Chapter XII: Introduction

The scheme is designed to furnish the student with a practical method of
recognizing the common minerals in rock sections.

The arrangement followed has been to group minerals having general optical characters in common, at the same time giving their specific characters so as to make it possible to distinguish one from another. In each rectangle the minerals are arranged in order of their indices of refraction.

A tabulation of the minerals, with a list of optical characters appended, is of aid to the skilled investigator, but of very little assistance to the beginner.

The more common minerals, or those which are important petrographically, are printed in heavy-faced type; the minerals of less importance in small capitals.

ABBREVIATIONS AND CONVENTIONS USED.

A. = Amorphous.

I. = Isometric system.

T. = Tetragonal system.

O. = Orthorhombic system.

M. = Monoclinic system.

Tri. = Triclinic system.

H. = Hexagonal system.

M(H). = Monoclinic, with hexagonal form or characters, as in the case
of biotite.

⟂ = At right angles to.

El. = Elongation.

Ex. = Extinction.

∥ Ex. = Parallel extinction, as when the crystals extinguish parallel
to cleavage lines or crystal edges. Extinction which is symmetrical to
intersecting cleavage lines is also included under this term.

The mean refractive indices are printed in heavy-faced type.

The term “grains” is used to describe not only minerals which occur in
typically granular form, but also those which have coarser
allotriomorphic form, as elæolite and sodalite in plutonic rocks, such
as syenite, etc.

GENERAL RULES FOR USE OF SCHEME.

The division of the scheme into two vertical columns is based on the values of the mean refractive indices, as determined by the “relief” and appearance of the surface. When the refractive index is above 1.60, the relief is fairly well to distinctly marked and the surface rough to very rough, depending on the value of the index.

Most of the rock-forming minerals with indices below 1.60 show no relief and a smooth surface, except in the case of a few of the rarer minerals (mostly isometric), which have very low indices and hence rough surface.

Mistakes may easily be made in the case of minerals near the limit; but practice and the use of the Becke test should soon make possible the classification into the two groups suggested by the scheme, and the appended descriptions will help to check errors.

When an unknown mineral lies adjacent to one that is known, use the Becke test for obtaining the relative refractive index of the unknown mineral; focus sharply on the line of contact between the known and unknown minerals,[158] then raise the objective slightly and the “bright line” will appear on the side of the mineral having the higher index. The method of Schrœder van der Kolk may also be employed, see p. 22.

The horizontal divisions of the scheme depend on the relative strength of the double refraction based on the observed interference colors. These colors to be of use in classification must be correctly recognized. The lower and middle colors of the 1° order, from bluish-gray through white to yellow, are easily known. The bright red, blue, green, etc., colors of the 1°, 2° and 3° orders can also be differentiated without trouble from the very high order colors (4° and above), which are essentially white in tone with no decided color tint.

When confusion arises, the exact order of the color can be determined by a quartz wedge, as given on p. 35. Furthermore, a ¼ undulation mica plate serves to quickly distinguish between the 1° order white and the practically colorless, high order tint of calcite, titanite, etc.; as after the insertion of the test-plate the 1° order white suffers a marked change in color, while the very high order (practically white) tint shows no appreciable change.

In the determination of these interference colors care must be given to considerations of orientation and thickness. The section must give the maximum interference color of all the obtainable sections of the mineral in the rock section. Such sections will be parallel to the _ć_ axis in the uniaxial minerals and to the axial plane in the biaxial minerals; and will, therefore, in convergent light never show the emergence of an optic axis or bisectrix. Crystal “form,” cleavage, pleochroism, etc., may at times aid in the selection of these sections. The thickness of the section must also be considered, and it is well in all cases to pick out some known mineral in the section, as quartz, and note its maximum interference color. Knowing how this varies from the color given by the scheme for a section 0.03 mm.[159] in thickness, due allowance can be made for a like variation in the colors given by the other minerals. In the case of minerals with strong absorption the interference colors may not be noticeable on account of the absorption of parts of the light.

Under the subhead of pleochroism, the vibration direction of the ray of a definite color is given and not the direction of transmission of that ray.

The interference figures in convergent light increase in clearness and distinctness with the strength of the double refraction. In uniaxial crystals sections at right angles to the optic axis, _i. e._, sections which remain dark during a revolution between crossed nicols, show the best interference figures. In biaxial crystals the most characteristic interference figures are shown by sections at right angles to the acute bisectrix.

Crystal sections which are too small do not give very satisfactory interference figures with convergent light.[160]

Any scheme, however designed, makes a more or less arbitrary classification of the minerals, and when in doubt it is always safer to look for the mineral on both sides of the scheme line.

The rarer minerals are not included in this scheme, so when the determination of a mineral is uncertain or not positive recourse should be had to more elaborate tables.

INDEX.

MINERAL NAMES IN HEAVY-FACED TYPE.

Abbreviations, ix

Abnormal interference colors, 30

Absorption, 26
directions, 26
tints, 17

▄Acmite▄, 83

▄Actinolite▄, 84

Acute bisectrix, 5

▄Ægirine▄, 83

▄Ægirine-augite▄, 84

Aggregate structure, 38

▄Albite▄, 103

▄Allanite▄, 96

Allotriomorphic, 15

Aluminium, test for, by Borichy’s method, 137
test for, by Behren’s method, 139

Amorphous bodies, 2

Amorphous substances, test for, 26

▄Amphibole▄, 84

▄Analcite▄, 56

Analyzer, 10

▄Andalusite▄, 72

Anhedron, 15

Anisotropic character, test for, 27

Anisotropic crystals, 2

Anomalies, optical, 27

▄Anorthite▄, 103

▄Anorthoclase▄, 113

▄Anthophyllite▄, 88

▄Antigorite▄, 114

▄Apatite▄, 68

Apatite, recognition of, by test for phosphorus, 139

Apparatus for petrographical laboratory, 127

Appendix, 141

▄Arfvedsonite▄, 88

▄Augite▄, 80

Automorphic, 13

Axes of elasticity, 4

Axial angle, 5
determination of, 47

Axial plane, 5

▄Basaltic Hornblende▄, 85

▄Bastite▄, 77

Becke’s method for determining relative values of refractive indices,
19

Behren’s method. (Hydrofluoric and sulphuric acids), 137

Bertrand lens, 11

Biaxial crystals, 4, 5
vibration directions in, 4

Biaxial interference figures, 43

▄Biotite▄, 89

Bisectrix, acute, 5
obtuse, 5

Borichy’s method. (Hydrofluosilicic acid), 135

Broken or strained crystals, 15

▄Bronzite▄, 76

▄Calcite▄, 65

Calcium, test for, by Borichy’s method, 136
test for, by Behren’s method, 138

“Cap” nicol, 10

▄Carbonaceous Matter▄, 61

Carbonaceous particles, test for, 132

Carbonates, test for, 130

Cement, 122
for mounting, 126

Cementing, 121

Centering stage, 10

▄Chalcedony▄, 64

Characters observed by, convergent light, 40
crossed nicols, 26
reflected light, 13
polarized light, 25

Characters of opaque minerals, 13
transparent minerals, 13

Chemical and mechanical tests, 129

Chemical (micro) reactions, 134
separation, 134
tests on crystal in section, 129

▄Chiastolite▄, 72

▄Chlorite group▄, 92

▄Chromite▄, 52

▄Chrysolite▄, 77

▄Chrysotile▄, 114

Circular polarization, 3

Classification into systems by optical determinations, 141

▄Clay▄, 115

Cleaning and finishing sections, 126

Cleaning microscope, 11

Cleavage, 23

▄Clinozoisite▄, 96

Color, 17
diagram (interference), 143
fringes, 48

Condensing lens, 9

Consolidation of minerals in plutonic rocks, order of, 144

Conventions, ix

Convergent light, 40
characters observed by, 40

Convergent and parallel light, résumé of uses of, 49

▄Cordierite▄, 78

Corroded crystals, 15

▄Corundum▄, 62

Crossed nicols, characters observed by, 26

Crossed twinning, 37

Crystallites, 16

Crystallizations obtained by Behren’s method, 137
Borichy’s method, 135

Crystals bounded by planes, 13

Crystals without bounding planes, 15

Cutting, 120

Cutting and grinding machines, 117

▄Cyanite▄, 113

▄Damourite▄, 92

▄Delessite▄, 94

Diagram, showing relation between strength of double refraction,
interference colors and thickness of section, 143

▄Diallage▄, 80

Diamond saws, 119

▄Dichroite▄, 78

▄Diopside▄, 80

▄Dipyre▄, 59

Directions of elasticity, ix

Dispersion, 5, 48

▄Disthene▄, 113

▄Dolomite▄, 67

Double image, 2

Double refraction, 2, 27
estimation of strength of, 30
strength of, measured by von Federow mica wedge, 35
table (maximum), 142

Effects produced by crystals on transmitted light, 2

▄Elæolite▄, 69

Elasticity, axes of, 4
directions of, ix

Electric motor for grinding machines, 117

Electro-magnetic separation, 134

Emery, grades of, for grinding, 124

▄Enstatite▄, 75

▄Epidote▄, 94

Etched figures, 130

Extinction, 31
angles, 5, 31
tests for, 32
wavy, 31

Extraordinary ray, 3

Eye-pieces, 11

Faster and slower rays, test for vibration directions of, 33

▄Fayalite▄, 78

▄Feldspar Group▄, 98

Feldspathoids, 69

▄Fibrolite▄, 73

Finishing and cleaning sections, 126

Focusing, 11

Form of crystals, 13

Fracture, 24

▄Garnet▄, 53

Gelatinizing silica, test for, 130

Glass slides, 125

▄Glaucophane▄, 88

▄Graphite▄, 61

Gravity (specific) separation, 133

Gridiron twinning, 37

Grinding, 123

Grinding and cutting machines, 117
apparatus for sections with parallel faces, 118
plates or laps, 121

▄Gypsum▄, 80

Gypsum test-plate, 33

Hardness of grains, 132

▄Haüyne▄, 56

▄Haüynite▄, 56

Haüynite and noselite, micro-chemical distinction between, 139

Heating sections to redness, 131

Heavy solutions, 133

▄Hematite▄, 61

Hexagonal crystals, 4

High relief, 17

▄Hornblende▄, 84

▄Hyalosiderite▄, 78

▄Hydrargillite▄, 115

Hydrofluosilicic acid test, 135

▄Hypersthene▄, 75

Idiomorphic, 13

▄Idocrase▄, 60

▄Ilmenite▄, 62

Inclusions, 24

Index of refraction, ix, 17
by Becke’s method, 19
by Schrœder van der Kolk’s method, 22
by Sorby’s method, 19

Indicators for specific gravity separation, 133

Indices of refraction (mean), table of, 142

Interference color, determination of order of, 35

Interference colors, 28

Interference colors (abnormal), 30

Interference figures, 40
biaxial, 43
uniaxial, 40

Investigation of microscopic and optical characters of minerals, 13

▄Iolite▄, 78

Iron, test for, by Borichy’s method, 137
test for, by Behren’s method, 139

Isolating crystals or mineral fragments for testing, 132

Isometric crystals, 2

Isotropic character, 26, 40
crystals, 2

▄Kaolin▄, 115

▄Labradorite▄, 103

Laps for grinding, 121

▄Leucite▄, 54

Leucoxene, 62

Light, ordinary, 1
plane polarized, 1

▄Limonite▄, 51

▄Lithia Mica▄, 92

Machines for cutting and grinding, 117

Magnesium, test for, by Borichy’s method, 137
test for, by Behren’s method, 138

▄Magnetic iron ore▄, 52

▄Magnetic pyrites▄, 51

▄Magnetite▄, 52

Measuring strength of double refraction by von Federow mica wedge, 35

Mechanical and chemical tests, 129

▄Melilite▄, 61

▄Menaccanite▄, 62

Methods of preparing sections, 117

▄Mica Group▄, 89

Mica plate (quarter undulation), 33

Mica wedge, von Federow, 35

Micro-chemical reactions, 134

▄Microcline▄, 102

Microlites, 17

Microscope (petrographical), 7

Microscopic and optical characters of minerals, 51

Minerals and thickness of section, determined by table and diagram, 36

Monoclinic crystals, 4
principal vibration directions in, 5

▄Monoclinic pyroxenes▄, 80

Mounting sections, 124
solutions, 125

▄Muscovite▄, 89

▄Natrolite▄, 79

Negative optical character, biaxial, 46
uniaxial, 42

▄Nepheline▄, 69

▄Nephelite▄, 69

Nicol prism, 7

▄Nosean▄, 56

▄Noselite▄, 56

Noselite and haüynite, micro-chemical distinction between, 139

Nose-piece for microscope, 10

Objectives, 10

Oblique extinction, 31

Obtuse bisectrix, 5

▄Oligoclase▄, 103

▄Olivine▄, 77

▄Opal▄, 51

Opaque minerals, characters of, 13

Optical, anomalies, 27
character, biaxial, 46
character, uniaxial, 42
classification into systems, 141
distinctions between orthorhombic, monoclinic and triclinic sections
(perpendicular to bisectrices), 48, 141
distinctions between tetragonal and hexagonal sections (perpendicular
to optic axis), 141
and microscopic characters of minerals, 51
principal section, 3, 4
scheme, 145

Optic axes, 4

Optic axis, 3

Optics for optical mineralogy, 1

Order of consolidation of minerals in plutonic rocks, 144

Order of interference color, determination of, 35

Ordinary light, 1
ray, 3

▄Orthite▄, 96

▄Orthoclase▄, 98

Orthorhombic crystals, 4
principal vibration directions in, 4

▄Orthorhombic pyroxenes▄, 75

Parallel and convergent light, résumé of uses of, 49

Parallel extinction, 31

Parallel face sections, grinding apparatus for, 119

▄Pargasite▄, 85

▄Perofskite▄, 57

▄Perovskite▄, 57

Petrographical apparatus, 127

Petrographical microscope, 7

Phenocryst, 14

▄Phlogopite▄, 89

▄Picotite▄, 53

▄Piedmontite▄, 95

▄Plagioclases▄, 103

Plane polarized light, 1

Plates, grinding, 121

Pleochroic halos, 59

Pleochroism, 25
test for, 25

▄Pleonaste▄, 53

Polarization colors, 28

Polarized light, characters observed by, 25
plane, 1

Polarizer, 7
test for vibration plane of, 9

Polishing sections, 124

Polysynthetic twinning, 37

Positive optical character, biaxial, 46
uniaxial, 42

Potassium, test for, by Borichy’s method, 136
test for, by Behren’s method, 138

Preparation of sections, 117

Principal section, optical, 3, 4

Principal vibration directions, 3, 4

Prism, nicol, 7

Pseudomorphic structure, 39

▄Pyrite▄, 51

▄Pyrites▄, 51

▄Pyroxenes, monoclinic▄, 80

▄Pyroxenes, orthorhombic▄, 75

▄Pyrrhotite▄, 51

Quarter-undulation mica plate, 33

▄Quartz▄, 63
wedge, 34

Reflected light, characters observed by, 13

Reflector, 7

Refraction, double, 2, 27

Refraction, index of, ix, 17

Refractive indices (mean), table of, 142

Relief, 17
test for, 17, 18

Resorption border, 15

Résumé of uses of parallel and convergent light, 49

Rotating stage of microscope, 10

▄Rutile▄, 57

▄Sagenite▄, 58

▄Sanidine▄, 102

▄Saussurite▄, 96, 113

Saws, 119

▄Scapolite Group▄, 59

Scheme of classification into systems by optical determinations, 141

Scheme, optical, 145

Schiller structure, 25

Schrœder van der Kolk’s method for determining refractive indices, 22

Sections, methods of preparing, 117
thickness of rock, 124

Selenite plate, 33

Separation, by chemical means, 134
by electro-magnet, 134
by specific gravity, 133

▄Sericite▄, 92

▄Serpentine▄, 114

Shagreened surface, 17

▄Shorl▄, 70

▄Sillimanite▄, 73

Simple twinning, 37

Single image, 2

Skeleton forms, 17

Slides, glass, 125

Slower and faster rays, test for vibration directions of, 33

▄Sodalite▄, 56

▄Sodalite Group▄, 56

Sodium, test for, by Borichy’s method, 136
test for, by Behren’s method, 138

Sorby’s method for determining index of refraction, 19

Special micro-chemical tests, 139

Specific gravity separation, indicators for, 133

Sphærulitic structure, 39

▄Sphene▄, 96

▄Spinel▄, 53

Stage of microscope, rotating, 10

▄Staurolite▄, 74

Strained or broken crystals, 15

Strength of double refraction, measure of, 30, 35

Structure, 37

Symmetrical extinction, 31

Systems, classification into, by optical determinations, 141

Table, double refraction, 142

Table, indices of refraction, 142

▄Talc▄, 94

Test for vibration plane of polarizer, 9

Tetragonal crystals, 4

Thickness of rock sections, 124

Thickness of section and minerals, determined by table and diagram, 36

▄Titanite▄, 96

Transmitted light, characters observed by, 13
effect on crystals, 2

Transparent minerals, investigation of characters of, 13

▄Tremolite▄, 84

Triclinic crystals, 4
principal vibration directions in, 5

▄Tridymite▄, 65

▄Topaz▄, 74

▄Tourmaline▄, 70

Twinning, 37

Uniaxial crystals, vibration directions in, 3

Uniaxial interference figures, 40

▄Uralite▄, 88

Uralitization, 88

Uses of parallel and convergent light, 49

van der Kolk’s, Schrœder, method for determining refractive indices, 22

▄Vesuvianite▄, 60

Vibration direction, 2
directions of faster and slower rays, test for, 33
directions, principal, 3, 4
plane of nicol, 9
plane of polarizer, test for, 9

von Federow mica wedge, 35

Wavy extinction, 16, 31

Wedge, mica, 35
quartz, 34

▄Wernerite▄, 59

Xenomorphic, 15

▄Zeolites▄, 79

▄Zircon▄, 58

▄Zoisite▄, 96
α and β, 96

Zonal structure, 38

ERRATA AND ADDENDA.

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.

-----

Footnote 1:

Often called a direction of maximum elasticity, c′ being a direction
of minimum elasticity.

Footnote 2:

Rosenbusch’s _Microskopische Physiographie_, p. 156.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 6:

It is interesting to remember in this connection that in the isometric
system there is also the greatest possible symmetry of “form.”

Footnote 7:

A. J. Moses, _Characters of Crystals_, pp. 85–97.

Footnote 8:

For this branch of optical physics, see A. J. Moses, _Characters of
Crystals_, pp. 97–100.

Footnote 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.

Footnote 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.

Footnote 11:

A. J. Moses, _Characters of Crystals_, pp. 98, 99.

Footnote 12:

The terms axes of elasticity are commonly used for these principal
vibration directions in text-books on petrography.

Footnote 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.

Footnote 14:

For most cases in observations with white light the “optics axes” may
be regarded as approximately fixed in position.

Footnote 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_.

Footnote 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.

Footnote 17:

For description of the ordinary microscope, eye-pieces, objectives,
magnification, etc., see _Manipulation of the Microscope_, by Ed.
Bausch.

Footnote 18:

Text Book of Mineralogy, by E. S. Dana, 1898 Ed., p. 176.

Footnote 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.

Footnote 20:

This condensing lens must be removed when very low power objectives
are used.

Footnote 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.

Footnote 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.

Footnote 23:

See pp. 33 and 34.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 30:

Partial resorption and recrystallization may produce a border of
secondary minerals, surrounding the original crystal.

Footnote 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.

Footnote 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.

Footnote 33:

_Mem. de l’Acad._, Paris, 1767–68.

Footnote 34:

_Min. Mag._, Vol. I., p. 193; Vol. II., p. 1.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 38:

In Seibert microscope use No. V, not sufficiently marked results being
obtained with No. II.

Footnote 39:

See Viola’s diagram, _Minn. Pet. Mitt._, Vol. XIV., p. 556.

Footnote 40:

Seibert, No. II. Fuess, No. 4.

Footnote 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.

Footnote 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.

Footnote 43:

Harker’s _Petrology for Students_, p. 306.

Footnote 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).

Footnote 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.

Footnote 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.

Footnote 47:

Moses’ _Characters of Crystals_, p. 106. Moses and Parsons’ _Min.
Cryst._ and _B. P. Analysis_, p. 163.

Footnote 48:

Iddings’ _Rock Minerals_, 1911, p. 172.

Footnote 49:

These sections always contain the principal vibration directions _a_
and _c_.

Footnote 50:

_Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p.
101.

Footnote 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_.

Footnote 52:

Iddings’ _Rock Minerals_, 1911, pp. 141, 183.

Footnote 53:

_Methods of Petrographic-Microscopic Research_, F. E. Wright, 1911, p.
132.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 58:

Described under next test.

Footnote 59:

In applying this rule count the 1° order white as green and the 1°
order gray as blue.

Footnote 60:

A. J. Moses, _Trans. N. Y. Acad. Sci._, Vol. XVI., p. 55, Jan., 1897.

Footnote 61:

E. von Federow, _Zeit. f. Kryst., etc._, Vol. XXXV., p. 340, 1895.

Footnote 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.

Footnote 63:

It is not safe to use minerals near the edge of the section, as the
thicknesses are apt to be unequal.

Footnote 64:

See at end of appendix.

Footnote 65:

In this way eliminate, so far as possible, the effect of the
orientation of the mineral section.

Footnote 66:

The different mineral sections are all supposed to have the same
thickness throughout the rock section.

Footnote 67:

Harker’s _Petrology for Students_, 1895, p. 14.

Footnote 68:

Iddings’ _Rock Minerals_, pp. 153, 173. Moses’ _Characters of
Crystals_, p. 115.

Footnote 69:

In the Seibert microscope use No. V objective, in Fuess microscope No.
7 objective, and in English microscopes a ¼″ or ⅕″ objective.

Footnote 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.

Footnote 71:

In the Seibert microscope there is a little slot _k_ for this purpose
just above the objective.

Footnote 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.

Footnote 73:

Iddings’ _Rock Minerals_, 1911, p. 172.

Footnote 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.

Footnote 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.

Footnote 76:

For construction of quartz wedge, see p. 34.

Footnote 77:

The wedge can be introduced in either of the several ways described
for the introduction of the test-plates on p. 33.

Footnote 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 2_E_ 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 2_E_ 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 2_E_ = 90°–100°.

Footnote 79:

For dispersion, etc., see A. J. Moses’ _Characters of Crystals_, p.
140.

Footnote 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.

Footnote 81:

C. W. Knight, _Canad. Rec. of Sci._, IX, No. 5. 265.

Footnote 82:

The interference colors of all minerals here recorded are those given
by sections 0.03 _mm._ in thickness (very thin sections).

Footnote 83:

Shown by Mügge, Joly, etc., to be caused by radiations emanating from
U, Th, R, etc. Iddings’ _Rock Minerals_, 1911, p. 189.

Footnote 84:

Iddings’ _Rock Minerals_, 1911, p. 392.

Footnote 85:

See page 31.

Footnote 86:

R. D. Irving, _Am. Jour. Sci._, June, 1883.

Footnote 87:

Granites of the central Alps, where the calcite crystals are
intergrown with quartz.

Footnote 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.

Footnote 89:

Oriented in conformity to the intergrowth with augite, etc., (010) and
(100) are reversed.

Footnote 90:

For other alteration processes, see Iddings’ _Rock Minerals_, p. 380.

Footnote 91:

May be difficult to determine in the case of prism zone sections,
showing large extinction angles.

Footnote 92:

See p. 88.

Footnote 93:

Iddings’ _Rock Minerals_, p. 456, 1911.

Footnote 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).

Footnote 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.

Footnote 96:

Test not easily made on account of the very high order interference
colors, resulting from the strong double refraction.

Footnote 97:

See under quartz, p. 63.

Footnote 98:

Iddings’ _Rock Minerals_, p. 208, 1911.

Footnote 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.

Footnote 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.

Footnote 101:

This change to kaolin or clay in granite is called by Dolomieu “_La
maladie du granit_.”

Footnote 102:

See p. 31.

Footnote 103:

Hatch’s _Introduction to the Study of Petrology_, p. 33.

Footnote 104:

J. W. Judd, _Geol. Mag._ [3], Vol. VI, p. 243, 1889.

Footnote 105:

The plagioclases have rather a complex composition; but may be
regarded as forming a series from the composition NaAlSi_{3}O_{8}(Ab)
to the composition CaAl_{2}Si_{2}O_{8}(An), consisting for the most
part of isomorphous mixtures of these types, with some replacement by
KAlSi_{3}O_{8}. The compositions of only a few of the common
plagioclases are given above.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 113:

These methods (both 2 and 3) are often not applicable on account of
the tendency of the crystals in an effusive rock to parallel
orientation, which may be so marked that the rock section does not
show any favorable sections of the plagioclase.

Footnote 114:

These extinction angles, as well as those previously given, are those
of only a few type feldspars of definite composition. As the
composition varies through a long series, so the extinction angle
changes, one being a function of the other.

For a complete list of compositions and related extinction angles, see
Iddings’ _Rock Minerals_ and Lévy & Lacroix’s _Les Minéraux des
Roches_.

Footnote 115:

Iddings’ _Rock Minerals_, p. 228, Wiley & Sons, 1911. _Étude sur la
Détermination des Feldspaths_ (troisième fascicule), Michel Lévy,
Paris, 1904.

Footnote 116:

In quartz ω is the refractive index of the ray with vibration
direction ∥ a [that is the direction of vibration of the faster ray
(the ordinary ray)]. Hence ω is direction ∥ a and ε ∥ c. In the
feldspars; α ∥ a, γ ∥ c and β ∥ b.

Footnote 117:

_Am. Jour. Sci._, May, 1906. This method is specially useful in
detecting presence of orthoclase, when plagioclase is the dominant
feldspar.

Footnote 118:

These tests are only possible on pure and fresh material. The specific
gravity increases with the Ca % (albite 2.62, anorthite 2.75).

Footnote 119:

In clear unstriated granules, which may be distinguished from quartz
by biaxial interference figure in convergent light.

Footnote 120:

The tendency of labradorite in gabbros to twinning, after both
_Albite_ and _Pericline_ laws, is to be noted.

Footnote 121:

Werveke’s (N. J. B., 1883, II, 97) theory is that a twin lamination
may be caused by the forces producing mechanical deformations, as
movement in the magma and mountain making pressure. Such lamellæ are
characterized by the fact that their extent and course seem to depend
on fracture lines in the crystal.

Footnote 122:

The derivation from pyroxene and amphibole appears to be doubtful, see
Weinschenk’s _Gesteinsbildenden Mineralien_, 1901, p. 121.

Footnote 123:

Harker’s _Petrology for Students_, p. 63, 1895.

Footnote 124:

Sections can be obtained from Voight & Hochgesang, Göttingen; C.
Marchand, rue Censier, 16ter, Paris; W. H. Tomlinson, Swarthmore, Pa.,
and G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y.

Footnote 125:

Made by G. D. Julien, 3 Webster Terrace, New Rochelle, N. Y. Price:
Power lathes (complete), $150.00 up; Foot-treadle lathes, $90.00 up.

Footnote 126:

A Crocker-Wheeler motor of ¼ to ½ H. P. will be large enough for an
ordinary laboratory machine.

Footnote 127:

Made by Voigt and Hochgesang, Göttingen. Price, $15.00.

Footnote 128:

Saws charged with diamond dust can be obtained from Elisha T. Jenks,
Middleboro, Mass. Price in 1910, $1.35 per diametrical inch. Foreign
saws 6″ and 8″ diam. can be bought from G. D. Julien, New Rochelle, N.
Y.

Footnote 129:

_School of Mines Quarterly_, Vol. XI, p. 32.

Footnote 130:

“Half a pound or less of ordinary shellac is melted in a flat-bottomed
open vessel over a Bunsen burner. Then an equal quantity of Venice
turpentine is carefully added under constant stirring. The mass should
be allowed to boil for about ten minutes, during which the stirring is
continued. Then small quantities are poured in separate heaps on an
iron plate or other cold surface and rolled into sticks about seven
inches long and half an inch thick.”

Footnote 131:

Square or standard 26 × 46 mm. glass slides are recommended instead of
the oblong slides (1 × 3 inches), which are very apt to project beyond
the edges of the stage and be struck by the fingers while rotating the
stage.

Footnote 132:

Reference can be made to the publications on this subject by: Borichy,
Behrens (Behren’s translation by Judd), Haushofer, Huysse, Klément,
Streng, Rénard, etc.

Footnote 133:

In case an acid is to be used which would attack glass, the
cover-glass can be replaced by a thin perforated disk of platinum.

Footnote 134:

The bases in solution can be determined by different methods of
analysis, for which the student is referred to more elaborate works on
this subject. In some cases it may be very advantageous to treat the
section with acid, to remove certain soluble constituents, when other
minerals not distinctly seen at first may be made more apparent.

Footnote 135:

The gas may be H_{2}S from a soluble sulphide, in which case the
solution containing the bubbles will color filter paper moistened with
lead water.

Footnote 136:

_Geol. and Nat. History Survey of Minn._, XIX., Ann. Rept., p. 42;
also A. J. Moses, _Characters of Crystals_, p. 147.

Footnote 137:

The symmetry of the etched figures would, of course, be related to the
system of crystallization.

Footnote 138:

A. J. Moses, _Characters of Crystals_, Chap. XVI.

Footnote 139:

When it is desired to preserve the section and at the same time to
study the surface covered with a film of air, the edges alone of the
cover-glass should be cemented.

Footnote 140:

_Geol. and Nat. Hist. Survey of Minn._, XIX, Ann. Rept., p. 50. Thin
sections of 2–3 sq. mm. area are of a convenient size, and they should
be subjected to a red heat for (1½)-3 minutes. Too long a continuance
of heat may render the sections too dark or lessen their transparency
or produce melting.

Footnote 141:

This test may vary, in many cases graphite not being consumed even
after long heating.

Footnote 142:

The isolation of material for investigation is of more interest for
the lithologist or chemist than for the student of optical mineralogy;
therefore only a very brief outline of some of the methods employed
will be given.

Footnote 143:

The grains passing through different meshes are investigated
microscopically to ascertain which size grains are homogeneous; the
rest of the sample should then be reduced to grains of this size.

Footnote 144:

Further reference can be made to _Iddings’ Rock Minerals_, pp. 25 and
95, 1911. A convenient form of apparatus is also described in _School
of Mines Quarterly_, Vol. X., p. 284, 1889.

Footnote 145:

S. L. Penfield, _Am. Jour. Sci._, Vol. L., p. 446, 1895. In this
article a convenient form of separating apparatus is also described.

Footnote 146:

A convenient list of minerals, arranged in the order in which they
would be attracted by increasing the force of the electro-magnet, is
given in Weinschenk’s _Tabellen_.

Footnote 147:

For micro-chemical work the reagents should be applied in very small
drops, which spread out on glass to discs 2 mm. in diameter. For
manipulating the reagents use platinum wires, 0.5 mm. in thickness.

Footnote 148:

_Elemente einer neuen chemisch-mikroskopischen mineral- und
Gesteins-analyse_, Pragg, 1877.

Translation of above by Winchell in _Geol. and Nat. Hist. Survey of
Minn._, Vol. XIX., Ann. Report, 1890.

Footnote 149:

The strength of the solution should be about 3½%; for if too weak many
minerals do not give satisfactory results, and if too strong a very
large number of fluosilicate crystals are formed together with the
separation of much silica, thus making it impossible to carefully
differentiate the crystals with a microscope.

Footnote 150:

As most of the rock-forming minerals that would be investigated are
silicates, the hydrofluosilicic solutions can also be obtained by
treatment with HFl.

Footnote 151:

After Lévy and Lacroix.

Footnote 152:

_Naturkunde_, Amsterdam, 2, Vol. XVII., 1881. _Chemical News_, Vol.
LXIII., No. 1647, June 1891, et seq.

_Micro-chemical Analysis_, Behrens (Judd), Macmillan & Co., London,
1894.

Footnote 153:

After Lévy and Lacroix.

Footnote 154:

After Lévy and Lacroix.

Footnote 155:

Compiled from Weinschenk’s Tables, as revised in _Petrographic
Methods_ by Weinschenk-Clark, 1912.

Footnote 156:

Pirsson and Robinson, _Am. Jour. Sci._, iv, Vol. X, Oct., 1900.

Footnote 157:

Harker, _Petrology for Students_, p. 28, 1895.

Footnote 158:

Have the convergent lens or condenser lowered and the analyzer out
during this test.

Footnote 159:

The minerals are grouped according to the maximum interference colors
given by sections of the thickness of 0.03 mm.

Footnote 160:

In the case of very small crystals, the centering must be accurate and
a cover with a small hole in the center, should be placed over the top
of the tube. In this way the eye is brought directly over the axis of
the microscope and figures can be observed from very minute crystals.

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

Page Changed from Changed to

80 occurs as an alternation product occurs as an alteration product
of anhydrite. Gypsum is soluble of anhydrite. Gypsum is soluble
in hydrochloric in hydrochloric

136 longer the higher the percentage longer the higher the percentage
of calcium in the solution of sodium in the solution

● Typos fixed; non-standard spelling and dialect retained. ● The author used the old chemical symbol (Fl) for Fluorine. ● Corrected the Errata with the exception of the last paragraph "Corrections for mean indices of refraction in Scheme (insert folder)". ● Used numbers for footnotes, placing them all at the end of the last chapter. ● Enclosed italics font in _underscores_. ● Enclosed bold font in ▄lower half block▄. ● The caret (^) serves as a superscript indicator, applicable to individual characters (like 2^d) and even entire phrases (like 1^{st}). ● Subscripts are shown using an underscore (_) with curly braces { }, as in H_{2}O.