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 14 of 35)
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regular hexagons and the cracks penetrating downward


A. High Isle Quarry, Maine.

B. Allen Quarry, Mount Desert, Maine.

(U. S. Geological Survey.)


make columns. This regular arrangement produces the
greatest amount of contraction with the least amount
of cracking, provided the centers are equally spaced.
But as the contractional centers are not always equally
spaced, three, four, five and even seven-sided columns
occur. The columns again, contracting lengthwise, break
into sections as they form. The same principle is also
seen in drying mud-flats which crack into polygonal
shapes and in the prisms of drying and contracting starch.
Such columns, however regular their appearance, are
not crystals but pieces of rock and should not be confused
with the hexagonal prisms produced by the crystallization
of certain minerals, such as quartz, beryl, etc., which are
due to an entirely different process.

Inclusions in Igneous Rocks.

Not infrequently there may be noticed in igneous
rocks masses which differ in mineral composition, color
and texture from the rock which includes them. They
may vary in size from a fraction of an inch to several yards
across. Study of them shows that sometimes they present
all the characters of distinct kinds of rock and by these,
and by their angular shapes, they show clearly that they
are fragments of pre-existent rocks which the magma on
its way upward has torn loose from the walls of its con-
duit and brought along, or blocks from the roof or sides of
the chamber, in which the magma came to rest, which were
loosened and sank into it. They may be composed of
other kinds of igneous rocks or of sedimentary ones, such
as shales, limestones, etc. In the former case they are
not usually much changed, but the fragments of stratified
rocks generally exhibit the results of intense metamorphic
action, such as described elsewhere, and are much altered.
In large intrusive masses inclusions of this character are
most apt to occur near the border. An inclusion in
granite is shown in Fig. 1, Plate 12.

In other cases the inclusions are composed of certain


minerals, especially the ferromagnesian ones, which occur
in the rocks and which by some process have been aggre-
gated into lumps, such as the masses of olivine crystals
often found in basalts. It is clear that such aggrega-
tion or growth of these minerals must have taken place
while the remainder of the rock was still in a liquid
condition. They have been termed segregations.

In still another kind the inclusions are indefinite in
form and often of boundary; they are apt to be drawn out,
lenticular, streaky in character and they may consist of
the same minerals as the main mass of the rock but in
quite different proportions, or they may contain dif-
ferent minerals. Thus one sees streaks in granite which
may be much richer in hornblende or biotite than the
enclosing rock. Some have held that these are due to
inclusions of other rocks which have been melted up and
then recrystallized and in some cases they may have had
this origin, but for the most part they are regarded by
the majority of petrographers as caused by streaks and
spots in the original magma of a different chemical com-
position from the main portion. The cause of such non-
homogeneousness in the magma is ascribed to differentia-
tion, as discussed elsewhere in this volume. Such streaky
portions are called by the Germans schlieren and in default
of anything better this word is often used for them in

Sometimes lavas show a streaky or even well banded
structure, portions differing from one another in com-
position or in texture having been drawn out in the
flowage. This is known as the eutaxitic structure.

Origin of Igneous Rocks Differentiation.

The fact that lavas differing decidedly from each other
in mineral and consequently in chemical composition
have been erupted by the same volcano at different
periods, early attracted the attention of geologists and led
to much speculation as to its cause. Thus felsites and


2 D


basalts have both been frequently noticed as the products
of eruption from a single vent. One explanation, which
used to be advanced, was that within the earth there were
two layers of magma, an upper one rich in silica, alumina
and alkalies, the other and lower, poor in silica but rich
in iron and magnesia; accordingly as the eruption came
from one or the other of these, felsites or basalts were
produced, while their mixtures gave rise to intermediate
products. It was soon seen, however, both on chemical
and geological grounds, that this view was insufficient to
explain the origin of all igneous rocks.

As the study of rocks progressed, other facts of a similar
nature came to light. Thus in the single rock mass com-
posing the core or neck of an old volcano,* where the
magma cooled under conditions favorable for the pro-
duction of the even-granular or granitic texture, it is not
infrequent to find that it is composed of two or more
distinct kinds of rock. The boundary between these
will sometimes show that one was erupted after the other
had solidified in its place, since fragments of the latter are
enclosed in the former. This is of course merely carrying
deeper down into the volcanic conduit the same facts
shown by the surface lavas previously mentioned. Other
cases, however, are of a different nature and of such
geological importance that they demand separate con-

Border Zones. In recent years the study of deep-seated
intrusive masses, such as stocks of granite, syenite, etc.,
which have become exposed by long continued erosion,
has shown that not uncommonly such masses have an
outer border or mantle of rock which differs in mineral
composition from the mass which it enfolds. The thick-
ness of such a border zone is very variable, even in the
same mass, and in places it may be lacking; it may be
several thousand feet thick or only a few hundred or even
less. While in general it bears some proportion to the

* See volcanic necks, page 138.


general size of the whole mass there is no rule about this
which can be stated.

In most cases this zone or border jades, as it is some-
times called, is produced by an enrichment of the rock
in the ferromagnesian minerals, such as pyroxene, horn-
blende, biotite and iron ore. Generally the enriching
minerals are the same as those more sparsely distributed
in the main rock body but very often different ones are
observed among them. From this it is clear that chemi-
cally the border zone is richer in iron and magnesia, and
to some extent in lime, than the main mass, with a corre-
sponding diminishing of silica, alumina and alkalies.
Since they contain less of silica, the acid oxide, they are
commonly called basic zones. Not all border zones,
however, are basic ones; a number of instances are known
where the margin of the intrusion is poorer in lime,
iron and magnesia and consequently in ferromagnesian
minerals than the interior rock body and therefore con-
tains more silica, alumina and alkalies, which expresses
itself mineralogically in greater abundance of feldspar
and sometimes of this and quartz. In this case they are
called acid border zones. Thus on the one hand intrusions
of syenite have been found which pass into pyroxenite at
the border while on the other hand syenite intrusions are
known which become granite towards the margin. It
must not be imagined that there is anything approaching
a contact between the two kinds of rock. The one kind
passes gradually into the other without change in texture
and all the facts indicate that this arrangement was not
produced by successive intrusions of different magmas
but by some process in a single body of magma after it
had entered into its chamber.

Zoned Laccoliths. The zonal arrangement just men-
tioned is still more strikingly shown in the case of certain
laccoliths which have been found in Montana and else-
where. Where these have been laid bare and dissected
by erosion the study of them shows that they consist of a


body of rock of one kind, generally one consisting mostly
of pyroxene, enclosing within a core of rock of a totally
different kind, usually a syenite, which is of course chiefly
feldspar. A cross section through such a laccolith is
shown in the accompanying diagram, Fig. 70.

6 .

Fig. 70. Diagram of a Zoned Laccolith: a, feldspar rock; 6, pyroxene rock;
c, shales and sandstone ; d, underlying sheet of intrusive basalt. Figures
in feet are heights above sea-level.

That the pyroxenic rock once had the extension shown
by the restoration in the figure is known from other
examples in the neighborhood where the erosion has not
been so great, and it is still found above, enwrapping the
interior syenite.

Associated Complementary Dikes. Another phenom-
enon, of the same category as those just described, is seen
in the dikes so commonly found associated with larger
intrusive bodies, such as stocks of granite, syenite, diorite,
etc., where these have become exposed by dissective
erosion. They are in origin subsequent to the main mass
which they accompany and are found cutting it and also
the surrounding rocks. In the latter, these minor intru-
sions may appear, not only in the form of dikes, but also
in intrusive sheets, laccoliths, etc. These rocks are
divisible into two classes; in the first they are very poor
or entirely wanting in ferromagnesian minerals (salic
rocks) and have been called aplitic dikes, since the dikes
of aplite usually found associated with granites are the
most common and best known representatives of this
class. They have also been called leucocratic dikes (from
the Greek, prevailing white) in allusion, to their general
light color, due to the fact that they are mostly composed
of feldspars or of these with quartz. They are generally


fine-grained rocks, sometimes of a sugar granular texture,
sometimes dense and to be classed as felsites. In some
cases they are porphyritic. They usually occur in narrow
dikes, a few feet wide and sometimes only an inch or even
less in breadth.

In the second class the rocks are heavy, dark or even
black, of basaltic aspect and composed chiefly of ferro-
magnesian minerals, iron ore, pyroxene, hornblende,
biotite and olivine, in variable amounts and with very
subordinate feldspar. They are very commonly por-
phyritic with good-sized phenocrysts of the minerals
mentioned above in a dense dark groundmass, though
these are often wanting. Such rocks have been called
lamprophyres (from the Greek, meaning glistening por-
phyry in allusion to the biotite), and are termed melano-
cratic rocks (/teAavos, black) . In our field classification
they would be named biotite melaphyre (or mica trap),
hornblende melaphyre, etc., according to the prevailing
phenocrysts. They also usually occur in narrow dikes
and are more apt to cut the surrounding rocks than the
main intrusive body they accompany, thus reversing the
custom of the aplites.

These two kinds of rocks, the aplitic, light-colored
feldspathic, and the lamprophyric, dark-colored, with
ferromagnesian minerals, are termed complementary
because taken together they represent the composition of
the main masses they accompany. If we could mix them
in amounts proportional to the bulk of their occurrence
we should obtain a rock whose chemical (and largely
mineral) composition would be that of these larger masses
upon which they appear to depend as satellite bodies.
In some cases this has been actually tested and proved.
When all the facts concerning their mode of occurrence are
taken into account they appear to have been formed by
secondary, later intrusions of the same magma producing
the larger stocks, which in some way has divided into
two unlike sub-magmas. If they should break through


to the surface they would give rise to lava flows also
unlike, to felsites and basalts, and thus explain in part the
phenomena noticed in many volcanoes.

It is to be understood of course that not all dikes,
sheets and laccoliths belong in this category of com-
plementary rocks. On the contrary we very often find
that the same magma which produces stocks, necks,
etc., occurs in intrusions of this character. They then
have the same minerals and composition as the larger
masses, or if independent bodies they usually contain both
ferromagnesian and feldspathic minerals in due amounts.
Only, as explained on page 153, they are liable to differ
in texture from the stocks and are very apt to be por-
phyries. Dikes, etc., of this kind have been called
aschistic, which means undivided, while the complementary
aplites and lamprophyres have been termed diaschistic,
which means divided, in allusion to their dual nature.

Differentiation. The varied lavas of volcanoes, the
marginal zones of stocks and necks, the zoned laccoliths
and the associated complementary rocks, which have been
described in foregoing sections, as well as other similar
features, present to us a body of geological facts that can
only be satisfactorily explained by the assumption that
in some way magmas, which form igneous rocks, have
the capacity of separating into sub-magmas, unlike the
original, but which, if mixed in proper proportions to a
homogeneous whole, would again reproduce it. Regard-
ing the division there seems to be in general two opposite
poles toward which the sub-magmas tend; to one con-
centrate the iron, magnesia and to a large extent the lime,
to the other the alkalies, alumina and to a great extent the
silica. The one gives us ferromagnesian rocks such as gab-
bro, the other feldspathic rocks such as granite. While
this is so in general, we find in detail the process infinitely
varied in nature; thus in some places one may observe a
division among the alkalies, an enrichment of potash
towards one pole as compared with soda or vice versa.


If the body of magma has come to rest in its chamber and
this process of differentiation takes place and it then,
crystallizing, solidifies and forms rock, it is evident that
such a rock body will be unlike in its different parts, and
marginal zones, zoned laccoliths, etc., will be produced; or
if further movements occur, producing new intrusions or
these with extrusions, then associated complementary
dikes, sheets and lava flows may occur.

This division into sub-magmas is termed the differen-
tiation of igneous magmas and the reality of it as a process
seems well established on geological grounds by a large
body of facts. That in some manner such a process
takes place and on the other hand the understanding of
how and why it does take place, are two entirely different
affairs, and while every one who is thoroughly conversant
with the facts is obliged to admit the former, a wide
diversity of views, owing to insufficient knowledge, pre-
vails in regard to the latter. Some phases of this subject
are discussed in the following paragraphs.

Formation of Zones and Ore Bodies. One partial
explanation that has been offered for the zoned structures
previously mentioned is of importance because it affords
at the same time an understanding of the origin of a
certain class of ore bodies which in some places are of
considerable extent and value. On page 148 it was shown
that there was a general order of crystallization of rock
minerals beginning with the iron ores, then passing into
the ferromagnesian silicates and finishing with the feld-
spars and quartz. In an enclosed body of magma,
crystallization would generally begin when the tempera-
ture had fallen to the proper degree. This would natu-
rally first occur at the outer walls where the effect of
cooling is felt. Against these the iron ores and ferro-
magnesian minerals, the earliest to crystallize, would
form and, if the process were extremely gradual, slow
convection currents in the magma would bring fresh
supplies of material to crystallize there until large


amounts of these minerals had formed. This might go
on until the temperature had fallen to a point where the
main body of magma was compelled to solidify and the
rock mass as a whole produced. The outer margin would
be much enriched in the earlier formed minerals, giving
a zoned arrangement to the whole mass. In such places
at the margin the iron ores are sometimes so locally con-
centrated as to yield workable deposits of value, though
very commonly the ore is titaniferous and therefore can-
. not be used commercially. The same explanation has
been offered for the occurrence of sulphide ores of iron
containing copper and nickel, of corundum and of other
useful minerals found in similar situations.

Origin of Salic Border Zones. The explanation given
above would show how marginal zones richer in ferro-
magnesian minerals might arise but it has been observed
that masses of granitic and syenitic rock are sometimes
poorer or deprived of these minerals at the margin of the
mass while the main part contains them in considerable
amounts, thus making salic zones. An explanation which
has been offered for this is as follows: If a solution of
a salt in a liquid (such as sea-water) be cooled down until
it is forced to crystallize (freeze) it is found that the sub-
stance in greatest excess, salt or liquid, will solidify first
until a certain definite proportion of dissolved salt and
liquid are obtained, called the eutectic mixture, when both
remaining salt and liquid will crystallize simultaneously
and the whole mass become solid. The proportion of salt
to fluid, forming the eutectic, varies with the kind of salt
and of solvent. Thus when sea water freezes the ice first
formed contains no salt, the latter forming in the remain-
ing water a brine of increasing strength until the eutectic
point is reached, when both solidify together. In the
case of granite and syenite the oxides composing the
quartz and feldspars are present in great excess and may
be considered the solvent for the others. It is possible
that under proper conditions these might solidify at the


outer margin, the other oxides, those of iron, magnesia,
etc., concentrating in the remaining portion and tending to
make an eutectic mixture. Thus when the whole solidifies
the inner part will contain ferromagnesian minerals, and
the outer part will be poor or wanting in them. In the
case of many diorites and gabbros, where the oxides of iron
and magnesia are in great excess, they would be the sol-
vent, and we should expect border zones of ferromagnesian
minerals. It is evident this explanation, and the one
previously given, which depends on the order of crystal-
lization, in the case of highly feldspathic rocks, are
opposed to each other; the first tends to make ferromag-
nesian zones around granite and syenite, the latter salic
ones. In the diorites and gabbros both tend to produce
margins richer in ferromagnesian minerals.

Zones by Absorption. It has also been suggested that
such zones are produced by the magma melting its con-
taining walls and thus, by absorbing foreign material,
becoming in composition, at its border, unlike the main
mass. Being thus unlike it would naturally have a
different mineral composition on solidification. It is
possible that this may have happened in some cases but
it cannot serve as a general explanation because in many
cases we find the border of an entirely different mineral
(and chemical) composition from that which it ought to
have if the rocks with which it came in contact had been
melted and absorbed.

General Explanation. It is obvious that the hypotheses
discussed above, while they may serve to explain border
zones and marginal ore deposits, do not give a general
explanation for the differentiation of igneous rocks. For
the occurrence of complementary dikes, of different lavas
from the same volcano, and the mixtures of different
types, which are not marginal, in the same stock, as well
as other facts, show clearly, that in general, differentiation
is not a division by a process of solidification, but one
which occurs in a magma in such a manner as to produce


separate bodies of differing liquids which may be inde-
pendently ejected or intruded. It must occur before
there is any solidification. While we see that this is so,
both from geological and chemical facts, no general
explanation, which is in all respects satisfactory, has been
offered for this process. Different hypotheses, which it
would be beyond the limits of this work to state and
discuss, have been suggested by various authorities, but
our knowledge of the physical chemistry of molten magmas
is yet too limited to know their proper value and appli-
cability. It is probable the processes of differentiation
are quite complex and that they are produced by a
variety of factors, the laws governing which must all be
taken into account in any general explanation. It is
known that molten artificial glasses and molten alloys
of metals, under conditions not yet well known, do not
remain homogeneous but undergo a kind of differentiation,
and it is along this line of experimental research that
light must be sought to explain the facts as we find them
in Nature.

Petrographical Provinces. Consanguinity. It has been
noticed in the study of rocks, that those belonging to
certain regions have particular features which to a greater
or less degree are found to be distinctive of all the members
of the group which occur there. This is shown, sometimes
in the presence of particular varieties of minerals, some-
times in peculiar textures, sometimes in peculiarities of
chemical composition and usually in a combination of
these things. They may be shown in varying degrees by
all the different rocks of the region: thus, for example, by
syenites which are chiefly composed of feldspar and by
dolerites in which ferromagnesian minerals prevail; in in-
trusive stocks of granular rocks with their associated com-
plementary dikes and sheets and in lava flows of felsites
and basalts. These common characters are sometimes
strongly marked and at other times only to be seen by
the experienced observer. The fact that such distin-


guishing features occur in the different types of a certain
region and serve to indicate their relationship to one
another and to show a common origin by differentiation is
termed the consanguinity of igneous rocks, and that region
over which the rocks thus show genetic relations is called
a petrographic province, or comagmatic region. Thus the
comagmatic region of South Norway is characterized by
the extremely high percentage of soda in the magmas,
which gives rise to certain minerals and peculiar rock
textures ; those of Italy and central Montana by very high
potash which shows itself in the formation of the mineral
leucite, common in such regions but rare or unknown else-
where; that of the western Mediterranean islands and
eastern Spain by an abnormally high amount of titanic
oxide in its rocks.

Such evidences of consanguinity in rock groups and the
proofs which they furnish of comagmatic regions cannot
usually be observed in field work and in the megascopic
study and determination of rocks. They generally
demand careful and complete investigation of thin sections
under the microscope, aided by chemical analyses in the
laboratory, together with a broad acquaintance of the
literature of this subject, in order to be perceived and
appreciated. The matter, however, is one of great interest
and although one may not be either a chemist or petro-
grapher, he may yet appreciate the significance of its
bearing on the solution of problems of the greatest impor-
tance in geology. It is evident that before we can safely
theorize as to the origin and history of the earth we must
first know the nature of its component parts and the laws
governing their distribution.

Post-intrusive Processes,

When a body of molten magma has come to rest in the
chamber it is destined to thenceforth occupy as a solid-
ified rock mass, cooling and eventually crystallization
begin. From this point on, so far as the magma is con-


cerned, those factors are at work which have been

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 14 of 35)