectrix can be recognized, because if the mineral is optically positive the trace of the axial plane is parallel to a and if negative parallel to c.
77. The wedge can be introduced in either of the several ways described for the introduction of the test-plates on p. 33.
78. For methods of measuring the axial angle, see Methods of Petrographic-Microscopic Research, F. E. Wright, 1911, p. 147.
For convenience in many cases only 2E is recorded, as then an indication is given as to whether the axial angle is visible with an ordinary microscope (arranged for observation with convergent light for interference figures). If 2E is very large the axial angle can only be observed by covering the section with some transparent, strongly refracting fluid. For the Seibert microscope with objective V the limit for good results is about 2E = 90°–100°.
79. For dispersion, etc., see A. J. Moses’ Characters of Crystals, p. 140.
80. The system of crystallization of leucite has been the subject of much discussion. Its habit is isometric. The consensus of opinion seems to be that leucite crystallizes in the isometric system, but that the isometric molecular arrangement, at least of the larger crystals, cannot exist for the temperature and pressure at the earth’s surface. Hence molecular displacement takes place, giving rise to a more or less complicated apparent twinning, and optical anomalies are noticed. The isotropic character returns if the section is heated to 500° C. Iddings’ Rock Minerals, p. 249, 1911.
81. C. W. Knight, Canad. Rec. of Sci., IX, No. 5. 265.
82. The interference colors of all minerals here recorded are those given by sections 0.03 mm. in thickness (very thin sections).
83. Shown by Mügge, Joly, etc., to be caused by radiations emanating from U, Th, R, etc. Iddings’ Rock Minerals, 1911, p. 189.
84. Iddings’ Rock Minerals, 1911, p. 392.
86. R. D. Irving, Am. Jour. Sci., June, 1883.
87. Granites of the central Alps, where the calcite crystals are intergrown with quartz.
88. The minerals Nephelite, leucite, sodalite (haüynite and noselite) and melilite are often grouped together under the name “feldspathoides”; on account of their relation in rocks being equivalent to that of the feldspars.
89. Oriented in conformity to the intergrowth with augite, etc., (010) and (100) are reversed.
90. For other alteration processes, see Iddings’ Rock Minerals, p. 380.
91. May be difficult to determine in the case of prism zone sections, showing large extinction angles.
93. Iddings’ Rock Minerals, p. 456, 1911.
94. Depending on whether the axial plane is parallel or at right angles to the clino pinacoid (010) (the plane of symmetry), we have micas of the second (biotite) or first order (muscovite).
95. The distinction between zoisite α and β (essentially orthorhombic, but may be composite triclinic twins) and clinozoisite (monoclinic close to orthorhombic) depends on differences in position of plane of optic axes; axial figures shown by cleavage plates; dispersion; anomalous interference colors; etc. See Weinschenk’s Die Gesteinbildenden Mineralien, p. 83. 1901.
96. Test not easily made on account of the very high order interference colors, resulting from the strong double refraction.
98. Iddings’ Rock Minerals, p. 208, 1911.
99. On account of the weak double refraction the interference figures are not very sharp or well defined in thin sections. In most cases only the black hyperbolas are seen, without any colored curves.
100. By heating feldspar crystals the axial angle decreases to 0° and then increases in the plane of symmetry (at right angles to its former position). On cooling the axial angle returns to its former position if the temperature has not exceeded 500° C. If the temperature has been 600°–1000° C. for some time the axial angle will not return to its former position. This fact may give some clew as to the temperature at which the feldspar crystals formed.
101. This change to kaolin or clay in granite is called by Dolomieu “La maladie du granit.”
103. Hatch’s Introduction to the Study of Petrology, p. 33.
104. J. W. Judd, Geol. Mag. [3], Vol. VI, p. 243, 1889.
105. The plagioclases have rather a complex composition; but may be regarded as forming a series from the composition NaAlSi3O8(Ab) to the composition CaAl2Si2O8(An), consisting for the most part of isomorphous mixtures of these types, with some replacement by KAlSi3O8. The compositions of only a few of the common plagioclases are given above.
106. The lath-shaped feldspars, moulding the augite, give to diabases the so-called “ophitic” structure, Fig. 12. The peculiarity of this structure is that the feldspars crystallized before the augite, which is contrary to the usual order of formation.
107. For the positions of the optic axes, bisectrices, etc., relative to the cleavage plates of the different plagioclases, see Iddings’ Rock Minerals, p. 222, 1911.
108. Die Gesteinsbildenden Mineralien (with tables), E. Weinschenk, Freiburg, 1901. Étude sur la Détermination des Feldspaths dans les Plaques Minces, Michel Lévy, Paris, 1894; and The Determination of the Feldspars, N. H. Winchell, Am. Geol., Vol. XXI, No. 1, 1898. Iddings’ Rock Minerals, Wiley & Sons, 1911. Étude sur la Détermination des Feldspaths (troisième fascicule), Michel Lévy, Paris, 1904.
109. Glass models of the feldspars (size 20 × 10 cm.) by F. Krantz, Bonn. Diagrams, showing optical orientation in the plagioclases, in Die Gesteinsbildenden Mineralien, E. Weinschenk, p. 133, 1901.
110. For this method of investigation little cleavage flakes or plates can often be obtained from the crushed mineral, but, on account of Albite twinning, plates are more apt to be obtained parallel to the twinning plane than to the best basal cleavage. If a fragment with only one cleavage surface is obtained, it must be cemented to a glass by this surface and ground down to a thin section with parallel sides.
111. This test is only possible when suitable sections of the given feldspar in the rock section can be found. The method, however, can be used with great accuracy with the aid of some form of apparatus for properly orienting the section. See “Klein’s Apparatus for the Orientation of Thin Sections,” Sitzungsber. Berlin. Akad., 1895, 1151; (also in N. Y. Acad. Sci., Vol. XVI, p. 51, 1897); and Von Federov’s “Universal Table,” Zeit. für Kryst, etc., Vol. XXV., p. 351.
112. In the determination of feldspar microlites it is well to remember the following facts: “Microcline is rarely, or never, seen in the condition of microlites, while the associations of labradorite and albite are so different that there is little danger of confounding them. Labradorite is the commonest product of the consolidation of the basic eruptives, and albite almost invariably results from metamorphism, frequently from the contact of igneous rocks on the calcareous clastics.” N. H. Winchell, Determination of the Feldspars, Am. Geol., Vol. XXI, No. 1, p. 33, 1898.