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 13 of 35)
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they may vary in size, some being relatively large while
the rest are minute. Or again the conditions may have
been such that the magma had no opportunity to crys-
tallize but solidified as a simple glass, or to only partly
crystallize and formed a mixture of glass and crystals.
Such variations for the most part are independent of
chemical composition, they depend upon the physical
conditions under which the molten mass has solidified, and
thus a magma of a given composition may appear in any
one of the states mentioned above if subjected to the
proper conditions.

The characteristic features which a rock exhibits in this
respect constitute its texture, and rocks are distinguished
and classified in one way according to their textures,
just as in another way they are distinguished and classified
according to their mineral composition.

Factors influencing Texture. If a strong hot solution
of a salt, such as common alum in water, is allowed to
cool very slowly and regularly, comparatively few centers of
crystallization will be set up, and the few resulting crystals
will have a long period of growth and will be of good size.
If, on the contrary, the cooling is very rapid a great number
of centers of crystallization will form, the period of growth
will be short and a great number of very small crystals
will result. The same is true in the molten liquids from
which the igneous rocks are formed. If the cooling has
progressed with great slowness and regularity then
coarse-grained rocks are produced; if the cooling is rapid
then they are fine-grained and the cooling may take place
so quickly that there is no opportunity for complete
crystallization, and rocks wholly or in part composed of
glass will result. The rate of cooling then is a prominent,
and in fact the most prominent, factor in the production
of rock texture. In addition to the temperature there
has been a tendency in the past to ascribe also a prominent
role to the pressure. The idea involved is that if a magma


remained liquid within the earth at a given temperature
and if for any reason the pressure increases, a point will
be eventually reached where it will be forced to crystallize
and become solid, since in so doing its volume would
be reduced. Decrease of temperature and increase of
pressure would then work together. While this may be
true in theory it does not seem probable that the pressures
obtaining in the upper region of the crust are a very
prominent factor in this direction, since geological obser-
vation has shown that a particular variety of texture can
be found unchanged through a range of 10,000 feet
vertical. Still it cannot be denied that pressure probably
has some influence on the process of crystallization and
the production of rock texture.

The presence of mineralizers, especially water, has
undoubtedly a strong influence on the texture, particu-
larly in the siliceous rocks, for this greatly increases the
fluidity of such magmas and, as they cool down and the
crystallizing points of the different minerals are reached,
they still retain their mobility instead of becoming stiffly
viscous. This increases the range of movement of the
mineral molecules forming, and enables larger crystals to
grow and a coarser texture to be produced. As we shall
see later this reaches its maximum in the pegmatite dikes.

In connection with what has just been stated chemical
composition of the magmas has a certain influence in
producing texture. This shows itself in two ways. Those
magmas which are deficient in silica and especially those
which contain much iron and magnesia and which are
shown in the right hand side of the diagrams given on a
previous page remain liquid, without becoming stiffly
viscous, to much lower temperatures than those with
high silica which are found expressed in the middle of the
diagrams. This liquid condition enables them to crys-
tallize more freely and to form in consequence coarse-
textured rocks under circumstances where the siliceous
magmas would produce only types fine-grained in texture


or even glassy from inability to crystallize completely,
owing to increasing viscosity. The other way in which
chemical composition influences texture is this. Dif-
ferences in composition in the magmas mean of course
differences in the kinds of minerals which they produce.
Different minerals crystallize in different shapes and
although, owing to interference with one another, they
may not form in perfect crystals, they tend to take such
shapes. Some form tabular shapes, others spherical or
cuboidal grains or elongated prisms. Thus, while the
general size of such grains may remain the same through-
out a mass of rock, such differences in shape will
produce corresponding differences in what we may call
the pattern or fabric of the rock and thus influence its

Relation of Texture to Geologic Mode of Occurrence. It
is evident that the condition most favorable for the pro-
duction of coarse-textured rocks conditions described
in the preceding discussion as slow cooling, pressure and
the presence of mineralizers will, in general, be best
realized when the magma is in large mass and deeply
buried in the earth's crust so that it is completely
enveloped by surrounding rock masses. The heavy
cover retains the heat and the mineralizers and gives in
part the pressure. Such igneous rocks, formed in depth,
will only become exposed to our observation when con-
tinued erosion has carried away the superincumbent
material. They are often therefore called plutonic or
abyssal and sometimes massive rocks, and referring to
what has been described as the modes of occurrence of
the igneous rocks it can be seen that bathyliths, stocks
and the lower part of volcanic necks may be particularly
expected to exhibit such texture and nearly always
do so.

On the other hand, when the magmas attain the surface
and are forced out in volcanic eruptions, lava flows, etc.,
entirely different conditions will prevail; there is no cover


to retain the heat and the cooling in consequence is rapid,
Also the pressure has been relieved, and with loss of cover
and pressure the mineralizers quickly depart. As a
result fine-grained, dense, compact textures are formed,
or the cooling may be so rapid that crystallization may
fail to occur, either wholly or in part, and rocks entirely
or partly composed of glass may be produced. When
rocks are more or less glassy it is in general very good
evidence that they solidified as surface lavas.

In the smaller intrusive bodies, such as the dikes,
sheets and laccoliths, the conditions in general are between
the two sets just described. The volume relative to the
surrounding rocks is less, the loss of heat and mineralizers
more rapid than in the stocks and bathyliths, and, since
in general the depth is less, the pressure is diminished.
Thus the textures are usually between those of the larger
abyssal masses and the effusive lavas. But the conditions
in these occurrences are apt to be very variable, and in
accord with this we find the textures sometimes dense
like the effusives but very rarely glassy and some-
times coarse-granular like the larger abyssal masses. In
them, too, the function of chemical composition described
in the preceding section is often most strongly displayed.
Thus highly siliceous dikes and sheets of fine grain will
be found associated under the same geological conditions
with other ones low in silica and high in iron and magnesia
of relatively much coarser grain.

It is especially in these occurrences and in the surface
lavas that the porphyritic texture, to be presently described,
is most liable to be found.

Textures of Igneous Rocks. Based on the principles
which have been enunciated in the foregoing sections the
textures of igneous rocks for megascopic study may be
classified as follows:

Grained. All sizes of grain large enough to be seen
with the unaided eye. Example, ordinary granite.


3. Coarse Grain.


Dense (aphanitic). The rock is crystalline, i.e. not
glassy, but the grains are too fine to be perceived by
the eye. Example, many felsites.

Glassy. The rock can be distinctly seen to be wholly
or in part composed of glass, as in obsidian.

The distinctions stated above relate in part to its
crystallinity or degree of crystallization, for all grades of
transition between rocks composed wholly of glass, partly
of glass and partly of crystals and wholly of crystals
exist, though to be perceived by the unaided eye the glass
must form a great or the greater part of the rock. It
relates also in part to the absolute sizes of the crystal
grains or what we may term the granularity.

The phanerocrystalline (Greek, <avepos, visible) rocks
according to the size of grain can be divided as follows:
(See Figs. 1, 2 and 3, Plate 6.)

Fine-grained, the average size of the particles less than
1 millimeter or as fine as fine shot.

Medium-grained, between 1 and 5 millimeters.

Coarse-grained, greater than 5 millimeters or as great as
or greater than peas.

But another very important feature of texture is that
of the pattern or fabric and this, for megascopic work, is
chiefly due to the relative sizes of the crystal grains in a
given rock. There are two chief kinds of fabric which
may be distinguished:

Even-granular fabric (or texture), grains of approxi-
mately the same general size.

Porphyritic fabric (or texture), grains of a larger size
contrasted with finer ones or with glass.

Even-granular Texture. While this means that in a
given rock the crystal grains have approximately the
same general size, as may be seen by referring to Plate 6,
it does not mean that they have necessarily the same
shape. Careful examination of granites which have this
texture will show that the dark mica is in many cases


present in well formed hexagonal tablets or crystals,
while the feldspars and quartz are in shapeless masses,
or the feldspar tends to have rough tabular or brick-like
shapes. This depends on the order of crystallization as
previously explained.

Porphyritic Texture. Porphyry. In this texture, when
typically developed, there is a sharp contrast between
larger crystals with definite crystallographic bounding
faces, which are termed phenocrysts (Greek, faivuv, to
show), and the material in which they lie embedded,
called the groundmass. This groundmass may have the
textural characters described on a preceding page, it
may be even-granular, coarse or fine, it may be dense
or wholly or partly glassy. A rock with this textural
fabric is called a porphyry.* Examples are shown in
Plate 7.

Great variations are seen in the phenocrysts ; they may
be extremely numerous and the amount of groundmass
small or the reverse; they may be an inch or more in
diameter or they may be so small as to require close
observation to detect them; they may be of light-colored
feldspars and quartz or dark-colored ferromagnesian
minerals, hornblende, augite and pyroxene, or of both
kinds of minerals. Again, they may be extremely well
crystallized and afford such striking specimens of perfect
crystal development that they find a place in mineral
cabinets or they may be very poorly defined in crystal
form. And with increase in numbers and poor crystal
form, all degrees of transition into the even-granular
texture may be found. The porphyritic texture is
extremely common in lavas and in intrusives of small
mass such as dikes, sheets and laccoliths; it is rarer in

* The porphyritic texture is not a contrast of colors of mineral
grains but of sizes. Care must be taken, therefore, not to confuse,
for instance, a white rock consisting of grains of light-colored minerals
such as feldspar, in which are embedded a few conspicuous black
grains of a ferromagnesian mineral of the same size, such as horn-
blende, with a porphyry.


A. With Phenocrysts of Feldspar.

B. With Phenocrysts of Augite.


the abyssal rocks, but is sometimes seen, especially in

Origin of Porphyritic Texture. In the case of many
effusive rocks or lavas it is easy to understand why they
have a porphyritic texture. The lavas of many volcanoes,
as they issue to the outer air, are full of growing crystals,
often of considerable size, suspended in the molten fluid.
The latter, however, subjected to new conditions, is
forced to cool rapidly and assumes a fine-grained, or
dense crystalline, or even a glassy, solid condition with
these larger crystals embedded in it, and thus the com-
pleted rock has a porphyritic fabric. The same process
may serve to explain this texture in some of the smaller
intrusives, such as dikes and sheets, but it cannot serve
as a general explanation for all cases because in some
dikes, laccoliths, etc., there is good evidence that the
phenocrysts have not been brought thither but have
formed, like the rest of the rock, in the place where we
now find them. It also fails to explain the porphyritic
border of many granites and the large phenocrysts found
in other granites; nor does it explain the origin of the
phenocrysts themselves, why a few large crystals have
formed while the rest of the magma fails to crystallize.
Evidently some more general explanation is needed.

It has been previously shown that molten magmas
must be considered as strong or saturated solutions of
some compounds in others. As the mass cools down it
may become supersaturated. Now it has been shown
that some saturated solutions cannot crystallize spon-
taneously but require to be inoculated with a minute
fragment of the substance in solution; this is called the
metastable state. Other saturated or supersaturated
solutions either crystallize spontaneously or can be
induced to do so by shaking or stirring with a foreign

Miers has shown that the same solution may pass from
one to the other of these states in accordance with changes


of temperature, and suggests that a magma may be in the
metastable condition in which a relatively few crystals
induced by inoculation from the surrounding rocks are
growing as phenocrysts and by cooling pass into the labile
condition when spontaneous crystallization of the remain-
ing liquid will ensue and form the groundmass. Or it
may start in the labile condition when the formation of a
crop of phenocrysts will reduce it to the metastable state,
in which condition it may be erupted as a lava, or remain-
ing and cooling down it may pass into a new labile state,
thereupon crystallize and form the groundmass. The
recognition of these states in cooling saturated solutions
(and we must regard the molten magmas as such) seems
quite sufficient to explain the different variations of por-
phyritic texture which occur.

Some Structures of Igneous Rocks.

The word texture is reserved for those appearances of
the rocks which are occasioned by the size, shape, color,
etc., of the component crystal grains. Certain larger
features exhibited by the rocks may be classed under the
term of structure and will now be described.*

Vesicular Structures. When a molten magma rises to
the surface and especially if it issues in the form of lava,
the pressure upon it is relieved and the water and other
vapors it may contain are given off. This has a tendency,
if it is still soft and stiffening, to puff it up into spongy
vesicular forms as illustrated in Plate 8. In the case
of very siliceous lavas it may be entirely changed into
a light glass froth called pumice. Such forms are espe-
cially produced in the lava in the throat of a volcano,
where the issue of gases is rapid or in the top portion of a
flow. Except in a rare and very limited way on the sides

* An example of the difference between the two usages would be
this. A certain lava from flowage might appear in layers; the
layers are of rock composed of exceedingly fine particles. We
would say then that the lava had a banded structure and a very
fine compact texture.





of dikes they never occur in intrusive rocks, and the
presence of well-marked vesicular structure may be taken
as pretty sure evidence that the rock exhibiting it was
originally a surface lava. In the throat of a volcano such
spongy forms of lava may, by explosions of steam, be
driven in fragments into the air to fall as dust, ashes,
lapilli, etc., making volcanic tuffs and breccias as described

Amygdaloidal Structure. Amygdaloid. When a lava
has been rendered spongy (vesicular, as described above),
it may be permeated by heated waters carrying material
in solution which may be deposited as minerals in the
cavities. This happens especially in basaltic lavas, and
the dark rock then appears filled with round or ovoid
whitish bodies which from a fancied resemblance to the
kernel of a nut are termed amygdules, from the Greek word
for the almond. The structure is called the amygdaloidal
and a rock exhibiting it is often termed an amygdaloid.
It is shown in Plate 8. While the smaller cavities are
usually filled solid, the larger ones are often hollow, the
minerals projecting in crystals from the walls as in geodes,
and from such amygdaloidal cavities some of the most
beautiful crystallizations are obtained. The minerals
most frequently occurring are quartz, which is sometimes
of the amethyst variety, calcite and particularly zeolites.
The basaltic lavas are an especial home of these latter
minerals, some of the more common kinds being analcite,
stilbite, natrolite, heulandite and chabazite. The basalts
of India, Iceland, Scotland, Nova Scotia and other
localities have furnished specimens which are known in
all mineral collections.

This structure is most commonly and typically
developed in surface lavas, that is, in effusive rocks, but
it is also seen at times in intrusive rocks, such as dikes
and sheets, especially at their margins.

Miarolitic Structure and Porosity. The volume which
a magma occupies in the molten condition is considerably


greater than that which it has when changed to a solid
crystalline rock. It is probably greater in the liquid state
than when cooled to a glass but how much we do not know.
This contraction in volume, in passing into the crystalline
state, is accompanied by a corresponding rise in specific
gravity. Thus an obsidian glass, consisting chiefly of
high silica with moderate amounts of alkalies and alumina,
has an average specific gravity of about 2.2-2.3, but the
same material crystallized into a quartz-feldspar rock
(granite) has a specific gravity of 2.6-2.7. There would
be a corresponding reduction in volume.

In general this contraction of volume, during the process
of crystallizing, produces minute interspaces or pores
between the mineral grains, and cracking and jointing of
the mass, a process described in the following section.
This production of pores accounts for the capacity of the
rocks to absorb moisture. It appears to be greatest in
the coarse-textured rocks, much less in the finer-grained
ones; greater in granites, less in diorites and other ferro-
magnesian rocks. In the case of porous vesicular lavas the
amount of pore space may be very great, but in ordinary
crystalline igneous rocks it is small, usually less than one
per cent of the rock volume.

In some cases, however, there may be distinct cavities
produced. These are commonly very small, sometimes
an inch or so in diameter and in rare instances as much
as several feet. It often happens that the crystal com-
ponents of the rock on the boundary walls of the cavity
are much larger in size than the average grain, and project
into it, bounded by distinct faces and of good crystal form.
One notices also, especially in granites, that the quartz
and feldspar crystals are often accompanied by those of
muscovite, topaz, tourmaline and others which are foreign
to the general mass of the rock but are common in peg-
matite veins. These are also well crystallized. The
presence of the water vapor, fluorine, boron, etc., necessary
for their production, as well as the larger size and





distinct form of the crystals, shows that such mineralizers,
excluded elsewhere from the magma during the process
of crystallization, collected in these cavities, possibly
helped to enlarge them, and promoted the formation of
the unusual minerals and the good crystal forms which
they and the ordinary rock minerals exhibit. Such
hollow spaces are called miarolitic cavities and a rock
which contains them is said to have miarolitic structure,
from a local Italian name (miarolo) for the Baveno
granite which shows it. Such drusy cavities are distin-
guished from geodes and others, in which the minerals
have been deposited from solutions, by the fact that they
have no distinct wall separating the minerals from the
containing rock. They often furnish fine mineral speci-
mens. An example of one is seen in Plate 9.

Jointing of Igneous Bocks. The most important way
in which the contraction of a body of magma, after
cooling and crystallizing into rock, manifests itself is in
the production of joints. These are the cracks or fissures
which, running in various directions, divide the mass into
blocks, fitted together like masonry and usually according
to more or less definite systems. Sometimes this shows
itself in the formation of rudely cubic or rhomboidal
blocks, as shown in granites and other abyssal rocks,
sometimes in a platy parting which may be quite thin and
cause the rock mass at first glance to resemble sedimentary
beds, and sometimes in concentric or spheroidal forms
which develop rounded or ovoid bodies like melons as
the weathering and rock decay progresses. Platy and
spheroidal partings, and jointing on a small scale by which
the rock body is divided into little blocks, are most com-
mon in small intrusions in dikes, sheets, etc. and in
surface lavas. Such jointing is a matter of great geologic
importance in permitting the entrance of air and water
to act in the weathering and decay of rocks and in the
processes of erosion, especially the splitting and breaking
of them by the action of frost. As can be readily


inferred it is also of great practical importance in the
work of rock excavation, in mining operations and in
quarrying. (See Plate 10.) Were it not for such joints
almost every igneous rock mass would furnish suitable
material for quarrying, whereas on the contrary it is diffi-
cult to find a granite jointed on so large a scale that it
will furnish solid blocks, for example, like those from
which the celebrated Egyptian obelisks were made.

Columnar Structure. The most remarkable way in
which the jointing of a cooling mass of igneous rock,
explained above, manifests itself is in the production of
columnar structure. This is found both in intrusive and
extrusive occurrences and in all kinds of igneous rocks, but
is usually best displayed in basalts. The whole mass is
made up of columns, regularly fitted together, from a few
inches to several feet in diameter and from one foot to two
hundred feet or even more in length. An example is
shown in Plate 11. The celebrated Giant's Causeway on
the north coast of Ireland is one of the best known
examples of this. In the most perfect cases, as in the one
just mentioned, the cross sections of the columns are
regular hexagons and the columns are divided lengthwise
at regular intervals by cross joints whose upper surfaces
are shallow cup-shaped. The columns are always per-
pendicular to the greatest extension or main cooling
surface of the igneous mass, hence in a lava flow or intru-
sive sheet they are vertical assuming the flow or sheet
to be horizontal while in a dike they tend to be hori-
zontal. Such a dike when exposed by erosion tends to
resemble a stretch of cord-wood regularly piled.

The cause of this structure seems to be as follows.
When a homogeneous mass is cooling slowly and regu-
larly, centers of cracking tend to occur on the cooling sur-
faces at equally spaced intervals. From each central
interspace three cracks radiate outward at angles of 120
degrees from each other. These intersecting produce

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 13 of 35)