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 12 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 12 of 35)
Font size
QR-code for this ebook

ples that are known are found in the region of the
Rocky Mountains, where in many places they are not

Cases have been described where the roof of a laccolith has been
ruptured and driven upward by the magma rising like a plug through
the strata. It has been suggested that such forms be called bys-
maliths (Greek, plug rocks). It has also been suggested that when
a body of magma is injected into the stratified rocks like a laccolith,
but of indefinite shape and without the relations to the planes of
stratification which a laccolith has, such a mass be termed a chonolith
(Greek, mold used in casting rock).


Necks. When a volcano ceases its activity and becomes
extinct the column of magma, occupying the conduit
leading to unknown depths below, may solidify and form
a mass of igneous rock. Erosion may cut away a great
part of the light porous ashes and lavas, leaving this more
solid and resistant rock projecting, as shown by the
heavy line abc in Fig. 65. Or the level of erosion may
continue to descend into the rocks which form the base-
ment on which the volcano
is built, all traces of the
ashes, lavas, etc., being
swept away and only this
mass being left to mark
its former site. Such a

Fig. 65. Section Through a Volcano , . . .

mass 01 rock is known as

a volcanic neck. It is commonly more or less circular
in ground plan and may be from a few hundred yards up
to a mile or more in diameter. The rocks about them
are apt to be fissured and filled with dikes and in many
cases, if stratified, with intrusive sheets.

Stocks. This term has been applied to large bodies of
intrusive rock which in the form of magma have ascended
into the upper region of the earth's crust and there solid-
ified. They have become visible by extended erosion and
tend to have a more or less circular or elliptic ground
plan. Their plane of contact cuts across the inclosing
rocks, is more or less irregular, and the mass may widen in
extent as it descends. Their size may be anything from
a few hundred yards to many miles in extent. Since
they are apt to form protuberant topographic features
through erosion they are sometimes, especially in Great
Britain, called bosses. The distinction from a volcanic
neck is not one of size alone, though necks tend to be
smaller than stocks, but lies in the fact that the term
neck is employed only when there is evidence that there
has been extrusive volcanic activity from it. Some
stocks were doubtless necks, but this cannot now be


r "3

o o

<J 02


proved. The granite hills of New England, of Scotland
and of other old eroded mountain regions are often stocks
or bosses.

Bathyliths. This term is used in a general way to
designate those huge irregular masses of igneous rock,
which, underlying the sedimentary and metamorphic
ones or sometimes cutting through them, have been
exposed by erosion. They are seen in the oldest exposed
areas of the crust where they are characteristically accom-
panied or surrounded by metamorphic rocks, as in eastern
Canada, or in mountainous regions where they form the
central cores or masses of the ranges, as in the Alps.
They differ chiefly from stocks in their much greater size,
as they are not infrequently many thousands of square
miles in surface area.

While some stocks are clearly intrusive and have displaced the
rocks whose site they occupy, the mode of formation of others and
of bathyliths is still a subject of speculation. Some have held that
they have attained their position by melting and assimilating the
previous formation and thus replacing it, while others have urged
the view that it has been ruptured, uplifted and driven out by the
invading mass and then eroded away. Various modifications of
these views have been suggested, but geologic science is not yet in a
position to pronounce definitely upon their correctness.

Extrusive Igneous Rocks. For petrographical purposes
two chief modes of extrusion may be recognized, the
quiet one, giving rise to outwellings of magma in the liquid
state which then solidifies to rock, and the explosive, in
which the material by the violent action of gases is pro-
jected into the air and falls in a solid but fragmental form.

Quiet Eruption; Lava Flows. When the magma rises
to the surfaces and outpours it is then called lava. The
solidified material is often called a sheet of lava or extrusive
sheet. Such flows often come from volcanoes; the
extrusions of some, like those now active in Hawaii, being
wholly of this nature, while in others they alternate with
or succeed projections of explosive fragmental material.



In other cases they are not connected with volcanic
eruptions but have taken place as quiet outwellings from
numerous fissures. This has sometimes occurred on a
huge scale, as in the Columbia River region of the north-
western United States, in western India and in the north
of the British Isles. In these regions the repeated lava
flows are thousands of feet in depth and cover areas of
from 100,000 to as much as 200,000 square miles.

Not infrequently sheets of lava have sunk below sea-level and
been covered by deposits, or they have originated on the sea floor and
have been covered. Such buried extrusive sheets are distinguished
from intrusive ones by the fact that they have not altered or changed
the sediments above them by contact metamorphism (qu. vid.), and
their upper surfaces usually show the structures common to the
surface of lavas, such as the vesicular, amygdaloidal, scoriaceous
and ropy ones described later.

Explosive Eruption; Tuffs and Breccias. When a
magma attains the surface in the canal of a volcano it
may give rise to quiet flows of lavas as mentioned above,
or if its viscosity is sufficient and it is charged with vapors
under great tension it will give rise to explosive activity,

Fig. 66. Diagram to Illustrate the Occurrence of Igneous Rocks : 6, bathylith;
s, stock; n, volcanic neck forming v, a. volcano with tuffs and breccias; /, I, lacco-
liths; i, intrusive sheet; 6, extrusive sheet; d,d, dikes. Horizontal distance
shown, thirty miles; vertical distance, three miles.

and the material will be projected into the air to fall in
solid fragmental form. Owing to the expansion of the
vapors, chiefly steam, the projected pieces usually have
a more or less pronounced vesicular structure, and vary
in size from those weighing perhaps several hundred


pounds to dust so fine that it floats for long periods in the
air. According to size these may be roughly classified
as follows. Pieces the size of an apple and upward are
called volcanic bombs; those the size of nuts are termed
lapilli; those the size of small peas or shot volcanic ashes;
while the finest is volcanic dust. The coarser material,
the bombs, ashes and lapilli, falls around the vent and
builds up the cone; the lighter ashes and dust, carried
by air currents, tend to fall after these and at greater
distances. The beds of coarser material thus produced
are termed volcanic conglomerate or more commonly vol-
canic breccia, while the finer is known as tuff.*

General Characters of Igneous Rocks.

Since igneous rocks are formed by the consolidation of
molten magmas it is evident that the nature of a rock
produced must in large measure depend upon the chem-
ical composition of the magma which forms it. For
most rocks are composed of mineral grains, and the kinds
and relative amounts of these must depend upon the
kinds and relative amounts of the chemical elements
which form the molten fluid. It is pertinent therefore
to inquire what the general chemical character of the
earth's magmas is like and if there are any general rules
which appear to govern their composition.

Chemical Composition of Magmas. We cannot of
course subject a molten magma directly to investigation,
but this may be essentially done if an average sample of
an igneous rock is subjected to chemical analysis. Several
thousand such analyses have been made of rocks from
all parts of the world, and these results show, as might be
expected from the discussion given on page 17 and
following, that the magmas and therefore the rocks are

* (Volcanic tuff was formerly commonly called volcanic tufa, but
at the present time it is customary to restrict the word tufa to
deposits from aqueous solution, especially those of a calcareous



made up of the following oxides: silica, SiO 2 ; alumina,
A1 2 3 ; iron oxides, both ferric, Fe 2 O 3 , and ferrous, FeO;
magnesia, MgO; lime, CaO; soda, Na 2 O, and potash, K 2 O.
Other oxides, including water, are also present but in
such relatively small amounts that they do not exercise
any controlling influence and may be neglected.

The variations in chemical composition which are
shown in the magmas are in a general way exhibited in
the following table of selected analyses.









SiO, .









Fe 2 3









FeO .









MgO .









CaO .









Na 2 O.
K 2 .









Rest .









Total .









I, Nephelite Syenite, Serra di Monchique, Portugal; II, Syenite,
Highwood Mountains, Montana; III, Granite, Castle Mountains,
Montana; IV, Quartz Diorite, Electric Peak, Yellowstone Park;
V, Diorite, Montgomery County, Maryland; VI, Gabbro, Red
Mountains, Montana; VII, Peridotite, Devonshire, England; VIII,
Dunite, Tulameen River, British Columbia.

Variation of Magmas and Mineral Composition. It is

not to be understood that all the different varieties of
magmas are represented by these analyses; they are only
selected to show the most prominent and general features
of variation. Certain of these can be readily seen by
observing the table. Thus in the first three analyses it
is evident that silica, alumina and the alkalies, potash
and soda, are the chief oxides composing them, while
lime, iron and magnesia play a very subordinate part.


It is therefore evident that if such magmas should
crystallize into minerals they would be mostly composed
of alkalic feldspars because these are composed of silica,
alumina and alkalies. Again, if we regard the amounts
of silica in these three and remember that orthoclase,
potash feldspar, contains about 65 per cent of silica and
albite, the soda feldspar, about 68, it is clear that in
No. Ill there is more silica than needed to form the alka-
lies and alumina into feldspars, and there will therefore
be extra silica which will crystallize as free quartz. In
No. I, on the contrary, there is not enough silica to turn
all of the alumina and alkalies into feldspar, and a certain
amount of some mineral, such as nephelite, which con-
tains these oxides in combination with a smaller amount
of silica must be formed to compensate this deficiency.
In No. II the per cent of silica is very nearly that required
for the pure feldspars, and these will make up the great
bulk of the rock with little either of quartz on the one
hand, or of nephelite on the other.

If now we turn our attention to the oxides of lime,
iron and magnesia, it is evident that the minerals which
they produce, such as biotite, hornblende and pyroxene,
will have but a subordinate role in the first three rocks,
but in Nos. IV-IX these oxides continually increase
while silica alumina and alkalies also decrease, and even-
tually the last two vanish and the silica becomes very
low. Expressing this in terms of minerals, if the magmas
crystallized, it is evident that in these four the ferro-
magnesian minerals those containing iron or magnesia
or more commonly both such as pyroxene, amphibole
or olivine, would play an increasingly important role, and
that the last rock would be wholly composed of them,
while feldspars correspondingly become less important
and ultimately disappear.

In this connection the variation of lime deserves a sepa-
rate word because lime has a dual function : it may form
a feldspar with alumina and silica which then commonly



combines with soda feldspar to form plagioclase (soda-
lime feldspar), or it may enter into the ferromagnesian
minerals, pyroxene and amphibole. It generally does
both and thus for a time as we follow the table of analyses
from left to right, as lime increases, the quantity of both
plagioclase and of ferromagnesian minerals increases also.
Coincident with this the alumina also increases somewhat.
Variation shown by Diagrams. The facts which have
been stated above may be shown in a graphic manner by

means of a simple dia-
gram, Fig. 67. Thus in
the place of the analyses
of the foregoing table
we may draw verti-
cal lines, one for each
analysis, at equal dis-
tances apart and on each
line set off a vertical
distance in millimeters
equal to the per cents
of each oxide in the
analysis. Through these
points lines are drawn
corresponding to each
oxide, the iron and
magnesia from the simi-
larity of function they

exhibit being united in one line. The equal distances for
each analysis at the foot of the diagram thus serve as
abscissas and the percentages are ordinates, while the
connecting lines approach curves which show the mutual
relations of the oxides. In the description of the variation
of the oxides it was pointed out how this caused a corre-
sponding variation in the minerals produced by the crys-
tallization of the magmas composed of these oxides. By
considering the relative amounts of the important minerals
which each type of analysis would produce we can con-

Fig. 67. Diagram to Illustrate Chemical
Variation of Igneous Rocks


B. Syenite, mostly Feldspar.

C. Diorite, some Feldspar.

D. Peridotite, no Feldspar.



struct a diagram, Fig. 68, which will show the variation
of the minerals in a general way in the common rocks.
It also shows the relative proportions of the minerals in
the more common and important kinds of igneous rocks,

Fig. 68. Diagram to Illustrate the Variations and Relative Proportions of the
Minerals Composing the Important Igneous Rocks.

and it serves as a basis for their classification as will be
explained later. The relative proportions of the minerals
are given in the vertical direction, the variation and pas-
sage of one kind of rock into another in the horizontal

It should be repeated that these diagrams and the
table of analyses are not to be taken in a hard and fast
manner as representing the limits of variation and all the
possible mineral combinations of igneous rocks. This
would be very wide of the truth. Other analyses might
be selected which would yield different diagrams, and if
of rare and uncommon rocks, they might be very different
indeed, but in a general way these may be accepted as
showing the more important chemical and mineralogical
features which distinguish the common kinds of igneous
rocks from one another.

Minerals of Igneous Rocks. From what has been


stated in the foregoing sections it is evident that the more
important minerals which compose the igneous rocks are
the feldspars, quartz and the ferromagnesian group. For
purposes of classification to be explained later it is con-
venient to contrast the ferromagnesian on the one hand
with the quartz and feldspars on the other. Recalling
that silica (Si0 2 ) and alumina (A/ 2 3 ) are prominent
substances in the composition of these latter minerals,
and following American petrographic usage we may term
this group the salic one. More specifically the prominent
minerals of the igneous rocks are given in the following

Alkalic Feldspar Pyroxenes
Plagioclase Feldspar Amphiboles
Quartz Biotite


Sodalite Iron Ores


The last three in the salic group are of much less impor-
tance than the first three on account of their restricted
occurrence; the iron ores, hematite, ilmenite and mag-
netite, though so widely distributed that nearly all igneous
rocks contain one or more of- them, are of less importance
than the other ferromagnesian minerals because they
usually form only a very small proportion of all the
minerals in the rocks. A mineral, like these, which may
be quite evenly distributed through a rock but makes only
a small part of its mass is called an accessory component
in contradistinction to those which form its main bulk
and are termed chief or essential components.

The chemical and physical characters of the minerals
mentioned in the above list have been described under
their appropriate headings in Part II, to which reference
may be made, for further information concerning them.

Order of Crystallization. If a polished surface of a


coarse-grained rock be attentively studied with a lens, or
better if a thin section be observed under the microscope,
it will usually be found that there are more or less distinct
evidences that all the minerals composing it have not
crystallized simultaneously but successively. Thus in
Fig. 69 the crystals of biotite mica (M) contain grains and
octahedrons of black iron ore, magnetite; they occur also
in the other minerals. They are evidently older than the
mica because they are inclosed by it. The mica is older
than the soda-lime feldspars or plagioclases (P) because
it abuts into them with its own crystal faces or is partly

Fig. 69. Diagram to Illustrate Successive Crystallization

inclosed by them as they grow around the already formed
crystals. In the same way the plagioclase has its own
form as regards the alkali feldspar, orthoclase (O) and
the quartz (Q), which surround it, and is therefore
judged to be older than they are. When the orthoclase
and quartz are considered they do not show any crystal
boundaries with respect to one another, and their crys-
tallization is therefore judged to be more nearly simul-


taneous. The order of crystallization as thus worked out
in this particular case is: first, magnetite, then biotite
mica, then plagioclase, and lastly orthoclase and quartz.

The studies which have been made of igneous rocks
teach that in general the order of crystallization is : first,
the oxides or ores of iron, then ferromagnesian minerals,
then soda-lime feldspars, then alkalic feldspars (and
feldspathoids) and lastly quartz. One observes from

this, as illustrated in the

1. Magnetite, Fe 3 4 . adjoining table that the

2. Pyroxene, (MgFe)Ca(SiO 3 ) 2 . process begins with metallic

Q P!o.ria * ( mCaAl 2 Si 2 8 - oxides which contain no

3. Plagioclase, I ,, .,J. i 8 ... ,

UNaAlSi 3 O 8 . silica, that next come the

4. Orthoclase, KAlSi 3 O 8 . ferromagnesian minerals,

5. Quartz. SiO 2 . ,, , .,.

ortho and metasilicates, then

feldspars which contain more

silica and finally quartz or free silica. Thus there tends
to crystallize out successively minerals richer and richer in
silica. It is not to be understood however that one mineral
necessarily finishes its period of crystallization before
another one begins as in A, but rather that they overlap

Flag. Orthoclase. Quartz.



as in B, that is, that one may begin before another has
finished, and continue after the former has ceased. Expe-
rience shows that with orthoclase and quartz the overlap
is so great that they crystallize nearly simultaneously,
only orthoclase usually begins and quartz finishes.

Insolubility vs. Infusibility. A molten silicate magma is
to be regarded as a complex solution of some compounds in
others, like a solution of mixed salts in some solvent such


as water. As the heated solution cools, a point is reached
where some compound, or mineral, becomes insoluble in
the resulting solution and it therefore crystallizes out.
The statement that is sometimes made that the minerals
crystallize in the order of their fusibility is entirely wrong;
thus from the preceding paragraph we see that pyroxene
crystallizes before quartz; now pyroxene is rather readily
fusible before the blowpipe, while quartz is infusible. It
is not therefore a question of infusibility but of solubility
which determines the order of crystallization.

Influence of Mineralizers. Experience teaches us that
those magmas which attain the surface in volcanoes and
in lava flows contain large quantities of volatile sub-
stances, especially water vapor, which they give off,
frequently with explosive violence. It was formerly
considered that the magmas imbibed these from the
moisture laden rocks with which they came in contact on
their way to the surface. At present these volatile sub-
stances are generally held to be wholly or in large part of
magmatic origin, that is, original constituents of the
earth's interior molten masses, contained therein under
pressure. Without further regard to the theories of how
they came to be there we know that the magmas contain
them and that they are of great importance in a number
of ways in the formation of igneous rocks. The most
important of these is water, but carbon dioxide, fluorine,
boric acid, sulphur and chlorine are also prominent and
may produce important results. The work of various
investigators, especially the French, has shown, that
while some minerals such as pyroxene, magnetite, lime
feldspar, olivine and nephelite may be artificially pro-
duced by fusing their constituents together and allowing
the molten mass to cool slowly, other minerals such as
hornblende, biotite, orthoclase and quartz do not form
in dry fusions in the same way. For their production
more or less of the volatile substances mentioned above
must be present, especially water vapor. These sub-


stances appear to act in two ways: in one in a chemical
manner since some minerals, such as biotite and horn-
blende, always contain small quantities of water (in the
form of hydroxyl, - OH) or fluorine or both, and these
are consequently necessary for their production; and
second, in a physical manner in that they lower the
melting point of the fusion and greatly increase its fluidity.
Thus orthoclase, albite and quartz which have extremely
high melting points but only crystallize at much lower
temperatures, in a dry fusion become so viscous on cooling
that they are unable to crystallize and therefore solidify
as glasses. The addition of water under pressure lowers
the temperature of solidification and increases the fluidity
or mobility of the melted mass and permits such move-
ment of the molecules that they can arrange themselves
in crystal form, and the above minerals are produced.
These substances then, such as water, fluorine, etc., which
exert so important a function in processes of crystalliza-
tion and on the formation of igneous rocks are called
mineralizers. As crystallization progresses, the amount
of them, beyond what is chemically (and to some extent
mechanically) retained in the minerals, is gradually
excluded from the solidifying rock mass to play an active
role in new and important processes, such as the forma-
tion of pegmatites, contact metamorphism and others
which will be described later in their appropriate places.

Texture of Igneous Rocks.

It has been pointed out that igneous rocks vary in the
kinds and proportions of the minerals that compose them
and that this variance is mainly due to the chemical
composition of the magmas from which they are derived.
Another important way in which these rocks vary is in
their texture. Thus one rock may be made up of mineral
grains so large that the different minerals are easily
distinguished, while in another the grains are so small as
to defy identification by the eye or simple lens. Again


the grains may be approximately all of about one size or

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 12 of 35)