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 15 of 35)
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described elsewhere, and which in time will produce the
completed rock. During this period of crystallization
.the volatile substances dissolved in the magma and
previously contained under pressure, such as fluorine,
boric acid, carbon dioxide and especially and chiefly
water, which have been already described as mineralizers,
are gradually excluded, except in so far as they may take
part in the chemical composition of some of the minerals.

This period in the history of the formation of a rock
body, when it is solidifying and giving off, as it crystal-
lizes, heat and vapors, is called the pneumatolytic (Greek,
gas, and to loosen), and these agents generate important
results. At the surface they give rise to hot springs,
fumaroles, solfataras, and similar secondary igneous
phenomena; in the depths they produce in the rocks
surrounding the igneous mass a variety of features known
under the term of contact metamorphism, and in the already
solidified parts of the igneous mass they bring about the
formation of pegmatite dikes, of greisen (described under
Granite) and in some cases of ore deposits; things which
are treated in the following sections.

Pegmatite Dikes or Veins. It has been previously
stated that when deeply formed stocks or masses of granite,
syenite, diorite, etc., have been laid bare by erosion they
are very frequently found to be cut by complementary
dikes of felsitic and basaltic aspect. In addition it is also
frequently observed that they are penetrated by dikes
which display certain definite characters, the most marked
of which is the very large and sometimes enormous size of
the individual minerals composing them. Such dikes
have been termed pegmatite dikes, from the name given
by Hauy to the intergrown masses of quartz and feldspar
found in them when they occur in granites. Dikes of
this character not only cut the stocks and batholiths, to
whose intrusion they owe their origin, but are also found
penetrating, as offshoots, the rock masses enveloping


them. There are a number of features which particularly
characterize them, as follows:

a. They consist in large part of the ordinary minerals
which compose the rock to which they belong, but these,
instead of having their regular order of successive crys-
tallization, show by their interpenetration that they
have crystallized more nearly if not entirely, simul-
taneously.* The size of the individual crystals is a
character that has been mentioned. Feldspar and quartz
may occur in crystals a foot or even several feet long,
apatite in dimensions like the handle of a broom, mica in
crystals yielding plates a foot or more in diameter and
other minerals in similar proportions. It is not to be
understood that these sizes represent the average; they
are the extremes which are, however, not infrequently
attained. Moreover, the essence of pegmatite structure
does not lie in mere size, for many rocks are very coarse-
grained which are not pegmatites, but rather in the other
qualities enumerated.

6. Another peculiar feature is that in many pegmatites
there is .an obvious tendency for the minerals to grow
outward from the walls of the dike on either side and
project inward toward the center. This may become
very marked and there may even be an empty space at the
center into which the minerals project showing crystal
faces as in miarolitic cavities (page 159) or in the vuggs of
mineral veins. The whole effect is to produce in a rough
way a zoned, banded or ribbon structure, which is often
so perfectly seen in mineral veins.

c. Another character is the extreme variability in the
relative proportions of the component minerals from
place to place, a variability not seen in the main rock
mass. Thus in granite pegmatites traced along the
outcrop of the dike great variations in the relative amount
of quartz and feldspar may often be observed; in tracing

* See in connection with this the description of graphic granite
in the granite pegmatite veins, p. 212.


them outward from the parent mass into the enclosing
rocks they may even pass into pure quartz veins. In
other cases they may turn into fine-grained granite
(aplite) or felsite, and this change in the character of the
dike may occur quite suddenly.

d. They are very apt to contain accessory minerals
which are either not found at all in the main rock mass
or which microscopic examination shows are sparingly
distributed in very minute crystals. These minerals may
be divided into two classes. In the one their chemical
composition shows that they consist of the ordinary
oxides which compose the magmas, alumina, lime, iron,
soda, etc., plus the volatile elements or oxides which form
the mineralizing vapors. Common ones are tourmaline,
which shows the presence of boric acid; topaz and fluorite,
which demonstrate that fluorine was present and many,
of which muscovite mica is perhaps the most prominent,
which prove the important role played by water vapor.
The other class are characterized by their containing in
larger or smaller amounts the oxides of rare elements,
such as lithium, caesium, beryllium, molybdenum, cerium,
lanthanum, niobium, etc., elements which are detectible
with difficulty as minute traces or not at all in the main
rock mass. In combination with silica, titanic oxide,
phosphoric acid, zirconia, carbonic acid, fluorine, etc.,
they give rise to a whole host of mineral combinations,
too numerous to mention, but of which beryl and spodu-
mene may be cited as examples. No sharp distinction
can be drawn between these two classes; many minerals
might be placed in either, but definite types of both, like
those mentioned, can readily be found.

These accessory minerals often occur also in crystals
of great size and sometimes aggregated together in places
in the dikes in very large amounts. It is due to this great
variety of minerals and the frequent size and perfection
of the crystals that the pegmatite dikes are mineralogically
of great interest and are, therefore, a favorite hunting


ground of mineral collectors. It is to be noted also that
each variety of magma (or rock) is characterized by
special mineral combinations in its pegmatite dikes, and
this applies not only to the ordinary minerals which com-
pose the rocks and distinguish them from one another
but also to the accessory ones as well.

Thus the mineral groups found in the pegmatites
associated with ordinary granites are quite different from
those with the nephelite syenites, as these in turn differ
from those of the gabbros.

Origin of the Pegmatites. The simultaneous method of
crystallization, the arrangements along the walls of the
dike, the variability in the proportions of the component
minerals and their frequent huge size all show that con-
ditions, different from those which attended the solidifica-
tion of the main rock masses, prevailed during the forma-
tion of the pegmatites. The presence of hydroxyl, fluorine,
boron, etc., also shows that mineralizing agents were
abundantly present. Bearing these facts, and those of
the geologic mode of occurrence, in mind, we can present
to ourselves a view of their origin which would be some-
what as follows:

When a body of igneous magma, such as will form a
stock or batholith, comes to rest in place it will commence
to cool. This will naturally take place first in the upper
and outer portions and here will begin the solidifying of
the mass by crystallization. As it becomes solid it breaks
up into jointed masses by contraction. The weight of
these masses, aided by the rock pressures from above upon
the still liquid material below, tends to force the latter
upward into the fissures in the solidified part and into
those of the surrounding rocks and produce dikes. If
differentiation is taking place and there is a concentration
of the iron, magnesia and lime towards the outer border,
as explained in previous sections, these dikes will be
complementary and we will find aplites (and felsites)
more commonly in the central mass, and the corresponding


basaltic lamprophyres more commonly in the outer portion
and in the surrounding rocks.

But as the process of crystallization goes on, the volatile
substances in the magma, and especially water in great
quantities, beyond what is retained by such minerals as
use them in their chemical composition, will be excluded,
and more and more as the gases accumulate they must
find their way outward. Thus they will tend to force
their way upward along the fissures in the solidified parts
above and at the sides. These cracks will therefore be-
come channel ways, not only for the still unconsolidated
magma as mentioned above (whether differentiated or not),
but also for the vapors which will collect in them and in
those of the immediately surrounding rock mantle under
pressures, which must often be enormous, until event-
ually they escape. It is evident from this that the
ascending magmas in the fissures will at various places
become supercharged with these vapors far beyond what
obtains in the normal rock. Now, both on experimental
grounds and what is observed in nature, it may be re-
garded as almost certain that no sharp line can be drawn
between igneous fusions of silicates (molten silicate mag-
mas) containing water under pressure and hot water solu-
tions. It appears that under pressure water will mix in
all proportions with magma so that at one end are molten
fusions, at the other hot solutions.*

At 360 degrees water reaches its critical point, that is,
heated to this degree, or above, its vapor cannot be
turned back into liquid by mere pressure, however great
this may be. At this temperature its expansive force is
almost 3000 pounds to the square inch, which would
require a vertical height of about 2500 feet of granite rock
to contain it. Above this temperature sufficient pressures
cause it to contract rapidly and it may even occupy less

* A good illustration of this is seen in a solution of thallium
silver nitrate which boils down, losing water, until a clear molten
fluid of the double salt remains which is anhydrous.


volume than it would in the liquid state (see p. 15).
The temperatures obtaining in molten rock magmas are
far above the critical temperature of water and it must
therefore be in the gaseous condition, though under the
enormous pressures obtaining under thousands of feet
or even several miles vertical of overlying rock, it may
well be much denser than water at the surface. Although
it has not yet been shown, so far as the writer knows,
that water in this state holds substances in solution just
as though it were a liquid, we can well imagine that at
high temperatures aided by the fluorine and other active
substances so commonly with it, its solvent action must
be enormously increased, especially its ability to dissolve

Under such conditions it is easy to see that the minerals
would crystallize quite differently from those in the
normal rock; in some places the magma would be in
excess and the results would more nearly approximate
those obtaining in the main rock; with diminished water
the dike might pass into an ordinary aplite or felsite
phase; with increased amounts, in another place, it
might pass from the state of a magma into an aqueous
solution and here would be favorable conditions for
crystallization on a large scale, for growth outward from
the walls and for the segregation of the rarer elements.
Finally passing onward the solution phase might become
more pronounced, only silica would be carried and the
dike turn into a quartz vein. Thus, as the degree of
differentiation of the magma and the proportion of
magma to water vary, we can see how dikes of ordinary
rock, of variable pegmatites and quartz veins may be
formed, which show in places very clearly their genetic
relationships. Also the slow cooling that would occur in
great masses of heated rock enclosing the fissure would
be favorable for the production of large crystals.

Contact Metamorphism. This term is applied to the
changes which are caused by a body of magma coming in


contact with other rocks already formed. The word
metamorphism, from the Greek, means a change of form
or body and is applied to those results, induced by a
variety of factors, by which rocks are recrystallized with
the formation of new minerals and textures. General
or regional metamorphism by which rocks are changed
over wide areas through various geological agencies is
considered in a later chapter; here only the results caused
by igneous magmas are treated. In several ways the
results of the two are alike and they often merge into one
another but in contact metamorphism the extent of the
masses involved is, in general, so much less than in regional
metamorphism, that from the standpoint of general geo-
logy, it is of much less importance. In respect to petro-
logy and to practical field work, however, it is a matter
of individual interest and great consequence and it is
therefore given separate treatment in this place.

The effects of the contact of a body of magma with
other rocks is seen in two ways: in one a change from its
general normal character is commonly observed in the
igneous rock itself and this is termed the endomorphic
effect; in the other, changes in the rocks with which it has
come in contact are seen and this is called the exomorphic
effect. We will consider the former one first.

Endomorphic Effects. It has been stated in a previous
section that a change in the mineralogical composition of
an intrusive rock body is not infrequently observed along
the contact, producing a border zone or facies. This is
due to a change in the chemical composition of the magma,
caused by differentiation, and has been fully discussed.
But at times also, even when this process has not occurred,
more or less of a change in the minerals of the igneous
rock may be seen directly at the contact or as one
approaches it, In this case it is due to the presence of
mineralizing vapors which, as previously described, tend
to be excluded as the mass cools and crystallizes and
to escape to the margin and into surrounding rocks.


Through their influence minerals are formed which do
not generally occur in the main part of the mass and which
are those which have been described as characteristic of
the pegmatite dikes. In granites the most characteristic
is perhaps tourmaline, whose presence is indicative of
boron, hydroxyl and fluorine. It is apt to take the place
of the biotite in the main rock and its occurrence as a
regular component of the granite should always lead to a
suspicion of approach to the contact, though it is also
found in the neighborhood of fissures which have served
as the conduit for pneumatolytic exhalations.

A variety of the granite of the Black Hills from Harney's
Peak illustrates this phase; in addition to the usual
quartz and feldspar, the rock contains black tourmaline,
abundant and well crystallized muscovite, green beryl
and red garnets: such minerals recall the associations
seen in pegmatites.

It may even happen that the accumulation of mineral-
izing vapors is so great at the outer margin before crystal-
lization begins that the conditions are favorable there for
the formation of a true pegmatite zone. The writer has
observed a granite stock in the White Mountains
enwrapped by a mantle of pegmatite; the large plates of
muscovite are set perpendicular to the contact and the
mixture is much enriched in quartz. Similar examples
are known from other localities, and in Pelham, Mass., a
pegmatite mantle partly enfolds a mass of peridotite in
the gneiss. Phenomena of the character described above
are most noticeable about the larger intrusions, such as
batholiths, stocks, etc.; in dikes, sheets and minor
intrusions they are not so conspicuous or are entirely

A much more common endomorphic contact effect is a
change in texture and this is independent of any change in
mineral composition, in fact, is largely observed where the
mineral composition remains constant. The most usual
feature of this kind is a change in the average size of grain


in the rock which grows smaller as the contact is
approached. The rock indeed may become exceedingly
dense at the contact and thus for instance a granite whose
average grain is of the size of coarse shot may turn into
a compact homogeneous appearing felsite. This is of
course due to a more rapid and general crystallization
produced by the chilling effect of the contact wall. In-
stances are even known where the cooling caused by the
cold rocks, with which the magma came in contact, was
so rapid that solidification took place at the margin before
crystallization could begin, with the production of a thin
selvage sheet of glass. Such instances are most liable to
occur in narrow dikes, in which the cooling of the con-
tiguous rocks is most strongly felt.

In other cases this denser contact facies may contain
larger distinct crystals or phenocrysts and thus be a
porphyry while the main mass is of even-granular texture.
The phenocrysts may be anterior in origin to the time
when the magma came to rest; in the main rock body they
may be of the same size as the rest of the later rock grains
but at the contact their contrast with the later dense
material produces a porphyry. On the other hand, it
has been observed that in many intruded masses of
porphyry occurring in dikes and sheets the phenocrysts
may be entirely absent at the contact margin, and in such
rock bodies they have been formed after the period of
intrusion, since if they had been brought up in the ascend-
ing magma they would be found at the contact as well as
in the interior of the mass.

The cases treated above are sufficient to illustrate the
chief endomorphic effects of contact metamorphism in
igneous rocks.

Exomorphic Effects of Contact Metamorphism in General.
The effect of the heat and vapors given off by an intruded
mass of magma upon the surrounding rocks with which
it is in contact varies with a number of factors. For one
thing it naturally varies with the size of the intruded


mass; it also varies with the nature of the vapors which
are given off, as described under pegmatite formation.
Another factor is the nature of the rock that is being
affected, some kinds being more susceptible than others,
and it also depends on the attitude of these rocks, that
is, in the case of sedimentary beds, on the position of the
planes of stratification toward the igneous mass. All of
these are important features and each deserves separate
treatment in order that the subject may be fully under-
stood. In general it may be said that the most noticeable
field evidence of the exterior effect is a baking, hardening
or toughening of the surrounding rocks. It not uncom-
monly happens as a result of this process that they
resist erosion better than the intruded mass or the un-
changed enveloping rocks and thus give rise to distinct
projecting topographic forms. This is admirably illus-
trated in the Crazy Mountains of Montana, where the
resistant rocks of the contact zone give rise to a series of
high ridges and peaks which encircle a more eroded mass
of intruded igneous rock and rise sharply from a sloping
plain of soft unchanged shales and sandstones. In the
case of a dike it may thus occur that the dike and the
surrounding beds are lowered more rapidly by erosion,
while the contact walls on either side are left projecting
as two parallel ridges.

The mineralogical effect is that, in general, where the
agencies have made themselves most strongly felt there
is a recrystallization of the rocks. This is produced by
an interchange of the molecules within short distances
whereby former chemical combinations are broken up and
new ones formed. In mass, that is, in sum total, the
chemical composition of the altered rock generally re-
mains the same, except that volatile compounds, water,
carbon dioxide, organic matter, etc., are driven out, and
in some cases, volatile components, fluorine, boron, etc.,
may be added by the mineralizing vapors from the
igneous mass.


Modes of Occurrence. The widest and most pronounced
contact zones as a rule are noticed about the great intrusive
stocks and batholiths. This is most natural, since the
vast size of the igneous body supplies heat and vapors for
a great length of time. Around them contact zones a
mile and even more in breadth have been observed in
many places. Next to them perhaps the most striking
are seen about old volcanic necks. The breadth and
intensity here often seem disproportionate to the size of
the igneous mass but this is to be explained by the fact
that the necks represent conduits through which fresh
supplies of highly heated matter have been successively
passing. This renewal of matter in the conduit may thus
induce a superadded effect. In such cases there may be
no endomorphic effect of cooling on texture as described
above; the conduit walls are so highly heated that the tex-
ture of the igneous rock remains the same up to the very
contact wall.

In the case of dikes considerable variations may be
seen; in small dikes the effect may be noticed only a few
inches or even less, while in large ones it may extend
many yards on either side. Again, some dikes have served
as conduits for magma passing up through them into
larger intrusions above, feeding sheets or laccoliths or
giving rise to extrusive outflows. About them the meta-
morphism will naturally be greater, other things being
equal, than where a fissure was filled by a single charge
of magma which immediately came to rest. For this
reason the metamorphism induced by intrusive sheets
and laccoliths is generally inconsiderable, since they also
represent a single charge of magma into the rocks about
them, which is not renewed. Immediately at the contact
and for a few feet or yards beyond, the rocks may be
altered but the effect soon dies out except in the cases of
very powerful sheets and large laccoliths. With extrusive
lava flows a small amount of baking or hardening of the
rocks and soils on which they rest is often seen.



Position of the Rocks. It is a common thing to observe
that the width of the contact zone varies considerably
from place to place about the intrusive mass. This may be
due to underground irregularities in the igneous rock body,
a wide extension of the zone pointing to a corresponding

extension of the mass be-
low, as illustrated in Figs.
7 1 and 72. In the stratified
rocks the position or atti-
tude of the planes of strat-
ification to the intrusive
mass is also important.
Thus in Fig. 73 the beds at
B dipping into the mass
of granite C tend to have
their bedding planes and joints opened by the upward
movement of the magma, and their position is such as to
facilitate the entrance and wide extension of the vapors

Fig. 71. Ground Plan or Map of an
Intruded Stock and its Contact Zone.

Fig. 73. Vertical Section along
Line A B in Fig. 71.

Fig. 73. Section showing width

of Contact Zone depending

on Position of Beds.

and heat, thus producing a broad contact zone. On the
side A on the contrary the conditions are just the reverse
of this and a much narrower contact zone is the result.

Effect on Different Kinds of Rocks. In a general way
the most notable effects are produced on sedimentary
rocks and these for purposes of consideration may be
divided into the sandstones, limestones, clay shales or
slates and their various admixtures. On pure quartz
sandstones the effect is relatively slight though for short


distances and in the near zone of most intensive action
they are sometimes found hardened into quartzites.
Pure limestones are recrystallized and changed into
marble and not infrequently in large masses and extending
over considerable distances. The most notable effects
are produced when the limestones are impure, containing
quartz sand and clay mixed with them. In this case the
Si0 2 drives C02 out and carbonates are changed to
silicates. If the limestone is a dolomite containing
magnesia the results are more complex. Some of the
simpler of these changes may be reaolily shown by equa-

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 15 of 35)