Louis V. (Louis Valentine) Pirsson.

Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online

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Online LibraryLouis V. (Louis Valentine) PirssonRocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools → online text (page 30 of 35)
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clear that the gneiss in such a case was originally a con-
glomerate, whose finer material has been metamorphosed,
leaving the larger pebbles mostly unchanged, save in

Color. The color of gneisses is too variable a feature to
be of any value as a special character. It depends on the
color of the quartz and feldspar, and on the relation of these
to the amount of biotite, or other dark colored minerals,
they may contain. Also, in gneisses of sedimentary origin,
carbonaceous material may be present and in the form of
graphitic material color the rock very dark. Hence we
find them from almost white passing through light shades
of red or gray into darker ones, into brown and green,
and even black.

Chemical Composition. As the sources of the material
from which gneisses have been made are varied, so do we


find great variability in their chemical composition, BO
much so that this character cannot be relied upon as
having any special value as one of their definite features.
Since they are composed of quartz and feldspar in
notable amount they must contain silica, alumina and
alkalies, and they usually have also more or less iron and
lime, but these oxides may vary within wide bounds, as
may be seen from the following table of analyses of a
few typical gneisses.

Si0 2

A1 2 O 3

Fe 2 O 3





K 2 O

H 2 O









5 6



?, 4


3 3

1 9













5 3



3 3

1 6

1 4


1 7

100 1





191 fi


3 3

3 fi

3 R

1 6


100 9

I, Granite-gneiss, Lincoln, Vermont; II, Garnet-biotite-gneiss,
Fort Ann, Washington Co., N. Y. ; III, Gneiss, fine grained, Great
Falls of Potomac River, Md. ; IV, Schistose gneiss, Marquette
region, Michigan. V, Plagioclase-Gneiss, Mokelumne River, Cali-
fornia; VI, Gneiss (Kinzigite), Schenkenzell, Black Forest, Baden.

On the other hand, as stated in the introduction to
metamorphic rocks, the analyses may suggest a clue to
the origin of the material. Thus analyses III, IV, and VI
above are quite unlike those of any igneous rock, and are
almost certainly of material of sedimentary origin, while
the others may be of igneous derivation.

Varieties of Gneiss. A very great number of varieties
of gneiss have been distinguished by geologists and
petrographers. These have been based, partly on dif-
ferences in texture, such as " banded-gneiss," " lenticular-
gneiss," " augen-gneiss," etc., partly on the presence of
some characteristic or unusual mineral, such as " biotite-
gneiss," " hornblende-gneiss/' " epidote-gneiss," etc., and


partly on general composition, such as " granite-gneiss,"
" diorite-gneiss," etc. In the latter case the term, as
explained in the introductory paragraph, is used in the
sense of a general textural modifier. It would not be
suitable in a work of this kind to give a description of all
these varieties, but a few of the most prominent may be
mentioned. The textural modifications have been already
sufficiently considered under the heading of texture.

Of mineralogic varieties, by common gneiss, or " gneiss " for short,
mica-gneiss is meant. If further distinction is required, the kind of
mica present may be stated, and we thus have biotite-gneiss, mus-
covite-gneiss, or biotite-muscovite-gneiss. If the mica is accompanied
or replaced by some prominent mineral, as is often the case, other
varieties are formed, such as hornblende-gneiss, epidote-gneiss, tour-
maline-gneiss, garnet-biotite-gneiss, etc. The different prominent
minerals, which may thus take part, have been already described
under composition. Of the varieties based on general composition,
it may be said that all of the different varieties of coarser-grained,
feldspathic, igneous rocks may occur with pronounced gneissoid
texture. In accordance with this we have granite-gneiss by far
the most common variety syenite-gneiss, diorite-gneiss, and even
gabbro- and anorthosite-gneisses. Sometimes this texture has been
imposed upon the igneous rocks after they had solidified, by intense
pressure and shearing, and sometimes while they were still soft, pasty
and crystallizing, by forced differential flowage, due to various causes.

Inclusions in Gneiss. It is very common to find
inclusions, or smaller rock masses, embedded in gneiss,
which differ in a marked degree in mineral composition,
texture, etc., from the main rock body which encloses
them. Thus lenticular masses of quartz frequently occur,
and of very variable size. They may be the remains of a
quartz pebble of a conglomerate, as explained under
texture, or they may have been deposited from solution
in some lenticular cavity, opened in the folding of the rock
masses. This case may sometimes be detected, in that
the quartz mass tends to possess a comb structure, being
composed of an aggregate of crystals whose prism direc-
tions are set perpendicular to the wall of the cavity.



In many gneisses irregular spots, streaks, and lines of
pegmatite occur, similar to those in granite. In addition
to the quartz, feldspar and mica, they often contain the
accessory minerals seen in granite-pegmatite dikes, such
as tourmaline, apatite, beryl, garnet, topaz, etc. In the
latter case they probably represent the remains of former
granite-pegmatite dikes, which have been folded up or
squeezed out in dynamic metamorphic processes, but
not all of the pegmatitic modifications seen in gneiss are
to be certainly ascribed to such an origin, for they may
have been produced by secretions from later solutions of
heated waters moving through the rock mass. The
beautiful crystals of orthoclase, of the varieties called
adular and moonstone, occurring in some gneisses, have
been probably produced in this way.

Also there are frequently seen in gneiss, spots, streaks
and irregularly curved and winding ribbons of white or
pink felsite, or fine-grained granite similar to the aplite of
granites. These may be former aplite dikes folded up,
or later granitic intrusions or secretions from heated solu-
tions. They are sometimes seen in the most complicated
systems of network passing through the rock, and
they may not have any definite wall against the gneiss,
as is the case with regular aplite dikes. By their fold-
ings, faultings, and contortions, they often show very
clearly the movements which the general rock body has

Included masses of other kinds are also frequently met
with in gneiss. Thus the streaks and smears, produced by
aggregates of the dark-colored or ferromagnesian minerals,
such as areseen in granites and are described as "schlieren,"
are found in gneiss, and may have a similar origin. Also,
irregular masses, strips, and lenticular bodies of other
schists occur, which, if the gneiss has been derived from a
former mass of igneous rock, may have been included or
enveloped fragments of the stratified beds, into which it
was intruded.


While the study of thin sections under the microscope is often of
great assistance to the field study of a gneiss, in the endeavor to
ascertain its origin and to thus understand better its relation to other
rocks, it is by no means always necessary. Very much may be done
by careful observation in the field of all the facts ascertainable, and
by the thoughtful correlation of these facts with one another.
From place to place the rock should be minutely studied with the
lens and any change in mineralogical composition or texture noted.
The following embody some of the chief points which should be looked
for, to distinguish rocks originally igneous from those of sedimentary
origin. The igneous ones are more apt to have a uniform composi-
tion and texture over large areas. The region of the contact with
other rocks should be carefully observed, to see if there are any
remains of a former endomorphic contact visible, such as a diminish-
ing of grain, or the assumption of a porphyritic texture, as well as
the appearance of pneumatolytic minerals, of which tourmaline
may be cited as a specially important example. The remains of
former aplite dikes and pegmatite veins, as described above, should
also be noted in this connection. The enveloping or bordering rocks
should be carefully studied to see if, by change in mineral composition,
in texture, and in the presence of tourmaline, or other pneumato-
lytic minerals, any remains of a former aureole of contact metamor-
phism may be discovered. The character of the plane of contact
of the gneiss and its neighboring rocks should be examined to see,
if possible, whether they are interwoven, as contorted interlaminated
beds might be expected to be, or whether the gneiss cuts directly
across them. In fact all of the field characters indicative of intru-
sion, which have been described under granite, should be looked
for, under the veil which metamorphism has cast over the region
under study. They may, of course, have been entirely obliterated,
but some of them may persist and be valuable indicators.

In sedimentary gneisses, on the other hand, more rapid changes,
from place to place, in composition and texture may be looked for,
both on a large and on a minute scale. The remains of former
pebbles, or small, lenticular masses of different composition indicative
of them, should be sought for. The presence of carbonaceous matter,
or graphite, diffused through the rock, or collected in spots or streaks,
is also of use in indicating this origin. The absence of any of the
signs of intrusion, and the character of the contact, as mentioned
above, may also be of value in this connection. Not too much stress
must be placed on the mere presence of felsitic and pegmatitic
veins or dikes, as these may have been injected into sedimentary
rocks, as well as into igneous ones. Their character, number, dis-
position, and contact wall must also all be considered in relation to
the rock mass they accompany.


If, to the facts observed in the field, a chemical analysis of a well
selected specimen, or series of specimens, of the gneiss can be added,
this may prove in addition of great value. This has been commented
on elsewhere and need not be repeated.

When all is said and done, however, it must always be remem-
bered, as Rosenbusch, the great German petrologist, has well said,
" there is no formula by which the derivation of a gneiss may
be invariably determined." It must not be done on any one
character alone, but all must be taken into account and relatively
balanced, and even when this is done, it is impossible in many cases
to say if the gneiss has been derived directly from an igneous rock,
or whether the material of the latter may not have passed through
an intermediate sedimentary stage.

General Properties and Uses of Gneiss. Those gneisses,
which under the action of metamorphic agencies have
been thoroughly recrystallized, form solid and massive
rocks, whose general properties closely resemble the
massive igneous ones. Thus granitic gneiss closely resem-
bles granite, and is used in the same manner for building
and structural purposes. But often gneiss contains so
much mica, that it has too easy a cleavage to be of much
value. In general a gneiss should be so placed, that the
plane of chief fracture lies in the mortar bed with the cross
fracture exposed; otherwise it is liable, like some sedi-
mentary rocks, to split and scale badly. Those gneisses
which have assumed their texture under conditions of dry
crushing and shearing are very tender and friable rocks,
which fall to pieces readily under the blow of the hammer,
and are of little value. The granite-gneiss of portions of
the Alps, and the anorthosite-gneiss of parts of the Adi-
rondacks, are examples of this. The jointing, erosion
forms, etc., of granite-gneiss are similar, in general, to what
is stated under granite. So also is the weathering, and
gneisses form fertile sandy soils, which pass into loamy
ones, as the decay of the feldspar and its alteration into
kaolin becomes more complete.

Occurrence of Gneiss. Gneiss, especially common or
mica-gneiss, is one of the most common and widely dis-


tributed of rocks. The occurrence of the metamorphic
rocks in general has been already commented on, and it
was stated that they are found in mountain regions and
in those areas where the sedimentary beds have been
eroded, as a basement upon which these later rocks rest.
In such places common gneiss is usually the most promi-
nent rock. . Owing to this, it is spoken of by many geolo-
gists as " the basal gneiss," or " fundamental gneiss," and
as, in many places, it is clearly the oldest rock of which we
have any knowledge, some believe that they see in it the
primitive crust of the earth. The Archaean, as it is now
used as a division of geologic time, is almost entirely com-
posed of gneiss, and to attempt to mention all the localities
of the rock, would be practically equivalent to a descrip-
tion of the occurrence of the Archaean. Gneisses are not
of course restricted to the Archaean; they occur in later
formations, into the Mesozoic. Gneisses are found all
over New England, and southward along the Piedmont
plateau into Georgia; in the Adirondacks; in the Rocky
Mountains' region, the Sierra, and other places in the
United States; they cover large parts of eastern Canada
and are prominent in Scotland, Norway and Sweden,
Finland, parts of Germany, and in the Alps. In all of these
regions different varieties, such as hornblende-gneiss,
occur associated with the common kind.

Granulite. Associated with gneisses in a number of localities is a
schistose, to thin schistose, rock composed almost wholly of quartz
and feldspar. It is nearly, or wholly, free from mica, and is usually
of fine to dense grain, so that, except for its schistose character and
place of occurrence, it is much like an igneous felsite or aplite. It is
apt to carry minute red garnets, and sometimes small quantities of
other minerals, such as cyanite, tourmaline, or hornblende, can be
detected with the lens. Chemically, it is similar in composition to
somefelsites orthe aplite varietyof granite, and it probably represents
in general former igneous rock of this nature which has been in-
volved in the metamorphic processes. Such granulites occur in
Saxony and other places in Germany, where they were first studied ;
in Sweden, Finland, Austria, etc., in Europe; in New England and in
the Adirondack region of New York.



Mica-schist is a rock which is closely related on the one
hand to gneiss, and on the other to quartzite. It is not
only a very common companion of gneisses, in regions of
metamorphic rocks, but in many places gneiss grades into
mica-schist, so that no definite line can be drawn between
them. It has also many other analogies with gneiss, some
of which will be presently mentioned. Of that great class
of rocks known as schists, it is, excluding gneiss, if the
latter be reckoned among them, the most widely dis-
tributed and important.

Composition Minerals and Texture. The essential
minerals of mica-schist are quartz and mica, and it is es-
pecially the latter which gives the rock its particular char-
acter. Different varieties of mica occur; the most common
is a silvery white muscovite; biotite of a dark color is com-
mon, while the soda-bearing mica paragonite is rare.
Muscovite and biotite occur alone, and also in combination,
as in gneiss. The micas are in irregular leaves or tablets,
without crystal boundaries, or in leafy or foliated aggre-
gates; biotite and muscovite are found intergrown, and
often so that they have a common cleavage. The micas
lie with their cleavage planes in the direction of schistosity,
and it is this which produces the extraordinary fissile
character of the rock. They are also very often curved,
bent, or twisted, as may be easily seen by the reflections
from their cleavage surface. The cleavage of the mica is
so marked that the surface of chief fracture, or the schistose
plane of the rock, appears completely coated by it, and it
may produce the impression that it is the only mineral
present; to see the quartz, the other essential component,
the cross fracture should be examined with the lens. The
quartz forms irregular grains, or aggregates of grains,
and these are sometimes arranged in small lenses, and
sometimes in thin layers, concordant with the layers of


Mica-schists, while they are very often composed of
these two minerals alone, also very commonly carry crys-
tals, often of large size, of other minerals. The most
common of these is a dark red garnet, sometimes sparsely,
but generally thickly, sprinkled through the rock, and
varying in size from that of coarse shot to that of a plum.
These garnets are often in the form of simple, rounded
nodules, but in most cases they show more or less distinct
crystal form, and sometimes they are beautifully crystal-
lized in the shapes mentioned in the description of this
mineral. This garnetiferous variety of mica-schist is a
very common metamorphic rock; in New England it is
widely distributed among the bowlders of the glacial drift.

Other minerals which occur in mica-schist, in a manner
similar to garnet, are staurolite, often with garnet, cyanite,
epidote, andalusite, and hornblende. These sometimes
are in large and well-formed crystals, which, especially
staurolite, andalusite, and cyanite, are not infrequently
colored dark, by included carbonaceous matter. Graphite
occurs in some mica-schists in quantity sufficient to pro-
duce a distinct variety. Graphite is such a strong coloring
matter, that a relatively small amount will cause the rock
to appear as if almost entirely composed of it; in conse-
quence unsuccessful attempts have been made in places
to exploit such schists for graphite.

Hornblende, when it occurs, is in dark-colored prisms;
by its increase in amount transitions into amphibolite or
hornblende schist are formed.

Cyanite, andalusite, and staurolite occur in prismatic
crystals, which may attain a length of several inches.
Their formation is contemporaneous with the metamor-
phism of the rock, and they produce a pseudo-porphyritic
texture as previously explained on page 342. Another
variety of mica-schist is one which contains more or less
calcite mingled with the quartz; it is readily detected by
its effervescence with acids. This variety is especially
apt to contain accessory garnet, epidote, hornblende, etc.



The parallel texture of the rock is its especial feature,
and its ready fissility is produced by the mica. If the
components are in thin, parallel layers, the surface of rock
cleavage is smooth and flat; if the lenticular arrangement
of the quartz is prominent, the surface is uneven or lumpy.
Frequently the surfaces of schistosity are bent, folded
and crumpled, showing pressures and shearing secondary
to its production.

Chemical Composition. As in the gneisses, the chemical
composition of these rocks is too variable a feature to be
of specific value. This comes from the natural variability
in the composition of the sediments from which they are
formed. In addition, not many of these rocks have been
chemically investigated, and some of the older analyses
have been very poorly executed. It is clear, however,
that they must contain silica, alumina, and potash, to
form the quartz and mica, and also magnesia and iron, if
biotite is present. The excess of magnesia over lime,
taken with the high silica, is a character foreign to igneous
rocks, and is clearly indicative of sedimentary origin.
They are probably formed mostly by the metamorphism
of feldspathic sandstones. Two analyses of typical sam-
ples carried out in the laboratory of the United States
Geological Survey are here appended.

Si0 2

A1 2 O 3

Fe 2 O 3





K 2

H 2 O


H 2














General Properties. The color of these rocks varies from
very light, through gray, yellow, or brown tones, into very
dark, depending on the proportions of light and dark mica,
the presence of carbonaceous material, and in part on the
amount of alteration of the iron-bearing biotite. Some
pure muscovite schists are almost silvery white or light


gray. The hardness and firmness of the rock depend on
the proportion of mica; the more this is present, the softer
and more easily cleavable it is. For this reason they are
of little or no value for practical purposes. Inclusions of
various kinds occur in mica-schists as in gneiss, thus veins
and lenticular masses of quartz, deposited from solution
in cracks and cavities opened by movement and foldings
of the rocks, are common. They also contain in places
lenticular masses of other schists, which may vary from
very small to huge dimensions. And sometimes they are
penetrated by seams and patches of granite, felsite, and
pegmatite as the result of granitic injections. With
respect to the alteration of mica-schist, the varieties com-
posed of muscovite are chiefly mechanically disintegrated
by the action of weathering without much chemical change.
The muscovite resists alteration energetically, and the
gravelly or sandy soils formed, are in consequence filled
with its sparkling flakes. Where much biotite is present it
alters easily; the rocks turn yellow or brown, lose their
luster, and eventually much limonite is separated out.

Varieties and Occurrence. The varieties composed
chiefly of muscovite, or with associated garnet, are the
most usual kinds, and are found all over the world as
common rocks in metamorphic regions, and are generally
associated with gneisses. They cover large areas in New
England and extend southward to Georgia. Biotitic
varieties are also very commonly found with them.
Staurolitic mica-schist occurs in many places in New
England, and in Maryland, and elsewhere along the Pied-
mont plateau; it is found in Scotland and various localities
in Europe, in Brazil and elsewhere. Cyanite-mica-schist
occurs in various places in New England; a variety in
which the mica is paragonite comes from the St. Gothard
region in the Alps, and is seen in mineral collections on
account of the beautiful crystals of cyanite it contains;
the common kind with muscovite is found in many places.
Hornblendic mica-schists occur as included lenticular


masses, often of large dimensions, in various places, in the
ordinary mica-schists. Graphitic mica-schist is found in
Connecticut and other places in New England, various
localities in Germany, Norway, etc. Andalusite-mica-
schist occurs in the White Mountains in New England,
in Scotland, Spain, Germany, etc.

An interesting variety is the conglomerate-mica-schist, in which the
rock contains pebbles of quartz, granite, and other rocks which are very
apt to be flattened, lenticular, or drawn out by pressure and shearing.
It is closely related to the conglomerate-gneiss previously described
and has had a similar origin. Such rocks occur in Massachusetts, in
Vermont, Scotland, Sweden, etc.

Transitions and Relation to other Rocks. The gneisses
formed from sediments and the mica-schists have both
been made from similar rocks; from feldspathic sand-
stones, shales and conglomerates. In the mica-schists the
feldspar has been converted into mica; in the gneisses it
has mostly persisted or been recrystallized. It is not in-
tended in this statement to affirm that this is the only
origin for mica-schists, only the most usual one; they may
have been formed in some cases from quartzose-f eldspathic
igneous rocks, though positive evidence on this point is
wanting. In this connection what is said elsewhere of
phyllites should be consulted. On the whole it would
seem most probable that the gneisses have been formed
most often from the conglomerates and coarser-grained
sandstones, the mica-schists from the finer-grained ones,

Online LibraryLouis V. (Louis Valentine) PirssonRocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools → online text (page 30 of 35)