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

. (page 29 of 35)
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 29 of 35)
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too small to be seen with the eye, and often even with the
lens; the rocks appear dense, but they have the capacity
of splitting into thin slabs, as seen in roofing slates.
The cause of this is discussed under the description of

Metamorphic rocks frequently contain large and well-
developed crystals of minerals, which have formed as a re-
sult of the processes to whichthe rocks have been subjected.
These may be very much greater in size than the average
grain of the rock, and this contrast, together with the per-
fection of their crystal form, produces a strong analogy
to the porphyritic texture of igneous rocks. They are not
true porphyries, however, not only because the texture
is not of igneous origin, but also because these large crystals
are not of an older generation, but are actually of later
formation than the minerals of the apparent groundmass
in which they lie. It is therefore termed the pseudo-
porphyritic texture. That these minerals or pseudo-pheno-
crysts are of later formation is shown by the fact that they
frequently contain as inclusions the other rock minerals,
and sometimes the inclusions, such as bits of quartz, graph-
ite, etc., pass through the large crystal in the lines of
original stratification, and out beyond it. Moreover, it
may be frequently noticed that where these pseudo-pheno-
crysts are not equi dimensional, but elongated, they may lie
in the rock pointing in all directions; their longer axes do
not necessarily lie in the direction of schistosity, like those
older minerals, which have been arranged by the pressure
and shearing. Having grown in the zone of pressure,
they are not oriented by it, unless subsequent and later
movement and shearing should take place after their for-



(Maryland Geological Survey.)


mation. The space in the rock in which movement of
material goes on to produce these larger crystals is clearly
shown in Plate 35, which is a photograph of a garnet
in gneiss. Around the garnet is a zone of feldspar,
from which all the ferromagnesian minerals, visible be-
yond it, have disappeared, having been used up in its

The crystals described above should not be confused
with larger crystals or crystal masses in the rock, which
may also give it a porphyritic, appearance, but which are
really remains of former structures. Such may be former
phenocrysts of some porphyritic, igneous rock, or large
grains from some former coarse-granular igneous rock, or a
pebble from a conglomerate. They are apt to form ovoid
masses, and they are then really a pronounced case of the
lenticular texture, which is sometimes termed, following
the German name, augen (eye) texture.

Relation to Previous Textures, etc. In proportion to the
degree of metamorphism which rocks have suffered do we
find that the characteristic textures described above have
been imposed upon them. But not infrequently, as
though looking through the veil which metamorphism
has cast over them, we can see back of these features
remains of original textures and structures which are
characteristic of igneous and sedimentary rocks. Thus,
as indicated above, we may see that the original texture
was that of a porphyry, or we may find remnants of the
spherulites, lithophysae, and flow lines of some felsite
lava,or of the amygdules of some basaltic one; on the other
hand, ovoid masses of different mineral composition may
indicate a former conglomerate, or parallel layers, differing
in general mineral and chemical composition, may show
former stratified material. Such indications may be very
useful in ascertaining the former origin of a metamorphic
rock, and in some cases may positively identify it, but
deductions from this source should always be made tenta-
tively, and used with great caution, for there are many con-



fusing appearances of this kind which may lead to serious
error, unless they are checked by microscopic examination
and chemical analyses.

Chemical Composition. The chemical composition of
the metamorphic rocks is extremely variable, and it is
evident that this must be the case, when one considers the
heterogeneous materials from which they may be derived.
If we take them together, as a class of rocks, the com-
position, therefore, cannot have the significance which it
plays in the igneous ones, in showing their mutual rela-
tions. It may, however, be of great importance as an aid
in helping to determine their origin. Thus, in examining
the chemical analysis of a metamorphic rock, we may be
able to say that it is similar to those of known igneous
rocks and it may therefore have been originally of
igneous nature, and on the other hand the analysis may
show definitely that it could not have been any igneous
rock, and consequently it must have been of sedimentary

SiO 2

A1 2 O 3

Fe 2 O 3




Na 2 O

K 2 O

H 2 O




































I, Gneiss, near Freiberg, Saxony; IT, Hornblende Schist (amphi-
bolite) Vestana, Sweden; III, Gneiss, near Rawdon, Quebec.

Thus, in the analyses given above, that of No. 1 might well be of an
ordinary granite, as may be seen by reference to those given under
granite; it might also, however, be that of an arkose derived from such
a granite. No. II has the composition of a gabbro; it might have
been such a rock originally, or a dolerite, or basalt ; it does not suggest
any ordinary sedimentary rock. No. Ill on the other hand has no
analogy among igneous rocks; the alkalies and alumina are too low
for the silica and the ferric oxide too high; it must be of sedimentary
origin and suggests an impure, ferruginous sandstone.


It is inferred, of course, that while movement among the
molecules within limited distances has occurred, whereby
exchanges among the oxides are produced, involving
recrystallization and the formation of new mineral com-
pounds, the chemical composition of a rock mass as a
whole has remained unaltered. That this is so, is shown
by the fact, that in innumerable occurrences stratified
rocks, although utterly changed in mineral composition
from their former state, still retain the spacing and relative
volume relations of the strata which they originally had.
Thus one band of strata, perhaps only a fraction of an
inch in thickness, is sharply marked off by its grain,
minerals, and texture from those above and below it.
There has been no melting and no formal transfusion of
substance, consequently the changes which have occurred
are, so to speak, inward, those which lie within the range
of molecular attraction. To this general statement that
there is no change in mass composition in metamorphism,
there is one exception, and that is, that volatile substances,
liquids and gases, may be driven out and, conversely, new
ones may enter and pass into mineral combinations, as
previously explained under the action of liquids and gases
as agents. This is most strikingly seen, perhaps, in the
metamorphism of impure limestones, as described in the
section dealing with marble, and is thoroughly analogous
to what has already been stated under contact metamor-

Injection of Gneisses and Schists. It has been pre-
viously mentioned that part of the heat of metamorphism,
and of the liquids and gases involved in the production
of minerals, is supplied by intrusions of igneous magma,
which are particularly liable to rise and invade those
areas where crustal movements are starting metamorphic
agencies at work. In such areas, of course, the effect of
contact merges into that of general metamorphism and no
definite line can be drawn between them. Indeed the
earlier formed intrusions may themselves become more


or less metamorphosed, or have metamorphic textures
imposed upon them by repetitions of the processes,
and this may happen while they are in a solid, or
yet partly plastic, condition. There is, however, another
function which t'hese magmas, rising under great pressure
into rocks already schistose and foliated, may perform;
they may squeeze themselves in thin veins, sheets, and
lenticles into the schists surrounding them, so that these
rocks may become partly igneous, partly metamorphic, in
composition. And, as previously explained under con-
tact metamorphism and pegmatite dikes, these effects
may be greatly aided by liquid and gaseous emanations
from the magma masses. This process has been termed
the injection of schists by magmatic material and, although
as it has been doubted by some geologists, just as it has
been given entirely too general an application by others, it
has undoubtedly occurred in many places. By it we are
enabled to understand the veins, stringers, and lenses of
granite penetrating the schists in the neighborhood of
larger granite intrusions in many places, which would be
otherwise incomprehensible. In this connection also, it
should be remembered that the intrusive effects in the
lower zone of rock flowage may be expected to be quite
different from those in the upper zone of rock fracture.

Occurrence and Age. The metamorphic rocks have a
wide distribution over the earth's surface, and in many
places they occupy great areas, over which they are the
*>nly ones exposed. There is good reason also for be-
lieving that they form the basement upon which all the
later unmetamorphosed, sedimentary rocks rest. The
reason for this is, that wherever these later strata are
sufficiently eroded away, this metamorphic basement has
come to light. The only exception to this general dis-
tribution over the continental areas is in those places
where later intrusions of igneous magmas have come up
through them and are now exposed as bathyliths, stocks,
dikes, etc. But these constitute but a subordinate part


of the total area. It is their extension over such wide
areas which has led to the processes, that have produced
them, being called regional metamorphism, in contrast to
the forming of the small zones around intrusive igneous
masses, which is therefore termed local metamorphism.
There is no difference in principle, however, between these
two, only in the relative intensity with which the varied
agents have operated. The metamorphic rocks are found
also in folded mountain ranges, of which they form the
interior core, and which subsequent erosion brings to light.
In proportion to the intricacy of the folding and mashing
of the strata, so is the degree of metamorphism increased.
This is so well established, that when we find areas where
the rocks are intricately folded and completely meta-
morphic, but not of any great elevation, we assume that
such an elevation formerly existed, but has been eroded
away, or in general that metamorphic rocks can only
become exposed at the surface through erosive processes.
It is these facts that led to the view, formerly held, that
metamorphic rocks must, geologically speaking, be of
very great age. This is, however, by no means necessarily
the case. For, on the one hand, we find unmodified
sands of Cambrian age in eastern Russia, and unaltered
beds of Ordovician age in the upper Mississippi valley,
which have not been changed from their original position,
while on the other, strongly folded strata of Tertiary age
in the Coast Range, in the Alps, and in other mountains,
are in places, profoundly metamorphosed. Rocks that are
metamorphic are likely to be old, but not necessarily so,
just as a battle-scarred soldier is likely to be a veteran,
rather than a recent recruit. It merely depends on
whether they have been subjected to metamorphic
processes or not, and the older they are, the more likely
they are to have suffered from them. Time, however,
is one of the great factors in metamorphism, and even in
the recent strata which have been changed, the time in-
volved, from our standpoint, is very long.



Classification of Metamorphic Rocks. It would be
natural to classify the metamorphic rocks according to
the origin of their material, and to separate those of igneous
from those of sedimentary formation. In some cases this
may be done. Thus it is clear that marble is not of igneous
origin, but when we attempt to carry this principle
through, it quickly becomes impracticable, especially
if we can use only megascopic means of determination.


A1 2 O 3






K 2 O

H 2 0,






















Thus in the two analyses given above, the upper one is
that of the Portland sandstone of Connecticut, a fine-
grained arkose full of feldspar, the lower one that of an
intrusive granite porphyry from the Crazy Mountains,
Montana. It is evident that if these two rocks, one sedi-
mentary, the other igneous, should be so thoroughly
metamorphosed as to lose all traces of their original
textures, it would be impossible to discriminate them from
one another, or to say what their original status was.

Remembering the simple primary classification of the
sedimentary rocks previously given, it is possible, in a very
general way, to show the relation between the most com-
mon ones and their metamorphic derivatives in the fol-
lowing table:


Compacted Strata.

Metamorphic Rocks.



Gneiss and various schists


Sandstone . .

Quartzite " "

Silt and Clay


Slate " "

Lime deposits


Marble " "


In the case of the igneous rocks, recalling that they may
be roughly divided into two main groups, the one chiefly
feldspathic, and the other mainly of ferromagnesian
minerals, we can illustrate also, in a very rough and
general way, the relation between them and their meta-
morphic derivatives in the following table :

Igneous Rocks.

Metamorphic Rocks.

Coarse-grained feldspathic
types, such as granite, etc.. .


Fine-grained feldspathic types,
such as felsite, tuffs, etc

Slate and Schists.

Ferromagnesian rocks, such as
dolerites and basalt

Hornblende-, Talc-,
(etc . ) , Schists and


A comparison of the two tables will show that gneisses
and schists may have diverse origins, and the reason for
this has been previously pointed out.

Another method of classification which has been recently sug-
gested is, disregarding the origin of the material entirely, to consider
only its chemical composition. According to this the metamorphic
rocks are divided into groups. The earth's crust is divided vertically
into zones, somewhat as described above, and the effect of the meta-
morphism in these zones upon each group is considered. It is found
that material of a given composition yields rocks, differing in mineral
composition and texture, according to the zone in which the meta-
morphism occurred. Thus the first grouping is a chemical one, while
the subdivisions are mineral and metamorphic, and in this way the
different rocks are produced' and classified.

While this method may be consistent and based on scientific
principles, it is not a practical one for field and megascopic use. We
cannot make analyses of rocks under ordinary circumstances, nor
can we, in most cases, even estimate megascopically the chemical
composition from the minerals they contain, as can be done with
the microscope and thin sections. And the different mineral com-
positions and textures pass into one another so gradually, that only


in a very general way, or in specific cases, can we say whether the
rocks have been metamorphosed in the zone of fracture, or the zone
of flowage.

At present we are obliged, for practical purposes of field
work and megascopic determination, to classify quite arbi-
trarily the metamorphic rocks according to their evident
mineral composition or texture, or a combination of both.
Sometimes, as in the gneisses, stress is laid upon the first
feature; sometimes, as in the slates, upon the second one,
in accordance with whichever one is the most evident and

We have in agreement with this the following main
groups of metamorphic rocks.

Grouping of Metamorphic Rocks.

1. Gneisses and Feldspar Rocks.

2. Mica-schist and Quartzite.

3. Slates and Phyllite.

4. Talc and Chlorite Schists.

5. Hornblende Schist.

6. Marble, Lime carbonate-silicate Rocks.

7. Dolomite, Magnesian carbonate-silicate Rocks.

8. Serpentine.

9. Iron oxides and other rocks.

By comparison it may be seen that the above is in the
main a combination of the two tables previously given.
The more important of the members are given in italics.



THE term gneiss is not only the name of a particular
kind of metamorphic rock, but also, in a wider sense, it is
used as an expression of a certain texture. Thus when we
use gneiss as a name in the limited sense, we mean a rock
which has the composition of granite quartz, feldspar,
and mica with a certain foliated texture; if we say
granite-gneiss, syenite-gneiss, diorite-gneiss, we use it in
the wider sense, and denote rocks whose composition is
indicated by the first word, and the texture by the second.
The only general definition of gneiss which will cover all
cases is, that they are metamorphic rocks, composed of
feldspar, with other minerals, which have a certain char-
acteristic texture. But, as everywhere generally used
when no qualifier is prefixed, common gneiss, which is
composed of quartz, feldspar and mica, as stated above,
is understood, and the term is so employed in this book.
If the wider sense is meant the qualifier is given.

Mineral Composition. Various kinds of feldspar are
found in gneisses, both the alkalic and soda-lime varieties,
but they can rarely be distinguished by megascopic
means. The mineral is white to gray in color, or reddish,
as in granite, and is apt to be in more or less round, or
elongated, lenticular, formless grains; this lack of definite
form makes it more difficult to distinguish from the quartz
than in most granites, and the cleavage should be carefully
sought. Sometimes large grains, the size of a pea, or even
larger, occur, giving the gneiss a porphyritic character; if
the cleavages of these are examined against the light, it



may be often observed that they are Carlsbad twins.
Such large crystals may indeed have been the phenocrysts
of a former porphyritic granite, or they may have been
feldspar pebbles of a conglomerate or arkose, or they may
have been made by injected material.

The quartz is also in more or less round grains or lentic-
ular masses, or in granular aggregates with the feldspar.
Its color is white or gray, sometimes reddish, rarely bluish.
In the larger grains it is easily recognized by its greasy
luster and conchoidal fracture.

The mica may be either biotite or muscovite, or a mix-
ture of both. The biotite is black or dark brown, the
muscovite is white or yellowish to light brown, sometimes
pale green. The mineral does not have any distinct crystal
form, but is in flakes, shreds or irregular leaves, drawn out
in bands, or in thin patches. It usually lies stretcher 1
out along the structure planes of the rock, and in large
part its easy cleavage, thus arranged in one direction, con-
ditions the schistosity or cleavage, and gives emphasis to
the gneissoid texture. Thus the surface of chief fracture
of a flake of the rock may appear to be largely coated
with mica, and, judging from this alone, one would be apt
to gain an exaggerated idea of the relative amount of it in
the rock; the surface of cross fracture should also be
examined to gauge correctly its relative amount, as com-
pared with the other mineral constituents. This is also
especially true, in the mica schists, and in those gneisses,
which, by decrease of feldspar and increase in mica, form
transitions into these latter rocks. This effect is also
more marked in many gneisses, because there is a tendency
for the quartz and feldspar to be collected in layers, which
alternate with layers of mica.

Hornblende may occur in gneisses, sometimes associated with the
mica, sometimes alone, forming a special variety. It is seen in dark,
prismatic crystals without good terminations, as in granite, syenite,
etc. Minute crystals may be aggregated into flattened lumps and



(U. S. Geological Survey.)


Besides these, many other minerals may occur in gneisses,
sometimes so prominently as to form special varieties. Of these
garnet, of a dark red common variety, is perhaps the most con-
spicuous. The crystals are sometimes large, as compared with the
size of the other rock constituents. Epidote may also be discovered,
as well as graphite, in some varieties. Sillimanite, a mineral with the
same composition as andalusite and cyanite, is sometimes seen in
gneiss, in bundles and brush-like groups of slender fibers or prisms.
Tourmaline occurs also under circumstances similar to those which
obtain in granite. In some gneisses the mica may be partly, or
wholly, replaced by chlorite, usually from alteration.

Texture. This has been already described in large, part
under the general remarks on metamorphic rocks and what
has been said above respecting the mica. The essence
of the texture consists in the layers of mingled quartz
and feldspar, which are separated by drawn out layers of
mica. Where the amount of mica is small, the gneissoid
texture is less evident, and it increases with the increase
of mica. Sometimes these layers are thick and coarse,
giving a pronounced gneissoid effect, sometimes the layers
are extremely thin. In some cases the layers continue
their individual character for considerable distances, in
others they are very short, lenticular, and are closely
interlaminated. According to these appearances, dif-
ferent varieties of gneiss have been named on a textural
basis. The gneissoid texture is sometimes scarcely per-
ceptible in a hand specimen, but clearly seen on a large,
exposed surface of the rock. This is especially the case in
rocks which were originally granites, but which, by pres-
sure and shearing, have been converted into gneiss.

The texture described above, the banding or schistosity,
may extend for long distances in straight, regular lines,
or it may be curved, folded, contorted, or faulted, often in
the most complex and remarkable manner, and on any
scale, even to a very minute or even microscopic one. Ex-
amples of such intricately folded and compressed gneisses
are seen on Plates 35 and 36. Such folding testifies ia
general to repeated dynamic movements, with shearing


and folding, the earlier ones producing the gneissoid
structure and the later ones crumpling it up, though it is
possible that in some cases the two things are simultaneous,
In some gneisses, as in some granites, a definite por~
phyritic texture may be present, with large and definite
crystals of feldspar, which show more or less distinct
crystal form.

Such gneisses are to be generally regarded as originally porphyritic
granites, which have had the gneissoid texture imposed upon them,
though in some cases, it may be, that the large crystals have been
formed in gneiss of a different origin by growth from injected material.
Such gneisses are allied to, and may pass over into, types, which, with
a short, thick, lenticular texture, contain ovoid masses of feldspar or
quartz. The ovoid bodies are called " eyes " (German, augen},
and the rocks containing them " augen-gneiss '* from the German
name, or " eyed-gneiss." As explained on a previous page on the
texture of metamorphic rocks, they may be of quite diverse origin.

In some gneisses are to be seen pebbles of various kinds
of previously existent rock masses, of granite, quartzite,
etc. They are apt to be drawn out into flattened lenticu-
lar masses, but their original character is evident, and it is

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 29 of 35)