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SCIENCES

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PETROLOGY FOR STUDENTS:



AN INTRODUCTION TO THE STUDY OF ROCKS
UNDER THE MICROSCOPE.



BY
ALFRED BARKER, M.A., F.R.S., F.G.S.

FELLOW OF ST JOHN'S COLLEGE, AND

DEMONSTRATOR IN GEOLOGY (PETROLOGY) IN

THE UNIVERSITY OF CAMBRIDGE.



THIRD EDITION



CAMBRIDGE:

AT THE UNIVERSITY PRESS.
1902



First Edition, 1895.
Second Edition, 1897.
Third Edition, 1902.



HFT



CARTH

SCIENCES
. UBRARY



PREFACE TO THIRD EDITION.

r I iHE following work, now offered in a further revised
- edition, has been written to serve as a guide to the
study of rocks in thin slices, and is of course assumed to
be supplemented throughout by demonstrations on actual
specimens. Since it is designed primarily for the use of
English-speaking students, examples are chosen, so far as
is possible, from British and North American rocks ; and
a like remark applies to the numerous references to original
authorities which are inserted in foot-no tes %

No systematic account is given of the crystallographic
and optical properties of minerals. This is rendered
unnecessary by such books as Iddings' translation of
Rosenbusch's well-known work and Hatch's translation
of the same author's tables. In particular, I have
made no explicit reference to the use of convergent
light.

In view of the difficulty of representing rock-sections
adequately by means of process-blocks, I have often cited

M909018



VI PREFACE.

the coloured plates in some standard works of reference,
to which most students will have access. The figures
given on the following pages are selected chiefly to
illustrate simple structural characters, and some of them
are necessarily rather diagrammatic. A number of new
figures have been added for the present edition, and a few
of the old ones have been withdrawn.



A. H.



ST JOHN'S COLLEGE, CAMBRIDGE.
October, 1902.



CONTENTS.



CHAPTER

I. INTRODUCTION ....... 1

A. PLUTONIC ROCKS ....... 23

II. GRANITES ........ 28

III. SYENITES (INCLUDING NEPHELINE-SYENITES) 44

IV. DIORITES ........ 5V

V. GABBROS AND NORITES ... .70

VI. PERIDOTITES (INCLUDING SERPENTINE-ROCKS) 87

B. HYP ABYSSAL ROCKS ...... 102

VII. ACID INTRUSIVES ...... 103

VIII. PORPHYRIES AND PORPHYRITES . . . 118

IX. DIABASES ........ 130

X. LAMPROPHYRES ...... 141

C. VOLCANIC ROCKS ....... 151

XI. RHYOLITES ....... 154

XII. TRACHYTES AND PHONOLITES . . . .170

XIII. ANDESITES ....... 181

XIV. BASALTS ........ 194

XV. LEUCITE- AND NEPHELINE-BASALTS, ETC. . 210

D. SEDIMENTARY ROCKS ...... 222

XVI. ARENACEOUS ROCKS ..... 223

XVII. ARGILLACEOUS ROCKS ..... 237

XVIII. CALCAREOUS ROCKS ..... 248
XIX. PYROCLASTIC ROCKS ...... 271

APPENDIX TO SEDIMENTARY ROCKS 283



Vlll CONTENTS.

CHAPTER PAGE

E. METAMORPHISM 287

XX. THERMAL METAMORPHISM .... 290

XXI. DYNAMIC METAMORPHISM 315

XXII. VARIOUS CRYSTALLINE ROCKS . . . 328

INDEX . 341



REFERENCES.

Berwerth, Mikroskopische Structurbilder der Massengesteine (chromo-
lith.), Stuttgart, 1895-1900.

Cohen, Sammlung von Mikrophotographien...von Mineralien und
Gesteinen (3rd ed.), Stuttgart, 1899.

Rosenbusch-Iddings, Microscopical Physiography of the Rock-forming
Minerals (with photographic plates), 1888.

Fouque and Levy, Mineralogie micrographique (with atlas of coloured
plates), 1879.

Teall, British Petrography (with numerous coloured plates), 1888.

Rosen busch-Hatch, Petrographical Tables.

Cole, Studies in Microscopical Science (coloured plates), 1882-3.

Watts, Guide Guide to the Collections of Rocks and Fossils be-
longing to the Geological Survey of Ireland^ Dublin, 1895.



ABBREVIATIONS.

G. M. = Geological Magazine.

M.M. = Mineralogical Magazine.

Q.J.G.S. = Quarterly Journal of Geological Society.

A. J.S. = American Journal of Science.



CHAPTER I.

INTRODUCTION.

IN this chapter will be included some notes on the optical
properties of minerals, which may be of use to a novice ; but
there will be no attempt to supersede the use of books dealing
systematically with the subject.

Microscope. We shall assume the use of a microscope
specially adapted for petrological work, and therefore fitted
with polarizing and analysing prisms, rotating stage with
graduated circle and index, and ' cross- wires ' of spider's web
properly adjusted in the focus of the eye-piece. The sub-stage
mirrors attached to such instruments usually have a flat and
a concave face. With day-light the flat face should be used ;
with artificial light things should be so arranged that the
mirror, used with the concave face, gives as nearly parallel
rays as possible.

A double nose-piece, to carry two objectives, is very useful,
although it usually gives very imperfect centring for high
powers. The most useful objectives are a 1 inch or Ij inch
and a J inch, but for many purposes a |- inch is also very
desirable. For minute objects, such as the * crystallites ' in
glassy rocks and the fluid-pores in crystals, a high power is
indispensable, and for very fine-textured sedimentary rocks an
immersion-lens offers great advantages.

A selenite-plate, a quartz- wedge, and other special pieces of
apparatus will be of use for various purposes. The methods

H. P. 1



2 FORM OF SECTION : MEASUREMENT OF ANGLES.

involving their use may be found in the mineralogical text-
books ; where too the student will find guidance as to the
examination of crystal-slices by convergent light.

Form of section of a crystal and cleavage-traces.

A well-formed crystal gives in a thin slice a polygonal section,
the nature of which depends not only upon the forms present
on the crystal, but also on the direction of the section and on
its position in the crystal, as, e.g. whether it cuts through
the centre or only truncates an edge or corner. Again, the
same shape of section may be obtained from very different
crystals. Nevertheless, if several crystals of one mineral are
present in a rock-slice, we can by comparison of the several
polygonal sections obtain a good idea of the kind of crystal
which they represent. Further, if by optical or other means
we can determine approximately the crystallographic direction
in which a particular crystal is cut, we can usually ascertain
what faces are represented by the several sides of the polygon.

For this purpose we may require to measure the angle at
which two sides meet, and this is easily done with a microscope
provided with a rotating stage and graduated circle. Bring
the angle to the intersection of the cross-wires, adjust one of
the two sides to coincide with one of the cross-wires, and read
the figure at the index of the circle. Then rotate until the
other side is brought to coincide with the same cross-wire, and
read the new figure. The angle turned through is the angle
between the two sides of the section.

This angle is the same as that between the corresponding
faces of the crystal only provided the plane of section cuts
these two faces perpendicularly. For a section nearly perpen-
dicular to the two faces, however, the error will not be great.

In consequence of the mechanical forces which affect
rock-masses, and also as a result of the process of grinding
rock-slices, the minerals often become more or less fractured or
even shattered. In a strictly homogeneous substance the
resulting cracks are irregular, but if there be directions of
minimum cohesion in crystals (cleavage), the cracks will tend
to follow such directions, and will appear in a thin slice as
fine parallel lines representing the traces of the cleavage-planes



CLEAVAGE-TRACES : TRANSPARENCY. )

on the plane of section. The regularity and continuity of the
cracks give an indication of the degree of perfection of the
cleavage-structure, but it must also be borne in mind that a
cleavage making only a small angle with the plane of section
will, as a rule, not be shewn in a slice.

In the case of a mineral like augite or hornblende, with
two directions of perfect cleavage, the angle which the two
sets of planes make with one another is, of course, a specific
character of the mineral, or at least characteristic of a group
of minerals, such as the pyroxenes or the amphiboles. In a
slice perpendicular to both the cleavages the traces will shew
the true angle ; for any other direction of section the angle
between the cleavage-traces will be different, but it will not
vary greatly for slices nearly perpendicular to both the
cleavages, and will often suffice for discrimination, as for
instance between the 87 of the pyroxenes and the f>f>J of
the amphiboles. In a slice parallel to the intersection of the
two cleavages the two sets of cleavage-traces reduce to one,
and a slice of a mineral such as augite or hornblende which
exhibits but one set of cleavage-traces may be assumed to be
nearly parallel to the intersection of the cleavages.

A mineral not possessing any good cleavage often shews
irregular cracks in rock-slices (e.g. quartz and usually olivine).
This is especially the case in brittle minerals.

Transparency, colours, and refractive indices of

minerals. Only a few rock-forming minerals remain opaque
even in the thinnest slices : such are graphite, magnetite,
pyrites, and pyrrhotite ; usually hematite, ilmenite, limonite,
and kaolin ; sometimes chromite or picotite. These should
always be examined in reflected light ; the lustre and colour,
combined with the forms of the sections and sometimes the
evidence of cleavage, will usually suffice to identify any of
these minerals. The great majority of rock-forming minerals
become transparent in thin slices. Those which seen in hand-
specimens of rocks appear opaque, are often strongly coloured
in slices, while those which in hand-specimens shew colours
are frequently colourless in thin slices. In the case of many
minerals these 'absorption-tints' are thoroughly characteristic,

12



4 REFRACTIVE INDEX.

but still more so are the differences of colour (pleochroism) in
one and the same crystal according to the direction of the slice
and the direction of vibration of a polarized beam traversing
it, as noticed below.

The colours ascribed to minerals in the following pages and
the epithet ' colourless ' apply to thin slices of the minerals.

Apart from colour, the aspect of a mineral as seen in thin
slices by natural light varies greatly according to its refractive
index 1 , and it is of great importance for the student to learn
to appreciate at a glance the effects due to a high or a low
refractive index.

If a thin slice of a single crystal be mounted by itself in
some medium of the same colour and refractive index as the
crystal, its boundaries and surface-characters will be invisible,
while its internal structure may be studied to the best ad-
vantage. Quartz mounted in Canada balsam (both colourless
and of very nearly the same refractive index) is almost invisible.
If olivine, a colourless mineral of much higher refractive index,
be mounted in balsam, its boundaries and the slight roughness
of its polished surface will be very apparent 2 . In ordinary
rock-slices, mounted in balsam, a roughened or ' shagreened '
appearance may be taken as the mark of a mineral having a
refractive index considerably higher than that of the medium
used.

Again, a highly refringent mineral . surrounded in the slice
by others less highly refringent is seen to be more strongly
illuminated than these, and this brightness is made more
conspicuous by a dark boundary which is deeper in proportion
to the difference in refractive index between the mineral in
question and its surroundings. For these reasons a highly
refringent crystal seems to stand out in relief against the rest
of the slice (fig. 1, ).

1 By this must be understood its mean refractive index. A crystal of
any system other than the regular has in any section two refractive
indices, the magnitudes of which depend further upon the direction of
the section ; but these differences in any one mineral are usually small as
compared with the differences between the mean indices in different
minerals.

2 Cohen (3), pi. XTATTII, compare figs. 1 and 2,



REFRACTIVE INDEX.



Such considerations must be borne in mind in examining
the minute inclusions in which many crystals abound. These
inclusions may be of gas, of liquid (usually with a gaseous
bubble), of glass, or a crystal of some other mineral ; and these
may be distinguished by observing that the depth of the dark
border depends upon the difference in refractive index between




FIG. 1. VARIOUS MICROSCOPIC INCLUSIONS, HIGHLY MAGNIFIED.
a. Gas-pores ; in obsidian. b. Fluid-pores with bubbles ; in
quartz. c. Fluid-pore with bubble and cube of salt; in quartz.
d. Fluid-cavity in form of 'negative crystal,' containing two fluids and
bubble ; in quartz. e. Fluid-cavities in form of ' negative crystals,'
with bubbles ; in quartz. /. Glass-inclusions in form of ' negative
crystals,' with bubbles; in quartz. g. Schiller-inclusions consisting
of three sets of flat ' negative crystals ' filled with opaque iron-oxide ; in
felspar. //.. Hchiller-inclusions consisting of ' negative crystals ' partly
occupied by a dendritic growth of iron-oxide ; in olivine. k. Zircon-
crystal enclosed in quartz and itself enclosing an apatite- needle.

the enclosing and the enclosed substance 1 (fig. 1). The most
strongly marked border is seen when a gaseous is enclosed by
a solid substance (a). A liquid-inclusion in a crystal has a

1 For figures of various inclusions in crystals see Cohen (3), pi. vm
xin ; Rosenbusch-Iddings, pi. vi, vn ; Sorby, Q. J. G. S. (1858) xiv,
pi. xvi xix ; Ward, ibid. (1875) xxxi, pi. xxx.



6 TABLE OF REFRACTIVE INDICES.

less marked boundary, but a bubble of vapour in the liquid is
strongly accentuated (b e). A glass-inclusion is still less
strongly marked off from its enclosing crystal, while a gas-
bubble contained in it shews a very deep black border (/).

When two minerals (or a mineral and Canada balsam) are
in contact with one another in a thin slice in such a position
that their surface of junction is cut approximately at right
angles by the plane of section, it is easy to determine which
of the two has the higher refractive index. For this purpose
the illumination should be limited by a diaphragm placed
below the stage, and a high-power objective focused upon the
line of junction at the upper surface of the slice. This line is
then seen to be bordered by a narrow bright band on the side
of the more highly refringent mineral and a narrow dark band
on the other side. If the objective be depressed until the
lower surface of the slice is in focus, these appearances are
reversed.

The refractive indices of the several rock-forming minerals
may be found in the tables or books of reference, but the
student will find it useful to carry in his mind such a list as
that given below.

Refractive indices of the common rock-forming minerals.

Very low (1*43 1'51) : tridymite, sodalite, analcime and
most other zeolites, (volcanic glasses), leucite.

Low (1'52 1'63) : felspars, nepheline, quartz, (Canada balsam),
micas, calcite, dolomite, wollastonite, actinolite, melilite.

Moderate (1/63 1'645) : apatite, tourmaline, andalusite, horn-
blende.

High (1/68 1'8) : olivine, sillimanite, pyroxenes, zoisite,
idocrase, epidote, garnets.

Very high (1'9 1'95) : sphene, zircon.

Extremely high (2'0 2' 7) : chromite, rutile.

Extinction between crossed nicols. When the
polarizing and analysing Nicol's prisms are used together,
with their planes of vibration at right angles to one another



AXES OF EXTINCTION. 7

('crossed nicols') 1 , if no object be interposed, there is total
darkness (' extinction '), and the same is the case when a slice
of any vitreous substance, such as obsidian, is placed on the
stage. If, however, a slice of a crystal of any system other
than the regular is interposed, there is in general more or less
illumination transmitted, and often bright colours. On ro-
tating the stage 2 carrying the object, it is found that extinction
takes place for four positions during a complete rotation, these
being at intervals of a right angle. In other words, there are
two axes of extinction at right angles to one another and the
slice remains dark only while these axes are parallel to the
planes of vibration of the nicols, which are indicated by the
cross-wires in the eye-piece. If we rotate the slice into a
position of extinction and then remove the nicols, the cross-
wires will mark the axes of extinction in the crystal-slice.

Without attempting to deal fully with this branch of
physical optics 3 , we may remark that all the optical properties
of a crystal are related to three straight lines conceived as
drawn within the crystal at right angles to one another (the
axes of optic elasticity) and to a certain ellipsoid having these
three straight lines for axes (the ellipsoid of optic elasticity).
The positions of the three axes may vary in different minerals,
but they must always conform to the symmetry proper to the
system, and the same is true of the relative lengths of the
axes of the ellipsoid. The plane of section of any slice cuts
the ellipsoid in an ellipse, the form and position of which
depend upon the direction of the section (ellipse of optic elas-
ticity), and the axes of extinction are the axes of this ellipse.

In certain cases the ellipse of optic elasticity may be a

1 In using the two Nicol's prisms, it should always be ascertained that
they are crossed. For this purpose the rotating prisms are usually
provided with catches in the proper positions, but the true test is total
darkness when no object is interposed.

2 In some microscopes, such as that devised by Mr A. Dick, the stage
is fixed, and the two nicols rotate, retaining their relative position, an
arrangement with several advantages. We shall assume for distinctness
that the stage is made to rotate, as in the most usual models.

3 The student is referred for this to such a book as Kosenbusch
(transl. Iddings), Microscopical Physiography of the Rock-makiny Minerals
(1888), London.



8 STRAIGHT AND OBLIQUE EXTINCTION.

circle. For this any diameter is an axis, and accordingly we
find that such a slice gives extinction throughout the complete
rotation. In crystals of the triclinic, monoclinic, and rhombic
systems there are two directions of section which give this
result. They are perpendicular respectively to two straight
lines in the crystal (the optic a,res), which lie in the plane of
two of the axes of optic elasticity, and are symmetrically
disposed towards them. In crystals of the tetragonal and
rhombohedral systems the two optic axes coincide with one
another and with the unique crystallographic axis, and only
slices perpendicular to this give total darkness. In the
regular system, the ellipsoid being a sphere, the ellipse is
always a circle, and all slices give total darkness between
crossed nicols.

Crystals of the regular system are spoken of as singly
refracting or optically isotropic, and their optical properties 1
are similar to those of a glassy or colloid substance. Crystals
of the other systems are doubly refracting or birefringent, and
they are divided into uniaxial or biaxial according as they
have one or two optic axes.

It is evident that the chance of a slice cut at random from
a birefringent crystal being perpendicular to an optic axis is
very small. If more than one crystal of a given mineral be
present in a rock-slice, and all remain perfectly dark between
crossed nicols throughout a rotation, it is a safe conclusion
that the mineral is a singly refracting one.

Straight and oblique extinction. By bearing in
mind that the ellipsoid of optic elasticity, and consequently
all the optical properties of a crystal, must conform to the
laws of symmetry proper to the crystal-system of the mineral,
we can foresee all the important points as regards the position
of the axes of extinction in crystals of the different systems
cut in various directions. For instance, a longitudinal section
of a prism of apatite (a hexagonal mineral) will extinguish
when its length is parallel to either of the cross-wires : this is
straight extinction. A longitudinal section of a prism of

1 That is, such of them as we are here concerned with.



MEASUREMENT OF EXTINCTION-ANGLES.

albite (a triclinic mineral) will, on the other hand, have axes
of extinction inclined at some angle to its length : this is
oblique extinction. It is to be noticed that these terms have
no meaning unless it is stated or clearly understood from what
direction in the crystal the obliquity is reckoned. In these
examples we reckoned with reference to one of the crystallo-
graphic axes defined by the traces of known crystal-faces.
Another character often utilised is the cleavage. Thus in a
monoclinic mineral with prismatic cleavages, such as horn-
blende, we select a crystal so cut that the two cleavages give
only one set of parallel traces. These traces are then parallel
to one of the crystallographic axes (the vertical axis), and we
examine the position of extinction with reference to this.
First we bring the cleavage-traces parallel to one of the
cross-wires, removing if necessary for this purpose one or both
of the nicols, and note the figure indicated on the graduated
circle. Then, with crossed nicols, we rotate until the crystal
becomes dark, and again note the figure. The angle through
which we have turned is the extinction-angle. Observe that
if a rotation through, say, 15 in one direction gives extinction,
a rotation through 75 in the opposite direction would have
given the same. For most purposes we do not need to
distinguish between the two directions of rotation, but take
merely the smaller of the two angles.

To obtain a measurement of use in identifying a mineral
we require more than the above. Slices of a crystal of
hornblende cut in various directions along the vertical axis
will give different extinction-angles, from zero (straight
extinction) in a section parallel to the orthopinacoid to a
maximum value in a certain other section. This maximum
extinction-angle is a character of specific value, being the
angle between the vertical crystallographic axis and the
nearest axis of optic elasticity. We may determine it with
sufficient accuracy for most purposes by noting the ex-
tinction-angles in two or three vertical sections of the same
mineral in a rock-slice and taking the largest value obtained 1 .

1 On the relation between this maximum extinction-angle and the
extinction-angle measured in a cleavage-flake of hornblende or augite,
see M. M. (1893) x, 239, 240 ; and Daly, Proc. Amer. Acad. Arts and Sci.
(1899) xxxiv, 311323.



10 CRYSTALLOGRAPHIC SYSTEMS : TWINNING.

By attention to the following points it is in most cases
possible to refer to its crystal-system an unknown mineral of
which several sections are presented in a rock-slice :
Regular system : singly refracting ; all slices extinguish com-
pletely between crossed nicols, as in glassy substances.
Tetragonal and Rhombohedral (including Hexagonal) : bire-
fringent and uniaxial ; straight extinction for longitudinal
sections of crystals with prismatic habit and for any
sections of crystals with tabular habit. The two systems
cannot be distinguished from one another by optical tests,
but in cross-sections of prisms the crystal outline or
cleavages will usually suffice to discriminate.
Rhombic (this and the remaining systems birefringent and
biaxial) : straight extinction for longitudinal sections of
crystals with prismatic habit ; sections perpendicular to the
vertical axis have axes of extinction parallel to pinacoidal
faces or cleavages and bisecting the angles between the
traces of prism-faces or prismatic cleavages. A section
nearly parallel to the vertical axis will give nearly straight
extinction, except in minerals which have a wide angle
between the optic axes.

Monoclinic : two important types may be noticed according as
the intersection of the chief cleavages (and direction of
elongation of the crystals) lies in or perpendicular to the
plane of symmetry. In the former case longitudinal



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